中国科学院南京地质古生物研究所机构知识库

The isotopic composition of seawater provides valuable information on how the Earth system has evolved. Here we present the stable tungsten isotopic composition (5186/184W) of seawater recorded in three ferromanganese (Fe-Mn) crusts spanning the past 80 million years in the Pacific. The 5186/184W of Fe-Mn crusts displays a pronounced decrease of & SIM;0.2%0 from 60 Ma to 40 Ma followed by a stable value of & SIM;0%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 5186/184W in the three Fe-Mn crusts also indicate limited alteration by deposition regime or diagenetic overprinting. Thus, the 5186/184W of FeMn crusts reflects mainly that of seawater. A simple mass-balance calculation and comparison to other proxies suggest that the early Cenozoic decrease of seawater 5186/184W 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 openocean sediments. Our results demonstrate the potential of 5186/184W as a novel paleo-proxy for global biogeochemical cycling.