Available online at www.sciencedirect.com Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89 – 129 www.elsevier.com/locate/palaeo Glacio-epochs and the supercontinent cycle after ∼ 3.0 Ga: Tectonic boundary conditions for glaciation Nick Eyles Department of Geology, University of Toronto at Scarborough, 1265 Military Trail, Scarborough ON, Canada M1C 1A4 Received 11 May 2006; received in revised form 5 September 2007; accepted 26 September 2007 Abstract Tectonic influences on long-term climate change are of considerable current interest and debate. This paper reviews the relationship between multi-million year periods of glaciation (glacio-epochs) over the last 3 Ga of Earth history and phases of supercontinent breakup and assembly. A preferred but not exclusive relationship is evident between glacio-epochs and their mostly glacially influenced marine record, with rifting. The earliest known glaciation (mid Archean ∼ 2.9 Ga) is recorded in the marine Mozaan Group of South Africa deposited along the passive margin of the Kapvaal Craton then part of the early continent Ur. The Paleoproterozoic glacio-epoch, exemplified by the Huronian Supergroup of Ontario, Canada (∼ 2.4 Ga) and strata in northern Europe and the U.S., is associated with rifting of Kenorland. A long Paleo-Mesoproterozoic non-glacial interval (c. 2.3 Ga to 750 Ma?) coincides with continental collisions and high standing Himalayan-scale orogenic belts marking the suturing of supercontinents Nena-Columbia and Rodinia. A near absence of glacial deposits other than at 1.8 Ga, may reflect lack of preservation. The extensive and prolonged Neoproterozoic glacio-epoch records either diachronous glaciations or discrete pulses of cooling between ∼ 750 and ∼ 580 Ma, and is overwhelmingly recorded by substantial thicknesses (1 km+) of glacially influenced marine strata stored in rift basins. These formed on the mid to low latitude (b 30°) oceanic margins of western (Panthalassa: Australia, China, Western North America) and eastern (Iapetus: Northwest Europe) margins of a disintegrating Rodinia. The youngest glacially influenced deposits formed about 580 Ma along the compressional Cadomian Belt exterior to Rodinia (Gaskiers Formation) possibly correlative with the classic passive margin Marinoan deposits of South Australia. A short-lived (1 to 15 Ma?) Early Paleozoic ice sheet about 440 Ma grew over highlands on the polar North Africa margin of Gondwana possibly likely triggered by uplift at high paleolatitudes as large terranes (e.g., Meguma, Avalonia) rifted away from North Africa. Incised valleys, coarse glacial fills and thick (1 km +) ‘postglacial’ shales suggest a continuing tectonic influence. Devonian cooling across the Frasnian-Famennian boundary (c. 376 Ma) is recorded by local ice covers in Brazil and Bolivia and is linked to elevated topography and enhanced erosion of continental crust. The Late Paleozoic glacio-epoch (∼ 350 and 250 Ma) coincides with a high paleolatitude positioning of Gondwana and the growth of high standing topography when Gondwana collided with Laurasia to create Pangea. Breakup after 180 Ma moved landmasses into higher northerly latitudes and was the backdrop to global cooling of the Cenozoic glacio-epoch that commenced after the Paleocene–Eocene Thermal Maximum (b 55 Ma). Earliest Antarctic ice at ∼ 40 Ma most likely nucleated on the high shoulders of the Transantarctic Rift coeval with opening of Drake Passage, and coincides with the earliest ice rafting in the Arctic Basin at 43 Ma, followed by another pulse at 34 Ma. Accelerated glacierization in both hemispheres occurred at ∼ 14 Ma during the middle Miocene Transition but Milankovitch-forced continental-scale ice sheets did not nucleate in the northern hemisphere until after 3.5 Ma on uplifted borderlands along North Atlantic passive margins. E-mail address: [email protected]. 0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2007.09.021 90 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 A preferred association of the deposits of Proterozoic and Phanerozoic glacio-epochs with rift basins reflects either a causal link between rift-related uplift and regional cooling, or simply enhanced preservation of glacial sediments. Glacial deposits are poorly preserved in areas of compressional tectonics. © 2007 Elsevier B.V. All rights reserved. Keywords: Earth's glacial record; Archean; Proterozoic and Phanerozoic glacio-epochs; Rifting; Tectonically created topography Contents 1. 2. Purpose of this paper: why does ice appear on planet Earth?. . . . . . . . . . . . . . . . . . . . . . . Glacio-epochs defined. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Structure of this paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Archean glacio-epochs (c. 4–2.5 Ga) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. A sparse record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Paleoproterozoic glacio-epoch (c. 2.4 Ga). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Rift related ice covers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Paleoproterozoic to Mesoproterozoic non-glacial interval (c. 2.3–0.75 Ga). . . . . . . . . . . . . . . . 5.1. Cycles of continental assembly and breakup but no glaciation? . . . . . . . . . . . . . . . . . . 5.2. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Neoproterozoic glacio-epoch (0.75 Ga to 545 Ma) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Stratigraphic record of Neoproterozoic cold climates: the ‘glaciated rift’ debrite–turbidite association 6.2. Neoproterozoic glacioeustasy or tectonics? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Diachronous response to regional tectonics or globally synchronous megafreeze events? . . . . . 6.4. An active hydrosphere; climate models and sedimentology . . . . . . . . . . . . . . . . . . . . 6.5. Carbon isotope excursions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Lower Paleozoic Saharan glacio-epoch (c. 440 Ma) . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Long lasting or short lived? And just how big? . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Geomorphic, sedimentologic and stratigraphic evidence of glaciation in North Africa . . . . . . 7.3. Sea level fluctuations; are they exclusively glacioeustatic? . . . . . . . . . . . . . . . . . . . . 7.4. Tectonic influences on glaciation in North Africa; the importance of black shales . . . . . . . . 7.5. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Late Devonian ice c. 374 Ma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1. Brief cooling triggered by continental collision . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Late Paleozoic Gondwanan glacio-epoch (c. 350–250 Ma). . . . . . . . . . . . . . . . . . . . . . . . 9.1. Long lived cooling triggered by continental collision . . . . . . . . . . . . . . . . . . . . . . . 9.2. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. Mesozoic non-glacial interval (c. 250 to 55 Ma); a role for small ice masses? . . . . . . . . . . . . . . Ma) . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Cenozoic glacio-epoch (b55 (b< 55 Ma) 11.1. Coupled tectono-climate models: the effects of plate collision and dispersal . . . . . . . . . . . 11.2. Glaciation at the tops of the world: Antarctic and Arctic cooling c. 40 Ma. . . . . . . . . . . . 11.3. Circum North Atlantic glaciation: Milankovitch-driven ice sheets . . . . . . . . . . . . . . . . 11.4. Pacific Northwest glaciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1. Assembly related glacio-epochs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2. Break up related glacio-epochs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3. Rifting and glaciation: causal link or simply preservational bias? . . . . . . . . . . . . . . . . 13. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 93 93 93 93 95 95 95 97 97 97 97 98 98 99 100 101 102 103 103 103 104 105 105 107 107 107 108 108 109 109 109 109 110 111 111 112 112 112 113 113 115 116 116 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 1. Purpose of this paper: why does ice appear on planet Earth? Ongoing climate change is at the forefront of current scientific and public debate. Turning to the remote geologic past, considerable interest is focussed on the record of cold climates and much-debated Neoproterozoic global freeze events (‘Snowball Earth’; Hoffman and Schrag, 2002). These putative climatic catastrophes are regarded by some as triggers for early metazoan life at around 580 Ma before present (abbreviated to Ma throughout); similarly, more recent Phanerozoic cold periods are credited with causing mass extinctions (Stanley, 1998; Brenchley et al., 2003). Against this backdrop of widespread interest in the causes and varied effects of cold climates, this paper contributes to discussion by reviewing the geotectonic setting of Archean, Proterozoic and Phanerozoic glaciations (Fig. 1). The objective is to see whether there are any broad generalizations that can be made about the tectonic boundary conditions required to grow extensive ice covers. 91 Assessment of the geodynamic setting of ancient glaciations was last attempted more than a decade ago (Eyles, 1993) and many data have emerged since to warrant a new look especially in the light of new knowledge on the timing of supercontinent assembly and breakup (e.g., Rogers and Santosh, 2004). Such a review is timely because longstanding (Le Roex, 1941) but formerly rejected (Schermerhorn, 1974) notions of globally extensive glaciations have re-emerged (Hoffman et al., 1998; Williams and Schmidt, 2004) supported by a wealth of geochemical and geophysical data. According to some, Precambrian glaciations are seen as being predominantly ‘near equatorial’ with radically different causes compared to those of the Phanerozoic which are regarded as exclusively mid to high latitude phenomena (Hoffman and Schrag, 2000, 2002, p.129; Evans, 2003a). According to another school of thought the broad timing of glaciations on planet Earth over the last 1 Ga can be linked primarily to extraterrestrial factors such as the periodic variability of the cosmic ray flux (CRF) and changing solar activity (Shaviv, 2003; Marcos and Marcos, 2004; Veizer, 2005; Fig. 2). In fact, Fig. 1. Schematic representation of glacio-epochs in Earth history and their relationship to phases of supercontinent assembly and break up. The timing of supercontinent growth and rifting is after Condie (2002a), Rogers and Santosh (2004). Earth's glacial record is after Crowell (1999). It remains unclear whether the passive-margin related Kaapvaal glaciation represents a glacio-epoch or a short-lived event. The timing and number of glacial events in the Neoproterozoic (3a, b, c) is uncertain (see text). The anomaly of the lack of a glacial record during the Paleo-Mesoproterozoic growth of Nena-Columbia is clearly evident though Williams (2005) reports evidence of glaciation at 1.8 Ga. The sedimentary record of most glacioepochs occurs in the geodynamic context of intracratonic rifting, crustal extension and the formation of passive margins. Paleoproterozoic (c.2.5 Ga) and Neoproterozoic glacio-epochs (c. 750–580 Ma) occurred during the breakup of Kenorland and Rodinia respectively. It is also possible that extension along high latitude continental margins and consequent uplift also played a role in triggering Ordovician glaciation at c. 440 Ma (when terranes rifted off Gondwana; see text). Most of the Gondwanan glacio-epoch deposits are stored in rift basins even though glaciation was initiated during the compressional growth phases of Gondwana. Tectonics played a major role in Cenozoic cooling after 55 Ma culminating in continental scale Northern hemisphere ice sheets only after 3.5 Ma. 92 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 Fig. 2. A: Glacio-epochs of the last billion years and their relationship to supercontinent cycle. Numbering of each glacio-epoch follows that of Fig. 1. The model of three distinct global pulses of glaciation in the Neoproterozoic glacio-epoch may be simplistic (see Section 6.3). B: Variation in 13C over last 1.5 Ga years (modified from Hoffman and Schrag, 2000, 2002; Lindsay and Brasier, 2002 with additional data). Note largest excursions occur in the Neoproterozoic are coincident with breakup of Rodinia. There is no agreement as to their cause. Prominent spikes in 13C during the midProterozoic non-glacial interval are interpreted as the consequence of rapid burial of carbon (Bartley et al., 2001); those of the Neoproterozoic are seen as driven by global glaciation and suppression of biologic activity (Hoffman and Schrag, 1999) or possible diagenesis in rift basins (Foden et al., 2001; Knauth 2005) divorced from the global ocean (e.g. Gammon et al., 2005). C: Variation in Cosmic Ray Flux (curve) a timing of Earth's periodic crossings of spiral arms of the Milky Way (after Shaviv, 2003; Marcos and Marcos, 2004). In this model, long episodes of glaciation occur at times of high CRF. Charged particles enhance cloud covers thereby increasing the planetary albedo and promoting overall climate cooling on geologic timescales (Veizer, 2005). D: Estimated global temperature trends (Veizer, 2005). E: Variation in atmospheric carbon dioxide (dashed line after Rothman, 2002, solid line after Berner, 2003; Veizer, 2005 with additional data from Sheldon, 2006). the relationship of CRF flux with terrestrial record of glaciation (and non-glacial periods) is not precise (Fig. 2) and additional causes such paleogeography, atmospheric composition and tectonics clearly play a role (Hay et al., 2002). As stressed by Shaviv (2003) and Evans (2003a) the key question in paleoclimatology is N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 93 how to identify each factor and its relative importance through time. The present paper assembles tectonic, stratigraphic, sedimentologic, paleontologic and other data appertaining to Earth's record of cold climates over the last 3 Ga. As we shall show, plate tectonic processes have played a role in the initiation of some but not all. During certain times, tectonic activity in both extensional and compressional settings created areas of high topography (‘tectonotopography’) on which ice could nucleate and expand once other thresholds were crossed (Crowell, 1999; Shaviv and Veizer, 2003; Marcos and Marcos, 2004; Veizer, 2005; Hay, 1996). lengthy episodes of glaciation extending (but not necessarily continuous) over millions of years are correspondingly termed glacio-epochs. Other terms that have been used are ‘glacio-eras’ (Chumakov, 1985), ‘glacial eras’ (Hambrey and Harland, 1985; Chumakov, 1985), ‘icehouses’ (Fischer, 1986; Frakes et al., 1992), ‘glacioepochs’ or ‘glacioperiods’ (Chumakov, 1978, 1985) and ‘ice age epochs’ (Shaviv, 2003). Glacio-epochs appear to be an intermittent feature of Earth's climate system as argued by Crowell (1999; Fig. 1). This conclusion may change in the future in the light of research focussed on apparently long non-glacial episodes within the Proterozoic and Phanerozoic. 