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The stabilizing properties of mineral-organic carbon interactions have been studied in many soil environments (temperate soils, podzol lateritic soils, paddy soils). Recently, interest in their role in permafrost regions is increasing as permafrost was identified as a hotspot of change. In thawing ice-rich permafrost regions, such as the Yedoma domain, 327-466 Gt of frozen organic carbon (OC) is buried in deep sediments. Interactions between minerals and OC are important since the OC is located in close contact with the mineral matrix. Mineral surfaces and elements could mitigate recent and future greenhouse gas emissions through physical and/or physico-chemical protection of OC. The dynamic changes of redox and pH conditions associated with thermokarst lake formation and drainage, trigger metal-oxide dissolution and precipitation, likely influencing OC stabilization and microbial mineralization. However, the influence of thermokarst processes on mineral-OC interactions remains poorly constrained. In this study, we aim to characterize Fe, Mn, Al and Ca minerals and their potential protective role for OC. Total and selective extractions were used to assess the crystalline and amorphous oxides or complexed metal pools as well as the organic acids found within these pools. We analyzed four sediment cores from an ice-rich permafrost area in Central Yakutia, which were drilled i) in undisturbed Yedoma uplands, ii) beneath a recent lake formed within Yedoma deposits, iii) in a drained thermokarst lake basin, and iv) beneath a mature thermokarst lake from the early Holocene period. We find a decrease in the amount of reactive Fe, Mn, Al and Ca in the deposits upon lake formation (promoting reduction reactions), and this was largely balanced by an increase in the amount of reactive metals in the deposits upon lake drainage (promoting oxidation reactions). We demonstrate an increase in the metal:C molar ratio upon thermokarst process, which may indicate an increase of metal-C bindings and could provide a higher protective role against microbial mineralization of organic matter. Finally, we find that an increase in mineral-OC interactions corresponded to a decrease in CO2 and CH4 gas emissions upon thermokarst process. Mineral-OC interactions could mitigate greenhouse gas production from permafrost thaw as soon as lake drainage occurs.

Abrupt permafrost thaw leads to changing redox conditions (Figure 1), thereby affecting manganese forms and their potential role in organic carbon stabilization. We analyzed four sediment cores from Central Yakutia (Russia) representing progressive thermokarst processes, from continuously frozen Yedoma deposits (Windirsch et al., 2020) to an Alas lake in a Holocene thermokarst basin that underwent multiple lake generations (Jongejans et al, 2021). We analyzed total manganese concentrations and manganese selectively extracted with dithionite-citrate-bicarbonate (DCB), oxalate and pyrophosphate. In addition, we analyzed organic carbon selectively extracted by oxalate and pyrophosphate. We also determined the main crystalline minerals by X-ray diffraction analyses. We found that i) selective extractions by DCB, oxalate and pyrophosphate are valid for quantifying manganese phases, and ii) thermokarst processes influence interactions between manganese and organic carbon in ice-rich permafrost.

Permafrost are permanently frozen ground found in northern latitudes. Deposits in permafrost regions store significant amounts of carbon, estimated at 1400 - 1600 GtC. Yedoma deposits, defined as ice-rich permafrost, contribute significantly to this carbon stock. With global warming, Yedoma deposits are likely to undergo abrupt thaw leading to thermokarst collapse, resulting in the formation of Alas deposits. The organic carbon stored in these frozen deposits could be exposed to microbial mineralization upon thaw, be released as CO2 or CH4 in the atmosphere and amplify global warming. However, many uncertainties remain on the subject, including the role of mineral elements on the mineralization rate of this carbon. This study focuses on the evolution of the different forms of iron (i.e., crystalline, amorphous or complexed forms) during abrupt thaw of an ice-rich permafrost profile and its impact on organic carbon release. We analyzed four sediment cores from central Yakutia (Siberia) representative for progressive stages of thermokarst processes (i.e., Yedoma dry, Yedoma lake, Alas dry and Alas lake). The concentrations of total iron and iron selectively extracted using dithionite-citrate-bicarbonate (DCB), ammonium oxalate, and Na-pyrophosphate were measured. The organic carbon bound to these iron pools was also selectively extracted. We found i) a lower concentration of well-crystallized iron in Yedoma deposits that have already undergone abrupt thawing during the Holocene, and ii) a lower concentration of complexed carbon in samples underneath lakes, under anoxic conditions, compared to samples from drier areas (Figure 1).