Climatic cycles during a Neoproterozoic “snowball” glacial epoch
Ruben Rieu Repsol YPF, Exploration & Production, C/Orense 34, 28020 Madrid, Spain
Philip A. Allen* Department of Earth Science and Engineering, Imperial College London, South Kensington Campus,
Michael Plötze
Thomas Pettke
London SW7 2AZ, UK
Institute for Geotechnical Engineering, ETH-Zürich, Schafmattstrasse 6, CH-8093 Zürich, Switzerland
Institute for Geological Sciences, University of Berne, Baltzerstrasse 1-3, CH-3012 Bern, Switzerland
ABSTRACT
The profound glaciations of the Neoproterozoic Cryogenian period (ca. 850–544 Ma) represent an extreme climatic mode when, it is claimed, Earth was fully or almost completely
covered with ice for millions of years. We show that the geochemistry and mineralogy of finegrained Neoproterozoic sedimentary rocks in Oman are best explained by climatic oscillations
that drove variations in the intensity of chemical weathering on contemporary land surfaces.
The cold climate modes of the Cryogenian were therefore cyclical, punctuated with welldefined warm-humid interglacial periods. The hydrological cycle and the routing of sediment
were active throughout the glacial epoch, which requires substantial open ocean water. This
reconstruction represents a significantly different target for numerical climate models at this
critical time in the evolution of Earth’s biosphere.
Keywords: snowball Earth, climate, weathering, glaciation, Neoproterozoic, Fiq, Oman.
INTRODUCTION
The possibility that Earth was repeatedly
completely frozen for periods of several millions of years in the Neoproterozoic (1000–
544 Ma) (Hoffman et al., 1998) is currently
among the most interesting and controversial
topics in Earth history. There is considerable
debate, however, as to whether the boundary
conditions required for Earth to enter or exit a
“snowball” state were reached (Crowley et al.,
2001; Lewis et al., 2004; Pierrehumbert, 2004).
Additionally, a growing body of sedimentary (e.g., Leather et al., 2002; Kellerhals and
Matter, 2003) and paleobiological (Olcott et al.,
2005; Corsetti et al., 2006) evidence suggests
less severe freezing than envisaged in the snowball Earth hypothesis and that open continental
shelves or equatorial oceans may have existed
even at times of glacial climax (Chandler and
Sohl, 2000; Hyde et al., 2000; Crowley et al.,
2001). It is crucial to remove some of these
uncertainties if the Cryogenian climatic mode is
to be used as an example of the Earth system at
its climatic limit (Hoffman and Schrag, 2002).
The Huqf Supergroup of Oman provides critical
evidence for the dynamics of glaciation during
the Cryogenian.
The Huqf Supergroup of Oman crops out in
three main areas (Fig. 1). In the Jabal Akhdar of
northern Oman, the Huqf Supergroup contains a
relatively thick (at least 1.5 km) succession (Fiq
Formation) of glacigenic and nonglacial marine
sedimentary rocks that filled a fault-bounded
basin formed by continental extension, overlain
by a thin (<8 m) cap carbonate, which has carbon
isotopic values depleted in 13C and is known as
the Hadash Formation (Leather et al., 2002; Allen
*E-mail: [email protected].
et al., 2004). The cap carbonate passes up gradationally into the marine shales and sandstones of
the Masirah Bay Formation (Allen and Leather,
2006). The assemblage of glacial diamictites,
debris-flow deposits, turbiditic sandstones,
hemipelagic shales, and wave-rippled shoreface
sediments is thought to be end-Cryogenian in
age (Brasier et al., 2000; Allen et al., 2004). A
low paleolatitude for Oman has been proposed
(Kempf et al., 2000; Kilner et al., 2005). The
Fiq Formation in Oman is an ideal test-bed for
the validity of key aspects of the snowball Earth
hypothesis, and whatever can be learned about
the snowball-type end-Cryogenian glaciation in
Oman is likely to be of generic importance in
assessing Neoproterozoic climate change.
BULK MINERALOGY AND
ELEMENTAL COMPOSITION AS AN
INDEX OF CLIMATE CHANGE
If severe climatic changes took place in the
Neoproterozoic era, a record of these changes
would be expected to be preserved in the bulk
mineralogical and chemical compositions of
the associated siliciclastic rocks, which depend
on the intensity of chemical weathering in the
source areas (Nesbitt and Young, 1982; Nesbitt
et al., 1996). Therefore, changes in the chemical
and mineralogical compositions of sedimentary
rocks can potentially be used as a proxy for climate change.
