南京大学Ruiyu Yang课题组研究了8000万年来海水中稳定的钨同位素组成。2023年5月30日出版的《地质学》发表了这项成果。

课题组人员展示了过去8000万年太平洋三个铁锰结壳记录的海水稳定钨同位素组成( δ^186/184W )。自ca. 40Ma以来，铁锰结壳的 δ^186/184W 在 60 Ma-40 Ma 间显著下降了约0.2‰，此后稳定在约0‰。多个证据表明，铁锰结壳和海水之间存在不变的平衡同位素分馏。三个铁锰结壳中 δ^186/184W 的一致变化也表明，受沉积机制或成岩叠加作用的影响有限。因此，铁锰结壳的 δ^186/184W 主要反映海水的 δ^186/184W 。

简单的质量平衡计算和与其他替代物的比较表明，新生代早期海水的 δ^186/184W 的减少很可能是由于海水和钨汇之间的钨同位素分馏降低引起的，与不断变化的沉积机制有关。课题组人员认为，有机物埋藏的增加和铁锰氧化物(FMO)沉淀的减少，限制水体中的钨在公海沉积物中铁锰氧化物上的吸附，可能会导致与钨汇相关的同位素分馏减少。研究结果证明了 δ^186/184W 作为全球生物地球化学循环新的古替代物指标的潜力。

据介绍，海水的同位素组成为地球系统的演化提供了宝贵的信息。

附：英文原文

Title: Stable tungsten isotopic composition of seawater over the past 80 million years

Author: Ruiyu Yang, Daniel Stubbs, Tim Elliott, Tao Li, Tianyu Chen, Adina Paytan, David B. Kemp, Hongfei Ling, Jun Chen, James R. Hein, Christopher D. Coath, Gaojun Li

Issue&Volume: 2023-05-30

Abstract: The isotopic composition of seawater provides valuable information on how the Earth system has evolved. Here we present the stable tungsten isotopic composition ( δ^186/184W ) of seawater recorded in three ferromanganese (Fe-Mn) crusts spanning the past 80 million years in the Pacific. The  δ^186/184W of Fe-Mn crusts displays a pronounced decrease of ~0.2‰ from 60 Ma to 40 Ma followed by a stable value of ~0‰ since ca. 40 Ma. Multiple lines of evidence indicate an invariable equilibrium isotopic fractionation between Fe-Mn crusts and seawater. The consistent variations in δ^186/184W in the three Fe-Mn crusts also indicate limited alteration by deposition regime or diagenetic overprinting. Thus, the δ^186/184W of Fe-Mn crusts reflects mainly that of seawater. A simple mass-balance calculation and comparison to other proxies suggest that the early Cenozoic decrease of seawater δ^186/184W was most likely caused by decreased W isotopic fractionation between seawater and W sinks linked to shifting sedimentation regimes. We propose that increased burial of organics and decreased Fe-Mn oxide (FMO) sedimentation may result in a smaller isotopic fractionation associated with W sinks by limiting the adsorption of W from the water column onto FMOs in open-ocean sediments. Our results demonstrate the potential of δ^186/184W as a novel paleo-proxy for global biogeochemical cycling.

DOI: 10.1130/G51208.1

Source: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G51208.1/623831/Stable-tungsten-isotopic-composition-of-seawaterredirectedFrom=fulltext