by

Cameron P. Wake, Paul A. Mayewski and Mary Jo Spencer
(Glacier Research Group, Institute for the Study of Earth, Oceans and Space,
University of New Hampshire, Durham, NH 03824, U.S.A.)

ABSTRACT

The glaciers of central Asia provide suitable locations from which to recover continuous. high-resolution glaciochemical records on a continental scale. Although the glaciochemical investigations undertaken to date in central Asia are few in number and limited in terms of spatial coverage and length of record, some preliminary observations can be made concerning regional and seasonaltrends in snow chemistry in this region.

The sodium chloride ratio for most snow samples collected in central Asia approaches the ratio found in sea water (0.86 in ueq/kg), reflecting a marine source for these constituents. Sodium and chloride concentrations are, on average, 3-10 times higher in the Himalayas than in the Karakoram, demonstrating the greater influence of monsoonal sources of moisture in the Himalayas. Very high sodium concentrations from Khel Khod Glacier probably reflect a local crustal source from surrounding ice-free areas. Low nitrate concentrations were found in snow collected from the southern margin of the Himalayas and high concentrations in snow deposited on the north margin of the Himalayas. This strong regional trend in the spatial distribution of nitrate suggests the influx of continental aerosols, rich in nitrate, originating from the and regions of central Asia. High calcium concentrations measured in snow from Mount Everest and the north-west corner of China are also indicative of dust derived from the arid regions of central Asia. Very high sulfate concentrations found in snow from the Tien Shan and the Bogda Shan most likely reflect local anthropogenic sources. The altitude effect on isotopic composition is not apparent from snow samples collected in central Asia.

Understanding the processes which control the chemical content of snow, the local - to-reg ional scale complexities, and the seasonal variability are fundamental steps necessary to assess the potential for recovering representative long-term glaciochemical records from central Asia.

INTRODUCTION

The recovery and analysis of snow and ice samples from appropriately located snow-pits and ice cores have proven extremely valuable in producing continuous records of atmospheric chemistry and climate (e.g. Lorius and others, 1985; Mayewski and others, 1986a). Furthermore, records from glaciochemical investigations are especially important when direct observations and measurements of the atmosphere are either spatially and/or temporally lacking. The vast extent of glacierized regions in the high mountains of central Asia (Fig.1) provide several suitable locations from which to recover glaciochernical records. The recovery of snow and ice samples from elevations in the range of 4500 to 7000 m above sea level (a.s.l.) provides a unique opportunity to describe and understand atmospheric chemistry and processes in the free troposphere. Glaciochemical,, records can be used to identify patterns of regional circulation, sources of moisture, sources and distribution of chemical species in the atmosphere and regional fluxes of chemical species relatable to biogeo chemical cycles (Mayewski and others, in press). Data sets describing the spatial and seasonal variation of snow chemistry in central Asia will provide useful input for ongoing global change research efforts such as the International Geosphere-Biosphere Program (National Academy of Science, 1986) and the Global Tropospheric Chemistry Program (National Academy of Science, 1984). This paper reviews the glaciochemical data which have been collected from central Asia and provides some preliminary observations concerning regional and seasonal trends in central Asian snow chemistry.

EXISTING DATA SETS

(Table1) provides a listing of the glaciochemical investigations that have been undertaken in the high mountains of central Asia and (Table2) provides a detailed summary of the chemical characteristics of central Asian snow. Perhaps the single most obvious fact that emerges for this vast region is that the existing data set is limited both spatially and temporally. However, data are now available also from the Dunde (Thompson and others, this issue) and Chongce (Nakawo and others, this issue) ice cores.

Glaciochernical data are currently available only for sites on the southern margin of the mountain ranges of central Asia (sites 1 through 4, (Fig.1)) and from the Tien Shan and Bogda Shan in north-west China (sites 6 and 7, (Fig.1)). Additionally, only the records from the Biafo, Sentik, Khel Khod and Yala glaciers extend for more than one year (Table3). Snow chemistry data recovered from the other southern margin areas (i.e. Kumjung village, Mount Everest and Xixabangma peak) represent samples of an individual, or a series of individual snowfall events covering a rather short period of time (i.e. less than one month). In addition, the types of chemical species and stable isotopes analyzed differ between studies, limiting comparison among the investigations.

SAMPLE LOCATIONS

The glaciochemical sample locations listed in (Table2) fall into three broad climatic zones. The Biafo, Khel Khod and Sentik glaciers are characterized by precipitation sources from both the west and the south. In this region, snow accumulation increases toward the west as a result of increasing influence of precipitation derived from westerly moisture sources. The upper Biafo Glacier consists mainly of extensive open and gently sloping accumulation areas covering over 500 km2. The percentage of ice-free area surrounding the accumulation zone is minimal. Glaciochemical samples were collected from snow-pits excavated from 4650 to 5520 rn a.s.l. The aspect of these collection sites varied from east to south to west. The northward- facing Sentik Glacier is one of several glaciers emanating from the Nun Kun Plateau (325 km2). While the plateau is largely ice-covered, extensive areas surrounding the plateau are ice-free. Khel Khod Glacier is one of several relatively small (< 10 km^2) glacier basins in Kashmir. It faces south-westward and is surrounded by ice-free areas.