The chemical index of alteration (CIA) is
potentially useful to evaluate changes in climate (Nesbitt and Young, 1982; McLennan
et al., 1993; Fedo et al., 1995; Nesbitt et al.,
1996; Scheffler et al., 2003). High CIA values
reflect the removal of mobile cations (e.g., Ca2+,
Na+, K+) relative to stable residual constituents
(Al3+, Ti4+) during chemical weathering, which
is enhanced during humid and warm climate
conditions. Low CIA values, on the other hand,
indicate the near absence of chemical weathering and consequently might reflect cool and/or
arid conditions. Since clay minerals form during progressive chemical weathering, largely
at the expense of plagioclase and potassium
feldspar, with quartz being relatively stable, the
ratio quartz/(quartz + K-feldspar + plagioclase)
(which is knows as the mineralogical index of
alteration [MIA]; see supplementary methods
in the GSA Data Repository1) is also expected
to be influenced by the intensity of chemical weathering (Johnsson, 1993; Nesbitt et al.,
1996; Nesbitt and Markovics, 1997).
Changes in clay mineral composition and
abundance may also reflect variability in source
rocks and/or hydrodynamic sorting during sediment transport. To ensure a well-mixed provenance and to minimize the effects of hydrodynamic sorting, this study is limited to mudstone
beds and the mudstone matrices of diamictites.
Subtle grain-size differences between the mudstone samples may exist, however. MIA values
are particularly useful in this case, since they
are largely unaffected by sorting and abrasion
(Nesbitt et al., 1996; Nesbitt and Young, 1996).
In addition, the absence of a positive correlation
between LOI (loss on ignition) and CaO content
implies that higher Ca contents reflect incorporation of a higher proportion of less weathered
material and not of carbonate (Fig. DR3 [see
footnote 1]).
We analyzed 76 bulk samples from continuous sections in the western Jabal Akhdar (Fig. 1;
Fig. DR1) that record the end-Cryogenian
glaciation and its direct aftermath, using
laser-ablation–inductively coupled plasma–mass
spectrometry (ICP-MS) for trace elements (32
samples), X-ray fluorescence (XRF) for major
elements, and, Rietveld analysis of X-ray diffraction (XRD) spectra to obtain quantitative
mineralogical compositions (Tables DR1 and
DR2). Analytical procedures are described in
the supplementary information (see footnote 1).
Corrections of CIA values for carbonate-derived
Ca were <0.1 CIA unit (<0.02 wt% CaO). Cor1
GSA Data Repository item 2007074, supplementary information on methods and additional geochemical data, is available online at www.geosociety.
org/pubs/ft2007.htm, or on request from editing@
geosociety.org or Documents Secretary, GSA,
P.O. Box 9140, Boulder, CO 80301, USA.
© 2007 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
GEOLOGY,
April
2007
Geology,
April
2007;
v. 35; no. 4; p. 299–302; doi: 10.1130/G23400A.1; 3 figures; Data Repository item 2007074.
299
B
U.A.E.
Jabal
Akhdar
Yemen
sampled
sections
SULTANATE
OF
OMAN
500km
SAUDI
ARABIA
20° N
Neoproterozoic outcrops
Sedimentary rocks
20o 30 Huqf
area
Basement
ARABIAN
SEA
SALALAH
YEMEN
54° E
100 km
Fara Fm.
PC/C
(542 ± 0.3 Ma)
Buah
Fm.
58 00 58° E
Limestone
v v Extrusive
Shale / siltstone
Masirah Bay Fm.
Khufai
Mas.Bay
Hadash
cap carbonate
(ca. 635 Ma)
Hadash
Hadash Fm.
Halfayn Fm.
(ca. 802 Ma)
Mirbat
Saqlah
Ghubrah
unconformity
712± 1.6 Ma
Shareef Fm.
Marsham Fm
(Upper Member)
Arkahawl Fm.
(Middle Member)
cap carbonate
Ayn Fm.
(Lower Member)
726 ± 0.4 Ma
Figure 1. Main outcrop areas of Neoproterozoic basement and sedimentary rocks in Oman
(A) and stratigraphic framework of Huqf Supergroup (B). UAE—United Arab Emirates.
rections for phosphate-derived Ca were typically <1.5 CIA units (rarely up to 5).
