Molybdenum drawdown during Cretaceous Oceanic Anoxic Event 2
Molybdenum drawdown during Cretaceous Oceanic Anoxic Event 2
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DOI：
10.1016/j.epsl.2016.02.006
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发表时间：
2016-04
影响因子：
5.3
通讯作者：
T. Goldberg;S. Poulton;T. Wagner;S. Kolonic;M. Rehkämper
中科院分区：
文献类型：
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作者：
T. Goldberg;S. Poulton;T. Wagner;S. Kolonic;M. Rehkämper
在白垩纪温室期间，广泛的海洋脱氧事件与全球范围内发生的黑色页岩沉积事件有关。这些海洋缺氧事件（OAE）中最明显的可能是塞诺曼阶-土伦阶 OAE2（∼94 Ma）。然而，尽管某些氧化还原敏感的痕量金属往往优先被隔离在缺氧条件下沉积的沉积物中，而钼的下降特别容易发生在常生环境中，但这些元素在OAE2期间沉积的沉积物中通常会有所消耗。为了了解导致微量金属减少的驱动因素，我们研究了原北大西洋（Tarfaya S57）的一个低纬度部分，其中现有的生物标记和铁硫数据表明主要为弱生水体，并周期性过渡到含铁（富铁）水体条件。我们利用各种氧化还原代理（铁形态、氧化还原敏感的痕量金属和钼同位素），它们相结合，使我们能够评估海洋氧化还原条件的详细性质，从而控制痕量金属的下降。结果表明，OAE2期间海水δ98Mo值可能在~0.6到1.1‰之间，这可能与当地钼储层的变化有关，这是由于海洋中钼浓度低且可能不均匀造成的。塔尔法亚和原北大西洋其他地方的 Mo/TOC 比率非常低，可能支持深水环流部分限制在北大西洋和其他海洋盆地内部和之间的模型。我们认为，海水Mo的低且可能异质的δ98Mo与低Mo/TOC比相结合，表明OAE2期间全球海洋Mo储库大幅减少，反映了持续性微环境条件下全球范围内Mo下降的大幅增加。
During the Cretaceous greenhouse, episodes of widespread ocean deoxygenation were associated with globally occurring events of black shale deposition. Possibly the most pronounced of these oceanic anoxic events (OAE's) was the Cenomanian–Turonian OAE2 (∼94 Ma). However, although certain redox sensitive trace metals tend to be preferentially sequestered in sediments deposited under anoxic conditions, with Mo drawdown being specifically prone to euxinic settings, these elements are generally somewhat depleted in sediments deposited during OAE2. To understand the driving factors responsible for this depleted trace metal drawdown, we have studied a low latitude section from the proto-North Atlantic Ocean (Tarfaya S57), where existing biomarker and iron–sulphur data point to a dominantly euxinic water column, with periodic transitions to ferruginous (Fe-rich) water column conditions. We utilise a variety of redox proxies (Fe-speciation, redox sensitive trace metals and Mo isotopes), which, in combination, allows us to evaluate the detailed nature of ocean redox conditions and hence controls on trace metal drawdown. The results suggest that seawaterδ98Mo values may have ranged between ∼0.6 and 1.1‰ during OAE2, likely connected to changes in the local Mo reservoir as a consequence of low and probably heterogeneous concentrations of Mo in the ocean. The very low Mo/TOC ratios at Tarfaya and elsewhere in the proto-North Atlantic may support a model in which deep-water circulation was partially restricted within and between the North Atlantic and other ocean basins. We propose that the combination of a low and possibly heterogeneousδ98Mo of seawater Mo, together with low Mo/TOC ratios, points to a large decrease in the global oceanic Mo reservoir during OAE2, reflecting a major global scale increase in Mo drawdown under persistent euxinic conditions.