Temperature dependence of calcite dissolution kinetics in seawater
Temperature dependence of calcite dissolution kinetics in seawater
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DOI：
10.1016/j.gca.2018.11.037
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发表时间：
2019-02
影响因子：
5
通讯作者：
J. Naviaux;A. Subhas;N. Rollins;Sijia Dong;W. Berelson;J. Adkins
中科院分区：
文献类型：
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作者：
J. Naviaux;A. Subhas;N. Rollins;Sijia Dong;W. Berelson;J. Adkins
摘要了解海水中方解石的溶解机制是理解全球碳收支变化的重要组成部分。为了实现这一目标，我们提供了海水方解石溶解动力学的温度依赖性的第一次测量。我们测量了13 C-标记方解石在海水中的溶解速率，在5，12，21和37° C下，在饱和状态0< Ω= Ca 2+[CO 32-] Ksp '<1的整个范围内。我们发现，溶解速率是非线性依赖于Ω和非线性的程度都随着温度的增加，并突然改变“临界”饱和状态（Ω临界）。在海洋学界最常使用的传统指数速率定律R= k（1− Ω）n，需要根据欠饱和度对k和n进行不同的拟合。虽然我们计算出的活化能与其他远离平衡的研究相似（25±2 kJ/mol），但指数速率定律不能用于从机械上解释我们接近平衡的结果。我们转向另一种框架，来自晶体成核理论，并发现我们的结果是一致的海水中的方解石溶解动力学被设置的后退预先存在的边缘/步骤从Ω= 1-0.9，缺陷辅助蚀坑形成从Ω= 0.9-0.75，并最终均匀蚀坑形成从Ω= 0.75-0。每种机制的Ω临界值比在稀溶液中更接近平衡，因此海洋酸化可能导致海洋碳酸盐进入更快的溶解状态，比以前的研究所预期的更快。我们使用所观察到的温度依赖性的每一种溶解机制，计算步骤动力学系数（β，cm/s），活性成核位点的密度（ns，网站/m2），和步骤边缘自由能（α，mJ/m2）。均匀溶解是很好地解释内的表面成核框架，但缺陷辅助溶解不是。溶解在所有温度下都是通过逐步扩展开始的，但在5° C下跳过了缺陷辅助机制，这可能是由于缺乏成核位点。表面成核框架增强了我们对海水中方解石溶解的理解，但我们的研究结果表明，一个完整的理论还需要将溶液/表面形态和络合的作用。
Abstract Knowledge of the mechanism of calcite dissolution in seawater is a critical component of our understanding of the changing global carbon budget. Towards this goal, we provide the first measurements of the temperature dependence of seawater calcite dissolution kinetics. We measured the dissolution rates of 13 C-labeled calcite in seawater at 5, 12, 21, and 37° C across the full range of saturation states 0< Ω= C a 2+[C O 3 2-] K sp'< 1. We show that the dissolution rate is non-linearly dependent on Ω and that the degree of non-linearity both increases with temperature, and changes abruptly at “critical” saturation states (Ω crit). The traditional exponential rate law most often utilized in the oceanographic community, R= k (1− Ω) n, requires different fits to k and n depending upon the degree of undersaturation. Though we calculate a similar activation energy to other studies far from equilibrium (25±2 kJ/mol), the exponential rate law could not be used to mechanistically explain our near equilibrium results. We turn to an alternative framework, derived from crystal nucleation theory, and find that our results are consistent with calcite dissolution kinetics in seawater being set by the retreat of pre-existing edges/steps from Ω= 1–0.9, defect-assisted etch pit formation from Ω= 0.9–0.75, and finally homogenous etch pit formation from Ω= 0.75–0. The Ω crit s for each mechanism are shifted significantly closer to equilibrium than they occur in dilute solutions, such that ocean acidification may cause marine carbonates to enter faster dissolution regimes more readily than would be expected from previous studies. We use the observed temperature dependence for each dissolution mechanism to calculate step kinetic coefficients (β, cm/s), densities of active nucleation sites (n s, sites/m 2), and step edge free energies (α, mJ/m 2). Homogenous dissolution is well explained within the surface nucleation framework, but defect-assisted dissolution is not. Dissolution is initiated via step-propagation at all temperatures, but the defect-assisted mechanism is skipped over at 5° C, potentially due to a lack of nucleation sites. The surface nucleation framework enhances our understanding of calcite dissolution in seawater, but our results suggest that a complete theory will also need to incorporate the role of solution/surface speciation and complexation.