1419
Early Silurian (Wenlockian) !13C profiles from the
Cape Phillips Formation, Arctic Canada and their
relation to biotic events
Paula J. Noble, Matthew K. Zimmerman, Chris Holmden, and Alfred C. Lenz
Abstract: Geochemical data from the Cape Phillips Formation, Arctic Canada, are examined in association with three
Silurian biotic crises in the graptolite community; the early Wenlockian Ireviken, mid Wenlockian Cyrtograptus
lundgreni, and end Wenlockian Colonograptus ludensis extinction events. Positive δ13Corg excursions are associated with
the Ireviken and C. lundgreni events, but not the Co. ludensis Event. The Ireviken and C. lundgreni excursions are recognized worldwide and are herein interpreted to be the result of carbonate weathering in response to eustatic sea-level
drop. The C. lundgreni excursion is of greater magnitude in the more proximal basin margin section at Abbott River,
Cornwallis Island, and is explained by the amplification of a more strongly positive δ13C signature in shallower parts
of an epeiric basin during increased exposure and weathering of the carbonate shelf. Excursion C5, within the
Co. praedeubeli – Co. deubeli Zone, is also of regional significance, as it occurs in both the Abbott River section and
Twilight Creek section on Bathurst Island, and is also recognized in Estonia, Poland, and England. Excursion C6 is
recognized in the Gorstian Stage, yet its regional significance remains equivocal. There is a reasonable general agreement between the shape of the δ13Corg and δ13Ccarb curves, yet the δ13Ccarb curve is largely a record of detrital carbonate derived from the shelf. The δ13Corg curve represents extraction of dissolved inorganic carbon by plankton and thus
is more indicative of ambient paleoceanographic conditions. These data are valuable in that they provide a detailed secular marine δ13C curve for the Wenlockian of Arctic Canada from relatively unaltered sections of varying facies whose
ages are well constrained by graptolite biostratigraphy.
Résumé : Des données géochimiques de la Formation de Cape Phillips dans l’Arctique canadien sont étudiées en
association avec trois crises biotiques siluriennes dans la communauté graptolite : l’événement Ireviken, au Wenlockien
précoce, et les extinctions de Cyrtograptus lundgreni au Wenlockien moyen et de Colonograptus ludensis à la fin du
Wencklokien. Des excursions positives δ13Corg sont associées aux événements Ireviken et C. lundgreni mais pas à
l’événement C. ludensis. Les excursions Ireviken and C. lundgreni sont reconnues à travers le monde et elles sont
interprétées ici comme étant le résultat d’une altération des carbonates en réponse à la chute eustatique du niveau de
la mer. L’excursion C. lundgreni est de plus grande magnitude dans la section plus proximale du bassin à la rivière
Abbott, sur l’île Cornwallis, et elle est expliquée par l’amplification d’une signature δ13C plus fortement positive dans
les parties moins profondes d’un bassin épicontinental durant la prolongation de l’exposition et l’altération de la plateforme carbonate. L’excursion C5, à l’intérieur de la Zone à Co. praedeubeli – Co. deubeli, a aussi une importance régionale, étant donné qu’elle a lieu à la fois dans la section de la rivière Abbott et celle du ruisseau Twilight sur l’île
de Bathurst; elle est aussi reconnue en Estonie, en Pologne et en Angleterre. L’excursion C6 est reconnue dans l’étage
Gorstien mais son importance régionale demeure équivoque. Les formes des courbes δ13Corg et δ13Ccarb concordent généralement bien, quoique la courbe δ13Ccarb représente surtout un enregistrement des carbonates détritiques provenant de
la plate-forme. La courbe δ13Corg représente l’extraction du carbone inorganique dissous par le plancton et elle est donc
plus indicative des conditions paléocénographiques ambiantes. Ces données sont importantes car elles fournissent une
courbe détaillée δ13C marine séculaire du Wenlockien de l’Arctique canadien à partir de sections relativement inaltérées
de faciès variés dont les âges sont bien cadrés par la biostratigraphie des graptolites.
[Traduit par la Rédaction]
Noble et al.
1430
Received 9 November 2004. Accepted 4 May 2005. Published on the NRC Research Press Web site at http://cjes.nrc.ca on
24 October 2005.
Paper handled by Associate Editor J. Jin.
P.J. Noble1 and M.K. Zimmerman. Department of Geological Sciences and Engineering, University of Nevada, Reno, NV
89557-1038, USA.
C. Holmden. Saskatchewan Isotope Laboratory, Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK
S7N 5E2, Canada.
A.C. Lenz. Department of Earth Sciences, The University of Western Ontario, London, ON N6A 5B7, Canada.
1
Corresponding author (e-mail: [email protected]).
Can. J. Earth Sci. 42: 1419–1430 (2005)
doi: 10.1139/E05-055
© 2005 NRC Canada
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Introduction
The Canadian Arctic is renowned for its thick, fossiliferous exposures of the Cape Phillips Formation carbonates
that were deposited along the broad outer shelf of northern
Laurentia from the Late Ordovician through the Early Devonian (Fig. 1). Silurian deposits of the Canadian Arctic represent some of the thickest and least structurally complicated
strata of this age known anywhere (de Freitas et al. 1999).
Abundant, well-preserved graptolites and conodonts provide
tight age control, making this an excellent location to reconstruct Early Silurian δ13C profiles, which may provide clues
to the environmental perturbations responsible for Silurian
oceanic and biotic extinction events (Jeppsson 1998; Melchin
et al. 1998).
