Interaction of Shibantan Biota and environment in the terminal Ediacaran ocean: Evidence from I/(Ca+Mg) and sulfur isotopes

Abstract

The terminal Ediacaran (∼551–539 Ma) was a crucial period in Earth’s history that witnessed the first appearance of skeletonized animals and complex ecosystems (e.g., the Shibantan Biota) in the ocean. However, the trigger for such biological innovations and their feedbacks to the environment are still poorly understood. In this study, to explore possible interactions between the Shibantan Biota and the oceanic environment, we carried out an integrated investigation of I/(Ca+Mg) along with sulfur isotopes of carbonate-associated sulfate (δ34SCAS) and pyrite (δ34Spy) in the Shibantan Member of the Dengying Formation at Wuhe (South China) where the Shibantan Biota was recovered. Our I/(Ca+Mg) profile yields fluctuating values in the range of 0.03 to 1.86 μmol/mol, corresponding to oxygen concentrations between 1–3 μM and 20–70 μM in the water column of the study area. This result indicates that the oxygen levels in shallow-water environments of the terminal Ediacaran ocean were generally low and the Shibantan Biota must have had low physiological requirements of oxygen. Variably high δ34SCAS values (+31.8‰ to +55.7‰, mean ± 1σ: +41.0 ± 5.2‰) were observed in studied unit, implying low sulfate concentrations in the terminal Ediacaran ocean, consistent with contemporaneous expansion of seawater anoxia. Furthermore, isotopically superheavy pyrites (i.e., δ34Spy > δ34SCAS) are present in the lower Shibantan Member, concurrent with the occurrence of bioturbation in the sediments, suggesting controls by microbial sulfate reduction (MSR) in sulfate-limited porewaters followed by preferential re-oxidation of 32S-riched H2S under the influence of burrowing organisms. This finding supports that burrowing activity of infaunas of the Shibantan Biota may have enhanced sediment porewater oxygenation, thus likely expanding the ecosystem space available to early animals. Our study thus provides new insights into the coevolution of early animals and terminal Ediacaran ocean environments.