Sample collection sites in the central Himalayas,. including Yala Glacier, Xixabangma peak, Mount Everest and Kumjung village are characterized by a monsoonal maritime type of climate (Li and Xu, 1980). This region experiences heavy precipitation during the summer monsoonal period. Yala Glacier covers an area of 2.6 km^2 flows in a north-easterly direction, and is surrounded by extensive ice-free areas. Fresh snow samples from Xixabangma peak and Mt. Everest were collected from relatively high elevation sites (6300 m and 5600-7100 m.a.s.l., respectively) on the north side of the mountain, in an elevation band where perennial snow and ice dominate. The Kumjung village fresh snow samples were collected at a much lower elevation (4900 m a.s.l.) adjacent to a small village.

The Tien Shan and Bogda Shan mountains in. north-west China experience an inland continental climate.; Moisture is derived primarily from the west and the north (Li and Xu, 1980). Both Glacier No. I at the headquarters~ of the Urumqi River and Bogda Feng Glacier are smalC (<2 kM^2), and surrounded by extensive ice-free areas.

The preservation of chemical records in snow and icC deposited in high altitude/low-mid latitude glaciers depends~ upon glacier basin morphology and local meteorological~ conditions. Sample locations which are removed from the 1; effects of avalanches, little affected by redistribution W snow by the wind, experience low mean annusl~~ temperatures, and high rates of annual snow accumulation! are most likely to develop relatively undisturbed glacio-~ chemical records. A qualitative assessment of the degree of14, post-depositional alteration of snow and ice samples collected in central Asia appears in (Table4).

DISCUSSION

Although the glaciochemical investigations undertaken to date are few in number and limited in terms of spatial coverage and length of record, some preliminary observations can be made concerning regional and temporal trends in snow chemistry and their importance in improving our understanding of regional atmospheric circulation and atmospheric chemistry in central Asia.(Fig.2)) illustrates the mean concentrations of sodium, chloride, nitrate and sulfate in snow collected from the mountains in central Asia. The columns in the histograms, from left to right, are roughly arranged west to east (except for the Tien Shan and Bogda Shan sample locations). In interpreting the data, more weight is given to the gtaciochemical data recovered from the Biafo and Sentik glaciers since the records from these two regions extend for more than one year and were recovered from areas that experienced minimal postdepositional alteration, and since sodium, chloride and nitrate concentrations were determined for both studies.

Both sodium and chloride show strong regional trends with the lowest values in the Karakoram, and progressively higher values toward the south-east. This regional trend clearly demonstrates the increased influence of monsoon-derived moisture in the Himalayas when compared with the Karakoram. Seasonal signals apparent in records from the Karakoram (Wake 1989, unpublished) and western Himalaya (Mayewski and others, 1984) are characterized by relatively higher concentrations of sodium and chloride in summer strata. This is indicative of the influx of moistore derived from the Arabian Sea which is then transported to the mountains by summer monsoonal circulation. Winter strata are characterized by overall lower concentrations of sodium and chloride, reflecting moisture derived from distant marine sources such as the Mediterranean Sea and/or Atlantic Ocean. The sodium:chloride ratios for samples collected from the Biafo and Sentik glaciers, Xixabangma peak and Mount Everest (Table.2) approach the ratio found in sea water (0.86 in ueq/kg). Thus, most of the sodium and chloride entering the region is derived from marine sources. The slightly higher sodium:chloride ratio for the Sentik Glacier samples implies an additional source of sodium. Snow from Sentik Glacier is also characterized by high concentrations of reactive silicate (Table.2). The elevated sodium and reactive silicate values suggest inputs of crus tally- derived material. The very high sodium values observed in samples from Khel Khod Glacier canno easily be interpreted in terms of a regional trend as they most, likely reflect a local crustal source from surrounding icefree areas. The high sodium and chloride concentrations of snow from the Tien Shan and the Bogda Shan most likely represent an input of sodium- and chloride-rich dust originating from the extensive evaporite deposits to the north and west of these mountain ranges.

Low nitrate concentrations were found in snow' collected from the southern margin of the Himalayas (Sentik, and Yala glaciers) and high concentrations in snow deposited on the northern margin of the Himalayas (Biafo Glacier, Xixabangma peak and Mount Everest). This strong regional trend in the spatial distribution of nitrate suggests the influx of terrestrial dust, rich in nitrate, originating from the extensive arid regions of central Asia. From an analysis of nitrate values in snow around the world, Lyons and others (in press) found that aerosols derived from continental interiors tend to be rich in nitrate. In addition, snow samples from Mount Everest and the north-west corner of China show extremely high calcium concentrations (Table.2), indicative of dust derived from the and regions in of central Asia.