In this paper we present δ13Ccarb, δ13Corg, and total organic
carbon (TOC) profiles together with paleontological and sedimentological data from two measured sections of the Cape
Phillips Basin that represent proximal (Abbott River,
Cornwallis Island) and distal (Twilight Creek, Bathurst Island)
facies. Two other studies have addressed the geochemistry of
the Cape Phillips Formation (Coniglio and Melchin 1995;
Märss et al. 1998), but only the latter study presented δ13Ccarb
profiles, which span the Wenlockian on Cornwallis and BaillieHamilton Islands. In that study, positive δ13Ccarb shifts were
found to coincide with the graptolite extinctions in the Cyrtograptus centrifugus (Ireviken Event) and Gothograptus nassa
zones (Märss et al. 1998). However, the study of Märss et al.
(1998) lacks isotopic data across the graptolite extinction
event in the Cyrtograptus lundgreni Zone. We provide more
extensive and more continuous coverage through the Wenlockian
and report on the first δ13Corg profiles for early Silurian
strata.
Within the time resolution allowed by available conodont
and graptolite biostratigraphy, these δ13Corg excursions correlate
with stratigraphically equivalent excursions in the shallower
facies of the Baltic region (Wenzel and Joachimski 1996;
Samtleben et al. 1996; Bickert et al. 1997; Kaljo et al. 1997,
1998; Munnecke et al. 2003), in England (Corfield et al.
1992), Australia (Andrew et al. 1994; Talent et al. 1993),
and the Great Basin of Nevada (Saltzman 2001). We review
the mechanisms for generating large positive δ13C excursions,
emphasizing the potentially important role of sea-level-forced
changes in the isotope composition of the C-weathering flux
(due to changes in the proportion of subareally exposed carbonate vs. silicate bedrock in the epeiric sea watershed),
which may be driven by glacio-eustacy.
Geological setting
The Cape Phillips Formation was deposited from the Late
Ordovician through the Early Devonian (Thorsteinsson 1958)
in an ocean-facing epeiric sea at or near equatorial latitudes.
The upper Llandoverian – lower Ludlovian strata of the Cape
Phillips Formation at Twilight Creek (Fig. 2) consist of
graptolite-bearing, laminated argillaceous lime mudstone and
calcareous shale that characteristically lack bioturbation and
shallow-water macrofossils, with exception of occasional ostracodes, trilobites, orthocone cephalopods, and brachiopods,
the last-named derived from debris flows. Graptolites and
radiolarians are common at the Twilight Creek and Abbott
Can. J. Earth Sci. Vol. 42, 2005
River sections indicating deposition in an open ocean, deepwater setting. However, the upper Homerian at the Abbott
River section (Fig. 2) is characterized by intermittent bioclastic
carbonate beds rich in shallow-water macrofossils indicating
deposition in a basin–margin setting within the Cape Phillips
basin (Fig. 1). Wenlockian “graptolitic facies” of the Cape
Phillips Formation can be correlated basinward with the clastic
Danish River Formation of the Hazen trough and correlated
also with the platform carbonate facies of the Allen Bay Formation (Figs. 1, 3).
Although four deformational pulses have been recognized
through the Late Silurian to the Mississippian (Trettin 1989),
both the Twilight Creek and Abbott River sections are only
mildly affected by early Carboniferous deformation associated with the Ellesmerian orogeny (Trettin 1989). Thermal
maturation studies suggest that an area encompassing the
Twilight Creek and Abbott River sections was subject to low
geothermal gradients that are within the range of normal
gradients for North American sedimentary basins (Goodarzi
et al. 1992).
In many localities globally, the Ireviken Event is marked
by a transition between shale (or relatively impure argillaceous carbonate) in the Pterospathodus amorphognathoides
conodont Zone and purer carbonates of the Kockelella
ranuliformis conodont Zone (Saltzman 2001). The facies shift
is interpreted to reflect a worldwide regression. A similar
facies shift from shale to argillaceous carbonate, and back to
shale again, occurs across the Gothograptus nassa Zone in
the east Baltic region, which is also interpreted as a regression
(Johnson 1996; Kaljo et al. 1997, 1998).
Biotic extinction events and graptolite
biostratigraphy
Early Silurian biotic extinction and (or) diversification events
addressed herein are the (1) Ireviken Event occurring near
the Llandoverian–Wenlockian boundary; (2) the late Wenlockian
C. lundgreni Event; and (3) the end-Wenlockian Colonograptus
ludensis Event. These events significantly affected the graptolite
community and are recognized worldwide (Lenz 1993, 1995;
Melchin 1994; Melchin et al. 1998). In addition, an excursion
occurring within the Co. praedeubeli – Co. deubeli Zone is
discussed.
The Ireviken Event is named after its type locality in
Gotland (Jeppsson 1987). It marks one of the more severe
graptolite extinctions (81% of species; Melchin 1994) in the
Paleozoic. Conodont diversity is also severely diminished at
this event (83% of species extinct), with brachiopods being
the least affected (Talent et al. 1993). The C. lundgreni Event
is younger than the Ireviken event, being coincident with the
lower–upper Homerian boundary. The C. lundgreni Event is
highly selective, strongly affecting graptolites, but having
little effect on shallow-benthic communities. In the graptolite
community, 95% of all species became extinct, including the
entire Cyrtograptus lineage (Lenz 1994; Lenz and KozlowskaDawidziuk 2002). Trilobites were also affected at the
C. lundgreni Event, with six genera becoming extinct globally
and six major clades being eliminated from northern Laurentia
(Adrain and Edgecombe 1997; Adrain 2000). Following the
C. lundgreni Event graptolite diversity is very low (two
species) in Pristiograptus dubius – Gothograptus nassa Zone
© 2005 NRC Canada
Noble et al.
