Thermokarst processes increase the supply of stabilizing surfaces and elements (Fe, Mn, Al, and Ca) for mineral–organic carbon interactions

Abstract

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 CO 2 and CH 4 gas emissions on thermokarst process.

Organic Carbon Burial With Reactive Iron Across Global Environments

“…For example, ligand exchange processes likely dominate sorption in tropical forest soils, which are rich in minerals with protonated hydroxyl groups (Shen, 1999), and depleted in the 2:1 phyllosilicates that dominate OC sorption in temperate forest topsoils (Kaiser & Guggenberger, 2003). In systems with high OC:Fe ratios such as peatlands, co‐precipitation likely dominates OC‐Fe R interactions (Joss et al., 2022; Patzner et al., 2020; Riedel et al., 2013), while complexation is important in thawing permafrost soils (Monhonval et al., 2022). Terrestrial environments also differ from most marine settings in the regularity and intensity of redox fluctuations, as in the association with wetting and drying cycles that can act to break down OC‐Fe R (Bhattacharyya et al., 2018).…”

“…The association of OC to reactive metals, particularly to reactive iron (Fe R ) oxy(hydroxide) phases such as ferrihydrite, provides physical protection (Figure 2) and prevents microbiological degradation (Keil et al., 1994). Complexation of organic molecules with metals such as iron is another stabilization mechanism, albeit one that has mostly been studied in terrestrial environments (e.g., Lützow et al., 2006; Monhonval et al., 2022). Therefore, the binding of OC to Fe R (OC‐Fe R ) represents an efficient mechanism by which OC escapes early diagenetic degradation in marine sediments and is buried to depths that are not in diffusive or advective connection with the overlying water column.…”

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

“…In ice‐rich sediments (Figure 7a), the proportions of OC forming complexes with metals are comparable, but the potential for association with poorly crystalline Fe oxides is more variable. More specifically, pyrophosphate‐extracted carbon (C p ) concentrations are in the same range between (i) the two ice complex units in Batagay (UIC—or Yedoma—and LIC), (ii) circum‐Arctic Yedoma sediments [92], (iii) undisturbed Yedoma in Yukechi [98], and (iv) Pleistocene‐aged ice‐rich tills in the Peel Plateau [46], even though there is more variability in the Yedoma at the Arctic scale (Table S3). The proportion of OC in the form of complexes with metals (C p /TOC; Figure 7a; Table S3) is also similar for the different studies.…”

Organic carbon burial with reactive iron across global environments

“…The association of OC to reactive metals, particularly to reactive iron (Fe R ) oxy(hydroxide) phases such as ferrihydrite, provides physical protection (Figure 2) and prevents microbiological degradation (Keil et al, 1994). Complexation of organic molecules with metals such as iron is another stabilization mechanism, albeit one that has mostly been studied in terrestrial environments (e.g., Lützow et al, 2006;Monhonval et al, 2022). Therefore, the binding of OC to Fe R (OC-Fe R ) represents an efficient mechanism by which OC escapes early diagenetic degradation in marine sediments and is buried to depths that are not in diffusive or advective connection with the overlying water column.…”

“…For example, ligand exchange processes likely dominate sorption in tropical forest soils, which are rich in minerals with protonated hydroxyl groups (Shen, 1999), and depleted in the 2:1 phyllosilicates that dominate OC sorption in temperate forest topsoils (Kaiser & Guggenberger, 2003). In systems with high OC:Fe ratios such as peatlands, co-precipitation likely dominates OC-Fe R interactions (Joss et al, 2022;Patzner et al, 2020;Riedel et al, 2013), while complexation is important in thawing permafrost soils (Monhonval et al, 2022). Terrestrial environments also differ from most marine settings in the regularity and intensity of redox fluctuations, as in the association with wetting and drying cycles that can act to break down OC-Fe R (Bhattacharyya et al, 2018).…”