Hoefs Jochen, Harmon Russell S
Geowissenschaftliches Zentrum, Universität Göttingen, Göttingen, Germany.
Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC, USA.
Isotopes Environ Health Stud. 2023 Aug-Dec;59(4-6):349-411. doi: 10.1080/10256016.2023.2271127. Epub 2023 Dec 12.
After the atmosphere, the ocean is the most well-mixed and homogeneous global geochemical reservoir. Both physical and biological processes generate elemental and isotope variations in seawater. Contrasting geochemical behaviors cause elements to be susceptible to different fractionation mechanisms, with their isotopes providing unique insights into the composition and evolution of the ocean over the course of geological history. Supplementing the traditional stable isotopes (H, C, O, N, S) that provide information about ocean processes and past environmental conditions, radiogenic isotope (Sr, Nd, Os, Pb, U) systems can be used as time markers, indicators of terrestrial weathering, and ocean water mass mixing. Recent instrumentation advances have made possible the measurement of natural stable isotope variations produced by both mass-dependent and mass-independent fractionation for an ever-increasing number of metal elements (e.g. Li, B, Mg, Si, Ca, V, Cr, Fe, Ni, Cu, Zn, Se, Mo, Cd, Tl, U). The major emphasis in this review is on the isotopic composition of the light elements based on a comparatively large literature. Unlike O, H and S, the stable isotopes of C, N and Si do not have a constant isotopic composition in the modern ocean The major cations Ca, Mg, and Sr fixed in carbonate shells provide the best proxies for reconstruction of the composition of the ocean in the past. Exhibiting large isotope enrichments in ocean water, B and Li are suitable for the investigation of water/rock interactions and can act as monitors of former oceanic pH. The bioessential elements Zn, Cd, and Ni are indicators of paleoproductivity in the ocean. Characteristic isotope enrichments or depletions of the multivalent elements V, Cr, Fe, Se, Mo, and U record the past redox state of the ocean/atmosphere system. Case studies describe how isotopes have been used to define the seawater composition in the geological past.
除大气之外，海洋是全球混合最为充分且最为均匀的地球化学储库。物理和生物过程都会导致海水中元素及同位素发生变化。不同的地球化学行为使得各种元素易于受到不同分馏机制的影响，而它们的同位素则为了解地质历史进程中海洋的组成与演化提供了独特视角。除了能提供有关海洋过程和过去环境状况信息的传统稳定同位素（氢、碳、氧、氮、硫）之外，放射性同位素（锶、钕、锇、铅、铀）体系可用作时间标记、陆地风化的指标以及海水团混合的指标。近期仪器技术的进步使得测量越来越多金属元素（例如锂、硼、镁、硅、钙、钒、铬、铁、镍、铜、锌、硒、钼、镉、铊、铀）由质量依赖型和质量非依赖型分馏产生的天然稳定同位素变化成为可能。基于相对丰富的文献资料，本综述主要关注轻元素的同位素组成。与氧、氢和硫不同，碳、氮和硅的稳定同位素在现代海洋中并不具有恒定的同位素组成。固定在碳酸盐壳中的主要阳离子钙、镁和锶是重建过去海洋组成的最佳替代指标。硼和锂在海水中表现出较大的同位素富集，适合用于研究水/岩相互作用，并且可以作为古海洋pH值的监测指标。生物必需元素锌、镉和镍是海洋中古生产力的指标。多价元素钒、铬、铁、硒、钼和铀的特征性同位素富集或亏损记录了过去海洋/大气系统的氧化还原状态。案例研究描述了同位素如何被用于确定地质历史时期的海水组成。