(Fig. 2), although abundances are high, and it is not until the
Co. praedeubeli – Co. deubeli Zone that diversity notably
increases (Lenz 1992, 1995). Interestingly, a “burst” in the
Osteostraci (ostracoderm fish) occurs in this interval (Kaljo
et al. 1995). The C. lundgreni Event has been studied closely
from a subsurface core in Poland, where it appears that the
graptolite extinction was stepwise, accompanied by a drop in
both abundance and diversity in the microphytoplankton
community (Por!bska et al. 2004).
The Colonograptus ludensis Event, coincident with the
Wenlockian–Ludlovian boundary, is seen largely in the
graptolite community, although acritarchs show a drop in diversity leading up to the event (Kaljo et al. 1995). The Co.
ludensis Event is marked by a mild extinction in the
graptolite community, where eight graptolite species become
extinct (Lenz 1992). No graptolite extinctions are seen at the
genus level (Lenz and Kozlowska-Dawidzuik 2002). The Co.
ludensis Event is also followed by a major resurgence and
radiation of new monograptid taxa (Lenz 1994; Koren and
Urbanek 1994).
Upper Homerian graptolite biozonations vary between the
Arctic and other locations discussed herein (i.e., British
Isles, Baltic region, and Poland) and require some explanation as to their correlation. In the Arctic, three upper
Homerian biozones are recognized: the P. dubius – G. nassa,
Co. praedeubeli – Co. deubeli, and Co. ludensis zones (Lenz
1995). The P. dubius – G. nassa Zone is a low-diversity interval zone that is generally only a few metres thick and is
part of the recovery stage following the C. lundgreni extinction event. The overlying Co. praedeubeli – Co. deubeli
Zone, defined by the first appearance of either of the nominal
taxa, constitutes the greatest diversity in the upper Homerian
of the Arctic. In the British Isles, only two graptolite zones
are recognized in the upper Homerian: the G. nassa and
Co. ludensis zones (Rickards 1976), the former equivalent to
the P. dubius – G. nassa Zone and the latter corresponding
to the combined Co. praedeubeli – Co. deubeli and
Co. ludensis zones of the Arctic. A subsurface core from Poland subdivides the upper Homerian even more finely with
the Co. praedeubeli Zone being equivalent to the lower part
of the Co. praedeubeli – Co. deubeli Zone of the Arctic
(Por!bska et al. 2004). The isotopic excursion recognized
within the Co. praedeubeli – Co. deubeli Zone of the Arctic
is thus believed to be time equivalent to a similar excursion
identified in the upper part of the G. nassa Zone or the lowest
part of the Co. ludensis Zone in England (Corfield et al.1992),
the Co. praedeubeli Zone in the Bartoszyce borehole in
Poland (Por!bska et al. 2004), and the G. nassa – P. parvus
Zone in the East Baltic region (Kaljo et al. 1998).
Sampling and analytical methods
Data presented in this study come from two measured
sections — the deeper water Twilight Creek section on
Bathurst Island and the shallower water Abbott River section
on Cornwallis Island (Fig. 1). Approximately 250 m of section
were measured at Twilight Creek ranging in age from uppermost Llandoverian through lower Ludlovian. At Abbott River,
43 m of late Wenlockian – early Ludlovian section were
measured. Graptolite field identifications were used to determine the age of the strata. Samples were collected for petro-
1421
Fig. 1. Inset map at top left showing location of study area
within the Arctic Islands. Map showing paleogeography of the
Cape Phillips embayment with the “graptolitic facies” representing the basinal deposits and the “carbonate facies” showing the
transition into shallow-water platform deposits. Dashed line represents the position of the shelf–slope break. Study localities:
TWC, Twilight Creek; ABR Abbott River.
logic and isotopic analysis at !2 to 4 m intervals with an increased sample density of 0.5 m intervals from 5 m below to
5 m above the C. lundgreni and Co. ludensis events. Radiolarian and sponge spicule bearing lithologies were collected
for micropaleontologic analysis wherever these microfossils
were identified in the sections. The petrology was characterized by standard thin section analysis, and X-ray diffraction.
A Phillips 3100 X-ray diffractometer was used and samples
were analyzed from 2°–60° 2θ. Major oxide analysis via
X-ray fluorescence spectrometry (SiO2, TiO2, Al2O3, Fe2O3,
MgO, MnO, CaO, Na2O, K2O, P2O5) was performed on 37
whole-rock samples from the Twilight Creek section and are
discussed to a limited extent herein.
Inorganic and organic δ13C analyses were performed in
the Saskatchewan Isotope Laboratory at the University of
Saskatchewan. Whole-rock powders for isotopic analysis
were drilled from a freshly broken rock surface with a diamond tip drill bit. Calcite veins and fossil material were
avoided in efforts to avoid diagenetic and vital effects of
organism-induced fractionation on the whole rock isotopic
signatures.
The δ13Ccarb analyses were measured using a Kiel III
carbonate device attached to a Finnigan MAT 253 dual inlet
mass spectrometer. Approximately 50–100 mg of carbonate
powder was reacted with three drops of anhydrous phosphoric
acid at 70 °C. Carbon isotope analyses are reported in the
© 2005 NRC Canada
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Can. J. Earth Sci. Vol. 42, 2005
Fig. 2. Measured stratigraphic sections from Twilight Creek and Abbott River showing lithology, age, fossil control, and position of
events discussed in text. Solid dots represent horizons with biostratigraphically useful graptolites, open dots represent horizons with
radiolarian recovery. Graptolite zones shown are those recognized in the Arctic (Melchin 1989; Lenz and Melchin 1990; Lenz 1995).