2. Glacio-epochs defined 2.1. Structure of this paper The overall timing and number of major glacial episodes in Earth history is known within broad bounds (Fig. 1) but there is no consensus regarding their causes. The words of Broecker (2000, p. 140) that ‘we do not understand how it is that Earth's climate is capable of achieving its glacial state’ were made in regard to short-lived (and relatively well known) Pleistocene glaciations but apply equally to the long 3 billion year pre-Pleistocene record. Despite the sparse overall sedimentary record of glaciations older than 750 Ma (Figs. 1 and 2) it is difficult to disagree with Crowell's suggestion (1999, p.2) that glaciers have always been present somewhere on planet Earth even at times of marked global warmth. Today, for example, ice survives in the tropics in many areas of high geodynamically-produced topography (e.g., Ehlers and Gibbard, 2004). The sedimentary deposits of many ancient ice masses were no doubt reworked, disguised or lost entirely and they remain (so far) invisible to sedimentologists. As will be shown, this is especially the case where glaciers occur in collisional tectonic settings. In contrast, the larger ice bodies that formed during long lasting episodes of extended glaciation were able to leave a sedimentary record because their margins reached sea level. These extended episodes of glaciation involve long-term climate change in response to processes such as plate tectonically altered paleogeographies (termed ‘tectonic scale climate change’; Ruddiman, 2001). These can be contrasted with the much higher frequency Milankovitch-driven and/or solar driven climate cycles that are bundled within them (relatively short lived Ice Ages or glacial/ interglacial stages). The term ‘epoch’ is widely used to informally designate an unspecified length of geologic time (Neuendorf et al., 2005) and in this paper such This paper commences with the tectonic setting of the poorly known Archean glacial record and then proceeds sequentially to examine Proterozoic and Phanerozoic glacio-epochs ending with the more familiar Cenozoic (Figs. 1 and 2). This review notes an essential continuity between all glacio-epochs in that glaciation is preferentially associated with plate tectonic processes that act to elevate areas of continental crust. In this light, the paper discusses the efficiency of extensional vs., collisional plate tectonic settings in generating tectonotopography and ice covers, and in preserving a sedimentary record of cold climates. 3. Archean glacio-epochs (c. 4–2.5 Ga) 3.1. A sparse record Geology is not much help in resolving Archean climates because the sedimentary rock record is so sparse. Local glaciation is evident at about 2.9 Ga and possibly again at 2.8 Ga in southern Africa. Both these deposits may represent ephemeral regional glaciations and may not strictly qualify as glacio-epochs. The results of climate modelling are contradictory. On the one hand is the notion that a ‘hot’ greenhouse' climate prevailed for much of Archean and Proterozoic time and prohibited widespread glaciation (e.g., Kasting, 1987; Kramers, 2002). Knauth and Lowe (2003) examined the oxygen isotope record preserved in Archean cherts and argued that climates were as much as 40 °C warmer at 3.5 Ga compared to the long term Phanerozoic norm as a consequence of a CO2 enriched atmosphere with concentrations 10,000 times greater than at present (Kasting (1993). This model was questioned by Shields and Veizer (2002) because of a lack of geologic evidence 94 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 and Veizer (2005) later proposed that global warming was facilitated not by CO2 but by a muted Cosmic Ray Flux (CRF) creating a reduction in cloud cover and a lowered planetary albedo. Others maintain that the Archean was much colder as a consequence of a ‘faint young sun’ with an output 25–30% lower than that of today. Caldeira and Kasting (1992) suggested that runaway glaciation resulting from condensation of CO2 clouds at low temperatures, was only prevented by greenhouse gases such as NH3 or CH4. Another ‘cold Archean’ model is that of Zahnle and Sleep (2002) who modeled CO2 fluxes in and out of the mantle. The rate of outgassing in the Archean was perhaps twice that of today which would result in CO2 rich atmospheres that would offset the effects of a fainter young sun and the tendency for irreversible glaciation. However, as Zahnle and Sleep (2002) indicate, rates of Archean subduction (and thus consumption of carbonate) were also substantially greater with yet additional CO2 consumed by weathering of the debris thrown out by massive meteorite impacts. They concluded that very cold, globally freezing surface temperatures could be expected for much of the Archean and early Proterozoic until about 2 Ga. Other authors too, have suggested the existence of ice-covered Archean oceans only occasionally melted by large bolide impacts; global freeze-thaw cycles are seen as a key influence on the synthesis of amino acids (Bada et al., 1994). Most recently, Kasting and Howard (2006) argue for a more temperate ‘warm’ early Earth somewhere between the hot and cold models reviewed above, suggesting that oxygen isotope data used to reconstruct surface temperatures instead reflect changes in the composition of seawater with time. Whilst the nature of Earth's early climates is ambiguous, the onset of Earth's glacial geologic record shortly after 3 Ga is broadly coincident with the first undisputed evidence of microbial life (Noffke et al., 2006) and the development of oxygenic photosynthesis (Ono et al., 2006). By 3 Ga, an early continent (Ur) is though to have included parts of the present day Kaapvaal, western Dharwar, Singhbhum and Pilbara cratons along with smaller crustal blocks now part of East Antarctica (Rogers and Santosh, 2004). Glaciation is recorded in the late Archean (∼2.9 Ga) Mozaan Group of South Africa (Young et al., 1998) on the passive margin of Ur (Fig. 1). Diamictites (up to 80 m thick) are present in the Odwaleni Formation near the top of the 5000 m thick dominantly marine Mozaan Group deposited on the southeastern passive margin of the Kapvaaal Craton. Diamictites1 containing striated clasts (von Brunn and Gold, 1993) are associated with turbidites, pebble conglomerates, volcanics and iron formations likely indicating a submarine setting where glaciclastic sediment was reworked downslope as debrites (von Brunn and Gold, 1993). Modie (2002) interpreted Archean diamictites and lonestones in shales within the 2.782 Ga Nnywane Formation of Botswana as glacially derived, but this is questioned by Evans (2003a,b, p. 370). If a ‘cold Archean Earth’ model is a reality, then glacially influenced strata should be widespread and preserved elsewhere other than on the Kaapvaal Craton and Botswana but this does not appear to be the case. The question then is where are they most likely to have been preserved? Glacial strata correlative with the Mozaan group of South Africa should be found in East Antarctica given that fragments of the Kaapvaal Craton were rifted and dispersed during the breakup of Gondwana (e.g., Wareham et al., 1998). Further clues are provided by recent studies of the Archean Slave Province in the 1 It is necessary here to define several terms used throughout this paper. The term ‘glaciclastic’ refers to sediment produced by erosion at the base of glaciers and ice sheets. Such sediment is uniquely fingerprinted by the occurrence of striated and glacially shaped ‘bullet’ clasts and far travelled extrabasinal lithologies. The greatest yields of glaciclastic sediment occur from temperate wet-based glaciers able to slide over (and thus abrade) their beds. Glaciers in areas of permafrost (polar glaciers) are frozen to their beds and are much less efficient producers unless parts of their mass are lubricated by wet sediment promoting fast ice flow (‘ice streams’). ‘Glaciomarine’ refers to processes or deposits in a very narrow zone in direct contact with the margin of a glacier (or ice sheet) that reaches sea level and which is affected by meltwaters issuing from the ice front. The term ‘ice contact marine’ or ‘proglacial marine’ are alternate terms with the same meaning. This environment is recorded by large, complexly structured ice contact deposits (‘morainal banks’) dominated by the conglomeratic deposits of energetic subaqueous meltwater fans. The term ‘glacially influenced’ was introduced by Eyles et al. (1985) as an umbrella term for deposits accumulating in more remote parts of a basin beyond the direct reach of glaciers (i.e., distal to the glaciomarine realm) but which nonetheless are composed of glaciclastic sediment reworked by currents or gravity. Areas affected by glacioisostatic or glacioeustatic sea level changes (or scour by drifting ice bergs) can also be regarded as glacially influenced. Within the subaqueous glacial realm, isolated clasts in fine-grained sediment (lonestones; descriptive term) are dropped by ice (dropstones; genetic term) but discrimination between dropstones left by glacial icebergs from other types of ice (sea ice, river ice) and the climatic ramifications therein is problematic. Many lonestones are not dropstones. Finally, the term diamict(ite) is a non-genetic term referring to any poorly sorted deposits regardless of depositional environment. A till(ite) is a diamict(ite) deposited directly from ice either terrestrially on land or in the glaciomarine realm. A debrite is the deposit of a debris flow. Benn and Evans (1998) and Eyles and Januszczak (2004a,b) review the processes and environments that result in the formation of till and diamict. N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 Northwest Territories in Canada (Polat and Kerrich, 2001; Bleeker, 2002). The Slave contains prominent basalt and turbidite dominated greenstone belt emplaced at about 2.6 Ga as autochthonous bodies on incipient continental blocks during massive rifting events. Bleeker (2002) argued that voluminous eruptions of basalt on rifted continental crust created unstable conditions because basalt has a density of 2.9–3.1 g/cm3 whereas the underlying gneissic and granitic crust is lighter (about 2.7 g/cm 3 ). This resulting Rayleigh–Taylor instability ultimately leads to ballooning of the lighter gneissic crust and the formation of large high standing basement domes up to 60 km across. Uplift is coeval with the appearance of turbidites and conglomerates in nearby basins deposited by sediment gravity flows (such as the Burwash Formation of the Slave Province) and are a marked feature of other Archean cratons (e.g., Zimbabwe, Yilgarn). In Canada, these poorly sorted infills are known as ‘Temiskaming-type’ deposits after the type locality (named the Porcupine Group) in the Abitibi Greenstone Belt of the Superior Province in Northern Ontario. It can be speculated that a record of Archean glacials could be preserved in these deposits as a consequence of ice masses growing on uplifting granite domes. Still older glacial deposits may be yet identified recording the earlier assembly of Ur. Ono et al. (2006) and Kasting and Howard (2006) suggest a short-lived photosynthetic oxidation event at 2.9 Ga destabilized the methane-rich Archean atmosphere triggering the Mozaan glaciation. As Kasting and Howard (2006) discuss (p. 1736) the presence of several small Archean glaciations is more easily reconciled with moderate Earth surface temperatures rather than the extremely hot conditions favoured by some workers. 3.2. Summary There are fundamental uncertainties regarding Archean climates because of the dearth of sedimentary deposits and climate modelling yields very different, opposed perspectives. Glaciation is recorded at about 2.9 and 2.8 Ga but is restricted to southern Africa. The geodynamic setting indicates a passive margin setting. A systematic search is needed for new deposits in other basins. 4. Paleoproterozoic glacio-epoch (c. 2.4 Ga) 4.1. Rift related ice covers The best-known earliest glacio-epoch is recorded at about 2.4 Ga within the Paleoproterozoic Huronian 95 Supergroup in Ontario, Canada (part of the Southern Province of Laurentia). Less voluminous and much less well-exposed strata are preserved within the Baltic Shield (in the Finnish sector of the Karelia craton), in South Africa (Kalahari craton) and in North America such as the Snowy Pass Supergroup of Wyoming (Marmo and Ojakangas, 1984; Kohonen and Marmo, 1992; see Eyles, 1993, pp. 59–67; Pesonen et al., 2003; Bekker et al., 2005). Other diamictites of glacial marine origin (the Makganyene Formation) occur in the ∼2400 Ma Transvaal Supergroup in the Griqualand West Basin of South Africa (Polteau et al., 2006). A close geographic clustering of these locations certainly appears likely within what has been called Kenorland (Pesonen et al., 2003; Fig. 1). Kirschvink et al. (2000) and Melezhik et al. (2005a,b) argue that the Huronian and other deposits are a record of one or more ‘global glaciations’ (Polteau et al., 2006) that occurred in low to middle latitudes (Williams and Schmidt, 1997; Kirschvink et al., 2000; Evans, 2000). Geodynamically, Paleoproterozoic glaciation occurred in the context of early, plume-related rifting of Kenorland (Fig. 1) that eventually resulted in Laurentia separating from Baltica at about 2.1 Ga (Visser, 1981; Ojakangas, 1985, 1988; Kohonen and Marmo, 1992; Heaman, 1997; Ojakangas et al., 2001; Bekker et al., 2001; Pesonen et al., 2003; Bekker et al., 2006). The overall plate tectonic context is strikingly similar to many of the later Neoproterozic glaciations where deposits were preserved for the most part in rift basins during the breakup of Rodinia (Section 6.1). The Huronian was deposited after 2.45 and is at least 12 km thick with four distinct tectonostratigraphic successions (Elliot Lake, Hough Lake, Quirke Lake and Cobalt). The most well known is the Gowganda Formation (up to 1.7 km thick) that occurs at the base of the Cobalt Group. It is little deformed and present over a wide outcrop area of northeastern Ontario largely within the structural confines of the so-called Cobalt Embayment which was a failed rift arm extending into the southern margin of the Superior Province (‘Superia’; Stott, 1997; Calvert and Ludden, 1999; Young et al., 2001; Bleeker, 2002) then part of Kenorland. Diamictites and associated facies of the Gowganda Formation have a long history of being interpreted in classic ‘bed for bed style’ as exclusively climate driven. ‘Tillite’ beds were argued to record ice expansion and non-tillite facies as interglacials (Young, 1981) but a more complex glaciomarine setting was recognised by Young and Nesbitt (1985). Miall (1985) emphasized the contextual importance of subaqueous mass flow facies interbedded with diamictites and argued that they had all been deposited in a deep marine glacially influenced 96 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 basin. Mustard and Donaldson (1987) found additional evidence of a marine setting. Miall pointed out the difficulty of recognizing climatically driven glacial/advance retreat cycles in deposits where active tectonics condition changing water depths, the supply of sediment to the basin and the timing of mass flow. Significantly, Yeo (1981) and Young and Nesbitt (1985) suggested that uplift of rift shoulders adjacent to a source of moisture such as the opening rift system, was instrumental in promoting Huronian glaciation. Indeed, it is now agreed that the Huronian stratigraphy likely records several episodes of pulsatory rifting, uplift-generated cooling and subsidence along the southern continental margin of the Superior Province (Eyles, 1993; Young