A FIRST ASSESSMENT OF THE PALEOREDOX UTILITY OF VANADIUM ISOTOPES USING BLACK SHALES FROM AN OCEANIC ANOXIC EVENT

The Cretaceous greenhouse experienced several climatic perturbations with the most recognized event at the Cenomanian-Turonian boundary (~93 million years ago), termed oceanic anoxic event II or OAE2. Most notably this event is marked by several ocean basins that deposit organic carbon as noted by black shales and by a 5-7‰ positive carbon isotope excursion. Recent work documents that a large igneous province (LIP) precedes the carbon isotope excursion as a potential mechanism for triggering a cascade of environmental perturbations that include increases in ocean and atmospheric temperatures, productivity, weathering rates, and marine reducing conditions. Recent estimates suggest an expansion of, but not whole ocean, euxinia (anoxic and sulfidic water column) at the onset. However, based on thallium isotopes and redox sensitive element (e.g. vanadium [V]), there is an expansion of reducing conditions that coincides with the LIP. This event and locality are an ideal target to test a new proxy, vanadium isotopes.

Vanadium has two stable isotopes, 51 (99.75%) and 50 (0.25%), and presents as three valence states in modern aqueous systems. In oxic environments, pentavalent vanadate is the dominant species. Tetravalent vanadyl and trivalent vanadium present in reducing environments which are sequestrated in sediments. Associated with the speciation is an isotope fractionation with more oxic environments documented to have the most negative values with progressively oxygenated environments, displaying that heavier vanadium tends to be captured in the authigenic fraction of reducing sediments. Here we present vanadium isotope compositions across the event with stable local redox conditions but variable V concentrations to constrain the controls on the V isotope signature. This proxy has the potential to refine the redox evolution for this event (e.g. non-sulfidic low oxygen) and other similar geologically important events.