Redox control on the tungsten isotope composition of seawater

Abstract

Significance The fate and transport of tungsten (W) in aquatic environments is still poorly constrained. To identify the processes that control the abundance of dissolved W, we applied a sophisticated analytical approach that enables the accurate determination of the seawater W isotopic composition. Our results indicate that the removal of W from seawater mainly occurs via adsorption onto oxide minerals. The marine inventory of W is therefore intimately linked to the areal extension of oxic marine conditions. Concurrently, the limited scavenging of W in anoxic marine settings seems a unique characteristic of W, highlighting that W isotopes can help to reconstruct the earliest rise of oceanic oxygen in Earth’s history. Free oxygen represents an essential basis for the evolution of complex life forms on a habitable Earth. The isotope composition of redox-sensitive trace elements such as tungsten (W) can possibly trace the earliest rise of oceanic oxygen in Earth’s history. However, the impact of redox changes on the W isotope composition of seawater is still unknown. Here, we report highly variable W isotope compositions in the water column of a redox-stratified basin (δ186/184W between +0.347 and +0.810 ‰) that contrast with the homogenous W isotope composition of the open ocean (refined δ186/184W of +0.543 ± 0.046 ‰). Consistent with experimental studies, the preferential scavenging of isotopically light W by Mn-oxides increases the δ186/184W of surrounding seawater, whereas the redissolution of Mn-oxides causes decreasing seawater δ186/184W. Overall, the distinctly heavy stable W isotopic signature of open ocean seawater mirrors predominantly fully oxic conditions in modern oceans. We expect, however, that the redox evolution from anoxic to hypoxic and finally oxic marine conditions in early Earth’s history would have continuously increased the seawater δ186/184W. Stable W isotope compositions of chemical sediments that potentially preserve changing seawater W isotope signatures might therefore reflect global changes in marine redox conditions.