古海水pH值代用指标——海洋碳酸盐硼同位素研究进展

Advances in Paleo-seawater pH Proxy:Boron Isotope in Marine Carbonates

仪器测量的海水pH记录太短,无法评估海水pH自然变化的频率和幅度,并预测未来大气CO2急剧增加后海水酸度的响应。海相碳酸盐的硼同位素是目前恢复古海洋pH的有效途径,倍受古气候—环境学家的重视。评述了近年来海洋碳酸盐的硼同位素的最新研究成果和研究现状,重点探讨了海相碳酸盐的硼同位素的测定方法、硼同位素—pH模型和古海水pH恢复等前沿内容,旨在提供一个系统的海洋碳酸盐硼同位素—pH系统的基本概念及研究思路,以利于气候学、地质学界了解这一交叉领域的发展动态。

Due to the tight coupling between ocean acidity and atmospheric concentrations of the greenhouse gas CO2, reconstruction of the former is one of the major goals in paleoclimatology. Given that instrumental records of seawater pH exceeding a single decade are not yet available, the boron isotopic composition of marine carbonates currently offers the only practical means to determine paleo-seawater through time. The boron isotope ratios are best measured by thermal ionization mass spectrometry （TIMS）. The boron isotope-pH proxy is founded on a theoretical model of carbonate δ^11B variation with pH that assumes that the boron isotopic composition of marine carbonates mirrors the boron isotopic composition of borate in seawater. Quantified, reconstruction of ancient ocean pH requires a precise definition of α （the Equilibrium isotope fractionation factor between borate and boric acid） and pKB （the apparent dissociation constant of boric acid） and further knowledge of δ^11B of ancient seawater, which is impacted by the rate of continental erosion. Complications aside, boron isotopes remain a powerful tool in the reconstruction of past seawater pH, however, various aspects of these parameters require rigorous evaluation. Therefore, given our current understanding, great care should be taken when interpreting long-term trends in δ^11B of marine carbonate when the ultimate goal is to reconstruct ocean pH and ancient atmospheric carbon dioxide levels. The paleo-pH reconstructions by δ^11B of marine carbonate show a higher pH in glacial ocean which is consistent with lower atmospheric carbon dioxide concentrations recorded ice cores and demonstrate the coupling between surface ocean chemistry and the atmosphere, allowing for quantitative estimation of atmospheric p（ CO2 ） beyond the reach of ice cores. However, high resolution δ^11B ratios of corals show rapid change of paleo-pH at decadal-millennial timescale indicating p（ CO2 ） between ocean and atmosphere may not be always in equilibrium and there are other forcing accounting for variability of ocean pH. Improvement the model of the boron isotope-pH proxy, high resolution paleo-pH reconstruction and relationships between ocean pH globe carbon cycling need to be pay more attention in future works of this field.