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 (delta C-13(org)) and whole-rock carbonate (delta C-13(carb)) fractions. The sections are well constrained biostratigraphically using graptolites, lithostratigraphically and palaeogeographically. delta C-13(org) data appear to provide a signal that mainly reflects chemical changes in the seawater, whereas the delta C-13(carb) data seem to have been variably affected by sediment reworking and diagenesis. Results show that a positive delta C-13(org) excursion of 3-6 parts per thousand begins just below the base of the Himantian Stage and peaks in the lower part of the Normalograptus extraordinarius biozone of lower Himantian. This is followed by an interval of reduced delta C-13 values and a second peak of similar magnitude, which occurs in the lower Normalograptus persculptus biozone (upper Himantian). These peaks appear to correlate well with episodes of glacial expansion described from West Africa. Global correlation between delta C-13 curves suggests that the timing of peak positive excursions is not completely synchronous between different regions. In particular, the lower Himantian peak seen in Arctic Canada and some other areas appears to be suppressed in sedimentary successions from the circum-lapetus region, where peak values occur in later Himantian time. Thus, no single, regional delta C-13 curve can reliably serve as a benchmark for high-resolution, global correlation. These data provide support for the hypothesis that the positive delta C-13 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 sealevel fall. This caused the isotope value of the C-weathering flux to shift towards the C-13-enriched carbonate end-member, increasing the delta C-13 value of carbon transported by rivers to both epeiric seas and the oceans. Magnitude differences between Himantian delta C-13 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 delta C-13 value of the Himantian oceans is predicted to be about 2-3 parts per thousand, which is about half the value of the larger excursions found in basin proximal settings of low latitude epeiric seas. (c) 2005 Elsevier B.V. All rights reserved.