INCREASED MICROBIAL CARBONATE ACCUMULATION FOLLOWING BIOTIC RECOVERY FROM THE END-PERMIAN MASS EXTINCTION SUGGESTS THAT THE CONTROLS GOVERNING MICROBIAL CARBONATE PRODUCTION SHOULD BE RECONSIDERED

Microbial carbonates are generally considered to dominate in situ carbonate production on subtidal platform slopes during intervals with high global temperatures, high carbonate saturation state, and absent or depleted metazoan-algal benthic ecosystems. To investigate the controls on microbial carbonate production on platform slopes, we analyzed the covariance in microbial and metazoan-algal carbonate content on the Xiliang slope of the Great Bank of Guizhou (GBG), an isolated carbonate platform in the Nanpanjiang Basin of South China. The GBG spans the Early to Middle Triassic recovery interval following the end-Permian mass extinction, providing an opportunity to assess modes of carbonate production across an interval transitioning from depleted to recovered benthic ecosystems, from unusually high to more normal global temperature, and from high to moderate carbonate saturation state. Field observations and petrographic analysis of hand samples and thin sections were used to estimate the volumetric abundance of Lower and Middle Triassic microbial and skeletal carbonates. Rather than enhancing slope microbial carbonate production, the Early Triassic interval of depleted benthic ecosystems, high global temperatures, and high carbonate saturation state is characterized by an absence of in situ microbial carbonate accumulation on the Xiliang slope. In contrast, the subsequent Spathian (late Early Triassic) to Anisian (early Middle Triassic) biotic recovery interval exhibits microbial carbonate accumulation. Further, when benthic ecosystem biodiversity returned to pre-extinction levels in the Anisian, microbial carbonate increased in volumetric abundance and physical complexity. Thus, metazoan-algal benthic ecosystems might enhance, rather than hinder, the production of microbial slope carbonates by providing physical and organic substrates for carbonate-precipitating microbial communities. Our findings imply that the controls on microbial slope carbonate production are poorly constrained and that the occurrence of microbial carbonates in the 3.5-billion-year carbonate sedimentary archive is an unreliable indication of inhospitable environments for metazoans and algae.