When plotted in A-CN-K (Al2O3–CaO +
N2O–K2O) space (Fig. 2), sediments produced
by intense chemical weathering appear in positions commensurate with high values of CIA
(80–100), whereas incipiently weathered sediments plot near the feldspar join (CIA of 50–70).
In Qtz-Pl-Kfs (quartz–plagioclase–K-feldspar)
space (Fig. 2), the position of incipiently weathered samples strongly depends on source rock
lithology. In both compositional and mineralogical space, however, changes due only to
increased chemical weathering cause samples
to move roughly parallel to the A-CN or P-Q
boundary, resulting in higher values of both
CIA and MIA. Diagenetic alteration, changes in
provenance, and, in A-CN-K space, grain size
Figure 2. Mineralogical and compositional variations of diamictite matrix and mudstone
samples of Fiq (black dots) and Masirah Bay (open circles) Formations. A: Q-P-K (quartz–
plagioclase–K-feldspar) mineralogical space (mineralogical index of alteration [MIA]). B:
A-CN-K (Al2O3–CaO + Na2O–K2O) compositional space (chemical index of alteration [CIA]).
MIA = [quartz/(quartz + K-feldspar + plagioclase) ] × 100 and CIA = [Al2O3 /(Al2O3 + K2O + Na2O
+ CaO*)] × 100. In B, arrow parallel to A-CN boundary is ideal chemical weathering trend of
granodioritic bedrock, which may be shifted toward K-apex (dashed arrow) due to potassium
metasomatism (illitization). Correction for potassium metasomatism is made by projecting
data points back onto ideal weathering pathway from K-apex. Projection of data points back
onto P-K join suggests source rocks of granodioritic composition on average. Qtz—quartz;
Pl—plagioclase; Kfs—K-feldspar; Cpx—clinopyroxene; Hbl—hornblende; Sm—smectite;
Bt—biotite; Ms—muscovite; Kln—kaolinite, Chl—chlorite.
300
may cause significant deviations from these
ideal pathways (Fedo et al., 1995; Nesbitt et al.,
1996; Nesbitt and Markovics, 1997).
Basement
Sandstone
Khufai Fm.
v v v
Mirbat area
Dolomite
Buah Fm.
Shuram Fm.
Shuram
Fm.
Diamictite
1 km
MUSCAT
Huqf
MIRBAT GROUP
Saudi Arabia
GULF OF OMAN
24° N
23 10
UAE Oman
ARA
ARABIAN GULF
Predominant lithologies
Jabal Akhdar
(Mirbat Sandstone Formation)
IRAN
Fiq Formation
Iran
HUQF SUPERGROUP
NAFUN GROUP
ABU MAHARA GROUP
A
RESULTS
Throughout the Fiq and Masirah Bay Formations, there are significant compositional and
mineralogical variations (Figs. 2 and 3). When
plotted in A-CN-K and Qtz-Pl-Kfs space, the
data define trends roughly parallel to the A-CN
and Pl-Qtz boundaries (Fig. 2), suggesting variability in the extent of chemical weathering of
the sediment in the source area (Fedo et al.,
1995; Nesbitt et al., 1996). Before interpreting
the data in terms of climatic variations, the influence of potential changes in grain size, hydrodynamic sorting, provenance, and diagenetic
alteration on the composition of the sediments
must be evaluated.
The first-order trend in CIA with stratigraphic
position is also seen in the mineralogical maturity (MIA) of the Fiq and Masirah Bay deposits (Fig. 3), as would be expected if the relative
enrichment in clay minerals (high CIA) resulted
from increased alteration of feldspars rather
than from hydrodynamic sorting. This suggests
that the first-order trend in CIA is not controlled
by the hydrodynamic sorting mechanism.
The composition of samples that have been
little affected by chemical weathering (low CIA,
Fig. 2B) suggests a granodioritic composition of
sediment sources, which is in agreement with
other major-element and trace-element contents
that suggest a constant granitic to granodioritic
source (Figs. DR4 and DR5 [see footnote 1])
(Taylor and McLennan, 1985; McLennan et al.,
1993; Fedo et al., 1997), and with felsic source
terrains indicated by clast lithologies in diamictites (granite, rhyolite, and volcanic tuff in addition to sedimentary clasts; Allen et al., 2004).