Dashed lines show positions of extinction and diversification events referred to in text. Section thicknesses in metres. Stratigraphic columns from Noble and Lenz (unpublished data, 1998).
© 2005 NRC Canada
Noble et al.
Fig. 3. Regional lithostratigraphic relationships of Late Ordovician –
Early Devonian strata of the central Arctic Islands. Figure from
Adrain and Edgecombe (1997). Fm, Formation.
standard delta notation relative to Vienna Pee-Dee Belemnite
(VPDB), calibrated against a δ13C value of 1.95‰ for US.
National Bureau of Standards NBS-19. The reproducibility
of the δ13Ccarb analyses is ±0.05‰ (1σ). δ13Corg analyses
were performed by combusting organic matter in a furnace
(ANCA-GSL) attached to a Europa Scientific 20-20 continuous
flow isotope ratio mass spectrometer in the Department of
Soil Sciences, University of Saskatchewan. Samples were
measured in duplicate to ensure optimum matching of reference and sample gas volumes for accurate isotope ratio
determination. Only the isotopic analyses for the optimized
runs are reported. The weight percent of TOC in the limestones was measured by combustion in the ANCA-GSL of
the residue remaining after HCl digestion of the whole-rock
carbonates. The organic fraction was calculated by comparing
the voltages for the sum of mass 44, 45, and 46 CO2+ ion
beams between the samples and a gravimetric standard with
a known wt.% carbon. Organic carbon isotope results are
reported in the standard delta (δ) notation relative to VPDB.
The reproducibility of the δ13Corg analyses is ±0.15‰ (1σ).
Results
Petrology and elemental geochemistry
Petrographic and X-ray diffraction analyses of several samples spanning the section at Twilight Creek (98TWC) show
that it is mineralogically homogeneous, silty microsparite
comprising dominantly quartz and calcite, with minor to
trace amounts of albite, pyrite, muscovite, and illite. Dolomite
is generally present in minor quantities, with exception of
three distinct horizons, where it is a major constituent.
Elemental analysis shows that the concentrations of CaO
1423
and SiO2 are inversely related and constitute 67% to 94% of
the rock mass.
Phosphorous is generally present in trace quantities, with
the exception of the C. lundgreni (1.3%) and Co. ludensis
(21.2 wt.%) event levels, where it is concentrated in fluorapatite.
At the Co. ludensis Event horizon, phosphatic nodules were
observed. Magnesium is present at low concentrations (up to
!5%), with exception of the anomalously high levels in
Twilight Creek samples at 107, 107.5, and 128.5 m, which
are assumed to reflect increased dolomite abundances.
δ13C excursions
Six positive δ13Corg excursions (denoted C1–C6) are found
in the latest Llandoverian through early Ludlovian strata of
the Cape Phillips Formation (Figs. 4, 5), three of which (C1,
C4, and C5) can be shown to have regional extent. The C1
excursion is associated with the Ireviken Event, and the C4
excursion is closely associated with the C. lundgreni Event.
The C5 excursion occurs within the Co. praedeubeli –
Co. deubeli Zone in the upper Homerian. The C4 and C5 excursions are found in both the basin and basin margin facies
of the Twilight Creek and Abbott River sections, respectively. The C1 and C2 excursions are found in the lower part
of the thicker Twilight Creek section, which is an interval
that was not sampled for geochemistry at Abbott River. The
C1 excursion, however, was reported from Baillie-Hamilton
Island (Fig. 1) by Märss et al. (1998). The C6 excursion occurs
in the overlying Gorstian stage at Twilight Creek only, and is
stratigraphically above the top of the Abbott River section.
The C3 excursion is identified only in the Abbott River section
and may have been missed at Twilight Creek as a result of
less dense sampling. As such, the regional extent of C2, C3,
and C6 excursions cannot be determined at this time.
The δ13Corg excursions are interpreted to reflect changes in
the δ13CDIC (dissolved inorganic carbon) of contemporaneous
seawater, but they differ from the true seawater value by a
fractionation factor that reflects the photosynthetic production
of organic carbon from dissolved inorganic carbon in seawater.
In contrast, the carbon isotope fractionation that occurs during
the formation of skeletal carbonate from seawater is very
small, and so the limestone δ13Ccarb value is generally accepted
to be close to the original seawater value. If the inorganic
and organic sedimentary carbon composing the Cape Phillips
rocks were both formed from the same seawater DIC reservoir, then stratigraphic fluctuations in δ13Ccarb and δ13Corg
should track each other very closely. In comparing the
shape-structures of the δ13Corg and δ13Ccarb profiles of the
Cape Phillips Formation, it is observed that their correspondence is generally not very close. Relatively good correspondence occurs, however, between some sections of the two
curves as, for example, through the Ireviken (C1) excursion
at the Twilight Creek section. In other excursion intervals,
like the C2 organic δ13C excursion at Twilight Creek, however, there is no counterpart in the inorganic δ13C profile.
Similarly, the C3, C4, and C6 organic δ13C excursions at
Twilight Creek are only weakly reflected in the δ13Ccarb profile.
