Carbon isotope chemostratigraphy in Arctic Canada: Sea-level forcing of carbonate platform weathering and implications for Hirnantian global correlation
Abstract Three sections through latest Ordovician strata in the Canadian Arctic Islands have been studied for carbon isotopes, derived from the organic matter (δ13Corg) and whole-rock carbonate (δ13Ccarb) fractions. The sections are well constrained biostratigraphically using graptolites, lithostratigraphically and palaeogeographically. δ13Corg data appear to provide a signal that mainly reflects chemical changes in the seawater, whereas the δ13Ccarb data seem to have been variably affected by sediment reworking and diagenesis. Results 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 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.