et al., 2001). Each succession oversteps the underlying succession (recording basin subsidence and expansion) and consists of ‘lower’ synrift submarine mass flow facies (glacially influenced diamictites, graded conglomerates), ‘middle’ thick turbidites and a coarser ‘upper’ stratigraphic cap of shallow marine to fluvial facies (Fig. 3). The tripartite tectonostratigraphic cycles of the Huronian are typical of marine rift basins where there is a close control on deposit type and geometry by repeated reactivation of basin boundary faults (e.g., Ravnas and Steel, 1998; Gawthorpe and Leeder, 2000; Hinderer and Einsele, 2002). Poorly sorted facies at the base of each cycle record renewed uplift of rift flanks and the shedding of coarse clastics from expanding ice covers into a deepening basin. Middle turbidites record maximum subsidence rates and thus minimum sedimentation rates. As the rate of subsidence decreases late in the rift cycle, so basin margin facies are able to prograde and produce a fluvial and shallow marine cap to each major tectonostratigraphic succession. Simple recognition of any short-lived glacial or longer-term eustatic signals is rendered unlikely (e.g., Miall, 2005; see Section 6.2). The key point here is that glaciers undoubtedly contributed to the sediment flux to the rifted basin but that ice contact strata were not widely preserved (Mustard and Donaldson, 1987). Kohonen and Marmo (1992) referred to Paleoproterozoic glacials of the Urkkavaara Formation in Finland, as ‘tectofacies’ because they accumulated in a failed arm where sediment influxes were controlled by episodes of faulting. In South Africa, banded iron formations occur below and well above diamictites of the Makganyene Formation. Diamictites are interbedded with and volcanic tuffs marking volcanic activity and the eruption of flood basalts during rifting (Polteau et al., 2006, p. 271). Young (2002) drew attention to the Fig. 3. Schematic glaciated rift basin during Rodinia breakup after 750 Ma (modified from Gawthorpe and Leeder, 2000). Primary glacial sediment is extensively reworked by mass flow processes and terrestrial glacial facies are seldom preserved. Sedimentation is markedly diachronous as a consequence of propagating faults and the non-synchronous formation and filling of different sub-basins. The fill of any one sub-basin comprises a tectonostratigraphic succession recording changing relationship between subsidence and sediment supply. Marked intrabasinal variability in the timing of rifting and the sedimentation response prohibits correlations of like facies (e.g., diamictites) and also wider extrapolation of age dates on any one stratigraphic horizon to other basins worldwide. N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 accumulation of banded iron formations (BIFs) with enclosed or at least semi-restricted rift basins separate from the global ocean. The formation and breakup of Kenorland accompanied marked changes in paleoatmospheric composition, distinct weathering styles, fluctuations in carbon isotopes and the accumulation of banded iron formations (e.g., the ‘Great Oxidation Event’ of Karhu and Holland, 1996) (see also Bekker et al., 2001, 2005; Lindsay and Brasier, 2002; Eriksson et al., 2005). It has been speculated that climatic cooling may have been initiated by chemical weathering of large exposures of continental crust (Knauth, 2005; Melezhik et al., 2005a, b). Kopp et al. (2005) and Kasting and Howard (2006) speculate that Archean and Paleoproterozic glaciations were triggered by rises in atmospheric oxygen concentration, counterbalancing the greenhouse warming of a methane-rich atmosphere. 4.2. Summary The well-developed relationship between Paleoproterozic rifting and glacial deposits suggests either a causal relationship between rift-related uplift and climatic cooling or selective preservation of glaciated rift deposits. Deposits are dominantly submarine debrites associated with thick turbidites. They are part of very thick marine tectono-stratigraphic successions, often associated with volcanics, recording the changing interplay of subsidence rates and sediment supply as rifting progresses. An association between glacials and banded iron formations may reflect deposition in semi-enclosed basins with incipient spreading centres. 5. Paleoproterozoic to Mesoproterozoic non-glacial interval (c. 2.3–0.75 Ga) 5.1. Cycles of continental assembly and breakup but no glaciation? Earth's known glacial record has a prominent gap between about 2.3 Ga and 750 Ma spanning the latter part of the Paleoproterozoic and most of the Mesoproterozoic (Eyles and Young, 1994; Brasier and Lindsay, 1998; Crowell, 1999; Kah et al., 1999; Fig. 1). Veizer (2005) proposed that the absence of glaciation could reflect diminished star formation in the Milky Way and a minimum in the Cosmic Ray Flux thereby producing a warmed planet (see also Shaviv, 2003; Marcos and Marcos, 2004). The absence of a glacial record may be more apparent than real given the recent discovery in the Kimberley district of Western Australia of 1.8 Ga old 97 glacially cut channels (Williams, 2005). Otherwise, the lack of any marked glaciation is anomalous because all other sedimentary environments found today on the Earth's surface are represented (Eriksson et al., 2005, p. 33) and geodynamic processes were no different from those of the Neoproterozoic and Phanerozoic. During this time, plate tectonics assembled several large continents such as Arctica, Nena and Columbia involving repeated supercontinent cycles of crustal rifting and collision (e.g., Starmer, 1996; Condie, 2002a,b; Rogers and Santosh, 2004). In nearly all continents, long midProterozoic orogenic belts encircle blocks of Archean crust and testify to protracted collisional events (e.g., Trans-Hudson, Capricorn, Trans North China, TransAmazonian, Yavapai/Mazatzal etc) during cratonization. Moreover, large landmasses such as Laurentia and Australia lay at high polar latitudes by 1.15 Ga (Pesonen et al., 2003). Throughout much of the Mesoproterozoic time interval, the necessary preconditions (tectonics, latitude) were in place for the formation of ice covers on tectonically elevated topography. The most conspicuous tectonic event capable of producing widespread ice covers was the assembly of Rodinia that commenced after 1.8 Ga. By 1.3 Ga this had built the largest orogenic belt known to date (Grenville Orogen) when a Himalayan-scale (glaciated?) topography 600 km wide and more than 10,000 km in length formed along the suture between early North America, South America (Amazonia) and Baltica (Karlstrom et al., 2001). Peneplanation of the mountains by c. 800 Ma produced large volumes of sediment that could have contributed to reducing global pCO2. Significantly, Sheldon (2006) reports a significant drop in CO2 sometime between 1.8 and 1.1 Ga. It may well be the case that glacial sediments survive as reworked deposits within highly metamorphosed successions requiring detailed sedimentological scrutiny. Rifting following the Grenville Orogeny occurs between about 1 Ga and 750 Ma but again, no glacial deposits are reported (e.g., Timmons et al., 2001; Brewer et al., 2002; Wingate et al., 2002). 5.2. Summary The absence of any extensive glacial record during the long Paleoproterozoic–Mesoproterozoic interval between about 2.3 Ga and 750 Ma represents a large gap in Earth's glacial history. Glaciation should have been a common phenomenon given the known formation of several large landmasses and the orographic effects of associated high standing orogens, but this does not appear to be the case other than briefly and locally in 98 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 Australia at about 1.8 Ga. Possibly the rock record has not been sufficiently well examined, deposits were extensively reworked and not preserved along active plate margins, or as yet unknown processes acted to suppress glaciation. 6. Neoproterozoic glacio-epoch (0.75 Ga to 545 Ma) The Snowball Earth hypothesis posits that severe Neoproterozoic glaciation occurred at low latitudes (Kirschvink, 1992) and currently ranks as the most controversial and polarized area of debate perhaps in the whole of the Earth Sciences (see Jenkins et al. 2004; Allen, 2006, 2007a; Fairchild and Kennedy, 2007, for succinct summaries of competing ideas). Thick and well-preserved Neoproterozoic glacial deposits occurs on all the continents formerly within Rodinia but despite the wealth of this sedimentary record, wider environmental reconstruction is hampered because of a lack of age data (Bowring and Condon, 2006; Kendall et al., 2006) and because of conflicting reconstructions of the supercontinent's configuration (Fig. 4; Karlstrom et al., 2001; Powell and Meert, 2001; Wingate et al., 2002; Pesonen et al., 2003; Meert and Torsvik, 2003; Torsvik, 2003; Meert and Lieberman, 2004; Rogers and Santosh, 2004; Cawood and Pisarevsky, 2006; Trindade and Macouin, 2007; Johnson et al., 2005). The Neoproterozoic drift of Rodinia from high latitudes to equatorial latitudes is incontrovertible but the timing of this movement with regard to glaciation(s) is still unclear. There is agreement that Neoproterozoic glaciations occurred against an overall tectonic backdrop of active crustal extension as Rodinia broke apart. This is clearly reflected in a glacial stratigraphic record dominated by the deposits of former rift basins. 6.1. Stratigraphic record of Neoproterozoic cold climates: the ‘glaciated rift’ debrite–turbidite association With few exceptions, the sedimentary record of Sturtian and younger land-based ice covers is largely (but not exclusively; see below) preserved in subaqueously deposited offshore strata preserved in rifts and along newly created passive margins (Fig. 3). As a consequence, an understanding of rift basin dynamics is fundamental to unraveling the nature of the stratigraphic record. Neoproterozoic ‘glaciated rift’ successions are dominated by thick, poorly sorted mass flow deposits (debrites) and associated turbidites found within successions marked by their great volume. Total thicknesses in Fig. 4. The Neoproterozoic glacio-epoch and the break up of Rodinia. The precise paleolatitudinal position of the supercontinent is not known and greatly controversial (see text). The purpose of this diagram is to simply show the very well-constrained association of thick glacially influenced marine deposits and rifting (see text; Fig. 8). A: ‘paleo-Pacific’ rifting after 750 Ma (Sturtian glacio-epoch; Figs. 1 and 2). B: ‘paleo-Atlantic’ phase after c. 610 Ma (Marinoan-Gaskiers glacio-epoch; Figs. 1 and 2). A: Arabia, Aus: Australia, EAnt; East Antarctica, Gr/Sc; Greenland, Scandinavia, In; India, NCB; North China Block, NWA; Northwest Africa, SCB; South China Block, T; Tarim. N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 excess of 2 km suggest a dominantly marine not lacustrine origin, and were accommodated in rift complexes extending through central Australia (Adelaide Rift Complex; Young and Gostin, 1989, 1991), western North America (Young, 1995; James et al., 2001; Clapham and Corsetti, 2005) and South China (Wang and Li, 2003; Jiang et al., 2003; Dobrzinski and Bahlburg, 2007) and Oman (Allen, 2007a). These rifts are regarded as contiguous in most Rodinian reconstructions and share a common tectonic history beginning as early as 800 Ma (Wang and Li, 2003) possibly related to mantle plume outbreaks below Rodinia (Li et al., 1999, 2006). New age constraints on the glacially influenced marine deposits in Australia indicate much younger ages than hitherto considered (Kendall et al., 2006; see below). Nonetheless, the sedimentology of these strata consisting of great thicknesses of debrites and turbidites (e.g., Young and Gostin, 1991; Eyles et al., 2007) indicate a rift setting. Thick rift-related glacial marine successions also occur in Central Africa (Wendorff and Key, 2006), Oman and Brazil (Young, 1992; Urban et al., 1992; Leather et al., 2002; Kellerhals and Matter, 2003; Allen et al., 2004; Tie-bing et al., 2006; Allen, 2007b) and around the incipient rifted margins of the Iapetus Ocean now preserved in several basins around the North Atlantic and Greenland (Crowell, 1999; Bingen et al., 2005). Many of these latter successions are characterised by large volumes of cannibalized carbonate debris (e.g. the Great Breccia of the Port Askaig Formation, Scotland and similar facies in Norway; Arnaud and Eyles, 2002a,b) recording instability of platform margins undergoing extension and collapse (Vernhet et al., 2006; Eyles and Januszczak, 2007). Elsewhere, thick glacially influenced marine strata of eastern Canada and Mongolia accumulated in volcanically influenced back arc basins (Gardiner and Hiscott, 1988; Eyles and Eyles 1989b; Lindsay et al., 1996; Allen (2007b)). Those in eastern Canada (Gaskiers Formation) formed on North African basement along the active northern Gondwana margin (Eyles, 1990). Deposits were subsequently rifted off as part of Avalonia–Cadomia during the opening of the early Paleozoic Rheic Ocean to be subsequently embedded within maritime Canada in the Silurian (Gutierrez-Alonso et al., 2003). Other related deposits (e.g., Squantum ‘Tillite” and the Granville Formation of northern France) have been recently cited as glacial in origin (e.g., Snowballearth.org) in support of a Marinoan global freeze event at about 580 Ma but this ignores a long history of previous investigation favouring a non-glacial origin involving debris flow and mixing of coarse and fine sediment to form diamictite (see Fairbridge, 1947; Dott, 1961; Eyles, 1990; Crowell, 1999 and 99 refs therein) referred to as ‘mixtites’. The wealth of glacial abrasional forms, such as striated pavements, ice-contact and glaciotectonically-deformed deposits typical of Pleistocene glaciations (e.g., Eyles, 1988; Benn and Evans, 1998; Boulton et al., 1996; Anderson, 1999; Clark et al., 2003) is not a characteristic of the Neoproterozoic record but some may be locally preserved around basin margins. The bulk of the ‘glacial’ record is biased toward offshore basin depocentres. Striated basement surfaces and true terrestrial tillites deposited below glaciers are rarely preserved (e.g., Wang and Li, 2003) with the major exception of the North African craton, which appears to have been tectonically stable (Proust and Deynoux, 1994) though Shields et al. (2007) report the widespread association of volcanogenic strata with postglacial deposits across a large portion of the Taoudeni Basin. In that basin, periglacial sandstone wedges indicate subaerial contraction and cracking of permafrozen ground (e.g., Deynoux et al., 1989) similar to the deposits of the classic Marinoan passive margin of South Australia (Williams, 1994). Other wedge-like forms (e.g., Spencer 1971; Nystuen, 1976) occur in strata that have marine characteristics and may record soft sediment deformation during rift-related earthquakes (Eyles and Clark, 1985). By and large, the global record of cold, arid conditions as proposed by the Snowball model is very restricted compared to the wealth of the offshore glacially influenced record. 