Calcite dissolution rates in seawater: Lab vs. in-situ measurements and inhibition by organic matter
Calcite dissolution rates in seawater: Lab vs. in-situ measurements and inhibition by organic matter
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DOI：
10.1016/j.marchem.2019.103684
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发表时间：
2019-09
期刊：
影响因子：
3
通讯作者：
J. Naviaux;A. Subhas;Sijia Dong;N. Rollins;Xuewu Liu;R. Byrne;W. Berelson;J. Adkins
中科院分区：
文献类型：
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作者：
J. Naviaux;A. Subhas;Sijia Dong;N. Rollins;Xuewu Liu;R. Byrne;W. Berelson;J. Adkins
化石燃料燃烧导致的海洋酸化正在降低全球海洋平均饱和状态（Ω= C a 2+ C O 3 2− K sp′），从而增加了碳酸钙矿物溶解的热力学驱动力。这一溶解过程最终将中和人为CO2的输入，但Ω和海水中方解石溶解速率之间的关系仍有争议。最近的进展还表明，海水pH值的分光光度测量，因此在现场Ωs，系统地低于pH/Ωs从碱度（Alk）和总溶解无机碳（DIC）的测量计算。方解石饱和层的位置，定义为水柱中Ω= 1的深度，因此根据用于计算Ω的参数，偏移约5-10%。“真正的”饱和水平线仍然是未知的。为了解决这些问题，我们开发了一种新的原位反应器，并测量了13 C-标记的无机方解石的溶解速率在四个站横跨一个断面的北太平洋。使用Alk-DIC（Ω（Alk，DIC））和Alk-pH（Ω（Alk，pH））对计算原位饱和度。我们将现场溶解速率与实验室条件下在5 ° C和21° C下过滤、中毒、紫外线处理的海水中测量的速率进行了比较。我们观察到高于Ω（Alk，DIC）= 1的原位溶解，但不高于Ω（Alk，pH）= 1。我们强调，海洋碳酸盐系统的平衡应重新评估，并应采取谨慎使用时，代理校准到历史Ω（Alk，DIC）。我们的研究结果进一步表明，方解石溶解速率在原位比在实验室中慢约4倍，但当拟合到经验速率= k（1-Ω）n方程时，它们各自具有相似的反应级数（n）。对于0.8< Ω< 1，反应级数为n< 1，对于0< Ω< 0.8，反应级数为n= 4.7，其中Ω crit= 0.8时的速率扭结与从台阶边缘后退到均匀蚀刻坑形成的机械转变一致。我们通过在正磷酸盐（20 μm）和溶解有机碳（DOC）浓度升高的情况下溶解方解石来调和实验室和原位速率之间的偏移，其中DOC以草酸（20 μm）、没食子酸（20 μm）和d-葡萄糖（100 μm）的形式存在。我们发现，可溶性活性磷酸盐对方解石溶解速率没有影响，从pH值5.5-7.5，但添加DOC的d-葡萄糖和草酸的形式减缓实验室溶解速率，以匹配原位观察，可能通过抑制方解石表面上的步骤的后退速率。我们的实验室和原位速率数据形成了以前的原位溶解测量的包络，可以被认为是低/高DOC沃茨溶解速率的外部边界。对于在地表沃茨中形成和下沉的颗粒，下限（高DOC）是最现实的，对于0.8< Ω< 1，其描述为R（mol cm-2 s-1）= 10-14.3±0.2（1-Ω）0.11±0.1，对于0< Ω< 0.8，其描述为R（mol cm-2 s-1）= 10-10.8±0.4（1-Ω）4.7±0.7。这些速率方程来自现场测量，可以很容易地实施到海洋地球化学模型来描述水柱方解石溶解。
Ocean acidification from fossil fuel burning is lowering the mean global ocean saturation state (Ω= C a 2+ C O 3 2− K sp′), thus increasing the thermodynamic driving force for calcium carbonate minerals to dissolve. This dissolution process will eventually neutralize the input of anthropogenic CO 2, but the relationship between Ω and calcite dissolution rates in seawater is still debated. Recent advances have also revealed that spectrophotometric measurements of seawater pHs, and therefore in-situ Ωs, are systematically lower than pHs/Ωs calculated from measurements of alkalinity (Alk) and total dissolved inorganic carbon (DIC). The location of the calcite saturation horizon, defined as the depth in the water column where Ω= 1, therefore shifts by~ 5–10% depending on the parameters used to calculate Ω. The “true” saturation horizon remains unknown. To resolve these issues, we developed a new in-situ reactor and measured dissolution rates of 13 C-labeled inorganic calcite at four stations across a transect of the North Pacific Ocean. In-situ saturation was calculated using both Alk-DIC (Ω (Alk, DIC)) and Alk-pH (Ω (Alk, pH)) pairs. We compare in-situ dissolution rates with rates measured in filtered, poisoned, UV-treated seawater at 5 and 21° C under laboratory conditions. We observe in-situ dissolution above Ω (Alk, DIC)= 1, but not above Ω (Alk, pH)= 1. We emphasize that marine carbonate system equilibria should be reevaluated and that care should be taken when using proxies calibrated to historical Ω (Alk, DIC). Our results further demonstrate that calcite dissolution rates are slower in-situ than in the lab by a factor of~ 4, but that they each possess similar reaction orders (n) when fit to the empirical Rate= k (1-Ω) n equation. The reaction orders are n< 1 for 0.8< Ω< 1 and n= 4.7 for 0< Ω< 0.8, with the kink in rates at Ω crit= 0.8 being consistent with a mechanistic transition from step edge retreat to homogenous etch pit formation. We reconcile the offset between lab and in-situ rates by dissolving calcite in the presence of elevated orthophosphate (20 μm) and dissolved organic carbon (DOC) concentrations, where DOC is in the form of oxalic acid (20 μm), gallic acid (20 μm), and d-glucose (100 μm). We find that soluble reactive phosphate has no effect on calcite dissolution rates from pH 5.5–7.5, but the addition of DOC in the form of d-glucose and oxalic acid slows laboratory dissolution rates to match in-situ observations, potentially by inhibiting the retreat rate of steps on the calcite surface. Our lab and in-situ rate data form an envelope around previous in-situ dissolution measurements and may be considered outer bounds for dissolution rates in low/high DOC waters. The lower bound (high DOC) is most realistic for particles formed in, and sinking out of, surface waters, and is described by R (mol cm-2 s-1)= 10–14.3±0.2 (1-Ω) 0.11±0.1 for 0.8< Ω< 1, and R (mol cm-2 s-1)= 10–10.8±0.4 (1-Ω) 4.7±0.7 for 0< Ω< 0.8. These rate equations are derived from in-situ measurements and may be readily implemented into marine geochemical models to describe water column calcite dissolution.