Molybdenum isotope signatures of mid-Cretaceous black shales: Implications on the spatial dimension of anoxic events?

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To evaluate the potential of Mo isotope compositions for the identification of global versus regional anoxia we propose to investigate the fractionation of Mo isotopes in sediments from regionally to globally distributed mid-Cretaceous black shales. The underlying idea is that during times of global or near-global anoxia, black shales can be expected to have served as the primary sink for Mo - as was the case during much of the Proterozoic. In contrast, under the predominantly oxic oceanic conditions prevailing in modern oceans, Fe-Mn-oxides appear to be the major sink for Mo. Adsorption to Mn-oxides, as it occurs under oxic conditions, leads to a significant fractionation of Mo isotopes, resulting in ¿isotopically heavy¿ modern seawater. However, the removal of Mo into black-shale sediments under anoxic conditions (e.g. Black Sea) does not seem to result in Mo isotopic fractionation. Hence, differences in the oceanic Mo mass balance should lead to a significant shift in Mo isotope systematics, depending on whether they result from the prevalence of either anoxic or oxic conditions. In fact, available Mo data from mid-Proterozoic and recent marine sediments support this view. Here we propose to test this hypothesis by analyzing the Mo isotope composition of mid-Cretaceous black shales that formed under different spatial and temporal boundary conditions. As a representative sample for an exceptional, presumably global and long-lived Oceanic Anoxic Event (OAE), black shales from OAE 2 (duration: ¿400 ka) will be analysed. To gain insights into the Mo isotope fractionation during a supraregional, short-lived (~45 ka) black shale event, sediments from OAE 1b (restricted to the western Tethys and Central Atlantic) will be investigated. To evaluate the Mo isotope signal under regional anoxic conditions prevailing over long (>10 ma) time scales, black shale sediments under- and overlying OAE 2 at Demerara Rise (ODP Leg 207) will be studied. The outcome of the proposed study will be (1) the evaluation of a new, widely applicable geochemical proxy to identify signals of global versus regional anoxia; (2) the possibility to validate Mo isotope-based models on ocean redox evolution during the Proterozoic; and (3) a characterization of the geochemical environment during mid-Cretaceous black-shale formation using Mo and other redox-sensitive metals as well as sulfur isotopes.