6.2. Neoproterozoic glacioeustasy or tectonics? Glacioeustatic sea level fluctuations of at least 1.25 km (an order of magnitude greater than those of the Pleistocene) are invoked for a Marinoan glaciation at c. 635 Ma based on assumptions of ice thickness on land and on the oceans (2 km and 0.4 km respectively; Hoffman et al., 2006). A corollary is that fine-grained marine strata up to 1 km thick resting on Neoproterozoic diamictites reflect postglacial glacioeustatic sea level recovery (see review by Eyles and Januszczak, 2004a) and are globally correlative (e.g., Christie-Blick et al., 1988, 1995; Young, 1992; Pyle et al., 2004; de Alvarenga et al., 2004; Pyle et al., 2004). In contrast, the thickness of these successions points to an underlying tectonic control involving thermal subsidence of newly formed passive margins (see Eyles and Januszczak, 2004a, pp. 31–2, 56–58). At times of maximum thermal subsidence, blankets of fine-grained sediment accumulate above coarser grained mass flow facies (‘rift climax deep water systems’; Leppard and Gawthorpe, 2006) and probably accounts for the thick, turbidite successions found above many Neoproterozoic glacials (see above 100 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 Section 6.1) and hitherto interpreted as postglacial ‘glacioeustatic’ strata. In contrast, postglacial sea level recovery returns water depths to their preglacial position and cannot create new additional accommodation space (see also Section 7.4 below). In general, intrabasinal tectonic processes (and accompanying relative changes in water depths) are more important than eustatic changes (Miall 1997; Miall and Miall, 2001; Miall 2005) resulting in marked variation in strata from one sub basin to another (see Martin et al., 1985; Thomas 1991; Gawthorpe and Leeder, 2000; Lund et al., 2003). 6.3. Diachronous response to regional tectonics or globally synchronous megafreeze events? Can Neoproterozoic glacial deposits be considered as the ‘global benchmarks’ of Calver et al. (2004) (Kirschvink, 1992; Hoffman et al., 1998) or are they tectonically conditioned and diachronous? On the one hand is the model of tripartite glaciations named from oldest to youngest Sturtian (c. 750 Ma), Marinoan (c. 635 Ma) and Gaskiers; c. 580 Ma; Chen et al., 2004; Halverson et al., 2005; Trindade and Macouin, 2007; Figs. 1 and 2). Recently however, Calver et al. (2004) correlated the Gaskiers with the Marinoan at c. 580 Ma and concluded that there are only two ‘truly global ice ages' (Sturtian and Marinoan; see Knoll et al., 2006, p. 15). A model of either two or three globally-correlated glacial intervals may be simplistic in the light of new ages on classic “Sturtian” black shales of 643.0 ± 2.4 Ma in Australia much younger than considered previously (Kendall et al., 2006) thereby raising the possibility of diachronous regional ice covers (Fanning and Link, 2004, 2006) throughout the Neoproterozoic. The limitation on testing the diachronous vs. discrete models still remains the lack of high precision, well-calibrated dates (Kennedy et al., 1998; Bowring et al., 2003; Bowring and Condon, 2006). The dominantly rift-related tectonic setting of much of the Neoproterozoic glacial record is central to the question of the synchroneity or diachroneity of individual glaciations but tends to be ignored in favour of simple lithostratigraphic correlations. For example, McCay et al. (2006) pigeonholed the glacial deposits of the Scottish–Irish Caledonides into a trinity of globally correlative glaciations without a single specific age date on any horizon. Yet, this succession is in part, tectonically controlled (Arnaud and Eyles, 2006) made up of distinct tectonostratigraphic successions (Dempster et al., 2002; Hutton and Alsop, 2004) recording intrabasinal tectonic controls on the initiation and preservation of glacial deposits. Adherents of the Snowball Earth model only briefly acknowledge the existence of ‘paleo- geographic boundary conditions to global climate’ (i.e., tectonics) (Halverson et al., 2004, 2005, p. 1201). The lack of a dating control looms large at a time of very rapid plate motions. Meert and Torsvik (2003) categorized existing reconstructions of Rodinia between 1110 and 500 Ma as ‘extremely fluid and controversial’ (p. 282) a finding echoed by Tohver et al. (2006) who argued that West Gondwana cratons were at middle to high latitudes between 700 and 575 Ma and did not reach an equatorial position until the Cambrian well after the end of Neoproterozoic glaciation. Consequently, Fig. 4 simply depicts the known association of rifted margins with glacial deposits rather than showing a specific paleogeography. Evans (2000, 2003a,b), Maloof et al. (2002), Li et al. (2004), Meert and Torsvik (2004), Kilner et al. (2005), Tohver et al. (2006) and Trindade and Macouin (2007) review a substantial paleomagnetic database. Of this, the Elatina Formation of South Australia is currently regarded as ‘the most thorough paleomagnetic determination of any Precambrian rock unit’ indicating glaciation at low paleolatitudes (Sohl et al., 1999; Raub and Evans, 2006). The presence of cold climate ice wedge casts in associated strata (Williams and Tonkin, 1985; Williams, 1994; Young, 2002) has been explained by marked changes in the angle of obliquity of the Earth's spin axis (Williams and Schmidt, 2004 but see Bingen et al., 2005). Nonetheless, there still remain major uncertainties about the precise depositional environment(s) recorded by the Elatina Formation in regard to the diamictites within it, the paleogeography of any ice covers at the time, and their age (Lemon and Gostin, 1990; Eyles and Januszczak, 2004a,b). That the Elatina is ice contact glacial in origin is based entirely (and tentatively) on the interpretation of a single thin (∼ 5 m) diamictite horizon, in contact with the underlying Trezona Formation, as an ‘ice-pushed tillite’ that is noted as the ‘only example’ in the entire succession (Lemon and Gostin, 1990, p.152; Eyles and Gammon, 2007). Despite uncertainty regarding the primary ice-contact origin of this classic deposit, it is the key global reference deposit for low latitude glaciation (see Knoll et al., 2006). The database of Trindade and Macouin (2007) includes deposits where a glacial origin has never been definitively demonstrated (e.g., Squantum Member of the Boston Bay Group). Namibian strata (‘Ghaub Tillite’) held to record Snowball conditions and the growth of glaciers on low latitude carbonate reefs (Hoffman and Prave, 1996; Hoffman et al., 1998; Hoffman and Schrag, 2002; Halverson et al., 2005; Hoffman, 2005) were the focus of several earlier studies (e.g., Schermerhorn, 1974; Porada and Wittig, 1983a,b; Miller, 1983; Martin et al., N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 1985) that could not recognise a glacial influence on sedimentation in a complex extensional basin (see Eyles and Januszczak, 2007). Advances in understanding paleoclimates is fundamentally dependent on detailed facies and basinal evaluations of deposits by glacial sedimentologists conversant with Pleistocene and modern glacial depositional systems. This is clearly evident in regard to climate models that invoke severe freezing whereas the rock record tells otherwise. 6.4. An active hydrosphere; climate models and sedimentology The Neoproterozoic geologic record provides no evidence as to planetary albedos or obliquity, ocean circulation, solar luminosity, or atmospheric carbon dioxide levels etc., but these are key inputs to climate models (Crowley and Baum, 1993; Frakes and Jenkins, 1998; Chandler and Sohl, 2000; Hyde et al., 2000; Baum and Crowley, 2001; Crowley et al., 2001; Poulsen et al. 2001, 2002; Pierrehumbert, 2002, 2004; Poulsen, 2003a,b; Lewis et al., 2003; Tajika, 2003; Ridgwell et al. 2003; Donnadieu et al., 2003, 2004a,b,c; Poulsen, 2003a,b; Peltier et al., 2004; Romanova et al., 2005; Sohl, 2006; Nédélec et al., 2007a,b; Godderis et al., 2007). Given the constraints in input data, model outputs relate more to the limitations of the models themselves and explain large discrepancies. Nearly all climate models stress the essential role of sea ice in lowering planetary albedos as a necessary condition for a snowball planet, but sedimentary evidence for its importance as a geologic agent is largely missing. Globally distributed ice rafted dropstone facies should be an integral part of any Snowball Earth with frozen ocean surfaces (with ice thicknesses of up to 400 m; Hoffman et al., 2006) and thick shore ice capable of freezing onto and exporting substrate materials. Marine deposits with far travelled ice-rafted debris should be widespread but this is not the case (e.g., Macquaker and Keller, 2005). Similarly, there is (as yet) no convincing record of the scouring and deformation of passive marine strata by the keels of sea ice such as occurs today over enormous areas around the margins of the Arctic Ocean, and which also occurred in water depths of as much as 1 km below Pleistocene icebergs (Polyak et al., 2001; Kristoffersen et al., 2004). The demonstrable absence of ice-rafted debris in successions that supposedly span assumed extreme climatic conditions (e.g., Polarisbreen Group of Svalbard) has been explained as a consequence of a ‘fully frozen ocean’ (Halverson et al., 2004, p. 316) and the inability of ice to drift. This scenario fails to consider the dispersal of 101 floating ice containing englacial debris at the end of any severe glaciation. In addition, ice rafted sedimentary facies similar to late Pleistocene Heinrich layers produced by ocean going armadas of icebergs travelling several thousand kilometres from their parent ice sheets (e.g., Andrews, 1998) are persistent features of Cenozoic (and possibly Paleozoic) glacio-epochs (Eyles et al., 1997; Crowell, 1999). Such layers may record a fundamental inherent instability of marine ice sheets and should be present in the Neoproterozoic. Their apparent lack of preservation indicates that the global reach of floating ice was limited. On the other hand, glendonite within Neoproterozoic cold water carbonates of the Windermere Supergroup in northwest Canada suggested to James et al. (2005) extensive freezing of ocean water. Glendonite is a calcite pseudomorph (after the mineral ikaite) forming today in cold, high latitude organic and carbonate-rich bottom sediments (see Marland, 1975; Suess et al., 1982; Buchardt et al., 1997). It occurs profusely within highlatitude, shallow marine shelf successions of the early Permian Sydney Basin in New South Wales when cooltemperate forest covers grew inland and cold Antarctic bottom currents flowed north from a late Paleozoic Antarctic ice cap (Eyles et al., 1997, 1998). These conditions cannot easily be compared with extensive freezing of Neoproterozoic ocean waters but ice wedge casts (Deynoux et al., 1989; Williams and Schmidt, 2004) provide convincing evidence of cold conditions well below − 6 °C close to sea level. Sedimentologic evidence for an active hydrosphere finds support from paleobiologists. The notion of a possible link between glaciation and biologic evolution is long standing (Rudwick, 1964) and recent views hold that extreme global cold had a profound effect on life (Hoffman and Schrag, 2000, 2002; Liang et al., 2006). This relationship is however currently categorized as ‘overstated’ (Corsetti et al., 2006a,b, p. 127). Complex metazoans (e.g., Ediacaran fauna) first appear in the rock record 4 million years after the so-called Gaskiers glaciation at around 582 Ma but the morphological complexity of this faunal group suggests they evolved preglacially (Vickers, 2006). No clear ‘cold effects’ are identified in the record of sensitive shallow dwelling marine organisms in glacially influenced and open marine environments (Grey et al., 2003; Gorjan et al., 2003; Olcott et al., 2005; Corsetti et al., 2006a, b). In fact, in that part of the Australian record which spans the classic Sturtian glacial event, the Acraman–Bunyeroo bolide impact event just after 580 Ma may be of greater environmental importance (Williams and Gostin, 2005; McKirdy et al., 2006). Data also suggest a key stimulus 102 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 on Neoproterozoic biologic evolution and in particular, the emergence of multicellular Ediacaran organisms was the breakup of Rodinia involving enhanced nutrient fluxes from mid-ocean spreading centres and generation of large newly rifted margins with shallow water. Neoproterozoic rifting and associated paleogeographic changes are increasingly recognised as a major stimulus on increased speciation rates and changing paleobiogeographic patterns (Lieberman, 1997, 2005; Lindsay and Brasier, 2002; Xiao and Kaufman, 2006). By analogy with the breakup of Pangea, global oceanic circulation (and thus climate) is profoundly altered by small changes in continental positioning and ocean basin shape and depth (Poulsen et al., 1998; Goddéris et al., 2007). Neoproterozoic glaciation may have been promoted by biotic weathering of newly ‘greened’ continental surfaces (Hedges, 2003). Neoproterozoic rift basins contain very substantial thicknesses many kilometres thick of glacially influenced submarine fan deposits dominated by muddy debrite/ turbidite facies. Thicknesses of such strata necessitate erosion by wet-based glaciers sliding over their beds (e.g., Hooke and Elverhoi, 1996; Eyles, 2006). Normal glacial and non-glacial sedimentary processes operated within a fully functioning hydrological cycle (McMechan, 2000; Leather et al., 2002; Condon et al., 2002; Allen et al., 2004; Arnaud, 2004; Eyles and Januszczak, 2004a; Arnaud and Eyles, 2006; Sohl, 2006; Allen, 2007b; Eyles et al., 2007; Dobrzinski and Bahlburg, 2007). It is not possible to create such thick sedimentary records of marine glacial environments without the ability to produce and flush large volumes of glaciclastic sediment to marine basins by water. Thick glacial marine deposits do not agree with a hard Snowball Earth scenario involving dry based ‘static’ glaciers permafrozen to their beds, thick sea ice and a non-functioning hydrosphere (see Gaucher et al., 2003, 2005). 