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Geochemical focusing and burial of sedimentary iron, manganese, and phosphorus during lake eutrophication

The redistributive transport of sedimentary, redox‐sensitive metals such as iron (Fe) and manganese (Mn) along depth gradients in lakes or marine basins, and in particular the concentration of these elements at local depressions, has been described as “geochemical focusing.” However, the impact of variable redox conditions at the sediment–water interface on this process is not entirely understood. In this study, we report that the enrichment and burial of Fe, Mn, and associated phosphorus (P) in dimictic Lake Arendsee, Germany, has been affected by a transition from oligo‐mesotrophic to eutrophic conditions in the first half of the 20th century. Eleven sediment cores (<50 cm long) were obtained from water depths ranging from 38.5 to 49.5 m, and analyzed for their elemental composition. The older, non‐varved section, which was deposited prior ca. 1940 during oligo‐mesotrophic conditions, contains up to 10 times higher Mn and 2.5 times higher Fe contents at the deep sites compared to corresponding intervals at shallower sites. Due to their high Fe content, sediments in this interval also contain up to eight times more P, at least partly owing to vivianite authigenesis. Burial of Mn occurred via Mn‐carbonate authigenesis, possibly as rhodochrosite. Since ca. 1940, calcite and diatom varves have formed under eutrophic conditions, whereas Fe enrichment decreased and finally stopped due to fixation as pyrite in the shallow sites. Our results suggest that redox‐controlled geochemical focusing determines the distribution of Mn and Fe, as well as the occurrence of authigenic minerals such as vivianite and Mn‐carbonate.