The stable tungsten isotope composition of sapropels and manganese-rich sediments from the Baltic Sea

Manganese (Mn) oxides preferentially adsorb isotopically light tungsten (W). The global deposition of Mn oxides in (hyp)oxic marine settings represents the most plausible reason for an open ocean stable W isotope composition ( δ 186 / 184 W = +0.543 ± 0.046‰) that is distinctly heavier than the main input source of marine W, the upper continental crust ( δ 186 / 184 W of +0.080 ± 0.053‰). If so, the seawater stable W isotope composition might be intimately linked to the areal extent of (hyp)oxic conditions, whereby earliest increases in the seawater δ 186 / 184 W might indicate earliest increases in marine oxygen concentrations in Earth history. However, stable W isotope data for sediments that might preserve the variability in the seawater δ 186 / 184 W are rare. We present the first stable W isotope data for a sedimentary record from the Landsort Deep, Baltic Sea, covering the last ∼1,700 years. Within this basin, distinct vertical stratification causes bottom water hypoxia or even euxinia that alternated with fully oxic conditions. Sediments deposited during long-lasting fully oxic periods such as the Little Ice Age are dominated by detrital W and Mn components and show crust-like δ 186 / 184 W values between -0.008 and +0.112‰. In contrast, during periods of pronounced bottom water hypoxia (e.g. modern warm period), sediments are authigenically enriched in Mn and W with δ 186 / 184 W values up to +0.226‰. Smooth depth trends in δ 186 / 184 W during hypoxic periods reflect temporal changes in the inflow intensity of O2 bearing Baltic seawater. Elevated inflow rates increase the sedimentary δ 186 / 184 W mainly because a slight O2 contribution fosters Mn oxide shuttling near the sediment-seawater interface and the preferential scavenging of isotopically light W. Consequently, the δ 186 / 184 W of the bottom water and subsequently deposited Mn oxides shift towards higher values. Thus, the variation in sedimentary δ 186 / 184 W values is related to the extent of Mn oxide formation that in turn depends on changing marine redox conditions. This emphasises the potential of stable W isotopes as a new tracer for early Earth redox reconstructions.