Stable boron isotope fractionation between dissolved B(OH)3 and B(OH)4
Stable boron isotope fractionation between dissolved B(OH)3 and B(OH)4
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DOI：
10.1016/j.gca.2004.12.011
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发表时间：
2005-06
影响因子：
5
通讯作者：
R. Zeebe
中科院分区：
文献类型：
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作者：
R. Zeebe
海相碳酸盐岩中稳定的硼同位素比值（11B/10 B）可作为古pH值的记录器，是最有前途的古碳酸盐岩化学指标之一。理解替代物的热力学基础是非常重要的，包括溶解的硼酸B（OH）3和硼酸根离子B（OH）4−之间的平衡分馏因子（[公式：见正文]，下文称为α（B3-B4））。然而，迄今为止，该系数还没有通过实验确定，因此广泛使用理论计算值（Kakihana和Kotaka，1977，以下简称KK 77）。我检查了这个值背后的计算。使用与KK 77相同的光谱数据和方法，我计算出300 K时的[公式：见正文] = 1.0193。不幸的是，事实证明，在一般情况下，结果是敏感的实验确定的振动频率和用于计算分子力的理论方法。利用分析技术和从头算分子轨道理论，在300 K下，α（B3-B4）的结果在1.020和1.050之间变化。然而，一些论据表明α（B3-B4）= 1.030。在各种10 B-，2D-和18 O-标记的同位素异构体中测量的同位素位移并没有提供对稳定的硼同位素分馏的限制。我的结论是，为了锚硼pH值代理的基本面，实验工作是必要的。然而，硼pH值代用指标的批评者应该注意到，如果使用生物体特异性校准，α（B3-B4）的不确定性不会使pH值重建产生偏差。
The stable boron isotope ratio (11B/10B) in marine carbonates is used as a paleo-pH recorder and is one of the most promising paleo-carbonate chemistry proxies. Understanding the thermodynamic basis of the proxy is of fundamental importance, including knowledge on the equilibrium fractionation factor between dissolved boric acid, B(OH)3, and borate ion, B(OH)4−( [Formula: see text] , hereafter α(B3–B4)). However, this factor has hitherto not been determined experimentally and a theoretically calculated value (Kakihana and Kotaka, 1977, hereafter KK77) has therefore been widely used. I examine the calculations underlying this value. Using the same spectroscopic data and methods as KK77, I calculate the same [Formula: see text] = 1.0193 at 300 K. Unfortunately, it turns out that in general the result is sensitive to the experimentally determined vibrational frequencies and the theoretical methods used to calculate the molecular forces. Using analytical techniques and ab initio molecular orbital theory, the outcome for α(B3–B4)varies between ∼1.020 and ∼1.050 at 300 K. However, several arguments suggest that α(B3–B4)≳ 1.030. Measured isotopic shifts in various10B-,2D-, and18O-labeled isotopomers do not provide a constraint on stable boron isotope fractionation. I conclude that in order to anchor the fundamentals of the boron pH proxy, experimental work is required. The critics of the boron pH proxy should note, however, that uncertainties in α(B3–B4)do not bias pH reconstructions provided that organism-specific calibrations are used.