Oxygen-isotope trends and seawater temperature changes across the Late Cambrian Steptoean positive carbon-isotope excursion (SPICE event)

The globally recognized Late Cambrian Steptoean positive C-isotope excursion (SPICE) is characterized by a 3 parts per thousand-5 parts per thousand positive delta C-13 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 (C-org) and pyrite. We analyzed delta O-18 values of apatitic inarticulate brachiopods from three Upper Cambrian successions across Laurentia to evaluate paleotemperatures during the SPICE. delta O-18 values range from similar to 12.5 parts per thousand to 16.5 parts per thousand. Estimated seawater temperatures associated with the SPICE are unreasonably warm, suggesting that the brachiopod delta O-18 values were altered during early diagenesis. Despite this, all three localities show similar trends with respect to the SPICE delta C-13 curve, suggesting that the brachiopod apatite preserves a record of relative delta O-18 and temperature changes. The trends include relatively high delta O-18 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 delta O-18 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 delta O-18 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 O-2 concentrations, which enhanced the preservation/burial of C-org causing the positive delta C-13 shift.