Stable tungsten isotope systematics on the Earth’s surface

Abstract

Stable tungsten isotopes (δ186/184W) show great potential for tracing the cycling of materials among geological reservoirs. This work investigates the behavior of stable W isotopes on the Earth’s surface by presenting new δ186/184W data of granitic rocks, loess, river water, fluvial sediments, regolith, estuarine seawater, and marine sediments. The investigated granites document significant W isotopic heterogeneity (−0.08 to 0.16‰) of the upper continental crust (UCC). In contrast, the eolian loess sourced from the vast crustal surface shows a high degree of homogeneity with an average δ186/184W of 0.01 ± 0.01‰ (mean ± 2 × standard error). The δ186/184W of river water (0.17 to 0.71‰) is significantly higher than that of the corresponding bedrock, which is likely to be caused by preferential uptake of light W isotopes by the authigenic precipitates. The δ186/184W and W concentration of river water also show a correlation with the lithological sources of weathering solute, the extrapolation of which results in a global riverine W flux of 17 × 106 mol yr−1 and a W flux-weighted average δ186/184W of 0.37‰. Data from the Yangtze estuary likely suggest a contribution of benthic W reflux from estuarine sediments to the ocean. However, benthic reflux, as well as hydrothermal W inputs, is insignificant compared to riverine input. The recalculated oceanic residence time of W is ∼4 kyrs, which is much shorter than previously thought. Sinks of W in the ocean are mainly associated with non-euxinic sediments, where the enrichment of W correlates with that of Mo. Constraints from the oceanic Mo budget give a W sink of 19 × 106 mol yr−1 in non-euxinic sediments, which nearly balances the riverine input. A simple box model shows that the steady-state δ186/184W of seawater (which can be achieved on time scales more than several times the residence time of W in seawater) is mainly controlled by the δ186/184W of riverine input and the partition of W into different sinks with varying fractionation factors.