Are recent changes in sediment manganese sequestration in the euxinic basins of the Baltic Sea linked to the expansion of hypoxia?

[摘要] Expanding hypoxia in the Baltic Sea over the past century has led to thedevelopment of anoxic and sulfidic (euxinic) deep basins that are onlyperiodically ventilated by inflows of oxygenated waters from the North Sea.In this study, we investigate the potential consequences of the expandinghypoxia for manganese (Mn) burial in the Baltic Sea using a combination ofpore water and sediment analyses of dated sediment cores from eight locations.Diffusive fluxes of dissolved Mn from sediments to overlying waters at oxic,hypoxic and euxinic sites are consistent with an active release of Mn fromthese areas. Although the present-day fluxes are significant (ranging up toca. 240 μmol m⁻² d⁻¹), comparison to published watercolumn data suggests that the current benthic release of Mn is small whencompared to the large pool of Mn already present in the hypoxic and anoxicwater column. Our results highlight two modes of Mn carbonate formation insediments of the deep basins. In the Gotland Deep area, Mn carbonates likelyform from Mn oxides that are precipitated from the water column directlyfollowing North Sea inflows. In the Landsort Deep, in contrast, Mn carbonateand Mn sulfide layers appear to form independently of inflow events, and arepossibly related to the much larger and continuous input of Mn oxides linkedto sediment focusing. Whereas Mn-enriched sediments continue to accumulate inthe Landsort Deep, this does not hold for the Gotland Deep area. Here, arecent increase in euxinia, as evident from measured bottom water sulfideconcentrations and elevated sediment molybdenum (Mo), coincides with adecline in sediment Mn content. Sediment analyses also reveal that recentinflows of oxygenated water (since ca. 1995) are no longer consistentlyrecorded as Mn carbonate layers. Our data suggest that eutrophication has notonly led to a recent rise in sulfate reduction rates but also to a decline inreactive Fe input to these basins. We hypothesize that these factors have jointlyled to higher sulfide availability near the sediment–water interfaceafter inflow events. As a consequence, the Mn oxides may be reductivelydissolved more rapidly than in the past and Mn carbonates may no longer form.Using a simple diagenetic model for Mn dynamics in the surface sediment, wedemonstrate that an enhancement of the rate of reduction of Mn oxides isconsistent with such a scenario. Our results have important implications forthe use of Mn carbonate enrichments as a redox proxy in marine systems.

[学科分类] 地球化学与岩石