Mineral Weathering in Thawing Permafrost: Causes and Consequences

Periodic Reporting for period 4 - WeThaw (Mineral Weathering in Thawing Permafrost: Causes and Consequences)

Okres sprawozdawczy: 2022-03-01 do 2022-08-31

Enhanced thawing of the permafrost in response to warming of the Earth’s high latitude regions exposes previously frozen soil organic carbon (SOC) to microbial decomposition, liberating carbon to the atmosphere and creating a dangerous positive feedback on climate warming. Thawing the permafrost may also unlock a cascade of mineral weathering reactions. These will be accompanied by mineral nutrient release and generation of reactive surfaces which will influence plant growth, microbial SOC degradation and SOC stabilization. Arguably, weathering is an important but hitherto neglected component for correctly assessing and predicting the permafrost carbon feedback. The goal of WeThaw was to provide the first comprehensive assessment of the mineral weathering response in permafrost regions subject to thawing. By addressing this crucial knowledge gap, WeThaw has augmented our capacity to develop models that can accurately predict the permafrost carbon feedback.

Specifically, the objectives were to provide the first estimate of the permafrost’s mineral element reservoir which is susceptible to rapidly respond to enhanced thawing, and to assess the impact of thawing on the soil nutrient storage capacity. To determine the impact of increased mineral weathering on mineral nutrient availability in terrestrial and aquatic ecosystems in permafrost regions, the abiotic and biotic sources and processes controlling their uptake and release were unraveled by combining isotope geochemistry (Si, Sr, Mg, Li) with soil mineral and physico-chemical characterizations. This frontier research which crosses disciplinary boundaries between cryospheric science, soil science and isotope geochemistry was a mandatory step for being able to robustly explain the role of mineral weathering in modulating the permafrost carbon feedback.

In conclusion,
(1) we demonstrate using silicon isotopes that the active layer is hydrologically connected earlier than previously considered in soils with a deeper active layer. We anticipate our findings to be a starting point to locate earlier pathways of hydrological connectivity, which need to be urgently monitored to quantify the seasonal flux of soil organic carbon released from permafrost soils.
(2) we show using radiogenic Sr isotopes that it is in saturated layers that mineral OC interactions remain undissociated and preserved since their formation, and that the redox interface in permafrost soils is a loci for the loss and gain in OC stabilization potential due to the dissolution and precipitation of the mineral OC interactions.
(3) we highlight using radiogenic Sr isotopes that micro-landscape variability in the exchangeable base cation reserve with soil depth represents a key source of readily available nutrients for both shallow- and deep-rooted plant species upon permafrost thaw. This finding lies beyond the common view that nutrient release at the permafrost thaw front preferentially benefits deep-rooted plant species.

The project was divided into four work packages (WP).

WP1 focused on the circumpolar mineral element content in the permafrost. We have collected data to build an estimate of the permafrost’s mineral element reservoir which is susceptible to rapidly respond to enhanced thawing. We have included regions of ice-rich permafrost (Yedoma-alas) sensitive to thermokarst processes, which may affect one third of the Arctic by the end of the century. Our database contains data for permafrost soil samples from major circum-Arctic regions (Siberia, Alaska, Canada, Greenland, Svalbard) (Monhonval et al, Frontiers, 2021a; Monhonval et al PANGAEA, 2021; Monhonval et al, Frontiers, 2021b; Monhonval et al, Permafrost Periglacial Processes, 2022; Monhonval, under review in Geoderma a; Thomas et al, under review in Geoderma).

WP2 aimed at determining controls on mineral element release from the permafrost. We collected permafrost characteristics in areas characterized by abrupt thaw and gradual thaw in order to develop a better understanding of the controls on mineral element release upon thawing. We combine information from lab thaw experiments in controlled conditions and field thaw experiment to determine the parameters controlling the soil pore water chemistry. We use isotope geochemistry on sediments, soils, plants, river waters, and soil pore waters from permafrost regions to trace sources and processes controlling mineral element release from the permafrost (Opfergelt, 2020; Monhonval et al, Frontiers, 2021b; Monhonval et al, Permafrost Periglacial Processes, 2022; Monhonval, under review in Geoderma b; Hirst et al Global Biogeochem Cycles, 2022).

WP3 focused on determining controls on the soil-to-plant transfer of mineral elements. We led field campaigns in Alaska and in Abisko to collect soil, plant, soil pore waters and river water samples at contrasted seasons: ice break-up, maximum thaw depth, and late shoulder season. Mineral element distribution in plants and soils were measured. The main vegetation types (sedge, forb, shrub deciduous and evergreen, moss, lichen) have been included in order to evaluate the influence of vegetation shift onto mineral nutrient cycling (Mauclet et al, Biogeosciences, 2022; Villani et al, 2022; Mauclet et al, ESSD, 2022; Mauclet et al, under revisions in Geoderma).

WP4 aimed at determining seasonal controls on the mineral element transfer to rivers in polar regions. In the Arctic, the seasonal variation of water chemistry and isotope compositions of a headwater stream from Alaska was analyzed and related with soil processes upon thawing. And the variation of a larger river chemistry associated with a rain event in a thermokarst area was investigated in Greenland. In Antarctica, seasonal Si isotope variations in rivers from McMurdo Dry valleys have been shown to depend on the influence of mineral weathering (Hirst et al, Frontiers, 2020; Hirst et al Global Biogeochem Cycles 2022; Hirst et al, under review in Communications Earth & Environment).

Overall, the project has led four field missions in the Arctic (three in Alaska, one in Northern Sweden), and involved samples from more than 30 different sites in the major circum-Arctic regions (Siberia, Alaska, Canada, Greenland, Svalbard, Sweden). A major synthesis effort is ongoing with the Permafrost Carbon Network.

The project has conducted to four breakthroughs contributing to advance the research field significantly beyond the state of the art. This opens a new avenue for the understanding of the influence of permafrost thaw on the release of nutrients and their transfer from soils to rivers.

The first breakthrough concerns the use of silicon isotopes as a tracer of freeze-thaw processes (Hirst et al, Frontiers, 2020; Hirst et al, under review in Communications Earth & Environment).

The second breakthrough concerns the seasonal changes in soil-river connectivity in permafrost landscapes (Hirst et al., Global Biogeochem Cycles, 2022).

The third breakthrough concerns the stability of mineral organic carbon interactions upon permafrost thaw (Opfergelt, 2020; Monhonval et al, Frontiers, 2021; Monhonval et al, Permafrost Periglacial Processes, 2022; Monhonval, under review in Geoderma).

The fourth breakthrough concerns the changes in plant nutrient sources upon permafrost thaw (Mauclet et al, Biogeosciences, 2022; Mauclet et al, ESSD, 2022; Mauclet et al, under revisions in Geoderma).

Periodic Reporting for period 4 - WeThaw (Mineral Weathering in Thawing Permafrost: Causes and Consequences)

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