Effects of a thaw slump on active layer in permafrost regions with the comparison of effects of thermokarst lakes on the Qinghai-Tibet Plateau, China

In this study, we monitored a thaw slump in the permafrost region of the Qinghai-Tibet Plateau in China, including its thaw settlement and solifluction creep characteristics, and analyzed the change in soil properties and hydrothermal process in the active layer. In addition, the change of the thaw slump active layer was compared with the change of the active layer in lakeshore areas, which were affected by thermokarst lakes, to study the relationship between thaw slump and thermokarst lake. Results showed that thaw slump solifluction creep displacement mainly occurred at the top 50 cm surface soil layer. Under the influence of the thaw slump, the active layer soil bulk density increases gradually along the slope; fine soil particles and soil organic carbon first deposit at the top 30-40 cm of the active layer at gentle slope area, and then are significantly leached at the slope bottom. Since the effect of thaw slump weakens the buffering effects of the active layer on heat transfer, the active layer gradually deepened along the slope and the ablation of the underlying ice-rich permafrost increased, which resulted in the increase of the thaw settlement. Concurrently, a large amount of thaw water from the thawed permafrost was released to the active layer, resulting in overland flow at the front part of the gentle slope area and ponding depression at the slope bottom. The active layer changes of the thaw slump and those of thermokarst lake shore were very similar. These results suggest that active layer soil properties were changed by thaw slump, leading to the increase in underlying permafrost ablation, causing large amounts of thaw water accumulating at poorly drained sites, to form sparse small-sized thermokarst lakes. For some large thermokarst lakes adjacent to a small thaw slump, lake coastal erosion caused the thaw slump, and further evolution of the thaw slump resulted in the changes of lakeshore active layer.