Manganese authigenesis in the Landsort Deep, Baltic Sea

Manganese mineralogy, because of its redox sensitivity, is a valuable parameter for interpreting water-column anoxia and sediment diagenesis. In the Baltic, and especially the Landsort Deep, the relatively widespread occurrence of Mn-carbonates and the unique observations of authigenic Mn-sulphides in sedimentary environments need to be accounted for within the models proposed for Mn authigenesis in diverse settings. In this study stratigraphic changes in the geochemistry and mineralogy are documented in two gravity cores using petrographic microscopy, XRD, SEM-EDS, analyses of C and S content, 13C isotopes, ICP-MS cation determinations, and electron microprobe element profiling. The lamination in the bottom sediments of the Landsort Deep suggests essentially continuous anoxic conditions at the sediment surface during the last ca. 4500 14C years. The most striking variations in the sediments are related to the occurrence of two, Mn(II)-precipitate-rich, sapropel intervals (recent and 1390–1990 14C years BP). Sapropel deposition is associated with relatively slow detrital sedimentation, allowing the concentration of organic matter. The sediment Corg content is influenced by the variations in the primary production as well as the supply of terrestrial Corg, but a major control appears to be detrital dilution. The precipitation of Mn-carbonate as individual aggregates of spherical particles, dispersed within clayey sediment matrix, and as laminae with semi-seasonal cyclicity is believed to be largely induced by the high alkalinity in connection with organic matter deposition and degradation. Where H2S has exceeded Fe availability, MnS has also precipitated, most commonly as well developed, hexagonal crystals, but also as the cubic variety, alabandite, which has not previously been reported from a sedimentary environment. An expansion of the anoxic bottom-water conditions during sapropel deposition is interpreted by correlation with laminated intervals on the slope of the deep basin. Budget calculations for the supply of Mn to the basin during Mn-rich sapropel formation can be better balanced by the addition of Mn from dissolution of oxides formed prior to the anoxic expansion. The importance of alkalinity for the precipitation of Mn-carbonates and sulphides does not exclude the need for considerable Mn concentrations, but the model argued in several recently published studies, stressing Mn availability as the major control, does not seem appropriate for the Landsort Deep, and may be too restrictive in many other settings as well.