CHANGES OF MICROBIAL COMMUNITIES IN 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 communities have received little attention to date. Here, we perform an organic geochemical investigation of a ∼95m core of the thermally immature New Albany Shale from Kentucky, USA. The New Albany Shale consists of the four members of the Blocher Shale, Selmier Shale, Grassy Creek, and Hannibal-Saverton Shale. Based on four positive excursions of stable carbon isotopes of organic carbon (δ¹³C_org) with amplitudes of 1.2‰, 2.0‰, 3.0‰, and 1.2‰, we tentatively identified four major bio-events, the Frasnes, Lower Kellwasser (LKE), Upper Kellwasser (UKE), and Hangenberg events. Low pristane/phytane ratios and elevated aryl isoprenoid concentrations during the UKE and Hangenberg events indicate the development of severe oceanic anoxia, including photic zone euxinia. The ratio of hopanes/steranes increased during the UKE and persisted in the following several millions of years. This pattern suggests that the UKE anoxia caused a shift in microbial communities, from being dominated by eukaryotes to prokaryotes, most likely due to anoxia created more ecosystem for prokaryotes by killing eukaryotes. Sterane compositions suggest frequent changes in algal communities but an overall increase in green algae and decrease in red algae from the UKE to Hangenber event. Collectively, these preliminary results demonstrate significant shifts in marine microbial communities.