Why Do We Need to Care About the Evolution of Mineral-organic Carbon Interactions in Permafrost Upon Thawing?

Abstract:

Evidence is mounting that permafrost thaw represents a tipping element in the Earth climate system due to permafrost carbon emissions. Yet uncertainties are associated with how this will unfold. Permafrost contains 1460-1600 Gt of organic carbon (OC), of which 15±3% could be emitted as greenhouse gases (GHG) by 2100. The evolution of mineral-organic interactions in permafrost upon thawing is a potentially important player for the modulation of permafrost C emissions. The interactions between OC and minerals influence the accessibility of OC for microbial decomposition and OC stability, and are therefore a factor in controlling the permafrost C emissions rate upon thawing.

Mineral protection of OC includes (i) physical protection, i.e., OC within soil aggregates spatially inaccessible for microorganisms, or (ii) physico-chemical protection, i.e., as organo-mineral associations (e.g., OC sorbed onto mineral surfaces) or as organo-metallic complexes (OC complexed with e.g., Al, Fe, Ca). Soil constituents are increasingly exposed to fluctuating water table in response to permafrost thaw. Mineral solubility and metal ions binding OC are highly sensitive to changing conditions such as water saturation or soil acidity.

This contribution aims at assessing the potential influence of changing from oxic to anoxic conditions or vice-versa for the evolution of mineral-OC interactions in thawing permafrost. Four scenarios of ongoing permafrost disturbances in a warming Arctic landscape will be discussed based on the current knowledge: (i) active layer deepening by gradual thaw, (ii) abrupt thaw creating lakes and wetlands, (iii) lake basin drainage, (iv) coastal erosion. Anticipated effects are on (i) the availability of OC for microbial decomposition which can amplify or mitigate permafrost C emissions, and/or on (ii) the CO₂ to CH₄ ratio of C emissions, with consequences for the net global warming potential.