Effects of Post-Depositional Processes on Chemical Species in a Snow-Firn Pack on Urumqi Glacier No. 1 in Eastern Tianshan, China

Chemical records from alpine ice cores provide an invaluable source of paleoclimatic and environmental information. However, not only are the atmospheric chemical composition and depositional processes recorded, but post-depositional processes within the snow/firn stratum, especially when melt occurs, are recorded as well. To investigate the effects of depositional processes and meltwater-related post- depositional processes on chemical species in the snow-firn pack, so as to answer the question that how much snow chemical information can be preserved as ice core record under the effect of those post- depositional processes on a alpine glacier, a research program, Program for Glacier Processes Investigation (PGPI), was launched in July 2002 by the Tianshan Glaciological Station (TGS), Chinese Academy of Sciences (CAS). An observational and experimental site (the PGPI site) was carefully located in a percolation zone at an altitude of 4,130 m a. s. l. on Urumqi glacier No. 1 in eastern Tianshan, China. Aerosol, surface snow, and snowpit samples were collected on a weekly basis at the PGPI site using established techniques to prevent contamination. Over the duration of the sampling campaign from 2002 to 2008, around 200 snowpit profiles were sampled which produced about 5000 snow/firn sample. In addition, six 10-20 m ice cores were retrieved in 2006 around the PGPI site. At the same time, a variety of observations of the snow-firn pack physical properties and experiments on meltwater were made. Snow-firn density and temperature were measured along a snow pit wall and in-situ air temperature was continuously observed using an Automatic Weather Station (AWS). Most of the samples were analyzed for major ions and insoluble microparticles while oxygen isotopic ratios, trace metals, and carbonaceous particles (organic and black carbon, OC and BC, respectively) were analyzed for selected samples. Up to the present, some primary results have been obtained as followings. The development of the microparticle and magnesium and Calcium stratigraphy in the snow pit are closely related to the physical development of the snow-firn pack. In addition to the effects of sublimation and wind erosion, melting plays a crucial role in both the physical and chemical evolution processes in the snow pack. The elution of ions is found to be driven primarily by air temperature and become evident when a diurnal mean temperature of -3.6 oC is attained. At 0.3 oC all of the year-round new ionic input can be leached from the snow. A significant linear relationship between ammonium concentrations in surface snow and aerosol during spring and summer indicates that the warm-wet condition facilitates the air-snow exchange of ammonium. Humidity is found to be a significant meteorological factor influencing ammonium in deposition in autumn and winter. The ammonium concentration in aerosol evidently shows a trend similar to that in surface snow, suggesting that the variation of atmospheric ammonium may have been preserved in the surface snow. Vertical nitrate stratigraphy in the upper snowpack is formed during wither by dry deposition and compaction and sublimation of snowpack. During the summer the snowpack nitrate is redistributed through the percolation of meltwater. Nitrate is found to be highly mobile in the snowpack relative to magnesium.