RESUMO
The stable-isotope data of ground ice from a deep borehole (~46 m) at the Tianshuihai (TSH) lake basin on the northwestern Qinghai-Tibet Plateau (QTP) are presented together with cryolithological information. Remarkable variations in the stable isotope composition of ground ice at depths allow a division of five clearly delineated stages. The remarkable deviations in stable isotopes of ground ice during each stage underline different initial source water and formation processes, indicating considerable fluctuations in paleo-lake conditions and multiple patterns of climatic-induced permafrost evolutions. In combination with the ground ice isotopes for two deep boreholes on the interior QTP, the position of the present permafrost table is found at 2-3 m. Two possible positions of paleo-permafrost tables at depths of 7-8 and 15-16 m are identified based on the ice isotopic composition of wellbores in combination with those from the other two deep boreholes on the interior QTP. The high uniformity in stable isotopic composition of ground ice below the depth of 16 m may have reflected the consistent regional climate transitions and the resultant permafrost evolution on the QTP. This study provides some new insights on the ground ice as an indicator for permafrost evolution on the QTP.
RESUMO
Under the influences of climate change and human activities, desertification has become widespread on the Qinghai-Tibet Plateau (QTP). However, the effect of desertification on frozen soil is still debated. Here, soil temperatures are observed through 14 boreholes at Honglianghe River Basin on the QTP to study the relationship between desertification and frozen soil. The results showed soil temperatures change with the thickness of sand cover. With increasing sand thickness, maximum soil temperatures at shallow depths (0.05-6.00 m) increase by 0.25-1.57 °C, but minimum temperatures decrease by 0.21-1.49 °C, on average. Temperatures at deep depth (≥6.00 m) exhibit a rising trend that temperatures increase by 0.01-0.05 °C on average with each increment of 10 cm in sand thickness. Furthermore, aeolian sand enhances seasonal thawing processes, resulting in an increase of 7.70-9.50 cm in active layer thickness with each increment of 10 cm in sand thickness. Meanwhile, aeolian sand weakens seasonal freezing processes, resulting in a decrease of 1.07-13.00 cm in seasonal freezing depth with each increment of 10 cm in sand thickness. Moisture contents of aeolian sand and vegetation coverages on the sand cover surface influence energy state and thermal regime of frozen soil. Annual heat budgets of soil under aeolian sand increase from -57.97 MJ m-2 to -26.28 MJ m-2 as water content of sand layer decreases from 13.42% to 3.61%. Annual range of ground temperatures of soil at shallow depths (0.05-1.60 m) increase by 2.19-6.17 °C on average as vegetation coverage increases from 5% to 20%. Due to the effects of aeolian sand on frozen soil, desertification accelerates, and can even cause, the degradation of frozen soil on the QTP. Our study provides an important reference for future research about the interaction between desertification and frozen soil in other regions.