海洋锂循环及其对表层碳循环的指示

Oceanic Lithium Cycling and Implications for Carbon Cycle

以地球系统科学的视角研究大气二氧化碳含量对地球宜居性的扰动是当今最为前沿的地球科学问题之一。大气二氧化碳浓度受控于火山、地幔排气、大陆风化和反风化等过程。大陆风化和高温热液作用是海洋锂的主要来源,而反风化作用则是海洋锂储库的汇。风化过程、热液蚀变和反风化过程中,锂同位素发生不同程度的分馏,而这些源和汇过程中的锂通量及其同位素的分馏共同控制了海水的锂同位素组成。因此,海水锂同位素组成可用于地表环境深时碳循环的示踪。近年来,通过海洋碳酸盐岩的锂同位素、87Sr/86Sr比值及碳同位素等指标的联用,来示踪新生代和前寒武纪等关键地质时期的构造—气候—风化共同影响的碳循环过程是研究热点。文章梳理了现代海洋锂元素循环的源汇过程,重新限定大陆风化和高温热液输入海洋的锂同位素组成。并在此基础上,分析了目前海水锂同位素质量平衡模型中的不确定性,探讨了海水锂同位素记录在示踪深时碳循环方面的应用和局限性,为锂元素及其同位素指标的应用和全球锂循环研究的深入提供参考。

How the carbon cycle influences the atmospheric carbon dioxide level is of a major concern for Earth’s habitability.The oceanic lithium(Li)cycle can be used to trace carbon cycle because both cycles are controlled by chemical weathering,hydrothermal-seafloor interaction,and marine clay authigenesis which also known as reverse weathering.The variations in seawater Li isotope compositions posit changes in the sink and source processes such as continental weathering intensity and/or rate as well as reverse weathering rate.This paper reviews the global cycle of oceanic lithium and the mass balance for seawater lithium isotopes,focusing on the present-day fluxes and associated isotope fractionation mechanisms.Challenges still remain to better constrain the budgets as well as isotope fractionation factor especially in hydrothermal alteration and reverse weathering.The review also extends to the use of seawater li isotope records in tracing carbon cycle during climate events and critical time periods in the earth history.Collectively,this review highlights the potential as well as limitations of utilizing seawater Li isotope records to trace global carbon cycle in deep time.