Collaborative Proposal: Molybdenum Isotopes as Paleoredox Proxies

本提案寻求资金，以继续发展和应用钼稳定同位素系统，以研究海洋氧化还原随时间的变化。海洋古氧化还原研究在古海洋学模拟和生物地球化学循环随时间变化的耦合模拟中具有重要意义。目前，通过对富有机碳沉积物中多种地球化学氧化还原指标的综合分析，可以较为确定地识别出缺氧-硫化物（含氧）底水沉积的沉积物。然而，除了这些沉积物的面积分布之外，没有任何方法可以估计在任何特定时间内缺氧的底水条件有多普遍。我们对黑海、锰铁结核、海水和陆相沉积物中Mo同位素的初步调查表明，海洋中Mo同位素的最大分馏发生在轻Mo同位素优先被mn -羟基氧化沉积物吸收的过程中，但Mo同位素在缺氧环境中去除时相对未分馏（Barling et al., 2001）。因此，海水中的Mo同位素轻于氧沉积和陆源进入海洋的Mo，但与缺氧条件下沉积的Mo相似。由于含氧和含氧沉积物都是海洋Mo收支的重要汇，这些发现使我们提出以下假设：黑色页岩的Mo同位素组成应反映海水的Mo同位素组成，并应随全球海洋缺氧程度的变化而变化。具体来说，97Mo/95Mo（以及其他Mo同位素比率）应该在海洋扩展的缺氧条件下向同位素较轻的值转变。作为EAR 0106712奖的一部分，我们已经探索了这一假设的关键部分，该奖项支持对主要储层进行为期一年的进一步调查，并设计了实验室实验，以验证Mo同位素在mn -氧氢氧化物吸收过程中分馏的推断。这项奖励即将到期。迄今为止的结果表明，在实验室中，Mo同位素在mn -氢氧化物的吸收过程中，按照预期的方向和预期的量级进行了分馏。此外，我们还发现卡里亚科盆地含氧沉积物的Mo同位素组成与黑海沉积物和海水的Mo同位素组成相似，这与卡里亚科盆地含氧沉积物提供了海水Mo同位素一级记录的观点一致。其他工作人员最近的发现（例如，Siebert等人，2001b）也增加了对Mo同位素古还原作用的信心。为了进一步验证这一假设，我们现在建议扩展我们的研究，以检查地质记录中Mo的同位素组成。具体而言，我们将研究中元古代（澳大利亚北部McArthur盆地Roper群）、古生代（阿巴拉契亚盆地北部中泥盆世Oatka Creek组）和中生代（Cenomanian-Turonian Boundary core, ODP Site 1138）三个约束良好的黑色页岩地层序列中的Mo同位素和其他古氧化还原指标。选择这三个层序是因为它们可能代表了与Mo海洋停留时间相当或更长时间的海洋氧化还原条件的大量但不同的扰动时期。这些层序还具有吸引力，因为它们可以通过其他地球化学技术很好地表征，并且具有良好的地层和沉积背景。从McArthur盆地、泥盆系黑色页岩和C-T边界获得的初步数据与我们的假设一致。我们还建议完善我们的实验室实验，以更好地了解自然界中Mo同位素分馏的机制。拟议中的研究将加强罗切斯特大学和密苏里大学的研究小组之间的联系，这将为共同感兴趣的古环境问题带来互补的方法。由于公众对可能的人为气候变化及其后果的兴趣，古环境研究在社会上的重要性日益增加。这两个小组的互补资源和方法将丰富两个研究生的培训，每个机构一个。此外，拟议的项目促进代表人数不足的群体的一名成员的参与。

This proposal seeks funding for continued development and application of the molybdenum stable isotope system for the examination of changes in ocean redox through time. Research into ocean paleoredox is of great importance in paleoceanographic modeling, and in the coupled modeling of changes in the biogeochemical cycles through time. At present, the integration of multiple geochemical redox indicators in organic carbon-rich sediments identifies, with reasonable certainty, sediments deposited under anoxic-sulfidic (euxinic) bottom waters. However, beyond the areal distribution of such sediments, there is no means of estimating how prevalent euxinic bottom water conditions were at any particular time. Our initial survey of Mo isotopes in sediments from the Black Sea, ferromanganese nodules, seawater and continental materials indicated that the largest fractionation of Mo isotopes in the oceans occurs during preferential uptake of light Mo isotopes to Mn-oxyhydroxide sediments, but that Mo isotopes are relatively unfractionated during removal in euxinic environments (Barling et al., 2001). Therefore, Mo in seawater is isotopically lighter than Mo in oxic sediments and continent-derived Mo entering the oceans, but is similar to that of sediments accumulating under euxinic conditions. Because both oxic and euxinic sediments are important sinks in the ocean Mo budget, these findings led us to develop the following hypothesis: The Mo isotope composition of black shales should reflect that of seawater, and should vary with the extent of global ocean anoxia. Specifically 97Mo/95Mo (and other Mo isotope ratios) should shif toward isotopically lighter values during extended periods of expanded euxinic conditions in the oceans.We have explored key parts of this hypothesis as part of award EAR 0106712 which supported one year of further investigation of major reservoirs, and laboratory experiments designed to test the inference that Mo isotopes are fractionated during uptake by Mn-oxyhydroxides. This award will soon expire. Results to date demonstrate that Mo isotopes are fractionated during uptake by Mn-oxyhydroxides in the laboratory, in the expected direction and to the expected magnitude. In addition, we have found that the Mo isotopic compositions of euxinic sediments from the Cariaco Basin are similar to those of Black Sea sediments and seawater, consistent with the idea that such sediments provide a first order record of seawater Mo isotopes. Recent findings of other workers (e.g., Siebert et al., 2001b) also increase confidence in the paleoredox utility of Mo isotopesTo further test this hypothesis, we now propose to extend our research to examine the isotopic composition of Mo in the geologic record. Specifically, we intend to study Mo isotopes and other paleoredox indicators in three well-constrained black shale stratigraphic sequences from the Mesoproterozoic (Roper Group, McArthur Basin, N. Australia), the Paleozoic (Middle Devonian Oatka Creek Formation, northern Appalachian Basin) and the Mesozoic (Cenomanian-Turonian Boundary core, ODP Site 1138) Eras. These three sequences are selected because they may represent periods of substantial, but differing, perturbation of ocean redox conditions for periods of time comparable to, or longer than, the ocean residence time of Mo. These sequences are also attractive because they are well characterized by other geochemical techniques and have good stratigraphic and depositional context. Preliminary data obtained from the McArthur Basin, a Devonian black shale and the C-T Boundary are consistent with our hypothesis. We also propose to refine our laboratory experiments to better understand the mechanisms of Mo isotope fractionation in nature.The proposed research will strengthen ties between groups at U. Rochester and U. Missouri, which bring complementary approaches to paleoenvironmental questions of mutual interest. Paleoenvironmental research is of increasing societal importance as a result of public interest in probable anthropogenic climate change and its consequences. The complementary resources and approaches of these two groups will enrich the training of two graduate students, one at each institution. In addition, the proposed project facilitates the participation of a member of an underrepresented group (Co-PI Barling).