| Summary: | Silicon isotope fractionation upon amorphous silica precipitation is sensitive to freeze-thaw cycles in arctic soils that are composed of carbon-rich permafrost (i.e. soil layer that remains frozen for at least two consecutive years) covered by an active layer (i.e. soil layer that thaws in summer in freezes in winter). The consequences of permafrost thaw for organic carbon decomposition are mainly studied during the growing season in summer, considering the soil as inert in winter. Here we show that biogeochemical processes involving organic carbon are present in early winter. We couple silicon isotopes with iron and dissolved organic carbon concentration measurements in soil pore water along a natural gradient of permafrost degradation (intact, intermediate, and thawed sites) and on a downstream river in Stordalen (Sweden) collected over two months during late summer and early winter. The data support that: (i) annual freeze-thaw cycles drive soil-water interaction and biogeochemical processes mostly at the intact site; (ii) early winter snowmelt results in soil water infiltration, water table increase and the dissolution of Fe-oxides at the intermediate site; (iii) early winter snow water infiltration increases lateral flow and export of dissolved organic carbon, especially between the thawed site and the downstream river. Combined, we show an extended period of soil-water interaction in early winter that destabilizes iron-organic carbon associations in permafrost soils and increases dissolved organic carbon transport to rivers.
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