Sulfate values are approximately 2 to 3 times higher in samples collected from Yala Glacier, Kumjung village, Xixabangma Peak, and Mt. Everest compared to samples from Biafo Glacier, Excess sulfate (non sea-salt) accounts for 90-95% of the total sulfate measured at all five sites. The elevated sulfate concentration in these samples could be due to an influx of anthropogenic emissions from the Indian subcontinent, transported to the Himalayas with monsoonal circulation. However, Mayewski and Lyons (1983) suggest that monsoonal air masses tend to dominate at lower elevations with more pristine air aloft. The calcium:sulfate ratio in snow from Mt. Everest is 0.17, suggesting that about one-fifth of the calcium and sulfate could enter the region in the form of gypsum-rich (CaS04*2H2O) dust from the arid regions of central Asia. Very high sulfate concentrations found in snow from the Tien Shan and Bogda Shan most likely reflect local anthropogenic sources (Wake and Mayewski, in preparation).

Strong seasonal signals are present in d180 records from both the Sentik (Mayewski and others, 1984) and Biafo (Wake, 1987, 1989, unpublished) glaciers, and have proven very useful in identifying seasonal stratigraphy. Strong seasonal variations of dD values were observed in precipitation collected in the Khumbu Himal; low dD values occurring in summer monsoonal rains are caused by removal of the heavy isotopic component in steady monsoon rains as they track across the Indian subcontinent (Wushiki, 1977a, b). d180 values derived from longer term records (i.e. greater than one year) and from individual samples of surface snow collected over a range of elevations, albeit from a limited number of sites, show no apparent trend with elevation (Fig.3). This is not surprising as factors other than temperature, such as transport history and local meteorological conditions, can strongly affect the isotopic composition of snow in mountain areas (Niewodniczanski and others, 1981; Grootes and others, 1999). There exists a good relationship between mean annual temperature and d180 values for samples collected from Biafo, Sentik and Yala glaciers (Fig.4). On the other hand, the Dunde Ice Cap experiences a very different climatic regime as it lies in the centre of a very and region of central Asia. It is classified as an inland continental glacier (Li and Xu, 1980). with a net annual accumulation of 0.2 m w.e. This might explain the poor relationship with the Biafo and Sentik data.

CONCLUSIONS AND PERSPECTIVES

While limited in spatial and temporal distribution, the glaciochernical data thus far collected reveal local and regional trends, including a marine influence (sodium and chloride), a local crustal dust influence (sodium and reactive silica), a regional dust influence (nitrate and calcium) and an anthropogenic influence (sulfate) on snow deposited in central Asia.

AS our knowledge of central Asian snow chemistry evolves, it is apparent that new programs ought to incorporate analyses of a wider variety of chemical species in order to d6termine the relative importance of various sources (e.g. marine, crustal, biogenic, anthropogenic, volcanic). For example, marine sulfate could be quantified by measuring methyl sulfonic acid. Inputs from the and regions of central Asia and local crustal debris could be identified by measuring the aluminium, sodium, calcium, magnesium, reactive silica and iron content of the samples. A potential volcanic source of sulfate could be identified by looking at microparticle, chloride and fluoride concentrations, and sulfur isotope ratios. The remaining excess sulfate would most likely be due to anthropogenic activity. Anthropogenic influence could be quantified by analyzing the snow for such constituents as ammonium, nitrogen and sulfur isotopes, and various trace metals. Stratospheric versus tropospheric inputs can be distinguished using 7Be as a tracer. In addition, we must also improve our understanding of the air/snow fractionation processes which dominate in the different climatic regimes of the area. We are in the process of developing this type of data set for snow samples collected from selected sites throughout central Asia.

The glaciochernical record in central Asian glaciers represents an extremely valuable, yet virtually untapped, resource. However, depending upon the sample location, glaciochemical records recovered from central Asian glaciers could potentially be swamped by high levels of marine, continental dust and/or anthropogenic inputs; a problem less prevalent in polar regions. Therefore, understanding the processes which control the chemical content of snow, the local to regional scale complexities, and the seasonal variability are fundamental steps necessary in order to assess the potential for recovering representative long-term glaciochemical records from central Asia.

REFERENCES

Davidson, C.I., S.F. Lin, J.F. Osborn, M.R. Pandey, R.A. Rasmussen, and M.A.K. Khalil. 1986. Indoor and outdoor air pollution in the Himalayas. Environ. Sci. Technol.,20(6), 561-567.

