Abstract

The globally recognized Late Cambrian Steptoean positive C-isotope excursion (SPICE) is characterized by a 3‰–5‰ positive δ13C shift spanning <4 m.y. Existing hypotheses suggest that the SPICE represents a widespread ocean anoxic event leading to enhanced burial/preservation of organic matter (Corg) and pyrite. We analyzed δ18O values of apatitic inarticulate brachiopods from three Upper Cambrian successions across Laurentia to evaluate paleotemperatures during the SPICE. δ18O values range from ∼12.5‰ to 16.5‰. Estimated seawater temperatures associated with the SPICE are unreasonably warm, suggesting that the brachiopod δ18O values were altered during early diagenesis. Despite this, all three localities show similar trends with respect to the SPICE δ13C curve, suggesting that the brachiopod apatite preserves a record of relative δ18O and temperature changes. The trends include relatively high δ18O values at the onset of the SPICE, decreasing and lowest values during the main event, and an increase in values at the end of the event. The higher δ18O values during the global extinction at the onset of the SPICE suggests seawater cooling and supports earlier hypotheses of upwelling of cool waters onto the shallow shelf. Decreasing and low δ18O values coincident with the rising limb of the SPICE support the hypothesis that seawater warming and associated reduced thermohaline circulation rates contributed to decreased dissolved O2 concentrations, which enhanced the preservation/burial of Corg causing the positive δ13C shift.

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