ASSESSING THE INFLUENCE OF ANOXIA ON THE LATE ORDOVICIAN MASS EXTINCTION USING BIOMARKERS IN THE VININI CREEK FORMATION, NEVADA

The Late Ordovician Mass Extinction (LOME) occurred during latest Katian to late Hirnantian time (~ 445.5-444.5 ma) and caused an estimated 85% of marine species to go extinct. Although the extinction was synchronous with a shift from greenhouse to icehouse climate, the proximate causes of the LOME remain uncertain. The Vinini Creek Formation, Nevada, formed in a deep oceanic environment and records profound changes in graptolite community diversity and composition through the onset of the LOME. We use lipid biomarkers for water column anoxia and stable nitrogen isotopes (δ¹⁵N) of organic matter to reconstruct water column redox to determine the extent of anoxia through the Late Ordovician. Isorenieratene, a biomarker sourced from anaerobic photoautotrophic green sulfur bacteria, occurs sporadically before the LOME but is consistently present following the abrupt turnover in graptolite communities that took place at the start of the Hirnantian Age, suggesting that the photic zone was not consistently euxinic until after the onset of species extinction among the Diplograptina and the positive excursion in δ¹⁵N and δ¹³C associated with the LOME. Pristane and phytane ratios <1.0 indicate anoxia, and our samples ratios are predominately <1.0 throughout the section. Preliminary results show aryl isoprenoid ratios between 0.07 and 0.77, which indicate persistent anoxia. Together, these ratios and the isorenieratene suggest that persistent anoxia existed in the water column at this site throughout the Late Ordovician, but the location of that anoxia in the water column changed through time. These results are consistent with a transition during the early Hirnantian from anoxia predominantly confined to sub-photic zone waters in a stratified basin that experienced near complete denitrification to photic zone anoxia and more incomplete denitrification as a result of strong upwelling conditions and increased deep ocean ventilation. Persistent anoxia in the photic zone and deep ocean ventilation closely coincide with the timing of the shift to dominance by the immigrant Neograptina at the Vinini Creek site and continue through the glaciation; these conditions in the water column may reflect the shift to an icehouse climate. The timing of the change in the water column suggests that environmental changes that culminated in photic zone anoxia played a critical role in the extirpation of the formerly resident diplograptine graptolite community from the Vinini Creek site.