Sediment redox dynamics in an oligotrophic deep-water lake in Tierra del Fuego: insights from Fe isotopes

Fe speciation and Fe isotopes have been widely used to reconstruct past basin dynamics and water redox conditions. However, sedimentation and early diagenesis of such proxies eventually alter any primary climate signal. In this work, we disentangled the processes occurring at the redox front below the sediment-water interface of a ventilated deep-water lake (Lago Fagnano, Argentina/Chile). A sequential extraction protocol was applied to characterize two reactive Fe pools: Fe oxyhydroxides and reduced iron. Subsequently Fe isotopes were constrained to determine the main processes mobilizing iron. At the redox front, ferric minerals reach a δ56Fe of -1.3 ‰ resulting from fluid upward migration and oxidation of dissolved Fe and likely following a Rayleigh distillation effect. Right below, low ferric Fe concentrations indicate Fe reduction which fuels the dissolved Fe pool. Consequently, as the sedimentation proceeds, light Fe is in constant upward migration along with the redox front. However, an increased sedimentation (e.g., turbiditic events) interrupts this dynamic cycle and the Fe rich redox front can be partially preserved. Additionally, oxidation of dissolved Fe is compromised and can be recycled in the formation of ferrous minerals, such as Fe monosulfides and amorphous phases, which show light Fe isotopes reaching a δ56Fe of -1.7 ‰