GEOCHEMICAL EVIDENCE FOR DYNAMIC MARINE REDOX CONDITIONS THROUGHOUT THE LATE ORDOVICIAN (HIRNANTIAN) MASS EXTINCTION

Many documented marine extinction events in Earth history seem to coincide with evidence for widespread reducing conditions, ranging from anoxia to euxinia (anoxic + sulfidic conditions) as a plausible kill mechanism. However, initial studies did not attribute widespread reducing conditions as the primary kill mechanism for the Late Ordovician Mass Extinctions (LOME), the first of the “Big 5” mass extinctions in the Phanerozoic. Several other paleoenvironmental factors have been proposed as causal factors including: sudden drop in global sea-surface temperatures and sea level, and an increase in global volcanism. An abrupt drop in sea level and increase in carbonate weathering patterns was previously proposed to cause the large magnitude positive δ¹³C excursion that is coincident with the LOME. However, paleoredox proxies have recently shown that an expansion in reducing marine conditions may have played a larger role in the LOME.

In this study we combine global and local redox proxies to investigate the role of marine anoxia and/or euxinia may be linked to the LOME. Here we analyzed two carbonate shelves on the eastern and western margins of Laurentia, and a third section on the southern margin of Baltica to test the hypothesis that portions of the Late Ordovician oceans became reducing during the Hirnantian. Here we have utilized δ³⁴S_CAS (carbonate-associated sulfate) to track changes in global burial rates of pyrite (e.g., euxinic conditions), and I/(Ca+Mg) ratios to document local changes in marine oxygen contents. We identified no changes in the fraction of the seafloor that were overlain by euxinia as δ³⁴S_CAS records were invariant throughout. However, we have identified widespread locally suboxic to anoxic conditions in all three settings utilizing I/(Ca+Mg) ratios. With the results from this study we hope to elucidate the currently enigmatic changes in low oxygen conditions and its role in the second largest mass extinction in Earth’s History.