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‰–5‰ positive δ 13 C 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 δ 18 O values of apatitic inarticulate brachiopods from three Upper Cambrian successions across Laurentia to evaluate paleotemperatures during the SPICE. δ 18 O values range from ∼12.5‰ to 16.5‰. Estimated seawater temperatures associated with the SPICE are unreasonably warm, suggesting that the brachiopod δ 18 O values were altered during early diagenesis. Despite this, all three localities show similar trends with respect to the SPICE δ 13 C curve, suggesting that the brachiopod apatite preserves a record of relative δ 18 O and temperature changes. The trends include relatively high δ 18 O 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 δ 18 O 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 δ 18 O 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 δ 13 C shift.