ENVIRONMENTAL CONTEXT FOR ANIMAL BIOMINERALIZATION AND FOSSIL PYRITIZATION IN THE TERMINAL EDIACARAN PERIOD

In terminal Ediacara 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 Gaojiashan Member of the Dengying Formation. Both fossils have funnel-in-funnel constructions, but Cloudina has biomineralized skeletons whereas Conotubus does not. In order to better understand the paleoenvironmental condition during this important transition, an integrative chemostratigraphic investigation was conducted. Coincident with this post-Shuram evolutionary milestone is a significant positive carbonate carbon isotope excursion up to +6‰ and a marked ³⁴S enrichment in pyrite. In contrast, ³⁴S abundances of carbonate-associated sulfate (CAS) remain steady at values of ca. +40‰ throughout Gaojiashan strata. As a result, sulfur isotope fractionations are large (up to ca. +70‰) in the lower half of the succession, and then decline to a steady value of ca. +30‰ during the positive carbon isotope excursion. Given the general invariance in CAS values and the occurrence of bedded gypsum associated with the carbon isotope peak, we interpret the stratigraphic trend in sulfur isotope fractionation to changes in microbial communities, with sulfur disproportionation dominating in the lower interval while in the upper interval sulfate reduction was the dominant process. Hence the coupled paleontological and biogeochemical anomalies may have coincided with an increase in weathering fluxes of sulfate, alkalinity, and nutrients to the depositional basin, which stimulated primary productivity, sulfate reduction, and the spread of the oxygen-minimum zone across subtidal and basinal environments. Enhanced production and burial of organic matter and pyrite are thus directly connected to the carbon isotope anomaly, and likely promoted pyritization as the main taphonomic mode of the Gaojiashan biota where sulfide and ferrous iron were abundant. With enhanced terrestrial fluxes of weathered products, elevated calcium concentrations and alkalinity levels in terminal Ediacaran oceans are envisioned as a facilitating factor for biomineralization.