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As global climate continues to warm, melting of glaciers releases a large quantity of mercury (Hg) originally locked in ice into the atmosphere and downstream ecosystems. Here, we show an opposite process that captures atmospheric Hg through glacier-to-vegetation succession. Our study using stable isotope techniques at 3 succession sites on the Tibetan Plateau reveals that evolving vegetation serves as an active “pump” to take up gaseous elemental mercury (Hg0) from the atmosphere. The accelerated uptake enriches the Hg pool size in glacier-retreated areas by a factor of ∼10 compared with the original pool size in the glacier. Through an assessment of Hg source–sink relationship observed in documented glacier-retreated areas in the world (7 sites of tundra/steppe succession and 5 sites of forest succession), we estimate that 400 to 600 Mg of Hg has been accumulated in glacier-retreated areas (5‰ of the global land surface) since the Little Ice Age (∼1850). By 2100, an additional ∼300 Mg of Hg will be sequestered from the atmosphere in glacier-retreated regions globally, which is ∼3 times the total Hg mass loss by meltwater efflux (∼95 Mg) in alpine and subpolar glacier regions. The recapturing of atmospheric Hg by vegetation in glacier-retreated areas is not accounted for in current global Hg models. Similar processes are likely to occur in other regions that experience increased vegetation due to climate or land use changes, which need to be considered in the assessment of global Hg cycling.

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Alpine cold ecosystem with permafrost environment is quite sensitive to climatic changes and the changes in permafrost can significantly affect the alpine ecosystem. The vegetation coverage, grassland biomass and soil nutrient and texture are selected to indicate the regime of alpine cold ecosystems in the Qinghai-Tibet Plateau. The interactions between alpine ecosystem and permafrost were investigated with the depth of active layer, permafrost thickness and mean annual ground temperature (MAGTs). Based on the statistics model of GPTR for MAGTs and annual air temperatures, an analysis method was developed to analyze the impacts of permafrost changes on the alpine ecosystems. Under the climate change and human engineering activities, the permafrost change and its impacts on alpine ecosystems in the permafrost region between the Kunlun Mountains and the Tanggula Range of Qinghai-Tibet Plateau are studied in this paper. The results showed that the permafrost changes have a different influence on different alpine ecosystems. With the increase in the thickness of active layer, the vegetation cover and biomass of the alpine cold meadow exhibit a significant conic reduction, the soil organic matter content of the alpine cold meadow ecosystem shows an exponential decrease, and the surface soil materials become coarse and gravelly. The alpine cold steppe ecosystem, however, seems to have a relatively weak relation to the permafrost environment. Those relationships resulted in the fact that the distribution area of alpine cold meadow decreased by 7.98% and alpine cold swamp decreased by 28.11% under the permafrost environment degradation during recent 15 years. In the future 50 years the alpine cold meadow ecosystems in different geomorphologic units may have different responses to the changes of the permafrost under different climate warming conditions, among them the alpine cold meadow and swamp ecosystem located in the low mountain and plateau area will have a relatively serious degradation. Furthermore, from the angles of grassland coverage and biological production the variation characteristics of high-cold ecosystems in different representative regions and different geomorphologic units under different climatic conditions were quantitatively assessed. In the future, adopting effective measures to protect permafrost is of vital importance to maintaining the stability of permafrost engineering and alpine cold ecosystems in the plateau.

The streamflow age is an essential descriptor of catchment functioning that controls runoff generation, biogeochemical cycling, and contaminant transport. The young water fraction (F yw ) of streamflow, which can be accurately estimated with tracer data, is effective at characterizing the water age proportions of heterogeneous catchments. However, the F yw values of permafrost catchments are not known. We selected a watershed in the permafrost region of the Qinghai-Tibet Plateau (QTP) as our study area. Daily interval stable isotopes (deuterium and oxygen-18) of precipitation and streamflow were studied during the 2009 thawing season. The results show that the stable isotope compositions of precipitation and stream water have significant spatial and temporal variations. HYSPLIT backwards trajectory results demonstrate that the moisture in the study area mainly derived from the westerlies and southern monsoons. Thawing processes in the active layer of the permafrost significantly altered the stable isotope compositions of the stream water. The soil temperature, soil moisture, and air temperature are the main drivers of the stable isotope variations in the stream water. We estimated the young water fractions of the five catchments in the study area, which were the first estimates of the F yw in permafrost catchments in the QTP. The results show that an average of 15% of the streamflow is younger than 43 days. Additional analyses show that the vegetation cover significantly controls the young water fraction of the streamflow. These results will improve our understanding of permafrost hydrological processes and water resource utilization and protection. KEYWORDS permafrost hydrology, Qinghai-Tibet Plateau, stable isotopes, watershed hydrology, young water fraction

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