6.5. Carbon isotope excursions A role for tectonics is indicated by recent work in regard to carbon isotope excursions. Isotope profiles are routinely used to for precise chemostratigraphic correlations of Neoproterozoic strata (see Halverson et al., 2005; Corsetti et al., 2006a,b) based on the assumption that strong negative shifts in 13C must record global freeze/thaw events (Hoffman and Schrag, 2000, 2002). An outstanding issue is that once again, ‘global isotope curves’ are not constrained by radiometric dating but are reliant on presumed lithostratigraphic correlations from one basin to another and from hemisphere to hemisphere (Halverson et al., 2005; Bowring and Condon, 2006; Knoll et al., 2006). Many isotope shifts after 1500 Ma are not associated with glacial deposits (e.g., Alene et al. 2006; Azmy et al. 2006; Kaufman et al., 2006; Polteau et al., 2006). Other Neoproterozoic records are of low stratigraphic resolution, show trends that diverge from the idealized Snowball model of negative 13C shifts, and which vary from basin to basin (e.g., Walter et al., 2000; Kennedy et al., 2001a,b; Shields et al., 2002; Hoffman et al., 2002; Lindsay and Brasier, 2002; de Alvarenga et al., 2004; Halverson et al., 2005; Le Guerroué et al., 2006; Corsetti et al., 2006a, and refs above). Where excursions occur in the Neoproterozoic they are regarded as being uniquely of climatic significance (Halverson et al., 2005) despite the widespread deposition of host sediments in enclosed rift basins possibly separated from the world oceans. Elsewhere in the Mesoproterozoic, prominent carbon isotope excursions are explained as a product of enhanced burial of organic carbon during the assembly of Rodinia (Bartley et al., 2001). Condie et al. (2001) emphasized the importance of changes in the rate of organic carbon burial in newly formed rift basins (e.g., Fig. 3) divorced from the global ocean during the breakup of Rodinia (see also Gammon et al., 2005). Isotope fluctuations may thus reflect tectonic rather than purely palaeoatmospheric and palaeoceanographic events (Brasier et al., 2002). Tectonic activity not only buries carbon but it may also under certain conditions fundamentally limit the use of carbon isotopes to infer paleotemperatures by altering the isotope system by circulating basinal fluids (Foden et al., 2001; Knauth 2005). Melezhik et al. (2005a) attribute major shifts in 13C in the Paleoproterozoic to intrabasinal geochemical factors arguing that the magnitude of excursions in the Proterozoic is much smaller than assumed. Other complicating factors identified by Kaufman et al. (2006) and Alene et al. (2006, p. 95) including diagenetic effects being very different in Neoproterozoic sediments where bioturbation and mixing is absent compared to Phanerozoic environments. There is growing evidence for a non-unique relationship between Neoproterozoic climate and carbon isotope excursions (Le Guerroué et al., 2006). In the Neoproterozoic Adelaide Rift Complex, positive excursions of 13C are not restricted to glacial intervals but are primarily associated with rifting events and volcanic activity (Foden et al., 2001). These workers reported significant disturbance of both C-and Sr-isotopic compositions of fine-grained clastic sediments, carbonates and basalts at several stratigraphic levels. There, the resetting of isotopic systems was attributed to uplift of basin margins and the recharge of basin fluids by low salinity meteoric waters. N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 6.6. Summary The bulk of the Neoproterozoic glacial record is stored within thick marine debrite-turbidite successions that accumulated within rift basins. Terrestrial ‘tillites’ and associated deposits are poorly represented. Neoproterozoic glaciers were wet based and produced abundant meltwater and sediment incompatible with catastrophically cold conditions of a hard Snowball Earth. The breakup of Rodinia took place over a 200 million year period and by analogy with other episodes of rifting there was significant along-strike diachroneity in the timing of rifting, basin formation and glacially influenced sedimentation (Kendall et al., 2006). Large-scale rearrangement of landmasses and oceanic configurations created by an evolving disintegrating supercontinent may have played a key role in climate change. There is growing recognition that Neoproterozoic glaciations were initiated as regional ice centres (Halverson et al., 2005, p. 1198) whose growth was diachronous (op cit., p. 1198) countering the longstanding use of glacial deposits as precise global time markers. Earlier ideas of ‘instant glaciation’ involving notional albedofeedback mechanisms and runaway refrigeration are now underplayed (see Halverson et al., 2005). In the light of the substantial gaps in knowledge identified above, and the emerging theme of diachroneity of Neoproterozoic glaciation, it is profitable to revisit the conceptual underpinning of current efforts to subdivide Proterozoic time using Global Stratotype Sections and Points (GSSP). Knoll et al. (2006, p. 14) believe that ‘the great ice ages that wracked the later Neoproterozoic world… were global in impact, and because they are associated with carbon isotopic excursions larger than any recorded in Phanerozoic rocks, the glaciations offer what are undoubtedly our best opportunities for the sub-division of Neoproterozoic time’. It can be argued in fact that the geologic consensus is moving away from catastrophic global freeze events and instantaneous deglaciations. In contrasts to ‘wracking’ the world, the Neoproterozoic rock record informs us that glaciers were wet-based and may have been part of diachronous events as tectonotopography evolved during the dispersal of crustal blocks. 7. Lower Paleozoic Saharan glacio-epoch (c. 440 Ma) 7.1. Long lasting or short lived? And just how big? After the breakup of Rodinia, the North Africa craton edged northwards within the south polar circle (Scotese 103 et al., 1999; Pharoah, 1999; Vercoli and Le Herisse, 2004; Fig. 5) but despite the high paleolatitude, only at the end of the Ordovician is there geologic evidence for ice over North Africa. This is the basis for the influential model of a single and short-lived (∼ 1 Ma) Late Ashgillian glacial event (see Brenchley et al., 1995). An apparent lack of long lasting ice covers could simply reflect the erosion or non-preservation of any older glacial strata. Indeed, some authors propose a 10 Ma long North African glaciation that started much earlier in the Ordovician (Ghienne, 2003) and lasted well into the Silurian (Grahn and Caputo, 1992; Caputo, 1998; Pope and Read, 1998; Crowell, 1999; Saltzman and Young, 2005). The last (and thus best known) Late Ordovician Saharan ice sheet formed during a time of high (16 × the modern value) atmospheric CO2 (Torsvik and Cocks, 2004; Fig. 2E). The ice sheet may have been comparable in size to the last North American Laurentide Ice Sheet (∼ 36 × 106 km3) and expanded eastward from North Africa onto the Arabian platform (Deynoux et al., 1985; Vaslet, 1990; Sutcliffe et al., 2000). There are however, continuing uncertainties over its true dimensions (and thus volumes). A Southern African ice mass is regarded by some as an entirely separate outlier (Young et al., 2004) whereas others suggest it was part of a continuous ‘pan-African ice sheet’ extending over more than 60° of latitude (Scotese et al., 1999; Veevers, 2004; Ruban et al., 2007). Traditional evidence for the volume (and timing) of ice covers, derived from so-called ‘glacioeustatic’ sea level fluctuations recorded on distant continental shelves is problematic (see Section 7.3). Crowley and Baum (1995) and Kump et al. (1999) modeled a notional North African ice sheet and argued that it responded like its Cenozoic counterparts to orbital Milankovitch variables (see also Williams, 1991). This finding has been used as the foundation of a sophisticated glacial depositional model by Sutcliffe et al. (2000) that sought to find lithostratigraphic correlations between horizons more than 6000 km distant in Northern and Southern Africa otherwise entirely unconstrained by age or biostratigraphic data. They argued that the deposits reflected two orbitally driven 100 ka cycles of ice sheet expansion and decay and could be correlated by ‘process based interpretation of sedimentation’ (p. 968) i.e., conventional bed-for-bed lithostratigraphy, from north to south. Elsewhere, outlying ice covers formed in areas of high topographic relief along the tectonically active proto-Andean margin of Gondwana in what is now Peru–Bolivia (Astini, 1999; Diaz-Martinez and Grahn, 2007). Diaz Martini and Grahn (2007, p. 77–78) were 104 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 Fig. 5. Palaeogeography of the Late Ordovician Saharan glacio-epoch at c. 440 Ma after various sources (principally Fortey and Cocks, 2003). The northern margin of Gondwana was the locus of active extension after 480 Ma and extension-related uplift of the Gondwanan Highlands may have triggered polar Saharan glaciation after 440 Ma. Outlying ice masses lay on the proto Andes and in southern Africa where they reached sea level (Cancanari Formation and Pakhuis Formation respectively). The Saharan ice sheet was short lived and disappeared by the Early Silurian but ice remained over the uplifted active margin of South America into the Devonian of Brazil and Bolivia but not over the pole (see text). When Gondwana collided with Laurentia to form Pangea beginning in the mid-Carboniferous this remnant ice would expand to form an extensive Gondwanan ice complex (Fig. 6). at pains to emphasize that existing global palaeoclimatic reviews (e.g., Raymond and Metz, 2004) fail to consider local tectonic controls on the existence and timing of ice covers along the western Gondwana margin. These glaciers left glacially influenced marine strata dominated by mass flow deposits. Because of local tectonic controls, the timing of these ice masses is thought to have been out of phase with the main North African glaciation(s) (Diaz-Martinez and Grahn, 2007). Ice covers extended well into the Silurian when Brazil lay at the South Pole and this area was to subsequently experience glaciation again in the Devonian (see Section 8 below). It can be ventured that ice was present along topographically elevated parts of the active proto-Andean margin throughout much of the Early Paleozoic. 7.2. Geomorphic, sedimentologic and stratigraphic evidence of glaciation in North Africa Late Ordovician glacial deposits are relatively thin (b200 m), mostly coarse grained and rest on a prominent channeled unconformity (e.g., Le Heron, 2007). Erosional stripping of overlying shale has exposed the original geomorphology of the glacial depositional systems below. Beuf et al. (1971), Trompette (1973) and Vaslet (1990) recognised terrestrial glacial and cold climate landforms such as eskers, moraines, drumlins, periglacial polygon structures, pingos, and channels cut by subglacial and proglacial meltwaters (see Tucker and Reid, 1973; Hirst et al., 2002; El-ghali, 2005; Armstrong et al., 2005). The presence of multiple bedding planes that expose striated surfaces across what was soft sand, is seen either as the product of repeated shear either below a grounded glacier (e.g., Sutcliffe et al., 2000, 2005; Deynoux and Ghienne, 2004; Le Heron et al., 2005) or grounding ice floes (Woodworth-Lynas and Dowdeswell, 1994). In this regard, Moreau et al. (2005) have mapped what they argued to be streamlined glacial lineations (drumlins) recording ice streams that flowed north from an area of higher topography in the interior (the ‘Gondwana Highland’ of Luning et al., 2000) north toward the Gondwanan continental margin (Ghienne and Deynoux, 1998; Ghienne, 2003). The assumption is that the modern ground surface across a large area of North Africa is an ancient exhumed landform and that the depth of post-Ordovician stripping has been precisely the same everywhere. N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 7.3. Sea level fluctuations; are they exclusively glacioeustatic? The size of an extensive North African Saharan ice sheet is traditionally constrained on the basis of thirdorder glacioeustatic sea level changes recorded on distant continental shelves and cratonic interiors (Dennison, 1976; Ross and Ross, 1992; Pope and Read, 1998; Izart et al., 2003). This model is deeply rooted in the long standing assumption that changes in relative water depths recorded in marine rocks (together with associated variations in 13C and 18O values; e.g., Brand et al., 2006) are driven exclusively by ice sheet growth and decay (see Zhang and Barnes, 2002 and refs therein). A well-cited paleobiological model attributes Late Ordovician extinctions to glacially driven fluctuations in sea level, ocean circulation and chemistry (see Webby and Laurie, 1992; Armstrong, 1995; Brenchley et al., 1995, 2003; Sheehan, 2001; Munnecke et al., 2003; Twitchett, 2006). For a number of reasons the ‘glacioeustatic’ model for reconstructing ice sheet volumes and explaining Lower Paleozoic extinctions is full of uncertainty. Late Ordovician glaciation had little or no effect on phytoplankton living along the northern glaciated margin of Gondwana (Vercoli and Le Herisse, 2004). Delicate organic walled microfossils (acritarch and chitinozoan assemblages) in Ashgillian strata show no extinction events, in fact, microphytoplankton became more diverse through the glacial interval, which is associated with new speciations and morphological innovations. Indeed, the principal extinction event when many taxa died out occurs earlier in the Middle Ordovician (late Llanvirn) (Vercoli and Le Herisse, 2004). This is an important finding because acritarchs occur at the base of the marine trophic system and are sensitive indicators of changes in the total biomass. Acritarchs constitute the ‘canary in the mine’ in regard to changes taking place in the numbers and diversity of primary producers. If the North African ice sheet had no negative effects on marine microorganisms living in close proximity, it is difficult to see how glaciation per se had any widespread global influence on other groups (Munnecke et al., 2003). Late Ordovician extinctions were highly selective and also diachronous occurring first in deeper water communities before affecting those of shallow water. This is the opposite of what might be expected had glacioeustatic sea level lowering been the prime cause as widely invoked. That a relatively small Saharan ice sheet, accompanied by small ice masses in South America perhaps could set in motion a series of global biological 105 crises begs the question of the role of other causative biogeochemical processes unrelated to glaciation. There appears to be scant recognition of a facies/substrate control on marine fauna during the Late Ordovician, viewed as responding primarily to changes in ocean water masses. Many so-called ‘extinctions’ appear to be times of altered diversity followed by reemergence of ‘Lazarus faunas’ (Munnecke and Servais, 2007). The possible role of tectonically modulated changes in water depths and thus on substrates and associated ecosystems, goes unremarked. The ‘dynamic topography’ model demonstrates that Paleozoic depositional successions (‘sequences’) on ‘stable’ shelves and cratons are not primarily the result of global sea level changes (as was originally thought) but variation in relative sea level conditioned by mantle processes below the overlying plate. These act to elevate or depress continental surfaces independent of global sea level (see Burgess et al., 1997; Mitrovica et al., 2000; Miall, 2005; Eriksson et al., 2005; Vakarelov et al., 2006). Thus the assumption of an exclusively glacioeustatic control on changing Late Ordovician and Early Silurian water depths (Brand et al., 2006) can be challenged. A clear illustration is provided by Zhang et al. (2006) who found significant differences in water depth changes recorded in coeval Late OrdovicianSilurian strata on Appalachian and Arctic coasts of North America. The rapid ‘glacioeustatic’ sea level rise during the persculptus Zone (supposedly the main Ordovician deglaciation event; Brenchley, 2004) could not be recognised in the south. They referred to this as a ‘paradox’ which indeed it would be if global eustasy were the only control on water depths. In reality of course, given the very different tectonic settings of these two widely separated margins, incongruence of relative sea level records is to be expected because their subsidence histories were different. Zhang et al. (2006, p. 266) argue that there is ‘significant evidence against a universal sea level curve in the latest Ordovician-Earth Silurian’. From the above, the timing, volume nor longevity of any North African ice sheet can be readily derived from changes in water depths recorded at any one site distant from centres of glaciation (cf. Nielsen, 2003). This represents a remarkable gap in our understanding of Earth's glacio-epochs. 