PATTERNS OF CHANGE IN BIOMARKER SIGNATURES SUPPORT A LINKAGE BETWEEN GRAPTOLITE AND PHYTOPLANKTON COMMUNITY CHANGES

The Late Ordovician, Hirnantian Age (444.7- 443.4 Ma) was a glacial period with varying climate and sea level changes that are marked by geochemical signatures. Climate change drove changes in deep-ocean circulation and upwelling zones during the concomitant mass extinction and it appears that the graptolites inhabiting the mesopelagic zone were the most vulnerable during these events. The goal of our study is to analyze biomarkers from the late Katian and Hirnantian Vinini Creek section based on a set of samples in which graptolite community change has been identified. Previous work at lower resolution in this section suggests a decrease in denitrification and increase in abundance of green algae relative to bacteria within the Hirnantian glacial lowstand interval, roughly synchronously with the mass extinction. Biomarkers, therefore, provide an opportunity to interpret changing ocean conditions and test these relations. We quantified C27- C29 steranes, which are produced by eukaryotic organisms (e.g., green algae), C27-C35 hopanes, which are specific to bacteria, and the 2-methyl-hopanes that are biomarkers for cyanobacteria. Many of our analyses are based upon samples that were previously analyzed for graptolite community structure, allowing us to directly track the biomarker signatures with the graptolite community structure. Our initial results indicate three distinct phases with variability superimposed within each phase. The first phase shows stability in the biomarker, ẟ15N (proxy to track denitrification), and 𝜀Nd (proxy to track relatively sea level), and graptolite community structure. The second phase occurs in a warm period of the Late Ordovician where globally high sea level is matched by εNd and complete denitrification. This period is accompanied by a proliferation of cyanobacteria and relative abundance of green algae with a decline in graptolite community evenness and mesopelagic species. The third phase occupies the Hirnantian glacial interval that shows lower degrees of denitrification and an overall dominance of green algae and lower cyanobacterial productivity with mesopelagic species essentially disappearing. These relationships suggest that climatically driven changes in nutrient cycling and phytoplankton communities that drove the mass extinction.