Stable isotopic evolutions of ground ice in permafrost of the Hoh Xil regions on the Qinghai-Tibet Plateau

Abstract

Climatic warming-induced permafrost (ground ice) thaw is expected to considerably alter hydro−geomorphology and the water cycle on the Qinghai-Tibet Plateau (QTP). A unique feature of permafrost is the substantial enrichment of ground ice, which can be used to directly indicate the past climate and hydrological cycle. However, essential characteristics of ground ice on the QTP are not yet understood. The stable isotope technique was used to investigate the formation processes and initial source of water in the ground ice in a borehole of Zhuonaihu (ZNH) lake at the Hoh Xil permafrost regions in the hinterland of the QTP. Variability in water migration, replenishment of water, and climatic conditions (freezing rate) have created a complex isotopic stratigraphy and ice growth history. The δD−δ18O relationship of ground ice samples showed similar characteristics to a local meteoric water line of regional precipitation on QTP, and the D-excess−δD relations of ground ice reveal different features from those of precipitation and freezing condition. The near−surface ground ice (0–3 m) was enriched in δ18O and δD which was formed from freezing of active layer water suffering evaporation, and deep layer ground ice (>3 m) was depleted in these isotopes. Stable isotope results were indicative of past−precipitation origin of ground ice between 2.5 and 5.5 m. Ground ice between depths of 5.5 and 10 m was formed under unstable freezing conditions. However, ice from 10 to 16 m was formed by freezing segregation of liquid water. Comparison of the results with other five boreholes indicates that differences in lithology, source water, and climatic conditions were responsible for the significant deviations in ground ice isotopes. This research provides an insight into the fundamental formation processes related to ground ice in the QTP, which will provide important guidance for future studies of permafrost hydrology.