Geochemical Evidence for Widespread Reducing Ocean during the Middle to Late Cambrian
Further evidence for increased anoxic deposition during the SPICE comes from the euxinic Alum Shale of Sweden. Molybdenum concentrations show a crash in enrichment coincident with the peak of the SPICE. The indication is a depleted seawater Mo inventory marking Mo sequestration over large areas of the ocean..
Evidence for persistent, reducing marine depositional conditions are not confined to the SPICE. Sulfur isotope data along with box modeling indicate a small and isotopically heterogeneous marine sulfate reservoir prior to the event. Additionally, the Mo/TOC ratios of the Alum are lower than other Phanerozoic euxinic black shales, suggesting a smaller seawater Mo reservoir before the onset of the SPICE. Put into the context of high rates of turnover in marine fauna, an abundance of low-oxygen tolerant organisms and a lack of large calcifying organisms, the sum of our data and modeling point to a more reducing and unstable redox state for Middle to Late Cambrian ocean than previously considered.