Lay summary
The aim of this project is the investigation of the geochemical lifecycle and the mineral structures of the authigenic ferromanganese layers occurring in the sediments of Lake Baikal. The riddle to be solved is: are the Fe/Mn layers formed under steady-state diagenetic conditions of reductive dissolu-tion and oxidative precipitation, or are they an indicator of changing sedimentary inputs? In the former case such concretions represent ideal model systems to study the kinetics and mechanisms of geo-chemical processes at redox boundaries. In the second case the prevalence of recent and older relict layers could be used as proxy indicators for environmental change in this unique continental archive.The Fe/Mn precipitates initially form at the redox interface of lacustrine sediments. In Lake Baikal, re-mains of such layers are buried in the sediment and bear the possibility to be used as proxy indicators of past global changes in climatic conditions, as relics of them have been found in sediment layers as old as 65'000 (and up to 85'000) years. At present it is not clear under what circumstances the crusts are buried. Among the possible forcing factors are: change of the MAR of organic carbon, change of the sedimentation rate and/or the fraction of allochthonous input changing the MAR of Fe and Mn, a decline in the provision of the bottom water with oxygen, deposition of turbidites, increase of crust density and subsequent decrease of sediment porosity and, thus, diffusivity due to ageing, and others.We will gather information on the dissolution, precipitation, and the timescale of diffusive processes within the sediment by sampling of sediment and porewater with high spatial resolution and subse-quent chemical analyses. Major and trace metal contents of undisturbed core sections will also be analyzed with spatial resolution in the sub-millimeter range by scanning methods such as micro-X-ray fluorescence spectrometry (µXRF) and laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS). In addition, solid phases will be characterized with X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) to gain information on the speciation and main mineral forms of Fe and Mn across the redox interface, and associated trace elements. With the rate estimates of diagenetic formation and dissolution of Fe/Mn layers, and the vertically changing diffusion coefficients of Fe2+ and Mn2+ across the Fe/Mn layers, the temporal dynamics of the formation and dissolution of Fe/Mn layers, and their dependence on key parameters will be evalu-ated using a dynamic sediment model.