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 (delta W-186/184 = +0.543 +/- 0.046 parts per thousand) that is distinctly heavier than the main input source of marine W, the upper continental crust (delta W-186/184 of +0.080 +/- 0.053 parts per thousand). 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 delta W-186/184 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 delta W-186/184 are rare. We present the first stable W isotope data for a sedimentary record from the Landsort Deep, Baltic Sea, covering the last similar to 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 delta W-186/184 values between -0.008 and +0.112 parts per thousand. In contrast, during periods of pronounced bottom water hypoxia (e.g. modern warm period), sediments are authigenically enriched in Mn and W with delta W-186/184 values up to +0.226 parts per thousand. Smooth depth trends in delta W-186/184 during hypoxic periods reflect temporal changes in the inflow intensity of O-2 bearing Baltic seawater. Elevated inflow rates increase the sedimentary delta W-186/184 mainly because a slight O-2 contribution fosters Mn oxide shuttling near the sediment-seawater interface and the preferential scavenging of isotopically light W. Consequently, the delta W-186/184 of the bottom water and subsequently deposited Mn oxides shift towards higher values. Thus, the variation in sedimentary delta W-186/184 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. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Manganese oxides preferentially adsorb light tungsten isotopes in marine environments, leading to heavier W isotope compositions in seawater. The stable W isotope data in sediments are related to the oxygenation level of seawater, indicating its potential as a tracer for early Earth redox reconstructions.