Abstract
Ice-rich permafrost has been subject to abrupt thaw and thermokarst formation in the past and is vulnerable to current global warming. The ice-rich permafrost domain includes Yedoma sediments that have never thawed since deposition during the late Pleistocene and Alas sediments that were formed by previous thermokarst processes during the Lateglacial and Holocene warming. Permafrost thaw unlocks organic carbon (OC) and minerals from these deposits and exposes OC to mineralization. A portion of the OC can be associated with iron (Fe), a redox-sensitive element acting as a trap for OC. Post-depositional thaw processes may have induced changes in redox conditions in these deposits and thereby affected Fe distribution and interactions between OC and Fe, with knock-on effects on the role that Fe plays in mediating present day OC mineralization. To test this hypothesis, we measured Fe concentrations and proportion of Fe oxides and Fe complexed with OC in unthawed Yedoma and previously thawed Alas deposits. Total Fe concentrations were determined on 1,292 sediment samples from the Yedoma domain using portable X-ray fluorescence; these concentrations were corrected for trueness using a calibration based on a subset of 144 samples measured by inductively coupled plasma optical emission spectrometry after alkaline fusion (R2 = 0.95). The total Fe concentration is stable with depth in Yedoma deposits, but we observe a depletion or accumulation of total Fe in Alas deposits, which experienced previous thaw and/or flooding events. Selective Fe extractions targeting reactive forms of Fe on unthawed and previously thawed deposits highlight that about 25% of the total Fe is present as reactive species, either as crystalline or amorphous oxides, or complexed with OC, with no significant difference in proportions of reactive Fe between Yedoma and Alas deposits. These results suggest that redox driven processes during past thermokarst formation impact the present-day distribution of total Fe, and thereby the total amount of reactive Fe in Alas versus Yedoma deposits. This study highlights that ongoing thermokarst lake formation and drainage dynamics in the Arctic influences reactive Fe distribution and thereby interactions between Fe and OC, OC mineralization rates, and greenhouse gas emissions.
Highlights
Upon ice-rich permafrost thaw, substantial amounts of organic carbon (OC) stored in frozen deposits become potentially available for microbial mineralization (Strauss et al, 2017; Nitzbon et al, 2020; Turetsky et al, 2020)
To test the influence of permafrost thaw on the distribution of Feoxides and on Fe involved in complexes, we performed selective Fe extractions on a subset of samples from Yedoma and Alas deposits (n 21) from three locations in Siberia: Sobo Sise Island, Buor Khaya Peninsula and Kytalyk
The dispersion (i.e., median absolute deviation (MAD)) of total Fe concentrations is more than two times larger for Alas deposits compared to Yedoma deposits (Figure 4)
Summary
Upon ice-rich permafrost thaw, substantial amounts of organic carbon (OC) stored in frozen deposits become potentially available for microbial mineralization (Strauss et al, 2017; Nitzbon et al, 2020; Turetsky et al, 2020). The OC can be 1) physically protected within soil aggregates (involving Fe-Al oxy-(hydr)oxides, clay minerals, or carbonates), which means that OC is spatially inaccessible for microorganisms; or 2) physico-chemically protected in organomineral associations and/or as organo-metallic complexes. These organo-mineral associations result from the interaction of OC with mineral surfaces such as OC adsorbed onto Fe-oxides or clay minerals, using cation bridges such as Ca2+ or Mg2+. Several stabilizing mechanisms exist between reactive Fe (Feoxides or complexed Fe) and OC
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