Dissimilar behaviors of the geochemical twins W and Mo in hypoxic-euxinic marine basins

Abstract

Redox-sensitive trace metals are powerful tools for the reconstruction of modern and past redox conditions in aquatic ecosystems. The most prominent example is molybdenum (Mo), which behaves conservatively as soluble molybdate in the oxygenated ocean, but forms particle-reactive thiomolybdates at sufficiently high sulfide concentrations. Previous studies proposed that the redox behavior of tungsten (W), the geochemical twin of Mo, is also affected by sulfide, suggesting its potential as a promising new redox proxy. Here we present a comprehensive W dataset from the highly sulfidic Black Sea and the weakly sulfidic Landsort Deep (Baltic Sea), as the type localities of modern euxinic basins, that combines water column, pore water, and sediment records. A dataset from the Gotland Basin obtained following oxygenation of the formerly euxinic water column during the major Baltic inflow in 2014 is used as the hypoxic example. The even stronger adsorption of W than Mo on Mn and Fe oxides at pelagic redoxclines identified these particles as important carrier transferring W from the oxygenated surface ocean to euxinic bottom waters. This shuttling was most pronounced during the inflow event in the Gotland Basin causing substantial deposition of Mn oxides that were highly enriched in W. Compared to the oxygenated surface waters, dissolved Mo was severely depleted in the highly sulfidic water column of the Black Sea, whereas the bottom water levels of dissolved W were elevated. An additional W source derived from pore-water reflux, as indicated by the exceptionally high concentrations of W in highly sulfidic pore waters from the Landsort Deep sediments. In addition to the decoupling from Mn, the pore water W enrichments are compatible with the previously proposed greater solubility of thiotungsate species than of tungstate. Despite the absence of a substantial sedimentary accumulation of authigenic W in the sapropels of the Black Sea, the comparable patterns of S, Fe, and W in an Eemian record may suggest sorption of internally cycled (thio)tungstate onto pyrite. Unexpected W enrichments in modern and medieval sediments from the central Landsort Deep indicate a close relationship with Mn carbonate formation occurring during long-lasting bottom water hypoxia. In contrast to the highly variable Mo content, the near-background variation of W in the sapropels of euxinic basins questions the utility of W as a redox proxy in euxinic systems. However, given the strong affinity between W and the Mn/Fe cycle, this element may be a useful proxy in hypoxic systems, especially its isotopic signature.