Behaviors of redox-sensitive tungsten and molybdenum in the northern South China Sea: From the Pearl River to the continental slope

To increase the proxy toolbox for the reconstruction of redox conditions, the transition metal tungsten (W) has been increasingly suggested as a promising new candidate during the last few years. However, compared to other trace metals such as molybdenum (Mo), studies on the redox behavior of W in the brackish/marine environment are relatively rare and mainly limited to euxinic settings. To contribute to our understanding of the behavior of W in modern aquatic ecosystems, we here present a water column W and Mo dataset that follows the salinity gradient of the Pearl River Estuary towards the adjacent coastal and offshore waters of the northern South China Sea (water depths range from 36 m to 3735 m) with the latter also including mostly anoxic porewater and rather oxic sediment surface data. Except for deviations of W in the low-salinity reaches of the Pearl River Estuary, both metals are conservatively introduced to the coastal ocean. The contribution of dissolved W via the Pearl River plume was identified in surface waters along the coastline. In line with previous open ocean studies, the conservative behavior of W proceeds towards the remote parts of the study area with surface waters that are possibly also affected by the South China Sea Warm Current. Within the short core porewater profiles, W concentrations increased with sediment depth and reached the highest levels at the more distal sites. These trends are not linked to the formation of more soluble thiotungstate species as indicated by generally non-sulfidic porewaters. Along with porewater gradients of W, the W concentrations in near-bottom waters suggest effective trapping of W at the sediment/water interface by Mn oxides especially at greater water depths and limited contribution to the open water column W inventory. In contrast to W, Mo concentrations show little variation in the reducing porewaters and remain close to or slightly above the ocean level. Enrichments of W and Mo in the surface sediments from the upper and lower continental slope reflect the known affinity of both trace metals to Mn oxides. Positive correlations of W with the heavy mineral-associated metals titanium (Ti) and zirconium (Zr) in sand-rich deposits of the outer continental shelf subjected to stronger near-bottom currents, further indicate a relation of sedimentary W abundances to the depositional environment.