MICROBIAL COMMUNITY RESPONSE TO MARINE ENVIRONMENTAL VOLATILITY THROUGH THE LATE DEVONIAN MASS EXTINCTION EVENTS

Marine environmental volatility is known to lead to several major biotic events during the Late Devonian (382.7−358.9 Ma), but its effects on marine microbial community structure have received little research attention to date. Here, we perform an organic geochemical investigation of a ∼95m thermally immature core of the New Albany Shale from Kentucky, USA. Based on four positive excursions of stable carbon isotopes of organic carbon (δ¹³C_org) and lithology, we tentatively identified four major bio-events, the Frasnes, Lower Kellwasser (LKW), Upper Kellwasser (UKW), and Hangenberg events. Low pristane/phytane ratios and elevated aryl isoprenoid concentrations during the UKW and Hangenberg events indicate the development of severely oceanic anoxia, including photic zone euxinia. The ratios of hopane/sterane increased during the UKW and persisted in the following several million years. This pattern suggests that a shift occurred in microbial community structure during the UKW, from being dominated by eukaryotes to prokaryotes, most likely due to organic matter accumulation that fueled blooms of heterotrophic bacteria, and that the response and recovery of microbial community to dramatic redox changes were delayed and prolonged. Sterane compositions suggest frequent changes in algal community but an overall increase in green algae and decrease in red algae from the UKW to Hangenber event. Collectively, these preliminary results demonstrate significant shifts in marine microbial community structure, although the timing and magnitude of these shifts may diverge from those of oceanic anoxia, reflecting the resistance and resilience of marine microbes to environmental volatility.