Persistent dysoxia in very shallow seas across the Late Cambrian SPICE Event, Durness Group, UK

The global late Cambrian Steptoean Positive Isotopic Carbon Excursion (SPICE; ca. 495−492 Ma) has been linked to oceanic anoxia followed by a pulse of atmospheric oxygenation that in turn may have facilitated the Ordovician Radiation. To provide new insight into the evolution of very-shallow-water redox conditions across the SPICE, we present an integrated multiproxy study of carbonate rocks from the Durness Group, Scotland, UK, combining Fe speciation, redox-sensitive trace element systematics, and I/(Ca + Mg) ratios. We interpret the Durness SPICE peak to have occurred during an early highstand interval, where shallow waters were impacted by either regional mobilization of Fe2+ under low-oxygen conditions or episodic upwelling of deep, ferruginous, anoxic waters that followed high-frequency cycles. Unlike other records, our data show that very low marine oxygen concentrations (dysoxia) persisted before, during, and after the SPICE, confirming that a very shallow redoxcline was maintained in this region, with no evidence for any sustained increase in oxygenation. We conclude that under the prevailing low-atmospheric-oxygen conditions of the Cambrian, dysoxia was prevalent even in very shallow waters.