7.4. Tectonic influences on glaciation in North Africa; the importance of black shales A regional, long-term tectonic influence on sedimentation patterns along the Late Ordovician North 106 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 African continental margin can be suggested. This is based on consideration of north draining paleovalleys identified as ‘tunnel valleys’ or ‘fiords’ (Ghienne, 2003; Armstrong et al., 2005; Turner et al., 2005) and their overlying thick shale cap. Paleovalleys are incised into underlying preglacial shelf deposits and are blanketed by black, graptolitic shales of Early Silurian age. This succession is classically interpreted as a consequence of glacial erosion of an shelf exposed during glacioeustatic fall accompanying ice sheet expansion, in turn, followed by postglacial sea level rise during the Parakidograptus acuminatus graptolite zone (Luning et al., 2000). Reported paleovalleys are however, broad shallow features that lack the overdeepened form of glacially cut channels and fiords typical of Pleistocene forms. It remains to be demonstrated that they are not simply ‘lowstand’ submarine valleys part of a widespread ravinement surface accompanying broad-scale uplift along the northern Gondwanan continental margin. The model of Seidler (2000) for submarine channels in the Tertiary of East Greenland is an appropriate analog. Overlying Early Silurian shales (that include lowermost ‘hot shales’ rich in organic matter) are exceptionally thick (1000 m +) and extend from Morocco to Saudi Arabia. The thickness alone indicates that a simple ‘postglacial flooding’ model is unrealistic because glacioeustatic recovery does not create additional accommodation space. Final ice melt simply returns sea level to its preglacial level. In certain cases, a short-lived (thousand years?) phase of increased accommodation occurs where glacioisostatic rebound lags behind the rapid rise in sea level during initial deglaciation (witness the widespread marine transgressions at the end of the last glaciation around Canada's Hudson Bay lowlands and St.Lawrence Valley). ‘Marine overlap’ is temporary however, and marine successions are rapidly raised above sea level and eroded as a consequence of rapid crustal rebound. One kilometre thick Silurian shales in North Africa point to the creation of new and tectonically driven accommodation space along that margin. The typical Late Ordovician tripartite stratigraphic motif of paleochannels, coarse clastic infill and thick overlying shale likely represents a distinct tectonostratigraphic succession. This succession can be interpreted in terms of an initial phase of crustal extension and upliftrelated climate cooling and glaciation along the North Gondwana margin, followed by deglaciation, thermal subsidence and flooding. This model suggests a lengthy early Paleozoic glacio-epoch lasting much more than 1 million years triggered by broad scale extension along the North African margin and the detachment of several terranes. During end Ordovician glaciation Laurentia was separated from North Africa by a wide ocean (Iapetus) (Fig. 5) that eventually closed when several terranes detached from the North African margin (e.g., Avalonia, Armorica including Iberia and Sardinia, Meguma; Mallard and Rogers, 1997; Sanchez-Garcia et al., 2003; Fortey and Cocks, 2003; Vercoli and Le Herisse, 2004; Torsvik and Cocks, 2004; Ruban et al., 2007) opening up the Rheic Ocean in their wake. The precise timing of separation is unclear (Scotese and McKerrow, 1991; Torsvik et al., 1996; Gutierrez-Alonso et al., 2003; Keppie et al., 2003; Cocks and Torsvik, 2006). Some terranes now found as the Paleozoic massifs of central and southern Europe may contain a glacially influenced marine record (Robardet and Dore, 1988) but deposits await sedimentological study and reevaluation (see Long, 1991). A glacial influence on sedimentation in Late Ordovician strata in terranes of eastern Canada is evident in central Newfoundland (McCann and Kennedy, 1974) and Nova Scotia (e.g., White Rock Formation; Schenk, 1972; MacDonald et al., 2002). The earlier literature had assumed that a North African ice sheet flowed across into eastern Laurentia at a time when these continents were thought to be immediately adjacent within Pangea. In fact, these glacially influenced marine strata accumulated autochthonous to North America as part of the sedimentary cover on terranes that rifted off from North Africa when the Rheic Ocean opened. Those in Newfoundland form part of the Dunnage terrane and, with other terranes (Gander, Avalonia), was obducted against the North American continental margin during the Taconic–Salinic Orogeny (Late Ordovician–Silurian). Those in Nova Scotia are part of the Meguma terrane emplaced during the midDevonian Acadian Orogeny (Schenk, 1991). The recent identification of significant post-Gondwana uplift across central and southern and central Africa in response to a mantle ‘superswell’ by Gurnis et al. (2000) provides an additional mechanism for creating an elevated dynamic topography on which ice could nucleate as an outlier ice cap. The development of a mantle superswell could possibly have been a precursor to uplift along the late Ordovician North African margin. Smith (1997, p. 176) suggested that extensional faulting and extensive footwall uplift was the trigger for widespread snow accumulation across the interior of Gondwana. Late Ordovician pCO2 levels were as much as sixteen times that of today (Crowley and Baum, 1995; Wang et al., 1997; Herrmann et al., 2004; Kaljo et al., 2004; Tobin et al., 2005; Fig. 2E) begging the question as to what processes might have drawn down pCO2. Increased organic productivity of plankton and elevated rates of carbon burial (Brenchley et al., 1995), accelerated N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 107 accumulation of carbonate on shelves (Villas et al. (2002) and chemical weathering of sediment released from the Taconic Mountains in North America (the Cenozoic ‘Himalayan model’ of Raymo and Ruddiman, 1992; Gibbs et al., 1997; Kump et al., 1999) have all been invoked. Such processes (if real; see Kerrick and Caldeira, 1999) could have amplified climatic cooling and snow accumulation accompanying the generation of rift-related dynamic topography across a polar North Africa sufficient to briefly override high global pCO2 pressures. 7.5. Summary Gondwana occupied a southern polar position throughout most of the Early Paleozoic but glaciation is only evident in North Africa after 440 Ma; an older record may have been eroded. The Saharan Ice Sheet has for many years been the villain for paleobiologists seeking to explain extinctions in Late Ordovician marine fauna as the result of glacioeustatic sea level and water mass changes. The volume of the ice sheet is constrained traditionally by the amplitude of fluctuating sea level recorded in distant shelf successions. Such fluctuations vary however, even in contemporaneous strata suggesting a cause other than glacioeustasy, such as dynamic topography. Thus the volume of the ice sheet remains unconstrained. In North Africa, a tripartite stratigraphy of preglacial channels, coarse glacial fill and postglacial Silurian shales (up to 1 km thick) suggest long-term tectonic controls, possibly accompanying rifting of terranes from the North Gondwanan margin. A tectonic trigger and associated uplift may explain why glaciation occurs only briefly during a long non-glacial, interval when northern Gondwana lay in polar latitudes for many millions of years. Clearly, a high paleolatitude is not by itself a sufficient requirement for glacierization (see Ruddiman, 2001). After the demise of the Late Ordovician Saharan glacio-epoch, Gondwana remained at high polar latitudes for another 100 million years but remained largely ice free until the onset of the Late Paleozoic Gondwanan glacio-epoch at about 350 Ma (Fig. 1). 8. Late Devonian ice c. 374 Ma 8.1. Brief cooling triggered by continental collision Uplift related cooling along the active margin of the South American plate spawned shortlived Late Devonian ice covers in what is now Bolivia and parts of Brazil (Caputo, 1998; Isaacson et al., 1999; Fig. 6). The suggested cooling role of a possible bolide impact at this Fig. 6. Ice growth phases during the Carboniferous-Permian Gondwanan glacio-epoch (After Crowell, 1999, Eyles, 1993). The nucleus was Devonian ice then located over the high topography along the active South American plate margin (Fig. 5). This expanded into southern Africa and then eastward across India, Australia and Antarctica concomitant with the drift of Gondwana across the south polar latitudes and the shift in the pole from North Africa to Antarctica and Australia by the Permian. The thickest rock record of Gondwanan ice covers occurs in marine intracratonic basins in Brazil, Oman and Southern Africa and in rifted margin basins of western Australia. 108 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 time has been argued by Streel et al. (2001) and evidence from fossil lycopsids suggests lowered levels of atmospheric CO2 (Beerling, 2002). Kaiser et al. (2006, p. 157) refer to Late Devonian ice volumes as being similar to that of Quaternary glaciations. A 3–4 °C cooling across the Frasnian–Famennian boundary has been linked to weathering-related consumption of C02 during collisional events leading to the building of Pangea resulting in ‘one of the greatest crises of the biosphere recorded during the Phanerozoic’ (Averbuch et al., 2005, p. 32). Evidence for changes in water depths on distant shelves is still related by paleobiologists to the waxing and waning of Devonian glaciers (e.g, Saltzman, 2002; Kaiser et al., 2006) but this fails to consider the effect of other processes on water depths such as dynamic topography as discussed above in reference to Late Ordovician glaciation (Sections 7.3, 7.4). 9. Late Paleozoic Gondwanan glacio-epoch (c. 350–250 Ma) 9.1. Long lived cooling triggered by continental collision Pujol et al. (2006) concluded that tectonic processes drove global paleoclimates of the Late Paleozoic. This is clearly evident after 350 Ma when large ice sheets formed across India, South America, Southern Africa, Australia and Antarctica (Crowell 1999; Veevers 2004). Veevers and Powell (1987) and Powell and Veevers (1987) showed that ice growth was a direct response to extensive uplift at high southerly paleolatitudes during the mid-Carboniferous Variscan and late Carboniferous Alleghenian collisions of Gondwana with Laurasia. These collisions coincided with large-scale thermal doming and uplift across Pangea (Speed et al., 1997; Veevers, 2000) in conjunction with lowered atmospheric CO2 levels (Beerling 2002). In general, the locus of ice covers progressively moved across Gondwana from South America to Australia (Crowell, 1999) tracking Gondwana's transpolar trajectory. The lack of any earlier ice cover across this area from the Late Ordovician through to the Carboniferous could be due to a lack of either continental topography high enough to promote significant cooling, or insufficient moisture in the supercontinental interior. Study of the deposits of the Gondwanan glacioepoch has been greatly facilitated by the presence of oil and gas in glacially related marine rocks and hence the availability of much subsurface data such as drill core and downhole and seismic geophysical data (e.g., Eyles et al., 1993, 1994; Tankard et al., 1994; Potter et al., 1995; Stephenson et al., 2005; Berthelin et al., 2007). Good quality age data allow identification of the timing of deglaciation sequences in Southern Africa (Bangert et al., 1999) with corresponding climate, floristic and relative sea level changes in distant basins (e.g., FalconLang, 2005; Feldman et al., 2005). In general, the landward terrestrial imprint of Late Paleozoic glaciation, in the form of subglacially deposited tillites and glacially eroded and striated basement surfaces, is locally prominent (e.g. Southern Africa) but is a very minor component of the record across Gondwana as a whole (e.g. Visser 1991, 1997; de Broekert and Eyles, 2001). Exceptionally, a thick ice-contact succession of glaciolacustrine, glaciofluvial and eolian facies is preserved in the intracratonic Cooper Basin of Southern Australia (e.g., Williams et al., 1987). The bulk of ‘glacial’ deposits accumulated offshore in a wide variety of tectonic settings (Bonorino and Eyles 1995; Lopez-Gamundi 1997). Past practice of simply drawing inferences regarding the size of the ice covers from the geographic extent of glacial strata across a reassembled Gondwana is therefore, of little value because deposits accumulated beyond the landward margins of ice sheets. In Oman, valley glaciation was triggered by uplift of rift shoulders associated with an incipient triple junction (the India/Madagascar/Arabia rift) marking early opening of Neotethys (Blendinger et al., 1990; Angiolini et al., 2003). In South America, marine glacial (and some terrestrial) strata were deposited within forearc basins along the western collisional plate margin (e.g., Tarija Basin; Lopez-Gamundi 1997) and within several small (Lopez-Gamundi et al., 1992) and large intracratonic basins (e.g., Parana Basin; Eyles et al., 1993). Glacially influenced marine and brackish water strata accumulated in southern Africa within the retroarc foreland Karoo Basin (Stollhofen et al., 2002; Catuneanu et al., 2002; Moore and Moore, 2004; Scheffler et al., 2006) and in interlinked intracratonic rift basins in central Africa (Visser 1997). Changes in relative sea level in these successions previously interpreted in terms of classic ‘ups and downs’ of glacioeustatically controlled sea levels, are increasingly seen as longer term tectonically driven changes in relative sea level (Eyles et al., 1993; Stollhofen et al., 2002). The thickest fills preserved on Gondwana accumulated in several rift basins clustered along the western extensional margin of the Western Australian plate. These contain exceptional thicknesses (2–5 km) of cold climate hydrocarbon-hosting marine strata. ‘Postglacial glacioeustatic’ shales that cap these successions can be shown to have accumulated diachronously along the Australian margin in response to increases in accommodation space N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 provided by tectonic subsidence (Eyles et al., 2002, 2003, 2006). These basins eventually widened and evolved into oceans during the break up of Gondwana (e.g., Harrowfield et al., 2005). As terranes rifted off from Gondwana they exported deposits of pebbly glaciomarine mudstone to what is now SE Asia (e.g., Sibumasu: China–Burma, Malaya and Sumatra; Veevers, 2000). 