Subtle variations in source rock composition
may have caused some spread in sediment compositions, but the lack of correlation between
CIA and provenance indicators (e.g., Th/Sc,
Al/Ti, Zr/Ti) (Taylor and McLennan, 1985;
McLennan et al., 1993; Fedo et al., 1997) (Fig.
DR4) demonstrates that the first-order changes
in CIA cannot be explained by provenance
changes alone. Importantly, the main trends in
CIA are largely unaffected by the lithological
facies of the beds sampled, and therefore they
are clearly superimposed on the detailed sedimentary architecture of the succession. Since
CIA values do not coincide with facies changes
(Fig. 3), we can rule out the dominance of provenance changes or changes in depositional environment on CIA values.
Relatively low Zr/Sc ratios (McLennan et al.,
1993) (Fig. DR4e [see footnote 1]) indicate that
Fiq and Masirah Bay rocks are the products of
a first-order cycle from erosion to deposition
without polycyclic reworking, although the high
GEOLOGY, April 2007
CIA (uncorrected: dashed line)
65
75
B
85
CIA (uncorrected: dashed line)
65
MB Fm.
MB Fm.
1600
55
cap carbonate
T6
glacial
F7
85
glacial
F7
70
80
CIA (corrected: solid line)
1200
40
60
80
100
MIA
T5
F6
800
Fiq Formation
Stratigraphic height (m)
75
cap carbonate
Fiq
A
Legend:
glacial
T4
F5
Cap carbonate
T3
Diamictites
T2
F3
Sandstones
glacial
Shales, siltstones,
thin sandstones
400
No exposure
Dropstones
T1
F3
increasing chemical weathering
incr. chemical weathering
T1
Diamictite unit
Transgression
0m
60
70
80
CIA (corrected: solid line)
60
80
100
MIA
Figure 3. Variations in chemical and mineralogical indexes of alternation (CIA and MIA)
with stratigraphic height for section in Wadi Sahtan (A) and critical section across glacialpostglacial transition at Hadash, Wadi Mistal (B). Numbering of diamictites and flooding surfaces is after Leather et al. (2002) and Allen et al. (2004). Errors in CIA due to uncertainties in
major-element concentrations are <1.5% (<1 CIA unit). Two sigma error bars are indicated for
MIA values. MB Fm.—Masirah Bay Formation.
proportion of sedimentary clasts (average 40%)
suggests that inheritance of a previous weathering history of the sediment is possible. Such
inheritance is supported by the fact that CIA
values in glacially influenced deposits are never
as low as would be expected for sediments produced solely by mechanical erosion. However,
minor amounts of chemical weathering also
may have occurred in glaciofluvial and other
periglacial environments.
During burial diagenesis, potassium metasomatism may strongly change the bulk composition, and consequently the CIA, of sediments
(Fedo et al., 1995). Potassium metasomatism is
suspected to have influenced the sedimentary
rocks studied because the samples that deviate
from the ideal weathering trend are enriched in
potassium, which is supported by the observation of potassium-rich overgrowths on quartz
and feldspar minerals (Fig. DR2) and the common presence of illite in the studied samples. To
allow for a maximum possible effect of potassium metasomatism on CIA values, we applied
a correction assuming that the ideal weathering
trend originated from a granodioritic source
rock composition (Fig. 3).
VARIATIONS IN CHEMICAL
WEATHERING DURING A
SNOWBALL EPOCH
Both uncorrected CIA values and those corrected for a maximum amount of potassium
metasomatism are plotted as a function of their
stratigraphic height in Figure 3. In both cases,
GEOLOGY, April 2007
a similar first-order trend is revealed, consisting
of three intervals during which chemical weathering was reduced as indicated by relatively low
CIA and MIA values. Reduced chemical weathering in these intervals is in agreement with
the presence of distinctive sedimentary facies
(diamictites, dropstone-bearing laminites) that
suggest a cold climate. These intervals alternate
with units that are characterized by relatively
high CIA and MIA values and that lack evidence
for any glacial influence during sedimentation,
which are interpreted to represent interglacial
periods. Because the lowermost diamictite unit
(F1) is not preserved in the sampled sections,
an older glacial period than that represented
in this section by the lowermost unit (F3) may
have existed (Allen et al., 2004). Importantly,
the end of the entire glacial epoch corresponds
to a major increase in CIA and MIA values in
the lowermost Masirah Bay Formation. These
values are the highest found in the succession
(CIA > 80, MIA = 100).
