RECONCILIATION OF THE PALEONTOLOGICAL EVIDENCE AND THE GEOCHEMICAL PROXIES: THE LOCAL WEAK OXYGENATION IN THE BOTTOM WATER DURING THE OAE2 AS TRACKED BY THE SEDIMENTARY V ISOTOPE COMPOSITIONS

Refining and more precisely constraining ancient oceanic redox perturbations are critical to our understanding of the connection between the oceanic redox conditions, the nutrient cycling, and the evolution of marine life. The current geochemical toolbox is relatively well developed to identify more reducing conditions such as anoxic (no oxygen), sulfidic (anoxic and free sulfide), and euxinic (sulfidic water column) conditions where the biological tracers are absent whilst the oxygen is substituted for by other oxidized elements such as nitrogen, iron, and sulfur compounds during consumption of organic matter. In contrast, the intermediate dysoxic-suboxic/anoxic condition (threshold: ~ 0.2mL/L [O₂]) is underconstrained particularly when the paleontological evidence is poorly preserved or biased by other factors such as the nutrient flux and sea-level changes. This is important as it has been difficult to constrain the influence of low oxygen stress during important biological turnovers or early extinctions. As a novel redox proxy, the early sedimentary V isotope (⁵¹V and ⁵⁰V) data suggest the ability to distinguish the currently ambiguous low oxygen redox conditions, especially discriminate the dysoxic condition from the suboxic/anoxic condition in the bottom water along the modern oxygen minimum zone, which makes the sedimentary V isotopic signature a promising proxy to constrain the dysoxic-suboxic/anoxic fluctuations in ancient oceans.

As one of the best-recognized Oceanic Anoxic Events (OAEs) in the Phanerozoic Era, OAE2 which occurred at the Cenomanian-Turonian boundary (~94 million years ago) has been relatively well constrained by geochemical and/or paleontological proxies at a range of different localities. We have determined the sedimentary V isotopic compositions during OAE2 at three sites in the proto-North Atlantic basin, and the results indicate spatiotemporal local redox heterogeneity for each site with bottom water redox conditions fluctuating at the three sites. Furthermore, the sedimentary V isotope fluctuations at ODP site 1258 correspond to similar changes in the abundance of low oxygen benthic foraminifera which provides the first geochemical evidence of fluctuations in low oxygen bottom water conditions.