运用硼同位素进行古气候重建的研究进展

海水中硼的同位素组成受海水pH控制,海洋生物碳酸盐形成过程中海水的硼主要以B(OH)4-形式进入碳酸盐晶格,该过程不产生同位素分馏,因此海洋生物碳酸盐的硼同位素组成能够反映海水中B(OH)4-的硼同位素组成,进而指示当时海水的pH值,再根据海水与大气间CO2溶解平衡关系可以间接获得当时大气CO2分压。最近十多年来成功地运用生物碳酸盐硼同位素研究了过去20 Ma以来不同时间段古海水pH演化历史以及大气CO2分压的变化历史,使得硼同位素成为研究古气候的新方法。由于大气CO2分压对海水pH变化的响应非常灵敏,必须对各参数的取值进行严格的论证,而且模型本身的合理性及精确性也要进一步讨论。本文详细地介绍了硼同位素-pH方法的理论模型以及近年来运用该方法研究古气候取得的主要成果,同时对模型本身的研究进展也进行了阐述。

REVIEWS ON PALEOCLIMATE RECONSTRUCTION USING BORON ISOTOPE

Boron isotopic composition is controlled by the pH value in the ocean.Boron in the ocean,predominately in the form of B(OH)4-,is incorporated into carbonate lattice during the formation of organic carbonate in the ocean,and no boron isotope fractionation took place during the carbonate formation.Thus,the boron isotopic composition of organic carbonate in the ocean can indicate the boron isotope.value of seawater B(OH)4-,and further reveal the contemporary seawater pH value,then the partial pressure of atmospheric carbon dioxide can be indirectly acquired according to the dissociation equilibrium between the seawater and CO2.In the recent decade,boron isotope has been successfully applied in studying evolution history of seawater pH and the partial pressure of atmospheric carbon dioxide during different periods in the past 20 million years,which makes boron isotope become a new tool in the field of paleoclimate.Since the partial pressure of atmospheric carbon dioxide is extremely sensitive to.the change in pH value of seawater,it is very necessary to strictly constrain the values of different parameters,and the reasonability and precision of the model also need to be further demonstrated.In this paper,the theoretical model,the progress of the boron isotope-PH proxy and the result in studying paleoclimate are discussed.