More than one third of the organic carbon exposed by the world’s largest thaw slump (Batagay, Siberia) is not directly available for mineralization but geochemically stabilized

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

A Third of Organic Carbon Is Mineral Bound in Permafrost Sediments Exposed by the World's Largest Thaw Slump, Batagay, Siberia

Organic carbon (OC) in permafrost interacts with the mineral fraction of soil and sediments, representing < 1% to ~80% of the total OC pool. Quantifying the nature and controls of mineral-OC interactions is therefore crucial for realistic assessments of permafrost-carbon-climate feedbacks, especially in ice-rich regions facing rapid thaw and the development of thermo-erosion landforms. Here, we analyzed sediment samples from the Batagay megaslump in East Siberia, and we present total element concentrations , mineralogy, and mineral-OC interactions in its different stratigraphic units. Our findings indicate that up to 34 ± 8% of the OC pool interacts with mineral surfaces or elements. Interglacial deposits exhibit enhanced OC-mineral interactions, where OC has undergone greater microbial transformation and has likely low degradability. We provide a first-order estimate of ~12,000 tons of OC mobilized annually downslope of the headwall (i.e., the approximate mass of 30 large aircrafts), with a maximum of 38% interacting with OC via complexation with metals or associations to poorly crystalline iron oxides. These data imply that over one-third of the OC exposed by the slump is not readily available for mineralization, potentially leading to prolonged OC residence time in soil and sediments under stable physicochemical conditions

Evolution of Fe oxides crystallinity in permafrost deposits from mid-Pleistocene to Holocene: implications for mineral organic carbon interactions

Mineral-organic carbon (OC) interactions are involved in the geochemical stability of OC and thus in the susceptibility of permafrost to release greenhouse gases. Those so-called stabilizing or protecting interactions - accounting for ~30 % to ~80 % of permafrost soil OC - includes organo-mineral associations, such as OC sorbed onto Fe-oxides, or organo-metallic complexes. Over timescales of soil development (millennia), the capacity of soils to stabilize OC is linked to soil development through changes in soil mineralogy. Specifically, weathering products such as short-range order minerals (e.g., poorly crystalline Fe-oxides) have an extensive surface area to bind OC. However, over time, these minerals evolve towards more crystalline phases with a lower surface area available to bind OC. Freezing conditions are considered to minimize changes in Fe oxides crystallinity at short time scale, but can we consider the mineralogy of Fe oxides in a frozen deposit as stable over millennial timescale? We investigate this question along a sequence of permafrost deposits from the headwall of the Batagay megaslump, Siberia, comprising sediment up to ~650 ka old. We analyzed the proportion of Fe as poorly crystalline and crystalline Fe oxides and organo-metallic complexes, and the proportion of total OC pool forming mineral-OC interactions in the different stratigraphic units. Our data show that: (i) the proportion of iron as poorly crystalline iron oxides significantly drops with increasing age of the deposit, from 28 ± 14% for Holocene deposits to 6 ± 2% for mid-Pleistocene deposits; (ii) the proportion of iron as crystalline oxides increases from 15 ± 20% to 34 ± 2% for the same deposits, respectively; (iii) the proportion mineral-bound OC relative to the total decreases over time from 45 ± 13% to 32 ± 6 %. These findings highlight that the mineral surfaces available for OC stabilization can evolve over millennial timescales in a frozen deposit. This raises the need to better constrain mineral OC interactions for older OC exposed by abrupt thawing of permafrost such as in megaslumps

Characterization of organic carbon-mineral interactions within a megaslump headwall and potential evolution following material export: case study in Batagaika crater, northern Yakutia, Siberia

Arctic is warming close to four times faster than the global average and, as a direct outcome, permafrost temperatures have increased by up to 0.39 ± 0.15 °C in the years 2007-2016. This increased warming is expected to generate a permafrost carbon feedback on the climate by promoting permafrost thaw and by emitting additional volumes of greenhouse gases into the atmosphere. Still, it is estimated that between 30% and 80% of soil organic carbon (OC) in permafrost is stabilized by geochemical interactions with mineral elements such as iron and thus less likely to be emitted as greenhouse gases. Quantifying the nature and controls of mineral-OC interactions is necessary to better frame permafrost-carbon-climate feedbacks, particularly in ice-rich environments that result in rapid thawing and the development of thermokarst landforms. Thaw slumps are amongst the most dynamic forms of slope thermokarst and expand through the years due to the ablation of an ice-rich headwall each summer. These phenomena are important to consider in the permafrost carbon budget since they expose a deep OC pool that may reach tens of thousands of years old and that would not have re-entered the modern carbon cycle if these disturbances had not occurred. Here, we collected samples from the Batagaika crater, Siberia, on a headwall reaching locally 55 m high - every half a meter for the upper 10 m of the headwall and then every meter. We present total element concentrations, mineralogy, and mineral-organic carbon interactions in the different stratigraphic units, i.e., from the top to the bottom, i) the organic surface layer, ii) the Holocene cover, iii) the upper ice complex, also called Yedoma, which is dominated by large ice wedges, iv) the woody debris layer which consists of macroscopic terrestrial plant remains, v) the lower sand unit of pore-ice-cemented aeolian-sourced fine sand, and vi) the lower ice complex which reveals ice-rich deposits of ice-wedges and provides access to ancient permafrost up to ~650 ka old. Our data show that the main mechanism of organic carbon stabilization through mineral-OC interactions is the complexation with metals, which stabilizes 35 ± 18% of the total organic carbon (TOC) pool. Associations to poorly crystalline iron oxides do not have a significant role in OC stabilization as we estimate a maximum of 5 ± 2% of TOC to be stabilized by this mechanism, with the exception of the Holocene cover which stabilizes up to 29 ± 14% of the TOC via associations with poorly crystalline iron oxide. From a budget perspective, we estimate that a mass of 1.65 × 107 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. Climatic and geochemical conditions at the time of deposition appear to be the key parameters influencing OC geochemical stability as the mineralogy in the deposits is very similar despite a sedimentary depositional series spanning ~650 ka old

More than one third of the organic carbon exposed by the world’s largest thaw slump (Batagay, Siberia) is not directly available for mineralization but geochemically stabilized

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