There seems to be slightly better correspondence between
overall shape structure of the δ13Corg and δ13Ccarb profiles at
Abbott River than at Twilight Creek, but a similar pattern is
observed with the excursions in that they are not strongly
reflected in the δ13Ccarb profile. The one exception to this
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Can. J. Earth Sci. Vol. 42, 2005
Fig. 4. Lithologic column and selected geochemistry of the Twilight Creek Section, Bathurst Island. Dashed lines show the positions of
the Ireviken, C. lundgreni, and Co. ludensis Extinction Event boundaries referred to in text. Graptolite Zonal abbreviations: greiston.,
M. griestonensis; sak., C. sakmaricus; du.-pra., P. dubius – G. nassa and Co. praedeubeli – Co. deubeli; lu., Co. ludensis. Hom.,
Homerian; l., lower; u., upper. TOC, total organic carbon; VPDB, Vienna Pee-Dee Belemnite.
pattern is the C5 excursion at Twilight Creek, which is pronounced in both the δ13Ccarb and δ13Corg profiles but appears
to be stratigraphically offset, occurring 2 m higher in the
δ13Ccarb profile. We interpret this offset as a delayed effect of
detrital carbonate delivery to the basin.
In three instances, the δ13Ccarb profile at Twilight Creek is
characterized by prominent negative δ13C excursions, most
notably at 22 m, and to a lesser extent at 107 and 128.5 m.
Stratigraphically, two of the negative δ13Ccarb excursions occur
just below the onset of the positive C1 and C4 excursions
preserved in the δ13Corg profile, thus complicating the direct
comparison of the shape-structure between the two curves.
The negative δ13Ccarb excursions occur coincidentally with
intervals of high dolomite abundance, and we consider them
to be diagenetic artifacts. These three points are plotted as
open circles (Fig. 4) and are excluded from the curve. An
anomalously negative point at 27 m in the Abbott River
section is also believed to be the product of diagenetic alteration (Fig. 5, open circle), but no geochemical analyses have
been performed on the Abbott River section to confirm this
assumption. The dolomitic beds may be detrital in origin,
but are more likely a result of fabric selective dolomitization.
Detrital carbonate phases are believed to make up a substantial fraction of the Cape Phillips sedimentary succession.
For example, the distal graptolite facies and rarity of carbonate
secreting planktonic organisms in the Silurian (Munnecke et
al. 2000, 2001), coupled with the high quartz contents and
the finely laminated and homogenous character of the sediment, point to a carbonate factory that was located in a more
proximal basinal setting. Some of the limestone in the Cape
Phillips Formation has features indicative of deposition by
turbidity currents, including normal grading and small-scale
© 2005 NRC Canada
Noble et al.
1425
Fig. 5. Generalized lithologic column and selected geochemistry of the Abbott River section, Cornwallis Island. Dashed lines showing
positions of the C. lundgreni and Co. ludensis event boundaries referred to in text. Graptolite zonal abbreviations: d., P. dubius –
G. nassa; lu, Co. ludensis. TOC, total organic carbon; VPDB, Vienna Pee-Dee Belemnite.
cross lamination (this study and Coniglio and Dix 1992)
consistent with a detrital origin. Bioclastic beds at Abbott
River are rich in pelmatozoan fragments, sponges, brachiopods fragments, and other shallow water carbonate debris.
We suggest that dissonance observed between δ13Ccarb and
δ13Corg profiles is because of the dilution of a minor authigenic carbonate component with a more substantial detrital
component that is older in age and characterized by an isotope
composition that is more characteristic of “background” δ13C
values, thus, diminishing the size of the positive isotope
excursions. Reasonable agreement between the δ13Ccarb and
δ13Corg curves in some parts of the succession, where the
detrital influence is lessened, shows that diagenetic disturbance is unlikely and that secular changes in the δ13CDIC
value of contemporaneous seawater are preserved.
In contrast with the questionable origin of the carbonate
fraction, there is little reason to doubt the paleoceanographic
significance of the organic fraction. The organic fraction
represents extraction of DIC by plankton and is a function of
chemical oceanography and paleoproductivity. Planktonic
remains, including graptolites, acritarchs, and chitinozoans
are abundantly preserved in Cape Phillips strata, and the relatively high organic content of the rocks (0.4% to 9%; this
study and Edwards 1989) suggests that the organic matter
originated as “organic rainout” from the surface water mass
overlying the depositional site. Rock-Eval and petrographic
analysis of the Cape Phillips Formation yielded Type 1
organic matter, consisting of Tasmanites, unicellular algae
(i.e., acritarchs and chitinozoans), graptolites, and bitumen
(Edwards 1989). Thus, in the Cape Phillips Formation, we
consider the δ13Corg curve to be the more reliable seawater
record of early Silurian C-cycle perturbations, as recorded in
the plankton.
Lastly, the TOC content of the samples measured at Twilight Creek and Abbott River ranged from 0.4 to 3.6 wt.%.
A consistent and clear relationship between δ13Corg and TOC
profiles is not present in the data. At times the two profiles
appear to be negatively correlated, and at other times positively
correlated. One notable positive correlation occurs through
the Ireviken Event, where δ13Corg and TOC values track each
© 2005 NRC Canada
1426
other across the excursion interval, although here the excursion
is stratigraphically offset occurring a few metres higher in
the δ13Corg than in the TOC curve.