It is possible that the observed compositional
and mineralogical variations in the Fiq Formation and lowermost Masirah Bay Formation are
due to variations in chemical weathering that
are unrelated to climate (Johnsson, 1993). For
example, the residence time of material exposed
to chemical weathering while stored in continental basins depends on catchment size and governing tectonics. As catchments enlarge, more
sediment is stored in trunk streams and alluvial
valleys. Evacuation of stored and chemically
weathered sediment from catchment valleys
would be favored by relative sea-level fall or by
a change to wetter climate following glaciation.
Increased cycling of sediment during periods
of higher tectonic activity in rifts, resulting in
lower CIA values, would normally be associated with increased footwall topography and
generally increasing water depths in hangingwall depocenters. The genetic stratigraphy of the
Fiq Formation (Leather et al., 2002; Allen et al.,
2004) and Masirah Bay Formation (Allen and
Leather, 2006), however, suggests an increase
in CIA values associated with transgression
and a decrease of CIA values associated with
shallowing- and coarsening-up trends into glacially influenced deposits (Fig. 3). Although
small-scale variations in CIA values observed
between transgressive surfaces T3 and T5 (Fig. 3)
seem to follow the trends expected to be associated with base-level variation, the major trends in
CIA are not closely linked to paleowater-depth
variations, and where there is a correlation (as
above diamictite F6; Fig. 3), rapid deepening is
associated with an increase in CIA values rather
than a decrease. We therefore rule out base-level
change as the mechanism responsible for the
major trends in CIA values, and we are confident
that the first-order trends in CIA are driven by
variations in the intensity of chemical weathering associated with climate change. Such climate
changes also influenced sea levels through the
build-up and melting of continental ice.
Since no chronometer for the climatic cycles
in the Fiq Formation is available, their duration
is unknown. A typical sediment accumulation
rate in rift basins of 0.1–0.2 mm y–1 implies
that cycles comprising 200–500 m of stratigraphy would represent 1–5 m.y., and that the Fiq
glacial epoch lasted a total of 10 m.y. or more.
We do not know the forcing mechanism or
internal system dynamics for climatic cyclicity
on this time scale. However, we note that a similar, dynamic climatic regime of multiple glaciations of short duration (<5 m.y.) alternating with
longer periods of globally warmer interglacial
or nonglacial conditions within a long icehouse
epoch is now the preferred view of the late
Paleozoic glacial epoch (Scheffler et al., 2003;
Fielding et al., 2006), rather than a single protracted glaciation between 320 and 265 Ma.
IMPLICATIONS FOR CRYOGENIAN
CLIMATE EXTREMES
The recognition of climatic cycles embedded
within the Fiq glacial succession is important for
the evaluation of climatic extremes during the
Neoproterozoic, since it is problematic for such
cycles to have been generated on a completely
frozen Earth characterized by a hydrological
shutdown or much-reduced water cycle driven
by sublimation. Consequently, the importance
for climatic reconstruction rests on the precise
points in time within the glacial epoch when
301
the sediments bearing evidence for climatic
cyclicity were deposited. While acknowledging
the possibility that the Fiq stratigraphy may represent deposition only during glacial recession,
the more plausible explanation for the Oman
succession is that the entire end-Cryogenian
epoch was climatically pulsed. In such a scenario, the Earth never froze completely from
pole to equator. This model would explain the
many other glacially influenced Neoproterozoic
sedimentary successions overlain by a single cap
carbonate, but which contain diamictite units
alternating with intervals lacking evidence for
glacial conditions. Intermittent climatic amelioration during “snowball” events would also help
to explain recently reported biomarker evidence
from Brazil, which suggests that periods of
increased primary productivity and photosynthesis existed during a Neoproterozoic glacial
epoch (Olcott et al., 2005).
The strong increase in chemical weathering
indices (CIA, MIA) in the postglacial period
is also in accord with the sedimentological
record worldwide, which suggests that the
abrupt change from diamictites to transgressive
cap carbonate and overlying shales primarily
reflects postglacial global warming (Hoffman
et al., 1998; Hoffman and Schrag, 2002) rather
than a local change in tectonic regime (Eyles
and Januszczak, 2004).
