MULTI-STAGE EDIACARAN OCEAN OXIDATION AND ITS IMPACT ON EVOLUTIONARY RADIATION

Using high-resolution paired carbon (δ¹³C_carb and δ¹³C_org) and sulfur (δ³⁴S_SO4 and δ³⁴S_pyr) isotopes, we can resolve a multi-stage record of Ediacaran ocean oxidation preserved in the strata of the Huqf Supergroup, Sultanate of Oman. Following the Marinoan glaciation, stage I oxidation lasts from ~ 635 – 610 million years ago (Myr) and records an increase in marine sulfate concentrations above ~ 200 μM, likely due to a rise in atmospheric oxygen. Stage II oxidation (~ 575 – 550 Myr) coincides with the Shuram excursion, a > 13‰ negative excursion in δ¹³C_carb created by the oxidation of a large reservoir of deep ocean dissolved organic carbon (DOC). Support for more oxidizing conditions comes from a decrease in the fraction of sulfur buried as pyrite (f_pyr) as well as a decrease in the abundance of both pyrite and TOC. The onset of stage II oxidation coincides with the evolution of acanthomorph acritarchs in Australia and China, and the appearance of the first Ediacaran metazoa, Charnia-type segmented fronds, while the first motile macroscopic metazoa (e.g., Kimberella) appear at ~555 Myr as the Shuram excursion draws to a close. Stage III oxidation (~ 550 – 547 Myr) is marked by presence of sulfur disproportionating metabolisms and the onset of co-variation in δ¹³C_carb and δ¹³C_org, which are absent in older strata. Coincident with the onset of stage III oxidation, we see the evolution of macroscopic multicellular algae, an increase in the diversity of the acanthomorph acritarchs, and the appearance of the weakly calcifying metazoa Cloudina and Namacalathus. Following stage III oxidation, there is a period of quiescence characterized by little perturbation in the carbon cycle and an absence of significant evolutionary events. At ~542 Myr, anoxia at the Ediacaran-Cambrian boundary causes the extinction of the Ediacaran fauna, setting the stage for the Cambrian radiation.