Impact of freeze-thaw cycles on organic carbon and metals in waters of permafrost peatlands

Despite the importance of soil and surface waters freezing in permafrost landscapes, the behaviour of dissolved organic carbon (DOC), nutrients and metals during periodic freeze-thaw cycles (FTC) remains poorly known. The on-going climate warming is likely to increase the frequency of FTC in continental aquatic settings, which could modify the chemical composition of waters. In this study, we conducted 9 repetitive cycles of overnight freezing (similar to 20 degrees C) and 5 h thawing (4 degrees C) in the laboratory using representative 0.22 mu m-filtered waters from NE European permafrost peatland: leachates of vegetation and soil, and natural surface waters (depression, thermokarst lake and river). Only minor (<5%-15%) changes of DOC concentrations, SUVA(254) and molecular weight were observed in all leachates and the depression water. In contrast, several trace elements (Fe, Al, P, Mn, As, and REE) exhibited sizable variations during FTC (>10%). The leachates and the depression water were enriched in trace elements, whereas the thermokarst lake and the river demonstrated a decrease in concentration of Fe (similar to 39 and similar to 94%, respectively), Al (similar to 9 and similar to 85%), and Mn (similar to 10 and similar to 79%) during FTC. Overall, the observations demonstrated an increase in aliphatic low molecular weight organic matter (OM), and the precipitation of Fe, Al hydroxides and organo-mineral particles. Therefore, enhanced of frequency of FTC can favour the release of metals and toxicants from acidic OM-rich surface waters and maintain stable OM-metalscolloids in large lakes and rivers, thus regulating aquatic transport of DOC and metals from soils to the Arctic Ocean. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

While dissolved organic carbon concentrations remained relatively stable during freeze-thaw cycles, certain trace elements exhibited significant variations, with enrichment in leachates and depression water, but a decrease in thermokarst lake and river water. The observations suggest an increase in aliphatic low molecular weight organic matter, as well as the precipitation of Fe, Al hydroxides, and organo-mineral particles, indicating potential effects on aquatic transport of DOC and metals in permafrost regions.