No abstract

Extensive marine anoxia in the terminal Ediacaran ocean was associated with the decline of the Ediacara biota.

In terminal Ediacaran strata of South China, the onset of calcareous biomineralization is preserved in the paleontological transition from Conotubus to Cloudina in repetitious limestone facies of the Dengying Formation. Both fossils have similar size, funnel-in-funnel construction, and epibenthic lifestyle, but Cloudina is biomineralized, whereas Conotubus is not. To provide environmental context for this evolutionary milestone, we conducted a high-resolution elemental and stable isotope study of the richly fossiliferous Gaojiashan Member. Coincident with the first appearance of Cloudina is a significant positive carbonate carbon isotope excursion (up to +6‰) and an increase in the abundance and (34) S composition of pyrite. In contrast, δ(34) S values of carbonate-associated sulfate remain steady throughout the succession, resulting in anomalously large (>70‰) sulfur isotope fractionations in the lower half of the member. The fractionation trend likely relates to changes in microbial communities, with sulfur disproportionation involved in the lower interval, whereas microbial sulfate reduction was the principal metabolic pathway in the upper. We speculate that the coupled paleontological and biogeochemical anomalies may have coincided with an increase in terrestrial weathering fluxes of sulfate, alkalinity, and nutrients to the depositional basin, which stimulated primary productivity, the spread of an oxygen minimum zone, and the development of euxinic conditions in subtidal and basinal environments. Enhanced production and burial of organic matter is thus directly connected to the carbon isotope anomaly, and likely promoted pyritization as the main taphonomic pathway for Conotubus and other soft-bodied Ediacara biotas. Our studies suggest that the Ediacaran confluence of ecological pressures from predation and environmental pressures from an increase in seawater alkalinity set the stage for an unprecedented geobiological response: the evolutionary novelty of animal biomineralization.

Early diagenetic silicification and phosphatization of the Ediacaran Doushantuo Formation (ca. 635 to 551 Ma) in South China offer extraordinary taphonomic windows into the early evolution of multicellular eukaryotes, including various algal groups and potentially animals. In order to understand how the ecological and taphonomic distribution of these Ediacaran eukaryotes was controlled by oceanic redox conditions, it is critical to reconstruct the redox architecture of the sedimentary basin. Recently two alternative redox models have been proposed to account for the geochemical and sedimentary features of the Doushantuo Formation. One argues that the unit was deposited on a continental margin where a metastable sulfidic wedge was dynamically maintained by a sulfate concentration gradient between shelf and basinal environments. The other contends that the sulfidic water mass was largely restricted to the intra-shelf basin behind a rimmed margin. These two models make different predictions about the stratigraphic completeness and correlation of the Doushantuo Formation. To test these predictions, we generated high-resolution time-series trends of multiple isotopic and elemental tracers, including δ 34 S, 87 Sr/ 86 Sr and Ce/Ce*, to facilitate an integrated chemostratigraphic correlation between inner shelf (Xiaofenghe), intra shelf (Jiulongwan), and outer shelf (Yangjiaping and Zhongling) sections. Our correlations suggest that both the inner and outer shelf sections are stratigraphically incomplete relative to the intra shelf section. The euxinic wedge model should be reconsidered insofar as it is based on a miscorrelation between sections. Viewed from our revised chemostratigraphic framework, euxinic conditions on the platform appear to have been largely restricted to the intra shelf basin. Carbonates in the upper Doushantuo Formation at Jiulongwan and their stratigraphic equivalents are characterized by a profound negative carbon isotope anomaly (i.e., the Shuram Excursion) coincident with a drop in pyrite sulfur isotope values and a significant rise in 87 Sr/ 86 Sr from 0.7080 to 0.7090. The integrated stratigraphic data from South China suggest that the onset of the Shuram Excursion is associated with enhanced oxidative continental weathering that delivered radiogenic strontium, as well as sulfate, to the Ediacaran basin.