Carbon isotope chemostratigraphy in Arctic Canada: Sea-level forcing of carbonate platform weathering and implications for Hirnantian global correlation

doi:10.1016/j.palaeo.2005.10.009

Palaeogeography, Palaeoclimatology, Palaeoecology

Volume 234, Issues 2–4, 18 May 2006, Pages 186-200

https://doi.org/10.1016/j.palaeo.2005.10.009 Get rights and content

Abstract

Three sections through latest Ordovician strata in the Canadian Arctic Islands have been studied for carbon isotopes, derived from the organic matter (δ¹³C_org) and whole-rock carbonate (δ¹³C_carb) fractions. The sections are well constrained biostratigraphically using graptolites, lithostratigraphically and palaeogeographically. δ¹³C_org data appear to provide a signal that mainly reflects chemical changes in the seawater, whereas the δ¹³C_carb data seem to have been variably affected by sediment reworking and diagenesis. Results show that a positive δ¹³C_org excursion of 3–6‰ begins just below the base of the Hirnantian Stage and peaks in the lower part of the Normalograptus extraordinarius biozone of lower Hirnantian. This is followed by an interval of reduced δ¹³C values and a second peak of similar magnitude, which occurs in the lower Normalograptus persculptus biozone (upper Hirnantian). These peaks appear to correlate well with episodes of glacial expansion described from West Africa.

Global correlation between δ¹³C curves suggests that the timing of peak positive excursions is not completely synchronous between different regions. In particular, the lower Hirnantian peak seen in Arctic Canada and some other areas appears to be suppressed in sedimentary successions from the circum-Iapetus region, where peak values occur in later Hirnantian time. Thus, no single, regional δ¹³C curve can reliably serve as a benchmark for high-resolution, global correlation.

These data provide support for the hypothesis that the positive δ¹³C shifts seen in these sections and many others worldwide are the result of increased rates of weathering of carbonate platforms that were exposed during the glacio-eustatically controlled sea-level fall. This caused the isotope value of the C-weathering flux to shift towards the ¹³C-enriched carbonate end-member, increasing the δ¹³C value of carbon transported by rivers to both epeiric seas and the oceans. Magnitude differences between Hirnantian δ¹³C excursions in shallower and deeper water parts of epeiric sea basins, as well as between different regions, may be explained by water mass differentiation between those regions. The positive shift in the δ¹³C value of the Hirnantian oceans is predicted to be about 2–3‰, which is about half the value of the larger excursions found in basin proximal settings of low latitude epeiric seas.

Introduction

The Late Ordovician was a time of mass extinction, associated with a widespread continental glaciation that occurred within what was otherwise a prolonged period of greenhouse climatic conditions (Sheehan, 2001). Evidence from sedimentological, faunal, and geochemical data all suggest that the major phases of glaciation occurred within the early to middle part of the Hirnantian Epoch (e.g., Brenchley et al., 1994, Ghienne, 2003). Knowledge of the temporal relationships of the biodiversity and palaeoenvironmental changes and how these correlate globally are central to understanding the processes that drove these changes. Carbon isotope chemostratigraphy has become an important tool in the study of bioevents because it can play a role both in their global correlation and also helps to constrain the possible nature and range of palaeoenvironmental changes. New carbon-isotope chemostratigraphic data from Arctic Canada are presented that are well-constrained by graptolite biostratigraphy. These results, combined with new biostratigraphic data for Dob's Linn, Scotland, and Anticosti Island, Quebec, yield insights into the relative timing of the carbon isotope excursions and graptolite biostratigraphy in those regions, as well as globally. The results cast doubt on the hypothesis that the timing and form of a composite curve based on the Baltic succession can be used as a benchmark for global correlation through the Hirnantian (Brenchley et al., 2003).

Section snippets

Materials and methods

Samples for the present study were collected from the lower member of the Cape Phillips Formation during the summer of 1998 from three localities in the central region of the Queen Elizabeth Islands, Nunavut, Canada: two sections from northeastern Cornwallis Island; and one from Truro Island (Fig. 1). The general stratigraphy and palaeogeographic setting of the Cape Phillips Formation in this region were described by Melchin (1989), Melchin et al. (1991), and Coniglio and Melchin (1995). The

Results

Biostratigraphic data record a succession of five regional graptolite biozones through the study interval (Fig. 2), the A. fastigata biozone, P. pacificus biozone, the Normalograptus extraordinarius biozone, and the Normalograptus persculptus biozone, which can be readily correlated with other global zonations. The Hirnantian Stage, as recently proposed by Chen et al. (2004) and employed by Cooper and Sadler (2004), includes strata of the N. extraordinarius and N. persculptus biozones.

