Carbon isotope chemostratigraphy in Arctic Canada: Sea-level forcing of carbonate platform weathering and implications for Hirnantian global correlation
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).
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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 δ13Corg), 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 δ13Ccarb excursion
The generally weaker and less consistent positive shifts in the δ13Ccarb data, as compared with the δ13Corg 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 δ13Corg 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 δ13Corg 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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