It is estimated that at least 25 % of the anthropogenic carbon dioxide (CO2) emitted to the atmosphere since the start of the industrial revolution has been absorbed and dissolved by the oceans. The uptake of CO2 by the oceans leads to an increase in the seawater proton concentration ([H+]), and decreases in seawater pH, carbonate ion concentration ([CO32–]), and saturation state (Ω) with respect to calcium carbonate (CaCO3) minerals; a process commonly referred to as “ocean acidification”. Shallow-water (<200 m), high-magnesium, biogenic calcites are expected to be amongst the first to respond to ocean acidification, and it has been proposed that they will dissolve selectively and sequentially according to their solubility in seawater. In this study, we test this competitive dissolution hypothesis by reacting a mixture of biogenic and synthetic carbonates of varying Mg content with acidified, natural seawater to simulate the progressive acidification of surface-ocean waters by anthropogenic CO2. The results of this study confirm the hypothesis that carbonates will dissolve sequentially according to their respective solubility. They also reveal that the dissolution of high Mg-calcites will proceed incongruently. The originality of this contribution rests with the demonstration that the presence of a single high Mg-calcite will generate, like in a sediment of mixed mineralogy, a continuum of transient states as lower Mg-calcites of greater stability are precipitated and dissolved. Consequently, in a semi-closed or closed system, the pH buffering of the acidified seawater solution will be progressive rather than occur in steps according to changes in the solubility of the individual carbonate phases that compose a sediment. Hence, we expect that, as the oceans take up more anthropogenic CO2 and further acidify, the average mineralogy and composition (Mg content) of shallow-water carbonate sediments and reef structures will change over the next few centuries as the most soluble carbonate phases (high-Mg calcites) are dissolved and no longer precipitated.

中文翻译：

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模拟海洋酸化作用下混合碳酸盐固体的竞争性溶解


据估计，自工业革命开始以来，排放到大气中的人为二氧化碳 （CO2） 中至少有 25% 已被海洋吸收和溶解。海洋对 CO2 的吸收导致海水质子浓度 （[H+]） 增加，而海水 pH 值、碳酸根离子浓度 （[CO32–]） 和饱和状态 （Ω） 相对于碳酸钙 （CaCO3） 矿物降低;通常被称为“海洋酸化”的过程。浅水 （x3C200 m）、高镁、生物方解石预计将是最早对海洋酸化做出反应的物质之一，有人提出，它们将根据它们在海水中的溶解度选择性地依次溶解。在这项研究中，我们通过将不同 Mg 含量的生物和合成碳酸盐的混合物与酸化的天然海水反应来测试这种竞争性溶解假设，以模拟人为 CO2 对表层海水的逐渐酸化。这项研究的结果证实了碳酸盐将根据各自的溶解度依次溶解的假设。他们还揭示了高 Mg-方解石的溶解将不一致地进行。这一贡献的独创性在于证明，单个高镁方解石的存在将产生一个瞬态的连续体，就像在混合矿物学的沉积物中一样，因为稳定性更高的低镁方解石被沉淀和溶解。因此，在半封闭或封闭系统中，酸化海水溶液的 pH 缓冲将是渐进的，而不是根据构成沉积物的单个碳酸盐相的溶解度的变化逐步发生。 因此，我们预计，随着海洋吸收更多的人为二氧化碳并进一步酸化，浅水碳酸盐沉积物和珊瑚礁结构的平均矿物学和成分（镁含量）将在未来几个世纪发生变化，因为最易溶的碳酸盐相（高镁方解石）被溶解并且不再沉淀。