Description: Mineral-organic carbon (OC) interactions account for 30 – 80 % of the total permafrost OC pool. Quantifying the nature and controls of mineral-OC interactions is necessary to better assess permafrost-carbon-climate feedbacks. This is particularly true for ice-rich environments that are impacted by rapid thaw and the development of thermokarst landforms. Retrogressive thaw slumps are amongst the most dynamic forms of slope thermokarst and they expand through the years due to the ablation of an ice-rich headwall. These phenomena are important to consider in the permafrost carbon budget since they expose deep OC sometimes tens of thousands of years old that would not have re-entered the modern carbon cycle if these disturbances had not occurred. Here, we analyzed sediment samples collected from the headwall of the Batagay megaslump, East Siberia, locally reaching 55 m high. The series of discontinuous deposits comprises also older sediment up to ~650 ka old. We present total element concentrations, mineralogy, and mineral-OC interactions in the different stratigraphic units. The mineralogy in the deposits is very similar across the sedimentary series. Our data show that up to 34 ± 8 % of the total OC pool is stabilized by mineral-OC interactions. For most of the analyzed samples, associations to poorly crystalline iron oxides do not have a significant role in OC stabilization. Hypothesizing a retreat rate of 26000 m²/yr and constant thickness of stratigraphic units within the headwall, we provide a first order estimate of ~2 × 10^7 kg of OC is exported annually downslope of the headwall, with ~ 38 % being geochemically stabilized by complexation with metals or associations to poorly crystalline iron oxides. These data support that more than one third of the organic carbon exposed by this massive thaw slump is not directly available for mineralization, but rather stabilized geochemically.

Description: The stabilizing properties of mineral–organic carbon (OC) interactions have been studied in many soil environments (temperate soils, podzol lateritic soils, and 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 OC is buried in deep sediments. Interactions between minerals and OC are important because OC is located very near the mineral matrix. Mineral surfaces and elements could mitigate recent and future greenhouse gas emissions through physical and/or physicochemical protection of OC. The dynamic changes in 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 on lake formation (promoting reduction reactions), and this was largely balanced by an increase in the amount of reactive metals in the deposits on lake drainage (promoting oxidation reactions). We demonstrate an increase in the metal to C molar ratio on thermokarst process, which may indicate an increase in 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 on thermokarst process.

Description: A compact scanning force microscope has been developed for in situ combination with nanofocused X-ray diffraction techniques at third-generation synchrotron beamlines. Its capabilities are demonstrated on Au nano-islands grown on a sapphire substrate. The new in situ device allows for in situ imaging the sample topography and the crystallinity by recording simultaneously an atomic force microscope (AFM) image and a scanning X-ray diffraction map of the same area. Moreover, a selected Au island can be mechanically deformed using the AFM tip while monitoring the deformation of the atomic lattice by nanofocused X-ray diffraction. This in situ approach gives access to the mechanical behavior of nanomaterials.