Regional and global correlation of δ13C excursions
Positive carbon isotope excursions associated with the
Ireviken Event (C1; early Wenlock) show a similar degree of
13
C-enrichment between the δ13Corg and δ13Ccarb for the
deeper water Twilight Creek section. The δ13Corg and δ13Ccarb
shifts are 2.6‰ and 2.2‰, respectively. These magnitudes
are similar for the Ireviken excursion in (1) Gotland, Sweden, using brachiopod calcite (3‰ Bickert et al. 1997; 3‰
Wenzel and Joachimski 1996; 3‰ Munnecke et al. 2003),
(2) Nevada, using whole-rock carbonate (3‰, Saltzman 2001),
(3) Latvia and Estonia, using whole-rock carbonate (3‰–5.2‰,
Kaljo et al. 1998), and (4) several locations worldwide, using
brachiopod calcite (3‰, Azmy et al. 1998; Heath et al. 1998).
An interesting feature of the Ireviken excursion in Arctic
Canada is its asymmetrical shape, which results from the
pre-excursion baseline δ13C values being higher than postexcursion baseline values. A similar shape-structure characterizes the Ireviken excursion reported in carbonates from
Baillie-Hamilton Island in Arctic Canada (Märss et al. 1998),
the carbonate succession on Gotland in the Baltic region
(Bickert et al. 1997; Samtleben et al. 2000), in Latvia and
Estonia (Kaljo et al. 1997, 1998), and in Nevada (Saltzman
2001).
Positive δ13Corg excursions associated with the C. lundgreni
Event (C4) and Co. praedeubeli – Co. deubeli Zone (C5) are
correlative between proximal (Abbott River) and distal (Twilight Creek) locations in the Cape Phillips basin and are
similar to δ13Ccarb profiles reconstructed for a section at BaillieHamilton Island (Märss et al. (1998). The magnitude of the
C. lundgreni isotope excursion (C4) is larger in the more
proximal basin margin section at Abbott River (δ13Corg =
3.4‰) than in the more distal basinal section at Twilight
Creek (δ13Corg = 2.3‰). At Abbott River, the C5 excursion
occurs in such close association with the C4 excursion that
together they may be considered a single, broad, bifurcated
excursion beginning just before the C. lundgreni Event and
passing into the Co. praedeubeli – Co. deubeli Zone. The
shape-structure of the curve is similar at Twilight Creek,
with the exception of nearly complete separation of the C4
and C5 peaks. Carbon isotope excursions in time-equivalent
strata have been reported in (1) whole-rock carbonate from
Latvia and Estonia (1‰–2‰ Kaljo et al. 1997, 1998),
(2) whole-rock carbonate and organic carbon from Poland,
showing a bifurcated excursion (!2.5‰) with maxima in the
final stage of the C. lundgreni Event and in the
Co. praedeubeli Zone (Por!bska et al. 2004), (3) brachiopod
calcite from Sweden — both single (!2.5‰ Bickert et al.
1997; Wenzel and Joachimski 1996; Samtleben et al. 1995)
and bifurcated (Samtleben et al. 2000) excursions of !3‰,
(4) England, where there is a large magnitude, bifurcated,
positive excursion spanning the C. lundgreni, G. nassa, and
Co. ludensis graptolite zones of 4‰ (Corfield et al. 1992),
with the negative apex of the bifurcated peak occurring in
the G. nassa Zone, similar to the example in Arctic Canada,
and (5) a composite brachiopod curve from many sites worldwide showing a single broad, positive excursion (2‰ re-
Can. J. Earth Sci. Vol. 42, 2005
ported by Azmy et al. 1998). An exception to the studies
previously mentioned is the δ13C profile in Nevada, which
does not show a positive carbon isotope excursion in the
C. lundgreni graptolite zone (Saltzman 2001), but does show a
positive δ13C excursion associated with the Co. ludensis
Event. We found no δ13C excursion in Arctic Canada directly
associated with the Co. ludensis Event, even though a
paleoceanographic event may be indicated by the anomalously high phosphorus contents of rocks coincident with the
extinction interval (21.2 wt.% at Twilight Creek). It is possible that erosion or nondeposition through this interval has
erased the δ13C excursion in Arctic Canada. A broad δ13C
excursion spanning the C. lundgreni, G. nassa, and Co.
ludensis zones occurs in England (Corfield et al. 1992), but
with this exception, and the occurrence in Nevada, a singular δ13C excursion associated with the Co. ludensis Event has
not been reported elsewhere.
Although we see no δ13C excursion associated with the
Co. ludensis Event in Arctic Canada, there is a positive δ13C
excursion (C6) in slightly younger strata of the Gorstian
Stage, in the Lobograptus progenitor graptolite Zone. A broad
excursion is found in England, and on the island of Gotland
in the Baltic Sea, where small positive δ13C excursions of
1‰ –2‰ have been reported from brachiopod calcite (fig. 2,
Corfield et al. 1992; fig. 3a, Samtleben et al. 2000). Two
positive excursions at Pete Hansen Creek, Nevada identified
as Co. ludensis and younger end-Polygnathoides siluricus
(Saltzman 2001) may be correlative to the Arctic C5 and C6
excursions. The Co. ludensis excursion at Pete Hansen Creek,
derived from whole-rock carbonate, may be equivalent to the
delayed C5 excursion recognized in the δ13Ccarb at Twilight
Creek, and the end-P. siluricus excursion may be equivalent
to the C6 Gorstian excursion. Pete Hanson Creek has sparse
accompanying age control, and it is difficult to tell if the
end-P. siluricus excursion is, in fact, younger (i.e., Ludfordian)
than the Gorstian excursion or correlative with it.
