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

为了评估潜在的钼同位素组成的全球与区域缺氧的识别，我们建议调查的分馏钼同位素沉积物从区域到全球分布的白垩纪中期的黑色页岩。基本的想法是，在全球或接近全球缺氧的时期，黑色页岩可以被预期为Mo的主要汇-就像在元古代的大部分时间里一样。相比之下，在现代海洋中占主导地位的有氧海洋条件下，铁锰氧化物似乎是钼的主要汇。吸附锰氧化物，因为它发生在好氧条件下，导致钼同位素的显着分馏，导致<$同位素重<$现代海水。然而，在缺氧条件下（如黑海）的黑色页岩沉积物中去除钼似乎不会导致钼同位素分馏。因此，海洋钼质量平衡的差异应导致钼同位素系统学的显着转变，这取决于它们是否导致缺氧或好氧条件的流行。事实上，从中元古代和现代海洋沉积物中获得的钼数据支持这一观点。在这里，我们建议测试这一假设，通过分析中白垩纪黑色页岩，在不同的空间和时间的边界条件下形成的钼同位素组成。作为一个特殊的，大概是全球性的和长期的海洋性事件（OAE）的代表性样本，从OAE 2（持续时间：<$400 ka）的黑色页岩将进行分析。为了深入了解Mo同位素分馏在超区域，短暂的（~45 ka）黑色页岩事件，沉积物从OAE 1b（仅限于特提斯西部和大西洋中部）将进行调查。为了评估区域缺氧条件下的Mo同位素信号，将对Demerara Rise（ODP Leg 207）OAE 2下和上覆的黑色页岩沉积物进行研究。拟议研究的成果将是：（1）评价一种新的、广泛适用的地球化学代用指标，以确定全球与区域缺氧的信号;（2）验证基于钼同位素的元古代海洋氧化还原演化模型的可能性;（3）利用Mo和其它氧化还原物质对白垩纪中期黑色页岩形成的地球化学环境进行了表征。敏感金属以及硫同位素。

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.