9.2. Summary A tectonic control on the onset of glaciation is particularly clear for the Gondwanan glacio-epoch where ice growth accompanies continental collisions and the growth of Pangea. Nonetheless, a glacial record was not preserved in many areas until the onset of extensional basin formation and subsidence. Thus the age of the glacial record does not necessarily fully reflect the onset of glaciation. It is now evident after many detailed studies, that the glacial record is overwhelmingly marine. The most pressing question regarding the Gondwana glacioepoch is understanding the apparent absence of any ice covers between the Late Ordovician and the Carboniferous; an almost 100 million year long period where a significant landmass lay at the pole but remained (apparently) ice free. This re-emphasizes that a polar position is clearly not a sufficient condition for glacierization. Key data are required on paleoelevations changes to assess the role of uplift in climate cooling. 10. Mesozoic non-glacial interval (c. 250 to 55 Ma); a role for small ice masses? Final deglaciation at the end of the Gondwanan glacio-epoch occurred in what is now northeastern Australia during the Late Permian (Kazanian; c. 256 Ma; Crowell, 1999; Fig. 2). This termination was followed by a lengthy largely non-glacial episode of more than 200 million years when no large ice sheets formed on the planet. Possible middle to late Triassic glaciation in Australia is contentious (Gore and Taylor, 2003). Jurassic and Cretaceous ice rafted debris is known (Frakes et al., 1995; Chumakov and Frakes, 1997; Lurio and Frakes, 1999) and sediments and erosional forms attributed to terrestrial ice masses are identified in Australia (Alley and Frakes, 2003). Any such ice masses were small. High frequency (sub million year) fluctuations in sea level and corresponding parasequences of the Cretaceous Western Interior Seaway of North America, were formerly interpreted either as glacio-eustatic (Plint, 1991; Plint et al., 1992) or Milankovitch forced (Gale et al., 2002) but are now viewed as tectonically driven (e.g., Vakarelov et al., 2006; see also Miall, 1997). It is 109 difficult to envisage ice masses large enough to create 50 to 60 m variations in early Cretaceous sea level proposed by Stoll and Schrag (1996, 2000) because they require a global ice volume equivalent approximately to the modern day ice cover over Antarctica or during the Pleistocene in North America. Moriya et al. (2007) concluded that there was no evidence of glaciation in high-resolution oxygen isotope records for the midCenomanian. Crowell (1999, p. 7–8) suggested that small Cretaceous glaciers could have formed on uplifted rift shoulders during early Atlantic opening. If so, any stratigraphic record now lies deeply buried at the base of thick passive margin sequences. 11. Cenozoic glacio-epoch (< 55 Ma) Earth began to cool after the Paleocene-Eocene Thermal Maximum at around 55 Ma (Figs. 2 and 7) and a range of tectonic influences are apparent on the formation of glacial ice covers. Plate tectonic modification of continental elevation, ocean configuration and oceanic gateways are recognised as keys to understanding the transition from a long warm Mesozoic to a cooler Cenozoic (e.g., Poulsen et al., 1998; Hay et al., 2002; Prothero et al., 2003; Meijer et al., 2004). 11.1. Coupled tectono-climate models: the effects of plate collision and dispersal Cooling after 55 Ma is increasingly being related to the tectonic and bathymetric evolution of the oceans as Pangea continued to disintegrate (Scher and Martin, 2006; Via and Thomas, 2006) in combination with shorter term, orbitally forced ocean circulation changes (Shevenell et al., 2004). The dispersal of Pangea moved large continental blocks (Eurasia and North America) to higher latitudes (Turekian, 1996; Wolfe, 1978) where climatic effects of Milankovitch variations were amplified by uplift. The northward obduction of India against Eurasia built the glaciated Himalayas and released large volumes of easily weathered sediment, changed upper atmosphere flow patterns, planetary albedos (Ruddiman et al., 1989; Kutzbach et al., 1989, 1993), monsoon intensity (and thus mechanical and chemical weathering) and atmospheric CO2 (Raymo and Ruddiman, 1992 though note Kerrick and Caldeira, 1999). In the southwest continental USA, crustal extension began in the Oligocene after 30 Ma resulting in broad scale uplift of the Basin and Range. This affected an area more than 900 km wide and produced elevations greater than 1.5 km above sea level across the Colorado Plateau (Parsons, 1995). Some models estimate an Earth that was 110 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 Fig. 7. The Late Cenozoic glacio-epoch after 55 Ma. The breakup of Pangea moved large landmasses into higher latitudes, isolated Antarctica and changed the configuration and bathymetry of ocean basins. The first ice appears at c. 40 Ma in both northern and southern hemispheres but Milankovitch-forced Northern hemisphere ice sheets only developed after 3 Ma. After Hay et al. (2002), Scher and Martin (2006), Moran et al. (2006), Via and Thomas (2006), Eldrett et al. (2007). Milankovitch-forced continental ice sheets in the northern hemisphere were the culmination of some 50 million years of tectonically-influenced cooling. This may provide a model for the generation of glacio-epochs during earlier episodes of supercontinent breakup (Fig. 1). warmer by 1–6 °C in the absence of uplift in Central Asia and elsewhere (Harris, 2006). Ramstein et al. (1997) argued that the closure of the epicontinental Paratethys was also important in determining Eurasian climates after 30 Ma. In the Pacific southeast, the climatic effects of Andean uplift have been recognised (Hartley, 2003) as well as the role of glacial erosion in promoting additional topographic (Lamb and Davis, 2003) and climatic feedbacks (Montgomery et al., 2001). Additional cooling effects may have resulted from widespread passive margin uplift and escarpment formation along newly rifted margins around the southern Atlantic Ocean (e.g., Brown et al., 2000) such as reflected in widespread periglacial and glacial deposits in southern Africa (e.g., Lewis and Illgner, 2001) though this is debated (Boelhouwers and Meiklejohn 2002). 11.2. Glaciation at the tops of the world: Antarctic and Arctic cooling c. 40 Ma It has been thought that glacierization occurred early in the Antarctic by c. 44 Ma followed by ice growth around the Arctic at 14 Ma (the unipolar ice sheet model of Perlmutter and Plotnick, 2003) but recent work continues to push back the onset of glaciation in circumArctic regions ranging from 45 Ma (Moran et al., 2006) to c. 38 to 30 Ma (Eldrett et al., 2007). Cenozoic glaciation in Antarctica possibly originated along the West Antarctic Rift System which is one of the largest areas on the planet of high standing, extended crust comparable in size to the East African rift. Abrupt uplift of the 3500 km long and 4 km high Transantarctic Mountains after 95 Ma records rifting above a mantle hot spot (ten Brink and Stern, 1992; Stump, 1995; Hamilton et al., 2001; Studinger et al., 2002; Winberry and Anandakrishnan, 2004). Major extension and uplift culminated just after 50 Ma and it cannot be just coincidental that the earliest ice sheet capable of reaching sea level (and thus recorded in the marine record) occurred at about 43 Ma before present (Lear et al., 2000). The separation of Australia and the opening of the Drake Passage at c. 40 Ma resulted in the thermal isolation of Antarctica at high polar latitudes and, in conjunction with changing ocean currents, set up the conveyor belt N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 of oceanic circulation that dominates the Cenozoic (Prothero et al., 2003; Scher and Martin, 2006). The onset of North Atlantic Deep Water production at about 33 Ma (Fig. 7) was tectonically triggered by subsidence of the Faroe–Icelandic Ridge (Via and Thomas, 2006). According to DeConto and Pollard (2003) Antarctic glacierization just before 40 Ma was aided by CO2 drawdown resulting from weathering of shallow water carbonates. Tripati et al. (2005) suggest at that time there was a global sea level fall of 125 m as a consequence of ice sheet growth in both hemispheres but the geologic record does not support this model because it requires global ice volumes similar to that of the classic Late Cenozoic ice ages after 3.5 Ma. Hitherto, the widely accepted model has been that significant landbased ice did not form in the Northern Hemisphere until about 14 Ma (e.g., Cecil and Edgar, 2003). Further resolution has until recently, been prevented by the lack of a detailed sediment record from the Arctic basin but this data gap has now been filled. Arctic drilling on the Lomonosov Ridge in 2004 identifies a lonestone in middle Eocene sediment (∼ 45 Ma) accepted as evidence of the onset of iceberg rafting in the Arctic, and the near synchronous start of glaciation in both hemispheres (Moran et al., 2006). Interestingly, the Lomonosov Ridge is a very large rifted piece of continental crust that was detached about 57 million years ago at the Paleocene–Eocene boundary by seafloor spreading along the Gakkel Ridge (Jokat et al., 1992). The overall geodynamic setting of this part of the Arctic during polar cooling after 55 Ma is directly analogous to that obtaining during the onset of the Late Ordovician Saharan glacio-epoch at 440 Ma along the north Gondwana polar margin (Fig. 5; Section 7.4). Significant Arctic cooling at about 45 Ma is indicated by abundant ice rafted debris in late Eocene to Oligocene sediments that indicates isolated calving glaciers in east Greenland (Eldrett et al., 2007). This is at a time when atmospheric CO2 concentrations were at least four times present. Later evidence for circum Arctic glacier ice occurs during the Middle Miocene Transition (Thiede and Myhre, 1996; Helland and Holmes, 1997) (recorded just after 16 Ma at the top of a prominent condensed interval in Arctic ocean cores; Moran et al., 2006), which coincides with a major expansion of the Antarctic Ice Sheet (Shevenell et al., 2004). By 14 Ma the abundance of Arctic Ocean ice rafted debris greatly increases, marking the onset of glaciation in Greenland and continues until about 5 Ma when there was a marked spike in warmth during the early Pliocene (involving an increase of as much as 10 °C) (Ballantyne et al., 2006). Renewed 111 cooling at 3 Ma ended this warm episode as incipient ice sheets began to wax and wane over Northern Europe and North America as part of classic Milankovitch-driven ice ages and interglacials. 11.3. Circum North Atlantic glaciation: Milankovitch-driven ice sheets The high latitude Atlantic margins are built of thickened Proterozoic crust underplated magmatically by the Iceland Plume (e.g., Wood et al., 1989; Eyles, 1996; Chalmers and Cloetingh, 2000; Mathieson et al., 2000; Faleide et al., 2002; Nielsen et al., 2002; Huuse, 2002; Rohrman et al., 2002; Redfield et al., 2005). In the cooling world after the Eocene, the increase in regional elevation, involving wholesale uplift of plateau surrounding the North Atlantic Ocean, was likely a major factor in promoting the growth and survival of perennial snowfields. These ultimately would develop into Milankovitch-modulated ice sheets after 3.5 Ma. In Scandinavia, episodes of uplift along the Norwegian continental margin are recorded by raised marine planation surfaces and are precisely correlated with offshore unconformities and influxes of glacial sediment in the marine record (Hendriks and Andriessen, 2002; Huuse, 2002; Hinderer and Einsele, 2002; Stoker, 2002) thereby providing a clear and well calibrated example of the causative climatic role of uplift along a passive margin. The North Atlantic borderlands share a common Mesoproterozoic geological and geomorphological heritage. Basement rocks in eastern North America, Greenland and Scandinavia were peneplaned by about 800 Ma BP. These shields experienced deep weathering during the Mesozoic (the ‘etch plain’ model of LidmarBergstrom et al., 2000; Lidmar-Bergstrom and Naslund, 2002). Deep clayey regolith was later stripped by Cenozoic uplift and reworked from the shields by ice sheets as till. Glacial erosion created the typical ‘glaciated shield topography’ of structurally controlled, overdeepened valleys (fiords, lakes) cut into little modified plateau surfaces on which tropical landforms of deep weathering (e.g., tors) survive locally (Dyke, 2004). 11.4. Pacific Northwest glaciation The role of tectonotopography in glacierization so well displayed along the Scandinavian passive margin is also evident in western North America but along the very different, compressional Gulf of Alaska margin. Pangean rifting after 200 Ma altered the trajectory of the North American plate creating an active plate margin what is 112 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 now British Columbia and Alaska characterised by terrane accretion and broad scale uplift. In the Pacific northeast, collision of the North America plate against the Yakutat block (an exotic terrane carried by the Pacific plate; Mazzotti and Hyndman, 2002) promoted rapid uplift along the Gulf of Alaska continental margin after 5 Ma. This created the highest mountain chain in North America (Chugach-St.Elias) and triggered regional glaciation of the far North Pacific Ocean (Haug et al., 2005) accompanied by the release of enormous volumes of glacially derived sediment to the Gulf of Alaska basin where the glacially influenced Yakataga Formation is at least 5 km thick (Lagoe et al., 1993). In contrast, the terrestrial record is meager (Eyles and Eyles, 1989a,b). The presence of this high topographic barrier along the coast contributed to cooling and permafrost in the interior of northern North America and subsequently, the development of sea ice covers in the Arctic Ocean after 3 Ma (White et al., 1997; Westgate 2003). At about the same time, final closure of the Straits of Panama by the building of a Central American volcanic arc after 3 Ma may have diverted warmth to the far northern hemisphere triggering changes in global ocean circulation (Driscoll and Haug, 1998). Ice sheet growth has also been linked to warm surface waters in the North Pacific increasing the availability of moisture over the northern interior of North America (Haug et al., 2005). 11.5. Summary The Late Cenozoic glacio-epoch highlights the interlinking of tectonically driven changes in ocean basin configuration and bathymetry, uplift adjacent to sources of oceanic moisture, and orbitally driven changes in ocean circulation. The broader geodynamic context was the breakup and dispersal of Pangea and the movement of large landmasses into more northern latitudes. Significant cooling in the Arctic just before 40 Ma is broadly coincident with the first appearance of ice in Antarctica following its divorce from South America and Australia and uplift along the Transantarctic Rift. 12. Discussion This paper has reviewed geodynamic factors that contribute to the appearance of the major glacio-epochs over the last ∼3 Ga. Glacio-epochs appear to be a response to complex interactions between changes in continental positioning and elevation, paleolatitude, ocean circulation and ventilation (e.g., Worsley and Kidder, 1991; Worsley et al., 1994; Poulsen et al., 1998; Crowell, 1999; Ruddiman, 2001; Rothman, 2002; Berner, 2003; Smith and Pickering 2003; Veizer, 2005; Figs. 1 and 2). Of all possible factors, variation in atmospheric CO2 appears to be the most poorly correlated with glacio-epochs, at least for the Phanerozoic (Fig. 2). The fundamental timing of glacio-epochs at least over the last 3 Ga appears may be related to celestial drivers such as variation in cosmic ray flux (CRF; Shaviv, 2003) suggesting that glacio-epochs are periodic phenomena. Other complicating factors are at work however insofar as some CRF highs are indeed matched by glacio-epochs whereas others are not such as about 900 Ma and after 200 Ma (Fig. 2C). Different processes combine to create a tipping point where climatic thresholds are crossed and ice can form over higher, tectonically generated elevations. A major challenge remains the lack of data regarding paleoelevations, as emphasized by Hay et al. (2002), Lamb and Davis (2003), Shuster et al. (2005), Harris (2006) and Ghosh et al. (2006). Being cognisant of these complexities, it is possible to group Earth's glacio-epochs into two major tectonoclimatic types; those that occur during supercontinent assembly and those that occur during or shortly after supercontinent break up. This is necessarily an oversimplification of earth's tectonic history given the diachronous nature of supercontinent assembly and breakup. Nonetheless, it identifies an important relationship between breakup and earth's glacial record. The bulk of Earth's glacial record is preserved within sedimentary basins formed on extended continental crust. 