In Summary, the Wenlockian δ13C secular curve in Arctic
Canada confirms the presence of positive δ13C excursions in
association with the Ireviken and C. lundgreni extinction
events observed in the Baltic region and elsewhere. In the
shallower water Abbott River section, the positive C. lundgreni
and Co. praedeubeli – Co. deubeli zone excursions appear to
represent a single, large bifurcated excursion. A bifurcated
excursion is also seen in correlative deposits in England,
Poland, and on the island of Gotland. In the Canadian Arctic,
the C. lundgreni excursion is larger in the more proximal
facies of the Cape Phillips Basin, and smaller in the more
distal facies, which is also a pattern observed in other early
Silurian δ13C excursions (Kaljo et al. 1997, 1998; Samtleben
et al. 2000; Munnecke et al. 2003). The youngest, positive
δ13C excursion from this study is in the Gorstian Stage,
which correlates with stratigraphically equivalent excursions
in the Baltic region (Kaljo et al. 1998; Samtleben et al.
2000). We find no δ13C excursion in Arctic Canada that is
associated with the Co. ludensis Event.
Discussion
The Ireviken and C. lundgreni event strata share a number
of common characteristics with the Hirnantian glaciation event,
© 2005 NRC Canada
Noble et al.
including the positive sign of the δ13C excursion, the association with some sort of biotic extinction or migration event
(Brenchley et al. 1995; Finney et al. 1999), coincidence with
widespread eustatic low stands and hiatuses in shallowwater environments (Johnson 1996; Baarli et al. 2003), the
relatively short duration of the event (Brenchley et al. 1995;
Brenchley 2004), and their relatively lean organic carbon
contents (see review in Samtleben et al. 2000). In addition,
the Ireviken Event is associated with glacial deposits (Grahn
and Caputo 1992; Caputo 1998). Such a long list of similarities
might be explained by a common environmental forcing.
One hypothesis involves changes to the carbonate weathering
flux, resulting from increased exposure of carbonates during
a eustatic sea-level drop (Kump et al. 1999). The net isotopic
composition of weathered carbon contributed to the oceans
is a function of the proportion of silicate to carbonate rock
weathering on the continents. Since there is little carbon in
silicate rocks, the carbon produced from silicate weathering
comes indirectly from the atmosphere as carbonic acid in
rain (which is about 7‰ lighter than the contemporaneous
surface ocean δ13C value). Therefore, marine carbonates will
always be about 7‰ more 13C-enriched than atmospheric
CO2, and consequently there should always be a minimum
of a 7‰ difference between the δ13C signature of the pure
silicate weathering flux and the isotope composition of recently
deposited carbonates. Kump et al. (1999) determined that a
16% increase in the fractional contribution of carbonates to
the isotope value of the net weathering flux to the oceans
would be sufficient to drive the 6‰ increase in δ13C values
recorded during the Hirnantian glacio-eustatic lowstand. This
16% estimate would be higher, except that atmospheric CO2
concentration rises through the Hirnantian glacial interval,
increasing the fractionation factor associated with the fixation
of organic carbon by primary producers, and the increased
fractionation allows some of the increase in the δ13C of seawater to be accounted for in this way. It seems reasonable
that a proportional increase in carbonate weathering flux
could account for the (+2.5‰) excursion associated with the
Ireviken and C. lundgreni events and provides a global mechanism to explain the geographically widespread occurrence
of the C1 and C4 excursions.
Although there are no glacial deposits known to be coincident
with the C. lundgreni Event, there is sedimentologic evidence
that supports a lowstand. In Gotland, Sweden, the end of the
C. lundgreni Zone is associated with the emergence and
erosion of parts of the carbonate platform indicating a minimum sea-level drop of 16 m (Calner and Säll 1999). This
lowstand also manifests itself in Estonia (Baarli et al. 2003)
and Latvia (Loydell 1998). This decline in sea level is noted
by Johnson (1996) in Avalonia, Baltica, Bohemia, and New
South Wales. At Abbott River, a subtle facies shift is recorded
in association with the C. lundgreni Event, suggesting that
either progradation of shallower facies or a lowstand accompanied the C. lundgreni Event boundary locally. There is an
increase in bioclastic carbonate beds at Abbott River several
metres below the event, starting at 12 m, and although the
C. lundgreni horizon is recessive and marked by 5 m of fissile
shale, allodapic beds with graded bedding and coarse bioclastic
texture resume above the event at 19.2 m. A qualitative look
at the siliceous microfossils at Abbott River shows an in-
1427
creased abundance of shallow water lithistid sponge spicules
accompanying the upward coarsening sequence (Zimmerman
2001).
Early Paleozoic δ13C enrichments have been alternatively
explained by increased productivity and burial of organic
matter during glacio-eustatically controlled lowstands (Brenchley
et al. 1994, 1995; Patzkowsky et al. 1997; Wenzel and
Joachimski 1996; Samtleben et al. 1996) or during changes
in global productivity patterns (Por!bska et al. 2004). This
widely held view purports that climate-induced seawater circulation changes caused increased burial of organic carbon
in the oceans, which shifted seawater δ13C to more positive
values both in epeiric seas and the surface ocean (Brenchley
et al. 1994; Wenzel and Joachimski 1996; Bickert et al.
1997). This interpretation is problematic with respect to the
Silurian δ13C excursions because there is little to no correlation between positive excursions and increase in TOC. In
some instances, TOC is negatively correlated with positive
isotopic shifts, and in the case of the Co. ludensis Event, the
most prominent increase in TOC and phosphate is not associated with any isotopic shift. Another problem with the
organic carbon burial hypothesis is that the magnitudes of
the Silurian δ13C excursions are quite large, which requires
massive amounts of organic carbon to be buried in ocean
sediments. Kump et al. (1999) determined that a 50% to
75% increase in organic carbon burial was needed to drive
the 6‰ positive δ13C excursion during the Hirnantian. The
strata composing these Silurian excursion intervals worldwide seem to fall short of the dramatic increases required by
Kump’s model.
