GSA Connects 2021 in Portland, Oregon

Paper No. 24-13
Presentation Time: 9:00 AM-1:00 PM


KULENGUSKI, Joseph1, GILLEAUDEAU, Geoffrey J.1, PITTS, Alan2, PIERANTONI, Pietro Paolo3, KAUFMAN, Alan4 and ELRICK, Maya5, (1)Department of Atmospheric, Oceanic, and Earth Sciences, George Mason University, 4400 University Drive, Fairfax, VA 22030, (2)University of Camerino Geology Division, Via Gentile da Varano III, Camerino, 20169-3321, ITALY, (3)University of CamerinoGeology Division, Via Gentile da Varano III, Camerino, 20169-3321, ITALY, (4)Department of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742, (5)Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131

Ocean Anoxic Events (OAEs) are discrete periods in Earth history of decreased marine oxygen concentrations, often associated with volcanism, increased organic carbon burial in the oceans, and positive δ13C excursions in marine carbonates. One such event, OAE 2, occurred ~94 Mya and is associated with widespread deposition of black shale and an accompanying positive δ13C excursion recorded globally. Whereas the presence of black shale is indicative of locally anoxic environments, it is difficult to determine the global extent of anoxia as many seafloor environments are not preserved in the geologic record. Here, we investigate uranium concentrations and isotope compositions of marine carbonates from two OAE 2 localities—the Umbria-Marche Sequence of Italy and the Guerrero-Morelos Platform of Mexico. Uranium concentrations are exceptionally low in pelagic carbonates of Italy—initially precluding isotopic measurement—but an order of magnitude higher in platform carbonates of Mexico. δ238U values in Mexico reveal an initial negative excursion that could indicate a brief pulse of anoxia pre-OAE 2, which is immediately followed by re-oxygenation. At the start of OAE 2, we see a large, progressive negative U isotope excursion that peaks at −0.6‰, before a final return to modern carbonate δ238U values. This indicates a large expansion of global marine euxinia during OAE 2. In comparison to a previously published coeval pelagic OAE 2 section from Eastbourne (UK) that showed two negative δ238U excursions, the δ238U record of Mexico reveals a single pulse of marine euxinia across OAE 2 with less overall δ238U scatter. Importantly, the Mexico negative δ238U excursion is accompanied by a positive δ13C excursion with a discrete temporal offset (δ238U change precedes δ13C change by ~200 kyr based on section-specific estimates of sediment accumulation rate). This indicates that organic carbon burial associated with the positive δ13C excursion is a direct result of the expansion of marine euxinia represented by the negative δ238U excursion. This, in turn, strengthens the link between the carbon cycle and ocean redox across OAEs.