Resolving controls on the marine nickel budget

镍(Ni)是碳、氮和氧循环中几种酶的重要痕量金属。它在海水中的浓度已被证明在数百万年内大致保持不变，但科学家们尚未能够令人满意地“平衡”现代海洋中镍的“收支平衡”。也就是说，已知的海水镍来源并没有被已知的去除过程所平衡。该项目将研究镍被转移到两种主要类型的海洋沉积物中的机制，以回答有关现代镍预算的关键问题。除了这一具体贡献，这项工作还将帮助那些希望重建海洋中骑自行车的古代历史的人。镍预算的戏剧性变化在地球和生命的共同进化中发挥了关键作用，例如24亿年前的大氧化事件之前，以及就在二叠纪-三叠纪大灭绝之前。此外，最近的研究表明，今天从河流到河口和海洋的大部分镍输入来自人类活动。拟议工作的结果可能有助于预测这种新的镍进入海洋后的命运。两名博士生和两名本科生将得到该项目的支持，既直接参与研究，也通过参与旨在增加地球科学多样性、公平性和包容性(DEI)的全系活动。学生们将邀请一些美国顶尖的地球科学家和活动家参观北亚利桑那大学校园，谈论他们的研究和他们对Dei的创新贡献，并作为大学社区成员的榜样。目前，已发表的镍循环模型要么极端偏离稳定状态，这是不可信的，要么必然建立在对某些类型海洋沉积物中镍行为的推测假设上。最近的一些研究旨在使用稳定的镍同位素作为平衡收支的额外限制：正如源和汇的通量应该平衡一样，流入海洋的源和汇的通量加权同位素组成之和也应该等于汇的通量加权同位素组成之和。到目前为止，对两个关键沉积产出的了解不足，排除了令人满意的解决方案：(1)富锰、自生粘土丰富的沉积物和(2)碳酸盐。为了解决最近文献中关于这两种产出的明显矛盾，并使镍的预算模型更准确，该小组将进行系统的实验，以量化镍在氢氧化锰(水钠锰矿)和蒙皂石上吸附过程中的镍同位素分馏。为了确定还原成岩作用带以下的远洋沉积物的镍同位素组成，并了解氢氧化锰/蒙脱石比值是否控制远洋沉积物中的镍同位素组成，他们将分析赤道太平洋沉积物岩心(RV Sonne Expedition SO240)的选定样品。研究小组还将收集特征良好的年轻碳酸盐沉积物的第一批镍同位素数据，以评估海水初级降水期间和早期成岩过程中的分馏作用。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Nickel (Ni) is a crucial trace metal in several enzymes involved in the carbon, nitrogen, and oxygen cycles. Its concentration in seawater has been demonstrated to have remained roughly constant for many millions of years, and yet scientists have not yet been able to satisfactorily “balance the budget” of nickel in the modern oceans. That is, the known sources of nickel to seawater are not balanced by the known removal processes. This project will examine the mechanisms by which nickel is removed to two major types of marine sediments, in order to answer key questions about the modern nickel budget. Beyond this specific contribution, the work will aid those who wish to reconstruct the ancient history of Ni cycling in the oceans. Dramatic shifts in the Ni budget have played crucial roles in the co-evolution of Earth and life, such as before the Great Oxidation Event 2.4 billion years ago and just before the Permian-Triassic mass extinction. In addition, recent studies have implied that most of the Ni inputs from rivers to estuaries and oceans today is from human activities. Results from the proposed work may help predict the fate of this new Ni once it enters the oceans. Two Ph.D. students and two undergraduates will be supported by this project, both directly in the research and through involvement in department-wide activities aimed at increasing diversity, equity, and inclusion (DEI) in geosciences. The students will invite some of the nation’s top geoscientist-activists to visit the Northern Arizona University campus, to speak about their research and their innovative contributions to DEI, and to appear as role models for members of the university community. At present, published models of the Ni cycle either deviate from steady state in the extreme, which is implausible, or rest necessarily on speculative assumptions about Ni behavior in certain types of marine sediment. A few recent studies aimed to use Ni stable isotopes as an additional constraint for a balanced budget: just as source and sink fluxes should balance, the sum of flux-weighted isotopic compositions of sources to the ocean should equal the sum of flux-weighted isotopic compositions of sinks. Thus far, inadequate knowledge regarding two critical sedimentary outputs precludes a satisfactory solution: (1) Mn-rich, authigenic clay-rich sediments and (2) carbonates. In order to resolve apparent contradictions in the recent literature about these two outputs and to enable more accurate models of the Ni budget, the team will conduct systematic experiments to quantify Ni isotope fractionation during sorption of Ni on Mn oxyhydroxide (birnessite) and on smectite. To determine the Ni isotopic composition of pelagic sediment below the zone of Mn-reducing diagenesis and to learn whether Mn oxyhydroxide/smectite ratios control Ni isotope compositions in pelagic sediments, they will analyze selected samples from equatorial Pacific sediment cores (RV Sonne Expedition SO240). The research team will also collect the first Ni isotope data for well-characterized, young carbonate sediments, to assess fractionation during primary precipitation from seawater and during early diagenesis.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.