Permafrost on the Qinghai-Tibetan Plateau (QTP) has been degrading in the past decades. While the degradation may mobilize previously protected material from the permafrost profile, little is known about the stocks and stability of mercury (Hg) in the QTP permafrost. Here we measured total soil Hg in 265 samples from 15 permafrost cores ranging from 3 to 18 m depth, and 45 active layer (AL) soil samples from different land cover types on the QTP. Approximately 21.7 Gg of Hg was stored in surficial permafrost (0-3 m), with 16.58 Gg of Hg was stored in the active layer. Results from six permafrost collapse areas showed that much of the thawed Hg is mobile, with decreases in total Hg mass of 17.6-30.9% for the AL (top 30 cm) in comparison with non-thermokarst surfaces. We conclude that the QTP permafrost region has a large mercury pool, and the stored mercury is sensitive to permafrost degradation. (C) 2019 Elsevier B.V. All rights reserved.

The understanding of temperature trends in high elevation mountain areas is an integral part of climate change research and it is critical for assessing the impacts of climate change on water resources including glacier melt, degradation of soils, and active layer thickness. In this study, climate changes were analyzed based on trends in air temperature variables (T-max, T-min, T-mean), and Diurnal Temperature Range (DTR) as well as elevation-dependent warming at annual and seasonal scales in the Headwaters of Yangtze River (HWYZ), Qinghai Tibetan Plateau. The Base Period (1965-2014) was split into two subperiods; Period-I (1965-1989) and Period-II (1990- 2014) and the analysis was constrained over two subbasins; Zhimenda and Tuotuohe. Increasing trends were found in absolute changes in temperature variables during Period-II as compared to Period-I. T-max, T-min, and T-mean had significant increasing trends for both sub-basins. The highest significant trends in annual time scale were observed in T-min (1.15 degrees C decade(-1)) in Tuotuohe and 0.98 degrees C decade(-1) in Zhimenda sub-basins. In Period-II, only the winter season had the highest magnitudes of T-max and T-min 0.58 degrees C decade(-1) and 1.26 degrees C decade(-1) in Tuotuohe subbasin, respectively. Elevation dependent warming analysis revealed that T-max, T-min and T-mean trend magnitudes increase with the increase of elevations in the middle reaches (4000 m to 4400 m) of the HWYZ during Period-II annually. The increasing trend magnitude during Period-II, for T-max, is 1.77, 0.92, and 1.31 degrees C decade(-1), for T-min 1.20, 1.32 and 1.59 degrees C decade(-1), for T-mean 1.51, 1.10 and 1.51 degrees C decade(-1) at elevations of 4066 m, 4175 m and 4415 m respectively in the winter season. T-mean increases during the spring season for > 3681 m elevations during Period-II, with no particular relation with elevation dependency for other variables. During the summer season in Period- II, T-max, T-min, T-mean increases with the increase of elevations (3681 m to 4415 m) in the middle reaches of HWYZ. Elevation dependent warming (EDW), the study concluded that magnitudes of T-min are increasing significantly after the 1990s as compared to T-max in the HWYZ. It is concluded that the climate of the HWYZ is getting warmer in both sub-basins and the rate of warming was more evident after the 1990s. The outcomes of the study provide an essential insight into climate change in the region and would be a primary index to select and design research scenarios to explore the impacts of climate change on water resources.