Redox-dependent changes in manganese speciation in Baltic Sea sediments fromthe Holocene ThermalMaximum: An EXAFS, XANES and LA-ICP-MS study

摘要

Manganese (Mn) enrichments in sediments of the deep basins of the Baltic Sea are believed to consist of Ca–Mn– carbonates that form from Mn oxides following periodic inflows of oxygen-rich North Sea waters. However, a range of Mn-bearing mineral phases, that besides Mn-carbonates (e.g. Ca-rhodochrosite), can include Mnsulfides are known to be present in marine sediments, with formation mechanisms that are sensitive to redox conditions. In this study, we use high resolution synchrotron EXAFS and XANES combined with LA-ICP-MS and micro-XRF, to investigate the nature of theMn enrichments in sediments fromthe Holocene ThermalMaximum (approx. 8000–4000 cal. yr BP) at a site in the northern Gotland Basin. Analyses were performed on epoxyembedded sediment sequences.Wespecifically address the role of changes in redox conditions in bottomwaters, as inferred fromsedimentmolybdenum(Mo) contents, forMnsequestration.Wefind that an up-core increase in Mo, indicating a transition into more anoxic and sulfidic (euxinic) bottomwater conditions, is accompanied by a decline in total sediment Mn contents. While Mn-carbonates dominate the Mn mineral fraction in the low-Mo interval, in the more sulfidic, high Mo interval, Mn is associated with framboidal pyrite. Mn/Fe ratios in the sulfidic interval vary between the investigated sequences but reach values of up to 7.7 mol%. This exceeds ratios previously reported for sedimentary pyrite and EXAFS spectra indicate that sulfide bound Mn is predominately tetrahedrally coordinatedwhenMn/Fe ratios are high. This suggests thatMn is incorporated in other Fe–S phases such as mackinawite besides pyrite. There is also evidence for the presence of a low concentration of Mn associated with detrital aluminosilicates throughout the investigated intervals. Our results suggest that increased bottom water euxinia inhibited the formation of Mn-carbonate at this site during the Holocene Thermal Maximum, possibly due to a more rapid reduction of Mn oxides in more sulfidic bottom waters.