Dissolution of calcite in water is fundamental to geochemistry. Laboratory methods generally give consistent dissolution rates under conditions far from equilibrium, but close to equilibrium the measurement of dissolution using isotopes is complicated by poorly understood calcite-fluid isotopic exchange processes. This near-equilibrium isotopic exchange occurs in laboratory experiments at rates of order 10 to 10 mol/m/s and decreases with experiment duration approximately as 1/time where time is measured from the start of the experiment. Such rates are fast enough to compete with dissolution in days-long experiments and consequently, net dissolution rates may not be measurable with isotopic tracer methods except in experiments carried out for long enough to allow the background exchange to dissipate sufficiently. To better define what should be expected for isotopic signals during dissolution, we develop a quasi-1D model for calcite dissolution that includes the mechanistically unconstrained isotopic exchange as well as solid state diffusion. We use the model, the observations reported in the literature, and the calcite dissolution rates in seawater reported by to illustrate ambiguities in measurement of near-equilibrium calcite dissolution rates. The results suggest that the reaction order of calcite dissolution close to equilibrium could be 2 at both room temperature and the lower temperatures of the seafloor. The reaction order of near-equilibrium calcite dissolution is important for understanding calcite mineral surface processes, long-term behavior of dissolution on the seafloor, and alkalinity fluxes from modern seafloor sediments. Distinguishing between net dissolution, which affects fluid saturation state, and “exchange,” which doesn’t, is important but typically cannot be done with isotopic tracers alone.

中文翻译：

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近平衡方解石溶解的反应级数：同位素示踪方法的不确定性和模糊性


方解石在水中的溶解是地球化学的基础。实验室方法通常在远离平衡的条件下给出一致的溶解速率，但接近平衡时，由于对方解石-流体同位素交换过程知之甚少，使用同位素的溶解测量变得复杂。这种接近平衡的同位素交换在实验室实验中以 10 至 10 mol/m/s 的速率发生，并随着实验持续时间而减少，大约为 1/次，其中时间是从实验开始测量的。这样的速率足够快，足以与长达数天的实验中的溶解竞争，因此，净溶解速率可能无法用同位素示踪剂方法测量，除非进行足够长的实验以允许背景交换充分消散。为了更好地定义溶解过程中同位素信号的预期情况，我们开发了方解石溶解的准一维模型，其中包括机械上不受约束的同位素交换以及固态扩散。我们使用该模型、文献中报道的观察结果以及海水中方解石溶解速率来说明近平衡方解石溶解速率测量中的模糊性。结果表明，在室温和海底较低温度下，方解石溶解接近平衡的反应级数可能为2。近平衡方解石溶解的反应级数对于理解方解石矿物表面过程、海底溶解的长期行为以及现代海底沉积物的碱度通量非常重要。 区分影响流体饱和状态的净溶解和不影响流体饱和状态的“交换”很重要，但通常不能单独使用同位素示踪剂来完成。