Grootes, P.M., M. Stuiver, L.G. Thompson, and E. Mosley Thompson. 1989. Oxygen isotope changes in tropical ice, Quelccaya, Peru. J. Geophys. Res., 94(DI), 1187-1194.

N Biato Glacier Jenkins, M.D., J.1. Drever, R.G. Reider, and T. Buchanan. 1987. Chemical composition of fresh snow on Mount Everest. J. Geophys. Res., 92(D9), 10,999-11,002.

Dunde Ice Cap Li Jijun and Xu Shuying. 1980. The distribution of glaciers on the Qinghai-Xizang plateau and its relationship to atmospheric circulation. In Miller, K.J., ed. The International Karakoram Project. Vol. 1. Cambridge, etc., Cambridge University Press, 84-93.

Lorius, C., and 6 others. 1985. A 150,000-year climatir; record from Antarctic ice. Nature, 316(6029), 591-596.

Lyons, W.B. and P.A. Mayewski. 1983. Nitrate plus nitrite concentrations in a Himalayan ice core. Geophys. Res. ;Lett., 10(12), 1160-1163.

Lyons, W.B,, P.A. Mayewski, M.J. Spencer, and M.S. Twickler. In press. Nitrate concentrations from remote areas: implications for the global NOx flux. Biogeochemistry.

Mayewski, P.A. and W.B. Lyons. 1983. Chemical composition of a high altitude fresh snowfall in the Ladakh Himalayas. Geophys. Res. Left., 10(l), 105-108.

Mayewski, P.A., W.B. Lyons, and N. Ahmad. 1981. Reconnaissance glacio-chemical studies in the Indian Himalaya. Proceedings of the Eastern Snow Conference, 38th annual meeting, 45-48.

Mayewski, P.A., W.B. Lyons, N. Ahmad, G. Smith, and M. Pourchet. 1984. Interpretation of the chemical and physical time-series retrieved from Sentik Glacier, Ladakh Himalaya, India. J. Glaciol., 30(104), 66-76,

Mayewski, P.A., W.B. Lyons, M.J. Spencer, and J.L. Clayton. 1986a. Correspondence. Snow chemistry from Xixabangma Peak, Tibet. J. Glaciol., 32(112), 542-543.

Mayewski, P.A., and 7 others. 1986b. Sulfate and nitrate concentrations from a south Greenland ice core, Science, 232, 975-977.

Mayewski, P.A., M.J. Spencer, and W.B. Lyons. In press. A review of glaciochernistry with particular emphasis on the recent record of sulfate and nitrate. Presented at Office for Interdisciplinary Earth Studies (OIES) - Global Change Institute-Trace Gases and the Biosphere, Snowmass, Colorado, August 1988.

Nakawo, M., Y. Ageta, and H. Jiankang. 1990. Climatic information from the Chongce Ice Cap, West Kunlun, China. Ann. Glaciol., 14, 205-207.

National Academy of Sciences. 1984. Global Tropospheric Chemistry Program. Washington, DC, National Academy Press.

National Academy of Sciences. 1986. Global change in the geosphere-biosphere. Initial priorities for an IGHP. Washington, DC, National Academy Press.

Niewodniczanski, J., J. Grabczak, L. Baranski, and J. Rzepka. 1981. The altitude effect on the isotopic composition of snow in high mountains. J. Glaciol., 27(95), 99-111.

Thompson, L.G., Wu Xiaoling, E. Mosley-Thompson, and Xie Zichu. 1988. Climatic records from the Dunde Ice Cap, China. Ann, Glaciol., 10, 178-182.

Thompson, L.G., and 9 others. 1990. Glacial stage ice core records from the subtropical Dunde Ice Cap, China. Ann. Glaciol., 14, 288-297.

Wake, C. 1987. Snow accumulation studies in the central Karakoram, Pakistan. Proceedings of the Eastern Snow Conference, 44th annual meeting, 19-33.

Wake, C. 1989. Glaciochemical investigations as a tool for determining the spatial and seasonal variation of snow accumulation in the central Karakoram, northern Pakistan. Ann. Glaciol., 13, 279-284.

Wake, C.P. Unpublished. The spatial and temporal variation of snow accumulation in the central Karakoram, northern Pakistan. (M.A. thesis, Wilfrid Laurier University, 1987.)

Watanabe, 0., S. Takenaka, H. lida, K. Kamiyama, K. Thapa, and D. Mulmi. 1984. First results from Himalayan Glacier Boring Project in 1981-1982. Bull. Glacier Res. 2, 7-23.

Wushiki, H. 1977a, Deuterium content in the Himalayan precipitation at Khumbu district, observed in 1974/1975. Seppyo, 39, Special Issue, 50-56.

Wushiki, H. 1977b. Ice cliffs and exposed stratigraphy of Kongma Glacier, Khumbu. Seppyo, 39, Special Issue, 22-25.