A multiproxy investigation of oceanic redox conditions during the Cambrian SPICE event

The research presented here is an effort to characterize changes in marine oxygen availability across a portion of the later Cambrian noted for unique evolutionary dynamics and which includes a significant global oceanographic event known as the SPICE event (Steptoean Positive Carbon Isotope Excursion). Previous studies have revealed the SPICE caused large changes to the global cycles of carbon, sulfur, uranium, molybdenum and the overall trace metal content of seawater. Furthermore, the initiation of these changes appears to have been temporally coupled with marine extinctions across several paleocontinents raising the possibility of a common causal linkage between all these features. In particular, expanding marine anoxia has been invoked as the most parsimonious explanation for these co-occurring features. The research presented here tests this hypothesis directly across a range of spatial scales using the iron speciation paleoredox proxy to characterize redox conditions within individual basins and to facilitate comparison of conditions between basins. In addition to these analyses, we apply a new proxy, thallium stable isotopes to this interval to assess potential global changes in deoxygenation across the event. These iron speciation analyses showed shallow environments deoxygenated coincident with the initiation of the SPICE and extinction horizons, and these conditions were dominantly ferruginous. Importantly, this work also shows deeper-water environments were deoxygenated prior to and remained so across the event and these environments were also largely. Last we looked at changes in thallium isotopes during this same interval to see if this deoxygenation would be recorded as a positive shift across the interval if expanded anoxia were to impact the areal extent of manganese-oxide sedimentation and burial. We found it did record these changes, but with a different expression than during other more recent events explored using the isotope system. We attribute these differences to the unique chemical structure of the oceans during the Cambrian, which as documented herein were widely oxygen-deficient in their deeper depths. Given this recognition we suggest that thallium isotope studies in deep time should account for this redox structure of ancient oceans likely common under the less-oxygenated atmospheres of the ancient Earth.