Stable tungsten (W) isotope systematics in marine sediments: a potential paleo-proxy for deep ocean oxygenation

Abstract

The stable tungsten (W) isotope system has emerged as a promising paleoenvironmental indicator. However, the modern oceanic budget of W and its behavior during sedimentary burial are to date not well understood. Specifically, the mechanisms governing W delivery to marine sediments and its behavior during early diagenesis remain unknown. In this study, we analyzed stable W isotopic compositions (δ^186/184W) in three sediment cores from various redox environments in the Gulf of California. Additionally, we investigated δ^186/184W compositions and major and trace elemental compositions in surface sediments from global ocean basins. Our findings indicate that continental margin sediments and hydrothermal precipitates lack significant authigenic W enrichment, whereas deep-ocean surface sediments exhibit notable W enrichment, primarily associated with Mn-oxides. Tungsten appears to be delivered alongside Mn but does not undergo burial under anoxic or euxinic conditions, unlike its geochemical counterpart, Mo. The tungsten output flux into total Mn oxides is estimated at 22.2 × 10⁶ mol/yr. Authigenic W (excess W relative to the detrital input) exhibits heavier W isotope compositions compared to equilibrium adsorption on Mn oxides from modern seawater, averaging a δ^186/184W of 0.255 ± 0.025 ‰. This is likely due to fractionation processes during continuous exchange between pore-water and the solid phase, or the recrystallization of Mn oxides during diagenesis. Finally, we propose an updated mass budget of W and its isotopes in the modern ocean suggesting that the benthic W recycling could be an important, so far underestimated source of W to the ocean. The exclusive association between W and Mn suggests that W isotopes could serve as a valuable indicator for identifying deep-ocean oxygenation in Earth's history. To advance this idea, further comprehensive investigations into W isotopic fractionation across various Fe and Mn oxide minerals, as well as additional downcore studies in authigenic W enriched pelagic core sites, would be essential.