It is significant to note that the magnitude of the C. lundgreni
isotopic shift appears to be facies dependent, increasing
inboard from the basin at Twilight Creek to the basin margin
at Abbott River. Likewise, excursions reported from other
parts of the world show a facies dependency with shallower
water settings recording greater C-isotope shifts (Kaljo et al.
1997, 1998; Samtleben et al. 2000; Munnecke et al. 2003).
This facies dependency will be most pronounced in epeiric
seas where a drawdown in sea-level shifts the δ13C value of
the locally acting C-weathering flux towards the carbonate
end member. Also, restricted circulation with the surface
ocean allows the weathering flux perturbation to become
quickly incorporated into local carbon cycling in advance of
the deep ocean, as mixing times for small basins are of
shorter duration than for the world ocean (Holmden et al.
1998; Panchuk and Holmden 2001). The effect of the change
in C-weathering flux will be most strongly felt in shallower
areas of epeiric seas where 13C-enriched weathered carbon is
delivered in precipitation runoff and by rivers. In deeper and
more distal areas of epeiric seas the 13C-enriched C-weathering
flux is diluted by mixing with the surface ocean. Thus the
C-weathering hypothesis predicts that Siluiran epeiric seas
in tropical and subtropical settings, should be characterized
by a gradient of decreasing δ13C excursion magnitudes in the
direction of the open ocean.
Conclusions
Six positive δ13C excursions are observed in Wenlockian
strata of the Cape Phillips basin, Canadian Arctic, three of
© 2005 NRC Canada
1428
which can be shown to be of global extent (C1, C4, C5).
Two of these excursions have been found to be associated
with biotic crises, the Ireviken Event (C1) and the C. lundgreni
Event (C4), and are correlative with many localities worldwide, including Sweden, Eastern Europe, and England. The
C. lundgreni Event (C4) and Co. praedeubeli – Co. deubeli
Zone (C5) excursions occur in both Abbott River and Twilight Creek sections and are correlative between proximal
and distal facies, respectively. No excursion was found to be
associated with the Co. ludensis Event, but a small positive
δ13C excursion (C6) is observed in the overlying Gorstian
strata. The global extent of the C6 excursion is equivocal; a
broad excursion covering this time interval has been found
in Baltic region, England and Australia, and in Nevada, a
large positive δ13C excursion assigned to the Co. ludensis
graptolite Zone (Saltzman 2001), may in fact be time equivalent to the C6 excursion.
In the Cape Phillips succession, the δ13Corg profiles are
more reliable than δ13Ccarb profiles because a large fraction
of the carbonate grains are detrital in origin. There is some
general agreement between the two profiles, yet the δ13Ccarb
seems more muted, particularly at Twilight Creek, which is
more distal from the carbonate source. In contrast, the δ13Corg
represents an in situ oceanographic carbon value preserved
in the fixed carbon of planktonic organisms.
The paleoceanographic nature of the δ13Corg profile is further
supported by major-element geochemistry and mineralogy.
Only three samples show alteration in the form of dolomitization, reflected by high MgO values. In these samples,
the isotopic values are anomalously negative and easily
excluded from the curve. The Cape Phillips succession at
Twilight Creek shows remarkable homogeneity in its majorelement and mineralogic content. Aside from fluctuations in
the percentage of quartz silt, reflected in SiO content and
quartz peak values, the section shows little change in source
material, indicating that isotopic shifts are not a function of
lithologic heterogeneity.
Sedimentary rocks deposited during the Ireviken and
C. lundgreni excursions share some physical characteristics
with each other, and with the Hirnantian Event, and both are
interpreted to be coincident with a sea-level drop. The Ireviken
event is associated with known glacial deposits and a positive
δ18O excursion that tracks the corresponding δ13C excursion
very well (Wenzel and Joachimski 1996; Bickert et al. 1997;
Azmy et al. 1998). A change in the carbonate weathering
flux that accompanies eustatic sea-level fall is interpreted as
the most plausible explanation for these two positive δ13C
excursions.
In our treatment of the carbonate weathering hypothesis,
we have included the effects of local C-cycling in epeiric
seas, which explains differences in the magnitudes of globally
correlated Silurian δ13C excursions. In particular, it has been
observed that shallow water δ13C excursions are larger than
their deeper water counterparts. This phenomenon is observed
with the C. lundgreni excursion between Abbott River and
Twilight Creek. We suggest that the proximal facies in epeiric
sea basins are more influenced by locally derived, 13C-enriched
DIC originating from weathering of subareally exposed carbonate platforms during sea-level lowstands, compared with
the more distal facies, which are better mixed with the surface ocean that is lower in δ13C value.
Can. J. Earth Sci. Vol. 42, 2005
Acknowledgments
This project was funded by National Science Foundation
research grants EAR 9870431, 9972845, and 0107139. M.
Desilets and B. Pecoraro assisted with X-ray diffraction analysis
at the University of Nevada Bureau of Mines and Geology.
Funding for A.C.L. was provided through a Natural Sciences
and Engineering Research Council research grant (Canada),
and partial funding form M.K.Z. came from the Geological
Society of America Grants-in-Aid. We particularly thank J.
Jin, J. Veizer, and an additional unknown reviewer for their
helpful reviews and comments.
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