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

为了评价钼同位素组成在识别全球和区域性缺氧方面的潜力，我们建议研究从区域分布到全球分布的中白垩世黑色页岩沉积物中钼同位素的分馏。基本的想法是，在全球或近全球缺氧时期，黑色页岩可以被认为是钼的主要汇--就像在元古界的大部分时间里一样。相比之下，在现代海洋中以含氧为主的海洋条件下，铁-锰-氧化物似乎是钼的主要汇。在氧气条件下发生的对锰氧化物的吸附，导致钼同位素的显著分馏，导致现代海水的重同位素。然而，在缺氧条件下(如黑海)黑色页岩沉积物中钼的去除似乎不会导致钼同位素分馏。因此，海洋中钼质量平衡的差异应该导致钼同位素体系的显著变化，这取决于它们是由缺氧还是缺氧条件造成的。事实上，来自中元古代和现代海洋沉积物的现有钼数据支持这一观点。在这里，我们建议通过分析不同时空边界条件下形成的中白垩世黑色页岩的钼同位素组成来检验这一假说。作为一次特殊的、可能是全球性的、持续时间较长的海洋缺氧事件(OAE)的代表性样本，将分析来自OAE 2(持续时间：？400ka)的黑色页岩。为了深入了解超区域、短暂(约45ka)黑色页岩事件期间的钼同位素分馏，将对OAE 1b(仅限于西特提斯和中大西洋)的沉积物进行调查。为了评估在长时间尺度(&gt；10 ma)的区域缺氧条件下普遍存在的钼同位素信号，将研究德梅拉拉隆起(ODP207段)OAE 2下和上覆的黑色页岩沉积。拟议研究的结果将是：(1)评价一种新的、广泛适用的地球化学指标，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界海洋氧化还原演化模型的可能性；(3)利用钼和其他氧化还原敏感金属以及硫同位素表征中白垩世黑色页岩形成期间的地球化学环境。

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.