Abstract

Lake Medard (LM), a post-mining lake in the Czech Republic with stratified, sulfate- and iron-rich bottom waters, serves as a natural laboratory to study sediment–water-interface (SWI) dynamics where steep aqueous redox gradients are present. This study investigates the interplay of redox conditions, microbial activity, and sedimentary processes, revealing that short-term Eh fluctuations (80–100 mV) in the bottom water significantly mobilize rare earth elements (REE) and influence the partitioning of other redox-sensitive elements such as vanadium (V) and arsenic (As) from reactive iron (Fe)- and manganese (Mn)-oxyhydroxides during early diagenesis. While carbonate phases like siderite primarily retain their REE signatures, they can incorporate REE released during these redox shifts. Spectroscopic analyses confirms the presence of FeOOH polymorphs (goethite and lepidocrocite) in organo-mineral aggregates in the upper sediments. Sequential extractions shows that under stronger reducing conditions (Eh ≈ –190 mV), As predominantly associates with carbonates, shifting to Fe(III)-oxyhydroxides at higher Eh (≈ –80 mV). Isotope analyses (δ¹³C) indicate that the bulk sediment carbonate is detrital, sourced from Miocene strata. Authigenic pyrite in LM sediments exhibits δ³⁴S_py values (–35.1 to –23.0‰), reflecting microbial sulfate reduction. However, the accumulation of the byproduct sulfide, and thus pyrite stabilization, is limited by the low availability of labile organic substrates and the reoxidation of sulfide by Fe(III)-oxyhydroxides. These findings highlight the sensitivity of geochemical signals in sediments to subtle redox shifts and improve our interpretation of ancient deposits formed under dynamic water column redox conditions.