Rare earth elements in the sediments of Lake Baikal

Lake Baikal is the deepest and probably oldest lake on Earth. Its water column is pervasively oxic and sedimentation rates are very low which leads to the formation of a dynamic iron (Fe) and manganese (Mn) enrichment below the Mn(II)/O2 boundary. These often massive accumulations can be buried within the reducing part of the sediments and give rise to complex and cryptic redox cycles. The mobility of rare earth elements (REEs) is influenced by the dissolution and reduction dynamics of the ferromanganese oxides. The present study offers an overview of the REE chemistry in Lake Baikal and its catchment area and more specifically REE distribution in five 11- to 26-cm-long sediment cores situated across the lake at different water depths. We analysed and discussed normalised REE patterns and their consequential cerium (Ce) anomalies. While particulate REE concentrations are mainly influenced by processes above or near the surface of Lake Baikal, such as the development of a widespread negative Ce anomaly, processes occurring during early diagenesis in the sediment are most reflected in pore water REEs. The dissolution of ferromanganese oxides at the Mn(II)/O2 boundary remobilizes significant amounts of REE into the pore water whereby some are likely adsorbed onto colloidal Fe oxides. However, besides the tendency of Ce being associated with Mn-oxides, pore water REEs fractionate predominantly around the buried Fe/Mn accumulation where light REEs preferentially adsorb onto Fe-oxides.