The evidence for cyclic climatic excursions
within long Neoproterozoic snowball glacial
epochs invites a closer comparison with the
extensive glaciations that took place during
Phanerozoic times, such as those of the late
Paleozoic. It also requires a reconsideration of
the limits of climate change represented by Neoproterozoic Earth history. The target for numerical models, and the background paleoenvironments hosting evolutionary developments in the
biosphere, should involve a markedly pulsed climatic history combined with remnants of open
oceans on a strongly glaciated Earth.
ACKNOWLEDGMENTS
Allen acknowledges ETH-Zürich for Ph.D. studentship TH-1/02-3 to Rieu. We thank Petroleum Development Oman (PDO) for long-term financial and
logistical support; J.L. Etienne, A. Cozzi, C. Fedo, and
M. Kennedy for comments and suggestions; Graham
Shields, Christopher Fielding, and an anonymous referee for their reviews; G. Kuper for field assistance;
and E. Reusser for help with microprobe analysis.
REFERENCES CITED
Allen, P.A., and Leather, J., 2006, Siliciclastic marine
sedimentation in the aftermath of a Marinoan
glacial epoch: The Masirah Bay Formation,
Huqf Supergroup, of Oman: Precambrian
Research, v. 144, p. 167–198, doi: 10.1016/
j.precamres.2005.10.006.
Allen, P.A., Leather, J., and Brasier, M.D., 2004,
The Neoproterozoic Fiq glaciation and its
302
aftermath, Huqf Supergroup of Oman: Basin
Research, v. 16, p. 507–534, doi: 10.1111/
j.1365–2117.2004.00249.x.
Brasier, M., McCarron, G., Tucker, R., Leather, J.,
Allen, P., and Shields, G., 2000, New U-Pb zircon
dates for the Neoproterozoic Ghubrah glaciation
and for the top of the Huqf Supergroup, Oman:
Geology, v. 28, p. 175–178, doi: 10.1130/0091–
7613(2000)028<0175:NUPZDF>2.3.CO;2.
Chandler, M.A., and Sohl, L.E., 2000, Climatic
forcings and the initiation of low-latitude ice
sheets during the Neoproterozoic Varanger glacial interval: Journal of Geophysical Research,
v. 73, p. 3–26.
Corsetti, F.A., Olcott, A.N., and Bakermans, C.,
2006, The biotic response to Neoproterozoic
snowball Earth: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 232, p. 114–130,
doi: 10.1016/j.palaeo.2005.10.030.
Crowley, T.H., Hyde, W.T., and Peltier, W.R.,
2001, CO2 levels required for deglaciation of a “near-snowball” Earth: Geophysical Research Letters, v. 28, p. 283–286, doi:
10.1029/2000GL011836.
Eyles, N., and Januszczak, N., 2004, ‘Zipper-rift’:
A tectonic model for Neoproterozoic glaciations during the breakup of Rodinia after
750 Ma: Earth-Science Reviews, v. 65, p. 1–
73, doi: 10.1016/S0012–8252(03)00080–1.
Fedo, C.M., Nesbittt, H.W., and Young, G.M., 1995,
Unraveling the effects of potassium metasomatism in sedimentary rocks and paleosols,
with implications for paleoweathering conditions and provenance: Geology, v. 23, p. 921–
924, doi: 10.1130/0091–7613(1995)023<0921:
UTEOPM>2.3.CO;2.
Fedo, C.M., Young, G.M., and Nesbittt, H.W., 1997,
Paleoclimatic control on the composition of the
Paleoproterozoic Serpent Formation, Huronian
Supergroup, Canada: A greenhouse to icehouse
transition: Precambrian Research, v. 86, p. 201–
223, doi: 10.1016/S0301–9268(97)00049–1.
Fielding, C.R., Frank, T.D., and Isbell, J.L., 2006,
The late Paleozoic ice age revisited: Eos
(Transactions, American Geophysical Union),
v. 87, p. 8, doi: 10.1029/2006EO080005.
Hoffman, P.F., and Schrag, D.P., 2002, The snowball
Earth hypothesis: Testing the limits of global
change: Terra Nova, v. 14, p. 129–155, doi:
10.1046/j.1365–3121.2002.00408.x.
Hoffman, P.F., Kaufman, A.J., Halverson, G.P., and
Schrag, D.P., 1998, A Neoproterozoic snowball earth: Science, v. 281, p. 1342–1346, doi:
10.1126/science.281.5381.1342.
