Iron Redistribution Upon Thermokarst Processes in the Yedoma Domain

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.

Seasonal Fluctuations in Iron Cycling in Thawing Permafrost Peatlands

“…With permafrost thaw, this mineral-bound OC is mobilized by reductive dissolution of Fe(III) minerals promoted by Fe(III)-reducing bacteria under water-logged and oxygen-limited conditions. 15 , 17 The resulting dissolved OC (DOC) can then be further metabolized and can lead to GHG emissions during permafrost thaw. 18 …”

Mineral Element Stocks in the Yedoma Domain: A Novel Method Applied to Ice-Rich Permafrost Regions

“…Iron oxides are also known to contribute to the formation of aggregates (Eusterhues et al, 2005;Kleber et al, 2015). Changing Fe/ TOC ratio between Yedoma and Alas in Alaska (Figures 7A,B) reinforces the need to further investigate the potential role of the evolution of aggregates for OC availability upon thawing (Monhonval et al, 2021). Changing conditions for mineral protection of OC upon thawing is likely to influence OC microbial degradation (Gentsch et al, 2015;Herndon et al, 2017;Kögel-Knabner et al, 2010;Opfergelt, 2020), thereby contributing to modulate the permafrost carbon feedback (Schuur et al, 2015).…”

Divergent changes in particulate and mineral-associated organic carbon upon permafrost thaw

“…Interestingly, this enrichment in OC-Fe after upland thermokarst distinctly differs from the observations from lowland thermokarst in the Arctic, which have revealed reduced OC-Fe upon permafrost thaw 28 . Such a discrepancy could be largely due to the different trajectories of soil moisture after thermokarst formation 56 – 59 , which acts as a critical driver for Fe(III) reduction and Fe(II) oxidation 28 , 58 – 60 . Specifically, water-logging is generally induced in lowland regions 28 , 61 , 62 , while soil drainage often occurs in upland areas upon permafrost collapse 12 , 13 , 63 .…”