Identification of winter biogeochemical connectivity in permafrost soils with silicon isotopes and redox-sensitive elements (Stordalen, Sweden)

Abstract

Climate change affects Arctic regions by exposing previously frozen permafrost to thaw and changing hydrological processes. As a result, permafrost soils in Arctic have recently developed unfrozen soil portions in winter. These unfrozen soil portions may increase the soil biogeochemical connectivity by creating lateral subsurface water flow, thereby contributing to the lateral transfer of nutrients including dissolved organic carbon. This winter connectivity is mainly expected if unfrozen soil portions are connected (open system). However, the proportion of connected (open system) relative to unconnected (closed system) unfrozen soil portions remains poorly quantified. Here, we investigate the silicon isotope composition (δ30Si) and the redox-sensitive element (e.g., Fe) concentrations in soil pore water collected from September to November 2021 on a natural gradient of permafrost degradation from a palsa (closed system) to a fen (open system) in Stordalen, Sweden. We use δ30Si measurements to distinguish between: a closed system in freezing soils where silicic acid concentration in soil pore water is increasing upon freezing, leading to amorphous silica precipitation that induces Si isotope fractionation due to the preferential incorporation of 28Si in colloidal amorphous silica; and an open system in freezing soils where silicic acid concentration in soil pore water is mixed with lateral contributions and amorphous silica precipitation is not induced. We then compare the evolution of the δ30Si values in soil pore waters where freeze-up has occurred (closed system) or where freeze-up is delayed or absent (open system). We couple our δ30Si data with variations in redox-sensitive element concentrations (e.g., Fe) to better constrain the biogeochemical connectivity with the atmosphere. The dual-approach of silicon isotope geochemistry with redox sensitive element analysis contributes to better understand the processes controlling the lateral transfer of water and nutrients from permafrost soils during winter months