Labile components as geochemical tracers at the soil column-scale in permafrost peatlands

Permafrost peatlands constitute a reservoir of highly labile components such as dissolved organic carbon (DOC), macro- and micro-nutrients, and toxic elements. Fast thawing of permafrost peatlands in most of climate warming scenarios will lead to the mobilisation of dissolved components from soils to rivers and lakes. Permafrost peatlands situated in western Siberia are especially vulnerable to thawing, as they contain the largest soil water and ice resources in the northern hemisphere.  Half of the volume of ice resources is in form of dispersed ice on the soil top layer (0-3 m) [1]. Dispersed ice is enriched in dissolved components, which may be highly reactive and even provide sizable contributions to the hydrological system via suprapermafrost flow [2]. However, contributions from local soil column-scale in the context of micro-environments potential deepening of the active layer remain to be assessed.

To characterize the dissolved fraction of dispersed peat ice, a study was performed in Tazovsky, a representative site located in a continuous permafrost zone in Siberia. Four peat cores (0-180 cm depth) were collected along a gradient from the bog to mineral-fen. DOC, nutrients and trace elements were analysed for both the porewater (active layer) and dispersed ice (frozen layer). Lateral and in-depth approaches were employed to quantify the pools of dissolved components. Preliminary results show an enrichment of highly labile components in the frozen layer, consistent with previous findings in the discontinuous permafrost zone [3]. We hypothesize that the highest concentrations of DOC and certain macro/micro-nutrients in the frozen layer are mostly driven by downwards colloidal migration during the freezing phase. These organic and inorganic nutrients may be released due to progressive thawing, and eventually become bioavailable for microbial uptake. Consequently, the production of CO₂ may increase, contributing to the ongoing climate change.

[1] W. H. Pollard and H. M. French, « A first approximation of the volume of ground ice, Richards Island, Pleistocene Mackenzie delta, Northwest Territories, Canada », Can. Geotech. J., vol. 17, n^o 4, p. 509-516, november 1980, doi: 10.1139/t80-059.

[2] A. G. Lim et al., « Dispersed ground ice of permafrost peatlands: Potential unaccounted carbon, nutrient and metal sources », Chemosphere, vol. 266, p. 128953, march 2021, doi: 10.1016/j.chemosphere.2020.128953.

[3] D. M. Kuzmina et al., « Dispersed ice of permafrost peatlands represents an important source of labile carboxylic acids, nutrients and metals », Geoderma, vol. 429, p. 116256, january 2023, doi: 10.1016/j.geoderma.2022.116256.