Collaborative Proposal: Molybdenum Isotopes as Paleoredox Proxies

本提案寻求资金，继续发展和应用钼稳定同位素系统，以检查海洋氧化还原随时间的变化。海洋古氧化还原的研究在古海洋学模拟和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，在富含有机碳的沉积物中综合多种地球化学氧化还原指标，可以合理确定沉积在缺氧-硫化物（缺氧）底沃茨下的沉积物。然而，除了这些沉积物的区域分布，没有办法估计在任何特定时间的富氧底层水条件有多普遍。我们对黑海沉积物、铁锰结核、海水和大陆物质中的Mo同位素进行的初步调查表明，海洋中Mo同位素的最大分馏发生在轻Mo同位素优先吸收到Mn-羟基氧化物沉积物中的过程中，但在富氧环境中去除Mo同位素的过程中，Mo同位素相对未分馏（巴灵等人，2001年）。因此，在海水中的钼是同位素轻于氧化沉积物中的钼和大陆来源的钼进入海洋，但类似于富氧条件下积累的沉积物。由于氧化和富氧沉积物都是海洋钼收支的重要汇，这些发现使我们提出了以下假设：黑色页岩的钼同位素组成应反映海水的钼同位素组成，并应随全球海洋缺氧的程度而变化。97Mo/95Mo（和其他钼同位素比值）应向同位素较轻的值偏移，在海洋中扩展的富氧条件下，我们已经探索了这一假设的关键部分，作为奖励的一部分，该奖励支持对主要储层进行为期一年的进一步调查，和实验室实验，旨在测试推断，钼同位素分馏吸收过程中锰氢氧化物。这个奖项很快就会过期。迄今为止的结果表明，钼同位素分馏在实验室中的锰氢氧化物吸收过程中，在预期的方向和预期的幅度。此外，我们已经发现，从Cariaco盆地的富氧沉积物的钼同位素组成相似的黑海沉积物和海水，与这样的沉积物提供了一个一级记录的海水钼同位素的想法是一致的。其他工作人员的最新发现（例如，Siebert等人，2001 b）也增加了对Mo同位素的古氧化还原效用的信心。为了进一步验证这一假设，我们现在建议将我们的研究扩展到检查地质记录中Mo的同位素组成。具体而言，我们打算研究钼同位素和其他古氧化还原指标，在三个良好的约束黑色页岩地层序列从中元古代（罗珀组，麦克阿瑟盆地，N。澳大利亚），古生代（中泥盆世奥特卡河组，北方阿巴拉契亚盆地）和中生代（新诺曼-土伦边界岩心，ODP站点1138）时代。选择这三个序列是因为它们可能代表海洋氧化还原条件的大量但不同的扰动时期，其时间段与Mo的海洋停留时间相当或更长。这些层序也很有吸引力，因为它们被其他地球化学技术很好地表征，并具有良好的地层和沉积背景。从麦克阿瑟盆地、泥盆纪黑色页岩和C-T边界获得的初步数据与我们的假设一致。我们还建议改进我们的实验室实验，以更好地理解自然界中钼同位素分馏的机制。罗切斯特和U.密苏里州，为共同感兴趣的古环境问题带来了互补的方法。由于公众对可能的人为气候变化及其后果的兴趣，古环境研究越来越具有社会重要性。这两个小组的互补资源和方法将丰富两名研究生的培训，每个机构一名。此外，拟议的项目还为代表性不足的群体的一名成员（共同主要研究者巴灵）的参与提供了便利。

ABSTRACTThis 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).