Molecular indicators of microbial community change linked to salinity variation and terrestrial inputs during the Ordovician-Silurian transition (East-Central USA)

Abstract

The end-Ordovician mass extinction (LOME), one of the largest biocrises of the Phanerozoic, was linked to a major climatic cooling event, continental glaciation, and oceanic environmental changes. To date, the effect of this biocrisis on marine microbial communities has received scant attention. Here, we generate biomarker records for Upper Ordovician to lower Silurian strata in eastern North America, along with geochemical proxies for watermass salinity and terrestrial fluxes, to investigate contemporaneous microbial community changes and possible controls thereon. Hopane-to-sterane ratios (H/S) exhibit reduced values related to enhanced eukaryotic inputs during the Hirnantian Ice Age. A decrease in the proportion of long-chain n-alkanes and an increase in moretane-to-hopane ratios reflect increased contributions from land plants to the organic fraction of marine sediments during the Ordovician-Silurian transition (OST). The high relative abundances of C₁₉-C₂₀ long-chain tricyclic terpenes in the lower Silurian imply increased inputs from marine algae, possibly in response to enhanced nutrient fluxes from terrestrial sources to the ocean. The salinity proxies B/Ga and S/TOC indicate a shift in watermass conditions from marine to slightly brackish in the pre-Hirnantian to marine to slightly hypersaline in the Hirnantian and early Silurian, probably linked to shallowing of the Lexington Platform and climate aridification resulting from Gondwanan continental glaciation. This increase in salinity is consistent with rapid variation in hopane/sterane ratios, showing the influence of watermass salinity on microbial community composition. Thus, marine environmental changes (i.e., in salinity, redox, and nutrient fluxes) were the immediate cause of microbial community evolution during the OST, although the ultimate driver may have been expansion and diversification of terrestrial floras.