Insights from GO-SHIP hydrography data into the thermodynamic consistency of CO2 system measurements in seawater
Insights from GO-SHIP hydrography data into the thermodynamic consistency of CO2 system measurements in seawater
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DOI：
10.1016/j.marchem.2019.03.006
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发表时间：
2019-04-20
期刊：
影响因子：
3
通讯作者：
Dickson, Andrew G.
中科院分区：
文献类型：
--
作者：
Fong, Michael B.;Dickson, Andrew G.
由于技术的进步，用于研究海洋二氧化碳系统的常规海水pH测量具有良好的重复性正变得越来越普遍。然而，pH测量的准确性受到了质疑，因为从不同航次的大量经过仔细校准的最先进的二氧化碳测量中广泛观察到，用分光光度法测量的pH与使用海水酸碱体系热力学模型同时测量的总溶解无机碳(C-T)和总碱度(A(T))计算的pH之间存在显著的pH依赖差异。对最近四个GO-SHIP重复水文数据集的分析表明，碳酸离解常数(K-1和K-2)、总硼盐度比以及C-T和A(T)测量中的小系统误差可能导致了观察到的部分(但不是全部)与pH有关的差异。只有在存在少量但有意义的(类似于4微克分子公斤(-1))对测量的A(T)有未确定且通常被忽略的贡献的情况下，才能完全解释剩余差异，这种贡献可能来自广泛存在于公海的有机碱基。这些误差的组合可以实现测量和计算的pH值之间的一致性，而不要求船上的任何测量都有明显的误差。未来的研究应侧重于确定开阔海洋中是否存在有机碱度，并限制二氧化碳测量和二氧化碳计算中使用的常量的不确定性。
Due to advances in technology, routine seawater pH measurements of excellent repeatability are becoming increasingly common for studying the ocean CO2 system. However, the accuracy of pH measurements has come into question due to a widespread observation, from a large number of carefully calibrated state-of-the-art CO2 measurements on various cruises, of there being a significant pH-dependent discrepancy between pH that was measured spectrophotometrically and pH calculated from concurrent measurements of total dissolved inorganic carbon (C-T) and total alkalinity (A(T)), using a thermodynamic model of seawater acid-base systems. From an analysis of four recent GO-SHIP repeat hydrography datasets, we show that a combination of small systematic errors in the dissociation constants of carbonic acid (K-1 and K-2), the total boron-salinity ratio, and in C-T and A(T) measurements are likely responsible for some, but not all of the observed pH-dependent discrepancy. The residual discrepancy can only be fully accounted for if there exists a small, but meaningful amount (similar to 4 mu mol kg(-1)) of an unidentified and typically neglected contribution to measured A(T), likely from organic bases, that is widespread in the open ocean. A combination of these errors could achieve consistency between measured and calculated pH, without requiring that any of the shipboard measurements be significantly in error. Future research should focus on establishing the existence of organic alkalinity in the open ocean and constraining the uncertainty in both CO2 measurements and in the constants used in CO2 calculations.