Carbon isotope chemostratigraphy in Arctic Canada: Sea-level forcing of carbonate platform weathering and implications for Hirnantian global correlation
Three sections through latest Ordovician strata in the Canadian Arctic Islands have been studied for carbon isotopes, derived from the organic matter (y13Corg) and whole-rock carbonate (y 13Ccarb) fractions. The sections are well constrained biostratigra-phically using graptolites, lithostratigraphi...
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| Format: | Text |
| Language: | English |
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2005
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.483.2527 http://sil.usask.ca/holmden/list_2/melchin_and_holmden_2006_pp.pdf |
| Summary: | Three sections through latest Ordovician strata in the Canadian Arctic Islands have been studied for carbon isotopes, derived from the organic matter (y13Corg) and whole-rock carbonate (y 13Ccarb) fractions. The sections are well constrained biostratigra-phically using graptolites, lithostratigraphically and palaeogeographically. y13Corg data appear to provide a signal that mainly reflects chemical changes in the seawater, whereas the y13Ccarb data seem to have been variably affected by sediment reworking and diagenesis. Results show that a positive y13Corg excursion of 3–6x 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 y13C 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 y13C 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 y13C curve can reliably serve as a benchmark for high-resolution, global correlation. These data provide support for the hypothesis that the positive y13C shifts seen in these sections and many others worldwide are |
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