ALBERT

Description: Iron (Fe) plays a key role in mediating organic carbon (OC) decomposition rates in permafrost soils. Fe-bearing minerals stabilize OC through complexation, co-precipitation or aggregation processes and thus hinder degradation of OC. In addition, Fe(III) reduction can inhibit methanogenesis and decrease warming potential of greenhouse gases release. Ice-rich permafrost is subject to abrupt thaw and thermokarst formation, which unlocks OC and minerals from deep deposits and exposes OC to mineralization. These ice-rich domains include Yedoma sediments that have never thawed since deposition and Alas sediments that have undergone previous thermokarst processes during the Lateglacial and Holocene warming periods. The post-depositional history of these sediments may affect the distribution and reactivity of Fe-bearing minerals and the role Fe plays in mediating present day OC mineralization. Here we quantify Fe concentrations, Fe spatial and depth distribution, and Fe mineralogy in unthawed Yedoma and previously thawed Alas deposits from the Yedoma domain (West Siberia, Laptev Sea region, Kolyma region, New Siberian Islands and Alaska). Total Fe concentrations of ice-rich Yedoma deposits and previously thawed Alas deposits were determined using a portable X-ray fluorescence (XRF) device. This non-destructive method allowed a total iron concentration assessment of Yedoma domain deposits based on 1292 sediment samples. Portable XRF-measured concentrations trueness were calibrated from alkaline fusion and inductively coupled plasma optical emission spectrometry (ICP-OES) measurement method on a subset of 144 samples (R² = 0.81). Fe extractions of unthawed and previously thawed deposits display that, on average, 25% of the total iron is considered as reactive species, either as crystalline or amorphous oxides, or complexed with OC, with no significant difference between Yedoma and Alas deposits. We observe a constant total Fe concentration in Yedoma deposits, but a depletion or accumulation of total Fe in Alas deposits, which experienced previous thaw and/or flooding events, suggesting that redox driven processes during the Lateglacial and Holocene thermokarst formation impact the present day distribution of reactive Fe and its association with organic carbon in ice-rich permafrost.