Exploring the Geochemical Fingerprints of an Oceanic Anoxic Event During the Late Cretaceous: the Global and Biological Implications
- Author(s): Owens, Jeremiah David
- Advisor(s): Lyons, Timothy W.
- et al.
Understanding the causes and consequences of oceanic anoxic events (OAEs) has been at the forefront of paleoceanography studies for the last several decades. The Mesozoic Era is noted for numerous OAEs that are diagnostically expressed by widespread organic-carbon deposition and coeval positive carbon-isotope excursions. OAEs have been extensively studied from angles but there is still minimal understanding of the global nature of these events. Through the work presented here I aim to quantify the global extent of euxinia (water column that is anoxic and contains sulfide) and seek to understand global extent of anoxia using a multi-geochemical-proxy approach. This work includes high-resolution studies spanning multiple sections with a global distribution for each proxy (sulfur isotopes, Mo, V and Cr trace metal, Fe isotopes) from OAE2, the Cenomanian-Turonian boundary event (~93.9 Ma).
Coupled carbon and sulfur isotopes show positive isotope excursions at each locality during OAE2; although, the peak magnitudes of these shifts are offset by approximately a few hundred thousand years due to a waning burial of organic carbon and pyrite burial. Geochemical box modeling suggests 2 to 7% of the seafloor sediments were deposited under euxinic conditions. While, Mo trace metal geochemistry suggests similar results with values of ~10% euxinia and V and Cr depletions prior to euxinia imply increased global anoxia prior to the OAE. An organic carbon compilation suggest the known burial of organic carbon during OAE2 may account for the entire isotope excursion observed unless there is an major change increase in volcanic or weathering fluxes. However, Fe isotopes suggest there was not a pervasive increased signal for hydrothermal delivery of Fe except to the know euxinic basins. Quantitative consideration of these cycles is of paramount importance for constraining the budgets of carbon and sulfur, but also oxygen and other key biological elements, as we seek to improve our understanding of the mechanisms behind the initiation and termination of OAEs.