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

Abstract

Abstract The isotopic composition of seawater provides valuable information on how the Earth system has evolved. Here we present the stable tungsten isotopic composition (δ186/184W) of seawater recorded in three ferromanganese (Fe-Mn) crusts spanning the past 80 million years in the Pacific. The δ186/184W 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 δ186/184W in the three Fe-Mn crusts also indicate limited alteration by deposition regime or diagenetic overprinting. Thus, the δ186/184W 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 δ186/184W 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 δ186/184W as a novel paleo-proxy for global biogeochemical cycling.