Factors controlling near surface ground-ice characteristics in a region of warm permafrost, Beiluhe Basin, Qinghai-Tibet Plateau

Abstract

Ground ice is a key component of permafrost, and its melt induced by climate change and anthropogenic disturbance has been causing increased ground surface subsidence, thermal erosion, and engineering problems. However, the distribution and quantity of ground ice in permafrost have yet to be investigated in detail on the Qinghai-Tibet Plateau (QTP), and consequently, an assessment of the nature of impacts associated with permafrost degradation is challenging. In this study, variation in near-surface ground ice content of the upper 2–3 m of the permafrost layer was examined by drilling 72 boreholes at eight sites in Beiluhe Basin, QTP, an area with relatively warm (near 0 °C) permafrost. High ground ice contents occur at most sites, but visible ice was absent at one site, where the vegetation cover has transitioned from a meadow to a sparsely-covered grassland. The moisture content within the active layer (surface to 2 m depth) increases with depth at most sites, and the higher moisture contents were associated with greater near-surface ground ice contents. The gravimetric moisture content (Mg) in permafrost typically ranged from 8% to 500%, and ~76% of samples were classified as ice rich (Mg ≥ 20%). The mean excess-ice content in near-surface permafrost was ~19% for all boreholes. At six flat sites, the minimum mean excess-ice content was about zero, and the mean maximum was ~22% at an alpine grassland site. The mean excess-ice content at a sunny sloping site was much higher (~27%) than at a north-facing shady site (10%) and the ice was distributed differently with depth. The mean subsidence ratio at the eight sites was from 0.05 to 0.44. The volumetric ice content varied from 1% to 70% in samples from the different sites, with an average value of ~16%. Topographically controlled moisture availability, slope direction, and fine-particle content are important controls on ground ice content in Beiluhe Basin. This study provides fundamental information about the spatial distribution of ground ice on QTP, which is important for future assessments of thermal erosion potential and infrastructure instability in the region.