In fewer than fifteen years, the study of Ni stable isotopes has advanced from early method development to application of a powerful tool for resolving a long-standing question: why does it appear that output fluxes of Ni from the global oceans far exceed input fluxes? The seawater concentration of Ni, a bioessential trace metal, is almost certainly at steady state on timescales comparable to its residence time of ∼20 kyr, so some of the current flux estimates must be inaccurate. Just as the input and output fluxes should balance, so should the flux-weighted isotopic compositions of the inputs and outputs. Thus, isotopic characterization of inputs and outputs provide an additional constraint on a balanced model of the marine Ni budget. Here, we report on experiments designed to elucidate fractionation mechanisms and magnitudes for sorption of Ni to Mn oxyhydroxide (birnessite), because Mn-rich sediments accumulating on abyssal plains represent the largest sink flux of Ni from seawater to marine sediments. Our results show remarkably large fractionations at low ionic strength (average ΔNi = +1.38 ‰). Neither closed-system equilibrium trends nor Rayleigh curves fit the data well. Fractionations are even larger at high ionic strength (ΔNi ranging from +2.0 to +4.0 ‰), and they decrease with experimental duration from 2 d (49 h) to 27 d. The high ionic strength data fit Rayleigh trends well. Here, we use X-ray absorption fine-structure spectroscopy (EXAFS) and results from previous studies to support interpretation of our data as combinations of kinetic and equilibrium isotope effects that vary in their proportional contributions to the total fractionation with time and with surface loading. One important consequence of this study is that none of the experimental results reported thus far, including ours, are directly applicable to building steady-state models of the Ni cycle. Even our longest duration experiments did not achieve equilibrium, which is likely to be manifest in the very slowly accumulating sediments on abyssal plains. Our work constrains further the mechanisms of Ni sorption to birnessite and clearly indicates that determination of equilibrium fractionation in this system, although challenging, will be a crucial step toward resolving the apparent marine Ni imbalance.

中文翻译：

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修正镍的海洋质量平衡模型：实验确定镍吸附到水钠锰矿过程中的同位素分馏


在不到十五年的时间里，镍稳定同位素的研究已经从早期的方法开发发展到应用强大的工具来解决一个长期存在的问题：为什么全球海洋的镍输出通量远远超过输入通量？镍（一种生物必需的微量金属）的海水浓度几乎肯定在与其 20 kyr 的停留时间相当的时间尺度上处于稳定状态，因此当前的一些通量估计肯定是不准确的。正如输入和输出通量应该平衡一样，输入和输出的通量加权同位素组成也应该平衡。因此，输入和输出的同位素表征为海洋镍预算的平衡模型提供了额外的约束。在这里，我们报告了旨在阐明镍向氢氧化锰（水钠锰矿）吸附的分馏机制和大小的实验，因为深海平原上积累的富锰沉积物代表了镍从海水到海洋沉积物的最大汇通量。我们的结果显示在低离子强度下有非常大的分馏（平均 ΔNi = +1.38 ‰）。封闭系统平衡趋势和瑞利曲线都不能很好地拟合数据。在高离子强度下，分级甚至更大（ΔNi 范围为 +2.0 至 +4.0 ‰），并且随着实验持续时间从 2 d（49 小时）到 27 d，分级减小。高离子强度数据很好地符合瑞利趋势。在这里，我们使用 X 射线吸收精细结构光谱 (EXAFS) 和先前研究的结果来支持将我们的数据解释为动力学和平衡同位素效应的组合，这些效应对总分馏的比例贡献随时间和表面负载而变化。 这项研究的一个重要结果是，迄今为止报道的实验结果（包括我们的实验结果）都不能直接适用于建立镍循环的稳态模型。即使我们持续时间最长的实验也没有达到平衡，这可能在深海平原上非常缓慢地积累的沉积物中得到体现。我们的工作进一步限制了水钠锰矿对镍的吸附机制，并清楚地表明，确定该系统中的平衡分馏虽然具有挑战性，但将是解决明显的海洋镍不平衡问题的关键一步。