Mo Isotopes During OAE2

Ocean oxygenation has varied through geologic time, but the timing and extent of these variations are not well understood. Mo stable isotopes have emerged as a valuable new tool in paleoredox investigations because (1) organic-rich shales deposited under sulfidic conditions in restricted basins can quantitatively remove Mo from seawater and record contemporaneous global marine Mo isotope ratios and (2) deposition of Mo in FeMn oxides induces a 2‰ equilibrium isotopic fractionation compared to seawater. Therefore, the marine Mo isotopic value varies with the relative magnitude of oxidized and reduced depositional sinks in the world ocean. A key question in using this proxy is the influence of local hydrographic conditions on Mo isotopes in sediments. We are exploring this issue in Late Cretaceous sediments from Demerara Rise. The Late Cretaceous experienced several periods of extensive marine anoxia. Laminated sediments spanning these events and the intervening intervals are recorded in ODP Hole 1258A (Demerara Rise, tropical western North Atlantic). Mo/Al and Fe/Al ratios, organic C content and DOP values for Demerara Rise all signify a persistently euxinic environment, with the OAEs marked by an increase in organic C content, Fe/Al ratios and δ13C values. However, because Demerara sediments were in continuous contact with the open ocean this setting provides an excellent test of the Mo isotope proxy in a setting where circulation was not restricted. We observe systematic variations in Mo concentrations and isotopes that correlate with positive carbon isotope excursions at the mid-Cenomanian OAE event and OAE2. During the OAEs, Mo isotope values are ~ 0.8‰, compared to 1.6‰ in modern seawater. Although Demerara Rise is not a restricted basin we hypothesize that Mo was quantitatively removed from seawater to sediments at this site due to drawdown of the global Mo ocean reservoir. This interpretation is supported by changes in Mo concentrations: from a background Mo concentration of 50 to 170 ppm, the onset of the OAE2 event is marked by an increase to more than 300 ppm, followed by a decrease to 20 ppm. This interpretation is further supported by data from the Furlo section of the Bonarelli in central Italy, which provides a comparison with sediments from a silled basin. Lithology, Mn/Al and Mo/Al ratios suggest that the Furlo depositional environment was anoxic or periodically euxinic during OAE2. The maximum Mo isotope value during OAE2 is 0.5‰. Hence, OAE2 at both Demerara and Furlo is characterized by Mo isotope values 0.8 - 1.1‰ lighter than modern seawater. This broad consistency supports the interpretation that these sites record a global Mo isotope signature consistent with widespread anoxia. In contrast, the Mo isotope compositions in sediments outside OAE2 are very different at the two locations. At Demerara, these values are ~ 0.4‰ while at Furlo they are as low as -1.0‰. This stark contrast suggests that isotope effects associated with locally non- quantitative Mo removal dominate the signal outside OAE2 in both sections.