New insights into the formation and burial of Fe/Mn accumulations in Lake Baikal sediments

Lake Baikal is the deepest and oldest lake on Earth. Extraordinary features of the lake are manganese and iron‐enriched layers and crusts occurring at different depths within the sediment. They can be broadly subdivided into an upper accumulation at the O2/Mn(II) redox boundary and one or more layers buried within the reducing part of the sediment. The processes leading to their formation and peculiar distribution within the sediment have remained subject of debate, in particular whether the burial of vast amounts of Mn and Fe-oxides results from a steady-state process or if it is the consequence of singular events, such as changes in sedimentation rate, bottom water oxygen concentrations, or the mass accumulation rate (MAR) of organic carbon (Corg), Mn or Fe. We retrieved short cores from the South basin, the North Basin, and Academician Ridge, determined sedimentation rates, contents of Corg, Mn and Fe, and estimated pore‐water fluxes from concentration profiles of O2, NO3−, Mn(II), Fe(II), SO42− and CH4. A consistent picture emerged from the data showing that the upper Fe/Mn layer formed at the lower end of the oxygen penetration depth as a dynamic pattern, moving upward with the growing sediment. Thereby, reductive dissolution of Mn(IV) occurred at the lower margin. Upward diffusing Mn(II) was oxidized with O2 forming the upper boundary of the Fe/Mn accumulation. The buried Fe/Mn layers were immobilized within the sediment and underwent slow reductive dissolution mainly driven by the anaerobic oxidation of CH4. The process leading to the detachment of the ‘active’ Fe/Mn layer from the top redox interface is not unambiguously clear. However, we suggest a cyclic pattern where the burial of a Fe/Mn layer is accompanied by the generation of a new enrichment at the O2/Mn(II) redox boundary, which is subsequently nourished by the slowly dissolving old layer.