12.1. Assembly related glacio-epochs Glacio-epochs associated with plate collisions and regional uplift associated with supercontinent assembly appears to be rare, or at least the record of such environments is sparsely preserved (Fig. 1). Regional cooling across the Frasnian–Famennian boundary after 380 Ma has been linked to collisional orogenesis (Averbuch et al., 2005) but the only glacio-epoch initiated in this fashion is that of the Late Paleozoic Gondwanan (325 to 250 Ma) triggered by uplift on active margin mountains as Gondwana collided with Laurasia (Section 8). Nonetheless, the fact remains that the stratigraphic record of this glacio-epoch is restricted with few exceptions, to marine extensional basins. The general lack of ‘assembly related’ glacio-epochs is surprising because orogenesis allows maximum exposure of terrestrial sediment to weathering processes during long overland transport to marine basins, thereby maximizing the drawdown of pCO2. Major glacioepochs should be associated with the Grenville Orogeny N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 at 1 Ga and others with the protracted orogenic activity during the Mesoproterozoic but this is not the case other than a single, short-lived event in Australia (Williams, 2005; Section 5). Averbuch et al. (2005) argued that cooler but non-glacial phases in Earth history are linked with enhanced mountain building and enhanced chemical erosion of continental crust such. In contrast, such effects do not appear at first blush, to have had a major effect on global cooling over the past 3 Ga (Fig. 1). Otherwise, glacio-epochs would be more commonly identified with phases of supercontinent assembly and mountain building particularly during the Mesoproterozoic and Neoproterozoic. This absence supports Kerrick and Caldeira's (1999) thesis that the effects of tectonically driven CO2 drawdown are overstated because the process is temperature dependent and its efficacy is greatly reduced during cooling. The apparent rarity of active margin glacio-epochs may also reflect the low preservation potential of glacial sediments in such settings. For example, Cenozoic glaciation of major mountain ranges created within continental interiors by compressional plate forces (such as the Himalayas) has left no primary glacial sediment (even ice-rafted debris) in the offshore sediment record (e.g., in the Indian Ocean). This is because glacial sediment is stored in floodplains or entirely reworked by rivers during long overland transport to the marine realm. The deposits of glaciated backarc basins along compressional margins are similarly underrepresented in the rock record. On the other hand, the longest and possibly most complete record of late Cenozoic glaciation anywhere on Earth is preserved in the 5 k thick Yakataga Formation of the Gulf of Alaska compressional margin (Lagoe et al., 1993; Eyles, 1993; Section 11.4). There, vast volumes of glaciogenic sediment released from rapidly uplifting coastal mountains and glaciers have been fed directly into deepwater submarine fans systems. The equivalent terrestrial glacial record inland is no more than a few hundred metres thick and only locally preserved (Eyles and Eyles, 1989a,b). Similarly, dramatic increases in sedimentation rates that accompanied glaciation at the same time along the Chilean convergent margin, choked offshore forearc basins with thick successions of glacially influenced marine strata (Melnick and Echter, 2006). The long-term fate of these thick successions is to be incorporated within the continental margin as highly deformed accretionary slivers. 12.2. Break up related glacio-epochs Most glacio-epochs have occurred in the geodynamic context of rifting and crustal extension (Figs. 1 and 2). 113 Paleoproterozoic (c. 2.4 Ga) and Neoproterozoic (c. 750-580 Ma) glacio-epochs are ‘break up related’ formed during the breakup of Kenorland and Rodinia respectively (Fig. 1). It is also possible that extension along high latitude continental margins and consequent uplift also played a role in triggering Ordovician glaciation at c. 440 Ma (when terranes rifted off Gondwana; Section 7.4) also Cenozoic ice covers after 55 Ma around the Arctic (rifting of the Lomonosov Ridge?) and in the Antarctic along the Transantarctic Rift. The protracted disintegration of Pangea still continues in Africa to the present day and glaciers occur along the uplifted but low latitude East African Rift system. Given the recurring association of glacio-epochs with continental break up throughout Earth history (Fig. 1) it appears that there is an essential continuity of tectonic boundary conditions for Proterozoic and Phanerozoic glacio-epochs. This begs the question as to the nature of possible causal links between rifting and climate change, or simply whether glacial sediments have been selectively preserved in this geodynamic setting. 12.3. Rifting and glaciation: causal link or simply preservational bias? Allen (2007a) provides a detailed basin analysis of Neoproterozoic glacial deposits in Oman (Huqf Supergroup), that supports the central thesis of this paper of a preferred relationship of glacial successions with rift basins. Allen (2007a) suggested this was clear evidence of a preservational bias rather than of causation between rift topography and cooling. Intracratonic extension is associated with significant regional uplift and thus cooling. Rift flank uplift is the consequence of flexural rebound at the edges of rifted plates combined with warming and thermal uplift arising from the upward movement of the lithosphere-asthenosphere boundary below the rift (e.g., Garfunkel, 1988; Bott, 1995; Van Der Beek, 1997). The magnitude of flexural arching varies however and depends among other things, on the thickness and rigidity of the rifted plate (Braun and Beaumont, 1989; Weissel and Karner, 1989; King and Ellis, 1990; Kooi and Beaumont, 1994; Fig. 8). Rifting can generate a dynamic topography of up to 1 km in elevation but is a transient condition followed by postrift thermal subsidence compounded by sediment loading. A lithospheric plate with a high effective elastic thickness (Te) characteristic of old thick continental crust produces a rift flank uplift that is high and broad (Fig. 8) sufficient to cross a climatic threshold allowing snow and ice to accumulate. Extension of cold and thick (e.g., 30–60 km) continental 114 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 Fig. 8. Geometry of continental extension (after Ebinger et al., 2002) as occurred during the Paleoproterozic and Neoproterozoic glacio-epochs (Fig. 1). The most extensive uplifts are created where crust is old, thick and thus flexurally rigid (creating a high effective elastic plate thickness: EEPT). The resulting isostatic rebound and uplift is distributed across a large area. This was the case during the break up of Rodinia after 750 Ma that resulted in the breaking of thick Paleoproterozoic continental crust that has amalgamated mainly after 2.5 Ga (Fig. 1). Additionally, mantle convection below the broken margin, and magmatic underplating contributes to uplift of passive margins and was instrumental in creating uplifted plateau on which late Cenozoic ice sheets grew on the margins of the North Atlantic Ocean after 3 Ma (e.g., Laurentide, European ice sheets). lithosphere such as in East Africa and on the margins of the Red Sea, has created a dynamic topography 1200 m above sea level across a zone more than 1000 km wide (e.g., Khalil and McClay, 2001; Ebinger et al., 2002; Morley, 2002). Partridge (1997) emphasized the climatic effects of uplift along the rift flanks. The magnitude of uplift in Kenya approaches 2 km (Morley, 2002) with tropical conditions along the rift floor and glaciated volcanic peaks of mounts Kilimanjaro and Kenya rising above. The breakup of Rodinia was likely marked by enhanced dynamic topography where rifts developed in old thick Archean and Proterozoic crust such as in Australia, NW Canada and in northern Europe. It is worth emphasizing that there can be a significant delay between rifting, uplift and any climate response. Illies (1977) and Garfunkel and Bartov (1977) showed that rift flank uplift commenced as much as 15 million years after rifting is initiated. The delay between rifting and climatic cooling increases to several tens of million years in the case of Cenozoic glaciations that developed around the North Atlantic borderlands long after Atlantic Ocean rifting was initiated (Eyles, 1993; Nielsen et al., 2002). Similarly, the diachronous nature of rifting along propagating spreading centers (e.g., Huchon et al., 2001; Morley, 2002) means that rift flank uplifts and corresponding regional changes in climate and a glacial influence on sedimentation, will be time transgressive. Other aspects of continental breakup may also favour glaciation. A rift creates several elevated zones flanking sources of moisture (an incipient ocean; Fig. 7 and 9), which is the key requirement for building ice sheets, not cold temperatures. This aspect was emphasized by Hay et al. (2002) in an excellent review of the links between tectonics and climate in the Late Cenozoic. In addition, rifting reverses plate motions creating active plate margins and elongate areas of glaciated mountainous topography on the outboard sides of the newly rifted plates. Above all, very short transport distances between uplifted source areas and rapidly subsiding basins (e.g., Fig. 3) minimizes sediment reworking promoting preservation of glacially influenced (mostly marine) facies. Uplifted source areas N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 115 Fig. 9. Simplified diagram illustrating principal differences between glacio epochs resulting from uplift resulting from continental collision (A1 and A2) and resulting from continental extension (B1, B2). In the first case, uplift occurs along a narrow intracratonic mountain belt in the dry interior of the continent. These glaciers leave only a minor terrestrial record of glaciation that is easily destroyed by large rivers. This contrasts with the much larger ice covers that grow on broad uplifted rift shoulders close to moisture sources provided by opening oceans. Ice lobes reach sea level and leave a record of glacially influenced marine strata in rift basins. Rifting is markedly diachronous resulting in a non-synchronous relationship between glaciation, the formation of fault bounded sub basins and sedimentation along growing rifts (Fig. 3). Additional tectonotopography is created by terrane accretion along the now active margins of the two new plates. are separated from basins only by steep slopes along the basin margin (Withjack et al., 2002). These slopes are rapidly incised by glacial erosion allowing direct delivery of sediment to the basin from upland source areas. Virtually the entire sediment flux in extensional settings is produced close to basin depocentres and is rapidly buried with minimal exposure to fluvial reworking and terrestrial weathering with limited potential to influence atmospheric composition. In this way, Earth's glacial record may be selectively associated with rifting not just through the effects of uplift-generated cooling and the availability of moisture, but also because of a preservational bias (see Fig. 9). Allen (2007b) has suggested that glacial deposits are selectively preserved in rifts at times when global climate was cooled by mountain building in collisional tectonic settings. As we have identified above, this was certainly the case in the long Permo-Carboniferous glacioepoch that affected Gondwana after 350 Ma (Section 9.1). The glacio-epoch was triggered by collision but its depositional record is found in later rift basin fills. This model however, does not fit well with the preferred relationship of glacio-epochs with times of supercontinent breakup (Fig. 1) where global tectonics were dominated by extension not collisional processes. 13. Concluding remarks A review of Earth's glacial record over the last 3 Ga indicates a close relationship between glacio-epochs and times of enhanced crustal extension during the Proterozoic and Phanerozoic; most of Earth's glacial record appears to be preserved in extensional basins. Tectonically generated topography produced by crustal extension may be an important control on cooling in conjunction with increased availability of moisture. Clearly there are times in Earth history of rifting with no ice, and ice with no rifting but the marked association between the two for most ancient glacio-epochs cannot be simply coincidental. This association could simply reflect better preservation of glacial strata in extensional basins and the tendency for glacial deposits to be entirely reworked by large rivers in compressional settings. Having recognised the importance of tectonic preconditions under which glacio-epochs develop and glacial deposits are preserved, detailed consideration of the role of tectonics in influencing climate and controlling water depths, sediment supply and the age of sedimentary successions, is essential in future basin investigations and climate models. 116 N. Eyles / Palaeogeography, Palaeoclimatology, Palaeoecology 258 (2008) 89–129 Acknowledgements The Natural Sciences and Engineering Research Council of Canada is thanked for generous research support. Several colleagues provided informative correspondence and discussions principally Paul Ramaekers, Carolyn Eyles, Jan Veizer, Louise Daurio, Angie Falcon, Nicole Januszczak, Vic Gostin, Andrew Miall, Kath Grey, Phil Allen, Paul Gammon, Jeff Lewis, Dick Peltier, Noel James, John Crowell and Tony Tankard. Any errors of interpretation or omission remain entirely mine. Two anonymous journal reviewers and Finn Surlyk are thanked for their very helpful comments. This is a contribution to IGCP 512 ʻNeoproterozoic Glaciations'. References Alene, M., Jenkin, G.R.T., Leng, M.J., Darbyshire, D.P.F., 2006. The Tambien Group, Ethiopia: an early Cryogenian (ca. 800–735 Ma) Neoproterozoic sequence in the Arabian–Nubian Shield. Precambrian Research 147, 79–99. Allen, P.A., 2006. Snowball Earth on trial. 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