Regional correlations

Using the combined biostratigraphic, lithological, and carbon isotope data (particularly δ¹³C_org), it is possible to propose a high-resolution correlation of the study sections through the interval from the upper P. pacificus biozone into the N. persculptus biozone (Fig. 3). This correlation suggests that strata equivalent to the uppermost P. pacificus biozone and some or possibly all of the N. extraordinarius biozone are missing at Cape Manning and that significant variations in thickness of

Impact of detrital carbonate flux and diagenesis on the δ¹³C_carb excursion

The generally weaker and less consistent positive shifts in the δ¹³C_carb data, as compared with the δ¹³C_org values, through the Hirnantian, and the generally more variable values throughout the study section may be accounted for by the fact that these analyses represent whole-rock carbonate data. Sedimentological observations and previous diagenetic studies (Coniglio and Melchin, 1995) suggest that the fine-grained calcite in these sediments derives from at least three sources: primary

Conclusions

Results of the study of three sections through latest Ordovician strata in the Canadian Arctic Islands show that a positive δ¹³C_org excursion of 3–6‰ begins just below the base of the Hirnantian Stage and peaks in the lower part of the N. extraordinarius biozone of lower Hirnantian. This is followed by an interval of reduced δ¹³C values and a second peak of similar magnitude, which occurs in the lower N. persculptus biozone (upper Hirnantian).

The peaks in δ¹³C_org values appear to correlate well

Acknowledgements

We thank E.W. MacDonald for his assistance in the field and the Polar Continental Shelf Project for logistical support. We also gratefully acknowledge the Natural Sciences and Engineering Research Council for the financial support to our research programs. C.H. thanks Tim Prokopiuk for his invaluable assistance in the laboratory, and for running some of the analyses. The manuscript has benefited from discussions with P. Copper, comments by M. Saltzman and an anonymous reviewer.

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Carbon isotope chemostratigraphy in Arctic Canada: Sea-level forcing of carbonate platform weathering and implications for Hirnantian global correlation

Abstract

Introduction

Section snippets

Materials and methods

Results

Regional correlations

Impact of detrital carbonate flux and diagenesis on the δ13Ccarb excursion

Conclusions

Acknowledgements

Geochimica et Cosmochimica Acta

Palaeogeography, Palaeoclimatology, Palaeoecology

Palaeogeography, Palaeoclimatology, Palaeoecology

Palaeogeography, Palaeoclimatology, Palaeoecology

Chemical Geology

Palaeogeography, Palaeoclimatology, Palaeoecology

Palaeogeography, Palaeoclimatology, Palaeoecology

Palaeogeography, Palaeoclimatology, Palaeoecology

Review of Palaeobotany and Palynology

Palaeogeography, Palaeoclimatology, Palaeoecology

Deep-sea sediments record the geophysiology of the Late Ordovician glaciation

Journal of the Geological Society (London)

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Geology

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Modern Geology

High-resolution stable isotope stratigraphy of Upper Ordovician sequences: constraints on timing of bioevents and environmental changes associated with mass extinction and glaciation

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Bulletin of Canadian Petroleum Geology

Reefs during the multiple crises towards the Ordovician–Silurian boundary: Anticosti Island, eastern Canada, worldwide

Canadian Journal of Earth Sciences

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Silurian system of the Canadian Arctic archipelago

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Late Ordovician mass extinction: a new perspective from stratigraphic sections in central Nevada

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Isotopic evidence for geochemical decoupling between ancient epeiric seas and bordering oceans: implications for secular curves

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Origin and significance of isotope shifts in Pennsylvanian carbonates (Austrias NW Spain)

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An alternative model for positive shifts in shallow-marine carbonate δ13C and δ18O

Sedimentology

Impact of detrital carbonate flux and diagenesis on the δ¹³C_carb excursion

An alternative model for positive shifts in shallow-marine carbonate δ¹³C and δ¹⁸O