Iron mineral dissolution releases iron and associated organic carbon during permafrost thaw

It has been shown that reactive soil minerals, specifically iron(III) (oxyhydr)oxides, can trap organic carbon in soils overlying intact permafrost, and may limit carbon mobilization and degradation as it is observed in other environments. However, the use of iron(III)-bearing minerals as terminal electron acceptors in permafrost environments and thus their stability and capacity to prevent carbon mobilization during permafrost thaw is poorly understood. We have followed the dynamic interactions between iron and carbon, using a space for time-approach, across a thaw gradient in Abisko (Sweden), where wetlands are expanding rapidly due to permafrost thaw. We show through bulk (selective extractions, EXAFS) and nanoscale analysis (correlative SEM and nanoSIMS) that organic carbon is bound to reactive Fe primarily in the transition between organic and mineral horizons in palsa underlain by intact permafrost (41.8 ± 10.8 mg carbon per g soil, 9.9 to 14.8% of total soil organic carbon). During permafrost thaw, water-logging and O2 limitation lead to reducing conditions and an increase in abundance of Fe(III)-reducing bacteria which favor mineral dissolution and drive mobilization of both iron and carbon along the thaw gradient. By providing a terminal electron acceptor, this rusty carbon sink is effectively destroyed along the thaw gradient and cannot prevent carbon release with thaw

Spatial and temporal evolution of groundwater arsenic contamination in the Red River delta, Vietnam: Interplay of mobilisation and retardation processes

Geogenic arsenic (As) contamination of groundwater poses a major threat to global health, particularly in Asia. To mitigate this exposure, groundwater is increasingly extracted from low-As Pleistocene aquifers. This, however, disturbs groundwater flow and potentially draws high-As groundwater into low-As aquifers. Here we report a detailed characterisation of the Van Phuc aquifer in the Red River Delta region, Vietnam, where high-As groundwater from a Holocene aquifer is being drawn into a low-As Pleistocene aquifer. This study includes data from eight years (2010–2017) of groundwater observations to develop an understanding of the spatial and temporal evolution of the redox status and groundwater hydrochemistry. Arsenic concentrations were highly variable (0.5–510 μg/L) over spatial scales of <200 m. Five hydro(geo)chemical zones (indicated as A to E) were identified in the aquifer, each associated with specific As mobilisation and retardation processes. At the riverbank (zone A), As is mobilised from freshly deposited sediments where Fe(III)-reducing conditions occur. Arsenic is then transported across the Holocene aquifer (zone B), where the vertical intrusion of evaporative water, likely enriched in dissolved organic matter, promotes methanogenic conditions and further release of As (zone C). In the redox transition zone at the boundary of the two aquifers (zone D), groundwater arsenic concentrations decrease by sorption and incorporations onto Fe(II) carbonates and Fe(II)/Fe(III) (oxyhydr)oxides under reducing conditions. The sorption/incorporation of As onto Fe(III) minerals at the redox transition and in the Mn(IV)-reducing Pleistocene aquifer (zone E) has consistently kept As concentrations below 10 μg/L for the studied period of 2010–2017, and the location of the redox transition zone does not appear to have propagated significantly. Yet, the largest temporal hydrochemical changes were found in the Pleistocene aquifer caused by groundwater advection from the Holocene aquifer. This is critical and calls for detailed investigations

Microbial iron cycling in permafrost peatlands affected by global warming - Impact on carbon mobilization and greenhouse gas emissions

Northern Hemisphere peatlands store vast amounts of carbon, particularly in permafrost regions where low temperatures inhibited organic matter decomposition since the last glacial ice age. With high latitudes warming faster than anywhere else on the planet, there is urgent concern about the impact of permafrost thaw on the stability of this carbon sink. It has been shown that iron(III) (oxyhydr)oxides can trap organic carbon in soils, underlain by intact permafrost, which may limit carbon mobilization and thus its degradation. Therefore, it is considered as a so-called rusty carbon sink. However, controls on the stability of iron-carbon associations in permafrost peatlands and their response to warming temperatures are poorly understood. Only little is known about the microbial iron cycle in permafrost peatlands and how it is impacted by global warming. Its consequences for carbon mobilization and ultimately greenhouse gas emissions such as carbon dioxide and methane prevail unexplored. Aiming to fill these knowledge gaps, we characterized the dynamic interactions between iron and carbon in a subarctic thawing permafrost peatland (Stordalen mire) in Abisko, Northern Sweden. Here, in the discontinuous permafrost zone, oxic palsa mounds with ice-rich cores are rapidly collapsing into acidic bogs before they ultimately transform into ice-free fen-type wetlands. We show that reactive Fe minerals such as iron(III) (oxyhydr)oxides bind significant quantities of organic carbon (up to 20% of total organic carbon) in areas of intact permafrost. However, these iron-carbon associations are not stable during permafrost thaw. Iron(III)-reducing bacteria, such as e.g. Geobacter spp., reductively dissolve iron(III) (oxyhydr)oxides coupled to carbon oxidation, and release aqueous iron (iron(II)) and the previously iron-bound, aliphatic-like organic carbon that becomes mobilized. The microbially driven iron(III) reduction thus directly contributes to greenhouse gas emissions such as carbon dioxide by iron(III) reduction coupled to carbon oxidation and indirectly by releasing bioavailable organic carbon which then can become further metabolized to carbon dioxide and/or methane by the present microbial community. Iron(III)-reducing bacteria increase in abundance soon after thaw initiates, as it results in increased water saturation and expanding reducing conditions. The loss of the rusty carbon sink in permafrost soils coincides with the highest measured dissolved organic carbon (535.75±133.74 mg C/L) and highly bioavailable acetate concentrations (61.7±42.6 mg C/L) along a permafrost thaw gradient, a significant increase in the abundance of methanogens and methanotrophs, and with increasing fluxes of the greenhouse gases carbon dioxide and methane. We found that permafrost thaw also increases the abundance of iron(II)-oxidizing microorganisms, such as Gallionella spp. and Sideroxydans spp. This suggests that post-thaw iron cycling and interlinked greenhouse gas emissions are highly dynamic, and that the measured iron redox state is a result of the net balance between reductive and oxidative processes. Indeed, seasonal re-precipitation of iron(III) (oxyhydr)oxides was observed in the active layer of partially-thawed bog areas. Ultimately, iron(II)-oxidizing microorganisms can not sustain or reform the rusty carbon sink after complete permafrost thaw in fully-thawed fen-type wetlands. This work has greatly expanded our understanding of microbe-mineral interactions in permafrost peatlands. It reveals an important and previously overlooked role of iron-cycling microorganisms in the release of iron mineral-associated organic carbon and its impact on greenhouse gas emissions of thawing permafrost peatlands – one of Earth’s most rapidly changing ecosystems.Dissertation ist gesperrt bis zum 20.10.2023 !