Resolving controls on the marine nickel budget

镍（Ni）是参与碳、氮和氧循环的几种酶中的重要微量金属。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”现代海洋中的镍。也就是说，已知的镍到海水的来源没有被已知的去除方法平衡。该项目将研究镍迁移到两种主要类型的海洋沉积物中的机制，以回答有关现代镍收支的关键问题。除了这一具体贡献之外，这项工作还将帮助那些希望重建Ni在海洋中循环的古代历史的人。镍收支的急剧变化在地球和生命的共同进化中发挥了至关重要的作用，例如在24亿年前的大氧化事件之前和二叠纪-三叠纪大灭绝之前。此外，最近的研究表明，大部分的Ni输入从河流到河口和海洋今天是来自人类活动。这项工作的结果可能有助于预测这种新Ni一旦进入海洋的命运。两个博士学生和两名本科生将支持这个项目，无论是直接在研究和通过参与部门范围内的活动，旨在增加多样性，公平和包容性（DEI）在地球科学。学生们将邀请一些全国顶尖的地球科学家活动家参观北方亚利桑那大学校园，谈论他们的研究和他们对DEI的创新贡献，并成为大学社区成员的榜样。目前，已发表的镍循环模型要么偏离稳定状态的极端，这是不可信的，或休息必然对镍的行为在某些类型的海洋沉积物的投机性假设。最近的一些研究旨在使用镍稳定同位素作为平衡预算的额外约束：正如源和汇通量应该平衡一样，源到海洋的通量加权同位素组成之和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物的认识不足，无法找到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输出的明显矛盾，并使镍预算模型更加准确，该团队将进行系统的实验，以量化镍在锰氢氧化物（水钠锰矿）和蒙皂石上吸附过程中的镍同位素分馏。为了确定锰还原成岩作用区以下远洋沉积物的镍同位素组成，并了解锰羟基氧化物/蒙皂石比例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索内远征SO 240）的选定样本。该研究小组还将收集第一批具有良好特征的年轻碳酸盐沉积物的镍同位素数据，以评估海水初次降水和早期成岩作用期间的分馏作用。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值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.