Hyde, W.T., Crowley, T.J., Baum, S.K., and Peltier, W.R., 2000, Neoproterozoic “snowball
Earth” simulations with a coupled climate/icesheet model: Nature, v. 405, p. 425–429, doi:
10.1038/35013005.
Johnsson, M.J., 1993, The system controlling the
composition of clastic sediments, in Johnsson,
M.J., and Basu, A., eds., Processes Controlling
the Composition of Clastic Sediment: Geological Society of America Special Paper 284,
p. 1–19.
Kellerhals, P., and Matter, A., 2003, Facies analysis
of a glaciomarine sequence, the Neoproterozoic Mirbat Sandstone Formation, Sultanate
of Oman: Eclogae Geologicae Helvetiae, v. 96,
p. 49–50.
Kempf, O., Kellerhals, P., Lowrie, W., and Matter,
A., 2000, Paleomagnetic directions in late Precambrian glaciomarine sediments of the Mirbat
Sandstone Formation, Oman: Earth and Planetary Science Letters, v. 175, p. 181–190, doi:
10.1016/S0012–821X(99)00307–6.
Kilner, B., Mac Niocaill, C., and Brasier, M.D.,
2005, Low-latitude glaciation in the Neoproterozoic of Oman: Geology, v. 33, p. 413–
416, doi: 10.1130/G21227.1.
Leather, J., Allen, P., Brasier, M., and Cozzi, A., 2002,
Neoproterozoic snowball Earth under scrutiny:
Evidence from the Fiq glaciation of Oman:
Geology, v. 30, p. 891–894, doi: 10.1130/0091–
7613(2002)030<0891:NSEUSE>2.0.CO;2.
Lewis, J.P., Elby, M., Waever, A.J., Johnston, T.,
and Jacob, R.L., 2004, Global glaciations in
the Neoproterozoic: Reconciling modelling
results: Geophysical Research Letters, v. 31,
L08201, doi: 10.1029/2004GL019725.
McLennan, S.M., Hemming, S., McDaniel, D.K., and
Hanson, G.N., 1993, Geochemical approaches
to sedimentation, provenance and tectonics, in
Johnsson, M.J., and Basu, A., eds., Processes
Controlling the Composition of Clastic Sediment: Geological Society of America Special
Paper 284, p. 21–40.
Nesbitt, H.W., and Markovics, G., 1997, Weathering of granodioritic crust, long-term storage of
elements in weathering profiles, and petrogenesis of siliciclastic sediments: Geochimica et
Cosmochimica Acta, v. 61, p. 1653–1670, doi:
10.1016/S0016–7037(97)00031–8.
Nesbitt, H.W., and Young, G.M., 1982, Early Proterozoic climates and plate motions inferred from
major element chemistry of lutites: Nature,
v. 299, p. 715–717, doi: 10.1038/299715a0.
Nesbitt, H.W., and Young, G.M., 1996, Petrogenesis of sediments in the absence of chemical
weathering: Effects of abrasion and sorting on
bulk composition and mineralogy: Sedimentology, v. 43, p. 341–358, doi: 10.1046/j.1365–
3091.1996.d01–12.x.
Nesbitt, H.W., Young, G.M., McLennan, S.M., and
Keays, R.R., 1996, Effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments, with implications for provenance studies: The Journal of Geology, v. 104,
p. 525–542.
Olcott, A.N., Sessions, A., Corsetti, F.A., Kaufman,
A.J., and de Oliviera, T.F., 2005, Biomarker evidence for photosynthesis during Neoproterozoic glaciation: Science, v. 310, p. 471–474,
doi: 10.1126/science.1115769.
Pierrehumbert, R.T., 2004, High levels of atmospheric carbon dioxide necessary for the termination of global glaciation: Nature, v. 429,
p. 646–648, doi: 10.1038/nature02640.
Scheffler, K., Hoernes, S., and Schwark, L.,
2003, Global changes during Carboniferous–Permian glaciation of Gondwana: Linking polar and equatorial climate evolution by
geochemical proxies: Geology, v. 31, p. 605–
608, doi: 10.1130/0091–7613(2003)031<0605:
GCDCGO>2.0.CO;2.
Taylor, S.R., and McLennan, S.M., 1985, The continental crust: Its composition and evolution:
Oxford, Blackwell Scientific Publications,
312 p.
Manuscript received 27 September 2006
Revised manuscript received 24 October 2006
Manuscript accepted 27 October 2006
Printed in USA
GEOLOGY, April 2007