Changes of Microbial Communities in Response to Marine Environmental Volatility through the Late Devonian Mass Extinctions

Abstract:

Marine environmental volatility is a known cause of the Late Devonian (382.7−358.9 Ma) mass extinctions, but changes of microbial communities in response to environmental volatility have received little study to date. Here, we perform a stable isotope (δ¹³C_org) and lipid biomarker investigation of thermally immature (Fig.1-A) New Albany Shale from a ∼95m core encompassing the entire Late Devonian bio-events at western Kentucky, United States. The results preliminarily have identified four bio-events of the Frasnes, Lower Kellwasser (LKE), Upper Kellwasser (UKE), and Hangenberg by four positive δ¹³C_org excursions (Fig.1-B). Changes in pristane/phytane and aryl isoprenoids indicate anoxia and euxinia drive environmental disturbances during the UKE and Hangenberg event (Fig.1-C and -D). The broad measure of bacteria/algae input, the ratio of hopane/sterane, elevates during and following the UKE, declines approaching the Hangenberg event, and elevates again during the Hangenberg event (Fig.1-E). The UKE anoxia caused a bacterial bloom that was more vigorous after the UKE. A small-scale bacterial bloom was also caused by the less intense Hangenberg euxinia. Environmental volatility created ecosystems for bacteria by killing algae during the Late Devonian. In the algal communities, particularly, the anoxic and euxinic environments were inhabitable to the dominant green algae (Fig.1-F), but H₂S in the euxinic environments was toxic to the red algae contrasting with the inhabitable anoxia (Fig.1-G). Biostratigraphic analysis including conodonts will be performed to date the excursions and provide a more comprehensive stratigraphic understanding of the study interval. Additional lipid biomarker data will be analyzed to further unravel the response of microbial communities to the Late Devonian environmental changes.