The effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: Implications for Eocene and Cretaceous ocean carbon chemistry and buffering
The effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: Implications for Eocene and Cretaceous ocean carbon chemistry and buffering
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DOI：
10.1002/2014gb004986
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发表时间：
2015-05-01
影响因子：
5.2
通讯作者：
Haug, Gerald H.
中科院分区：
文献类型：
--
作者：
Hain, Mathis P.;Sigman, Daniel M.;Haug, Gerald H.
海水中钙和镁浓度（[Ca 2 +]，[Mg 2 +]）的重建变化可预测地影响海洋的酸/碱和碳化学。然而，目前使用不准确的化学平衡常数公式来解释这些变化。在这里，我们开发了一个有效的实现迈阿密离子相互作用模型预测所有化学平衡常数下变量[Ca 2 +]和[Mg 2 +]的碳化学计算所需的。本文研究了[Ca 2 +]和[Mg 2 +]对海洋pH、CO2、溶解无机碳（DIC）、CaCO 3饱和状态和缓冲能力之间关系的影响。增加[Ca 2 +]和/或[Mg 2 +]增强离子配对，这通过增加总碳酸根离子与游离（未络合）碳酸根离子的浓度比来增加海水缓冲。然而，[Ca 2 +]的增加也会导致碳酸根离子的下降，从而压倒离子配对效应并降低海水缓冲。考虑到始新世[Ca 2 +]和[Mg 2 +]的重建（[Ca 2 +]类似于20 mM; [Mg 2 +]类似于30 mM），始新世海水将需要与今天基本相同的DIC来同时解释类似于现代和估计的始新世大气CO2类似于1000 ppm。在白垩纪，在类似的4倍现代[Ca 2 +]，海洋缓冲将在最低限度。总体而言，在高海水[Ca 2 +]，CaCO 3饱和度，pH值和大气CO2的时间更容易受到全球碳循环的扰动。例如，考虑到始新世和白垩纪海水[Ca 2 +]和[Mg 2 +]，大气中二氧化碳的加倍将需要比现代海水成分更少的碳添加到海洋/大气系统中。此外，自白垩纪以来，增加海水缓冲可能是进化的驱动力，因为它提高了生物控制的钙化和二氧化碳浓度机制的能量需求，有助于光合作用。
Reconstructed changes in seawater calcium and magnesium concentration ([Ca2+], [Mg2+]) predictably affect the ocean's acid/base and carbon chemistry. Yet inaccurate formulations of chemical equilibrium constants are currently in use to account for these changes. Here we develop an efficient implementation of the MIAMI Ionic Interaction Model to predict all chemical equilibrium constants required for carbon chemistry calculations under variable [Ca2+] and [Mg2+]. We investigate the impact of [Ca2+] and [Mg2+] on the relationships among the ocean's pH, CO2, dissolved inorganic carbon (DIC), saturation state of CaCO3 (), and buffer capacity. Increasing [Ca2+] and/or [Mg2+] enhances ion pairing, which increases seawater buffering by increasing the concentration ratio of total to free (uncomplexed) carbonate ion. An increase in [Ca2+], however, also causes a decline in carbonate ion to maintain a given , thereby overwhelming the ion pairing effect and decreasing seawater buffering. Given the reconstructions of Eocene [Ca2+] and [Mg2+] ([Ca2+]similar to 20mM; [Mg2+]similar to 30mM), Eocene seawater would have required essentially the same DIC as today to simultaneously explain a similar-to-modern and the estimated Eocene atmospheric CO2 of similar to 1000ppm. During the Cretaceous, at similar to 4 times modern [Ca2+], ocean buffering would have been at a minimum. Overall, during times of high seawater [Ca2+], CaCO3 saturation, pH, and atmospheric CO2 were more susceptible to perturbations of the global carbon cycle. For example, given both Eocene and Cretaceous seawater [Ca2+] and [Mg2+], a doubling of atmospheric CO2 would require less carbon addition to the ocean/atmosphere system than under modern seawater composition. Moreover, increasing seawater buffering since the Cretaceous may have been a driver of evolution by raising energetic demands of biologically controlled calcification and CO2 concentration mechanisms that aid photosynthesis.