The Influence of Organic-Soil Horizons on Thermal Dynamics in High-Latitude Soils: Identifying Thresholds for Permafrost State Change

Abstract:

Organic-soil horizons exert significant control on soil temperature and permafrost dynamics in high-latitude regions. Ecosystem protection of permafrost is governed by the low thermal conductivity of organic soils, which is sensitive to changes in horizon thickness (OHT), moisture content, and decomposition extent (and thus, porosity, and density) of organic matter. At broad spatial scales, the occurrence of permafrost is positively correlated with OHT when organic horizons are relatively thin (< 30 cm). Across sites where OHT is deeper, this correlation reverses and becomes negative. We hypothesize that this bi-modal relationship between OHT and permafrost occurrence is primarily governed by the contrasting thermal properties of upper organic-soil horizons and the underlying deep organic-soil and mineral-soil horizons. As documented with prior investigations on snow thermal properties, we find that that the underlying layers can have a profound impact on the insulating effect of the overlying layer. To evaluate this hypothesis, we examine the sensitivity of permafrost to soil properties (OHT, moisture content, and texture) and their variations across landscape positions and drainage class using field–based observations and generalized simulations using the Geophysical Institute Permafrost Laboratory model (GIPL). We observed significant negative correlations between minimum daily ground-surface temperature during summer and OHT across upland forest sites in interior Alaska. In peatlands, ground-surface temperature and OHT appear to be decoupled, which is likely due to variation in deposit thickness as determined by the timing of peatland formation across the region. Model results highlight the role of moisture content and water table position, both as controls on organic matter accumulation and on permafrost extent and thermal state.