Massive Mn carbonate formation in the Landsort Deep (Baltic Sea): Hydrographic conditions, temporal succession, and Mn budget calculations

The sediments of the Landsort Deep and Gotland Basin in the central Baltic Sea are strongly enriched in Mn carbonate. However, conceptual models attempting to explain the intense Mn carbonate precipitation in both basins are in part conflicting. In the Gotland Basin model, deposited Mn oxides are converted to Mn carbonate after the oxygenation of euxinic bottom waters enriched in dissolved Mn by major Baltic inflows (Huckriede and Meischner, 1996). By contrast, according to the Landsort Deep model, Mn carbonate precipitation occurs independent of oxygenation events below a euxinic water column (Lepland and Stevens, 1998; Lenz et al., 2015). In this study, we investigated Mn solid-phase signatures in recent/sub-recent well-dated sediments from the Landsort Deep and compared them to long-term observations to identify the hydrographic conditions favoring Mn carbonate formation. The comparisons of water column O2 and sulfide time series with sedimentary Mn carbonate enrichments identified long-lasting bottom water oxygenation as an important environmental factor in the enhancement of Mn carbonate precipitation (up to 32wt% Mn) in the Landsort Deep. Thus, by preventing the escape of dissolved Mn from still reducing sediments into the open water column, these conditions allow the accumulation of large amounts of Mn-oxide particles at the sediment/water interface, with their subsequent conversion to Mn carbonate. The euxinic conditions that have prevailed almost continuously in the Landsort Deep since roughly AD 2000 do not favor Mn enrichment (maximum 0.9wt% Mn), highlighting the importance of the recurring oxygenation of bottom water, most likely via medium-intensity inflows. The Mn abundances in seven sediment cores from water depths of 190–437m together with Mn balance calculations indicated that the Mn inventory in the water column in response to porewater Mn reflux and detrital Mn input is sufficient for Mn carbonate enrichment only in the deepest part of the basin.