NSFGEO-NERC: Constraining the oxic marine sink of novel metal isotope proxies to underpin paleoceanographic reconstructions

生命的进化与海洋化学密切相关。随着时间的推移，生物已经适应并推动了海洋化学环境的变化。生命史上最深刻的例子之一是含氧光合作用的增殖。较高的氧气水平对许多现有生物有毒，并改变了微量营养素金属在海洋中的循环方式，从而对生命的进化途径产生直接和间接的影响。重建这些海洋生物地球化学变化的时间和顺序对于理解地球历史上生命的进化及其对全球气候的依赖和影响至关重要。由于海洋和大气氧合的变化控制了海洋生物地球化学的演变，因此建立能够在大范围氧化还原（例如，从高度还原到高度氧化）范围内监测这些变化的代理是必要的。为了了解过去氧化还原模式及其对养分循环和生命进化的相关影响，一个新兴的领域正在研究对这些变化敏感的新型金属同位素系统，并可作为这些变化的替代品。特别有希望的是Mo， Tl， U和Zn同位素代用物，它们各自对氧化还原条件的变化做出独特的反应。来自古代沉积物和岩石档案的记录揭示了这些同位素系统的重大和系统变化，这些同位素系统已被用于估计全球海洋缺氧条件程度的变化。对代理记录的准确解释要求详细了解海水收支中每一次输入和输出的同位素组成。尽管人们通常认为缺氧汇的变化是改变Mo、Tl、U和Zn的海洋同位素收支的主要手段，但这些元素的很大一部分被移到了广阔的深氧海底。海洋中Mo、Tl、U、Zn的深海氧汇主要由铁锰氧化物（Fe-Mn）组成，以包被形式存在于深海中上层红粘土中，还有少量纯铁锰结壳和结核。这些元素的大量同位素分馏已被记录在地壳和结核中，大多数古海水研究假设这些分馏因子与远洋沉积物中的氧化物相同。然而，深海沉积物的分馏因子从未被明确和系统地分析过。本研究的初步数据表明，上层沉积物与铁锰结壳和结核之间的分馏因子是不同的，这表明可能需要对古代沉积物档案中的同位素代用数据进行相应的重新解释。在此，我们建议通过对深海含氧海洋沉积物中Mo、Tl、U和Zn同位素的首次全球调查来约束关键古海洋学代用物的氧汇。同位素数据将与已公布的海水同位素组成进行比较，以确定每种元素在掺入不同深海含氧沉积物过程中的稳健同位素分馏因子。我们将把这些结果与基于Th同位素衰变对这些元素进入氧汇的全球埋藏通量的估计结合起来。最终，该项目提供的约束条件将成为未来新型金属同位素研究的基准，并显著改善对海洋氧合和生物地球化学过去变化的定量估计

The evolution of life is intimately linked to ocean chemistry. Throughout time, organisms have adapted to - and driven changes in - their marine chemical environments. One of the most profound examples from life's history was the proliferation of oxygenic photosynthesis. Higher oxygen levels were toxic to many existing organisms and changed how micronutrient metals cycle in the ocean, thus having direct and indirect effects on the evolutionary pathway of life. Reconstructing the timing and sequence of these changes in ocean biogeochemistry is vital to understanding the evolution of life and its dependence and effects on global climate over Earth history. Because changes in ocean and atmosphere oxygenation control much of the ocean's biogeochemical evolution, it is imperative to establish proxies that can monitor these changes across wide ranges of redox (e.g. from highly reducing to very oxygenated). To understand redox patterns and associated effects on nutrient cycling and life evolution in the past, a burgeoning field is investigating novel metal isotope systems that are sensitive to - and can be used as proxies for - these changes. Of particular promise are the Mo, Tl, U and Zn isotope proxies, which each respond uniquely to changes in redox conditions. Records from ancient sediment and rock archives reveal significant and systematic variations in these isotopic systems that have been used to estimate changes in the extent of anoxic conditions in the global oceans. Accurate interpretations of the proxy record mandate a detailed understanding of the isotopic compositions of every input and output in the seawater budget. Although it is often assumed that changes in the anoxic sink are the primary means to change the marine isotopic budgets of Mo, Tl, U and Zn, a significant portion of these elements are removed into the vast expanses of deep oxic seafloor. The deep-sea oxic sink of Mo, Tl, U and Zn in the ocean is mainly comprised of ferro-manganese (Fe-Mn) oxides in the form of coatings on abyssal pelagic red clays as well as a minor amount of pure Fe-Mn crusts and nodules. Large isotopic fractionations of these elements have been documented for crusts and nodules and most paleo-seawater studies assume that these fractionation factors are the same for the oxides in pelagic sediment. However, fractionation factors for abyssal pelagic sediment have never explicitly and systematically been analyzed. Preliminary data generated for this proposal suggests that fractionation factors are distinct between pelagic sediment and Fe-Mn crusts and nodules, suggesting isotope proxy data from ancient sediment archives may need to be re-interpreted accordingly. Here, we propose to constrain the oxic sink of key paleoceanographic proxies by performing the first global-scale investigation of Mo, Tl, U and Zn isotopes in deep-sea oxic marine sediment. The isotope data will be compared with published seawater isotope compositions to determine robust isotope fractionation factors for each element during incorporation into diverse deep-sea oxic sediments. We will combine the results with estimates of global burial fluxes for these elements into the oxic sink, based on the decay of Th isotopes. Ultimately, the constraints provided by this project will become a benchmark for future research of novel metal isotopes and significantly improve quantitative estimates of past variations in ocean oxygenation and biogeochemistry