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 (δ 13C org) and whole-rock carbonate (δ 13C carb) fractions. The sections are well constrained biostratigraphically using graptolites, lithostratigraphically and palaeogeographically. δ 13C org data appear to provide a signal that mainly reflects chemical changes in the seawater, whereas the δ 13C carb data seem to have been variably affected by sediment reworking and diagenesis. Results show that a positive δ 13C 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 δ 13C 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 δ 13C 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 δ 13C curve can reliably serve as a benchmark for high-resolution, global correlation. These data provide support for the hypothesis that the positive δ 13C 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 δ 13C value of carbon transported by rivers to both epeiric seas and the oceans. Magnitude differences between Hirnantian δ 13C 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 δ 13C 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.