ASSESSING THE ROLE OF ANOXIA IN THE END-MARJUMAN EXTINCTION AND SPICE EVENTS

The second half of the Cambrian Period was a dynamic interval in Earth history, characterized by high rates of faunal turnover and large perturbations of the global carbon cycle. Understanding the Earth system interactions responsible for these patterns can provide important information about the co-evolution and dynamics of the biosphere and surface environments. Here we investigate the environmental conditions associated with the end-Marjuman extinction (~499 Ma), which coincides with the initiation of the global SPICE event. These events serve as the best example (largest, most well-documented) of the larger pattern of extinction coincident with changes to the carbon cycle. In particular, the role of anoxia in these events is explored, using well core material from Ohio and Kentucky (Nolichucky and Eau Claire Formations of the Conasauga Group). This succession reflects sediments deposited in and around the Rome Trough, a rift graben within the intrashelf basin on the Laurentian margin. This framework enables investigation of water conditions along a paleodepth gradient across the extinction and SPICE event. Our data records the SPICE event in both d¹³C_carb and d¹³C_org and TOC values are generally low (0.07 to 0.31 wt%). Iron speciation data show a transition from oxic to anoxic conditions in the shallower water environments that is coincident with the extinction horizon and the initiation of the isotope excursion. While, pyrite sulfur contents also increase following the SPICE initiation, they are considerably below the ratio characteristic of euxinic environments, possibly owing to low local organic productivity.