Geochemical Evidence for Widespread Reducing Ocean during the Middle to Late Cambrian

Conventional thought concerning the history of the redox state of the Earth is that after the Ediacaran-Cambrian transition, the ocean was generally well-oxygenated, which paved the way for the proliferation of macroscopic multicellular life. However, the extent and persistence of this increase in oxidation is not well understood or constrained. We present geochemical data from the SPICE (Steptoean Positive Carbon Isotope Excursion) event that point to a reducing Late Cambrian ocean. Carbonate-C, sulfate-S and pyrite-S isotope records from globally distributed sections through the SPICE show parallel, positive isotope shifts that suggest enhanced organic C and pyrite S burial consistent with widespread anoxia. This suggestion is confirmed by geochemical box modeling of the event.

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.