THE AMOUNT EFFECT AND OXYGEN ISOTOPE RATIOS RECORDED IN HIMALAYAN SNOW

CAMERON WAKE
Glacier Research Group, Institute for the Study of Earth, Oceans and Space
University of New Hampshire, Durham, NH 03824 USA

and

MICHELE STIEVENARD
Laboratoire de Moddlisation du Climat et de I'Environnement,
Centre d'Etudes Nucleaires de Saclay, 91191 Gif-sur-Yvette Cedex, France.

Early research on the isotopic content of precipitation (e.g., Dansgaard, 1964) showed that annual mean d180 and dD values measured in precipitation were closely related to annual mean surface air temperature for mid and high northern latitude coastal stations. This in turn led to the extensive use of stable isotopic ratios measured in firn/ice cores recovered from polar regions for the reconstruction of past variations in climate (e.g., Jouzel et al., 1987; Dansgaard et al., 1993).

While the link between the isotopic content of precipitation and climate in polar regions is well understood, this is not the case in temperate and tropical regions (Rozanski et al., 1992). Analysis of data derived from the International Atomic Energy Agency (IAEA)/World Meteorological Organization (WMO) global survey of mean monthly stable isotopic ratios in precipitation reveals that, at tropical marine stations, there exists a strong inverse relationship between d180 and the amount of precipitation at these stations (Dansgaard, 1964-, Rozanski et al., 1993). Furthermore, at tropical stations in monsoon climates (e.g., New Delhi and Hong Kong) the mean monthly d180 is inversely correlated with monthly temperatures, confirming the dominant role of the 'amount effect' in controlling the observed seasonal variations of d180 in precipitation at these stations (Rozanski et al., 1993). In a study of snow accumulation in the Peruvian Andes, Grootes et al. (1989) found that the strong seasonaA signals in 6180 values in snow from the Quelccaya Ice Cap (13.90S latitude) were opposite to the trend frequently observed in middle and high latitudes, and were predominantly the result of the depletion of d180 during heavy precipitation over the Amazon Basin during the austral summer.

There has been only limited research on the'seasonal cycles of stable isotopes in precipitation in the Himalayan/Tibetan Plateau region. Wushiki (1977) reported on dD analyses of precipitation samples collected on a daily basis over the course of an entire year at the Lha~jung meteorological station (4420m, 27.90N latitude) on the southern slopes of the Himalaya in the Khumbu region. dD shows a strong depletion in deuterium (i.e. more negative values) during the summer monsoon season (July to September), indicating that the amount effect is the dominant control on the isotopic content of precipitation during this period of the year. Wushiki (1981) also investigated the spatial variation in isotopic ratios in precipitation on the southern and western margins of the Tibetan Plateau. Mayewski et al. (1983: 1984) report on dD signals in fresh snow and a 17m ice core recovered from the Ladakh Himalaya.

More recently, d180 was measured in precipitation collected at the Delingha meteorological station (37.3N latitude) in the northeastern region of the Tibetan Plateau (Yao et al., 1995). At this more northerly location, the seasonal variation in monthly mean d180 values and surface air temperature are similar, suggesting that the temperature effect predominates in the northeastern portion of the Tibetan Plateau.

In this paper, we extend the spatial coverage of the isotopic content measured in central Asian precipitation through the investigation of seasonal variations in d180 values recorded in snow at four high elevation glaciers which cover a broad region influenced by summer monsoon precipitation.

The oxygen isotope and major ion data presented below was derived from analysis of samples collected from fresh snowfall events and snowpits at four sites in the Himalayas and the southern/central region of the Tibetan Plateau (Fig.1).

Ngozumpa Glacier lies in the Khumbu Region on the southern slopes of the eastern Himalaya. Ile region is strongly influenced by the Indian summer monsoon and receives the majority of annual precipitation during the summer (Inoue, 1976). Winter precipitation at higher elevations is derived from low pressure systems which are steered along the southern slope of the Himalayas by the westerly jet stream (Barry, 198 1). Snow samples were collected from a 3 meter deep snowpit excavated at 5700m in the accumulation zone.

The Xixabangma massif is located on the northern slopes of the Himalaya. The main crest of the Himalaya represents the climatic boundary between a region dominated by the influence of the Indian summer monsoon to the south, and the relatively cold, dry, continental climate which characterizes much of the Tibetan Plateau. Xixabangma is still influenced by precipitation derived from the summer monsoon, but receives less precipitation than the southern slopes of the Himalayas. Snow samples were collected from two fresh snowfall events on the Kangwure Glacier on the northeast margin of Xixabangma. Unfortunately, accumulation on the Kangwure Glacier occurs via superimposed ice - even at 6140m. Only a few tens of centimeters of snow lay on top of glacier ice. We were therefore unable to recover a reliable annual record from this site.

The Qlang Yong Glacier lies in the Gangdise mountains, a range which parallels, and lies north of, the Himalaya. The climatic regime is similar to that described for Mt. Xlxabangma. Snow samples were collected from a 1.7 in snowpit at 5850m. The snowpit was dug down to glacier ice.

Mt. Geladaindong lies in the central region of the eastern Tibetan Plateau. The region is characterized by grassland steppes to the south and east, and and regions to the north and west. Summertime precipitation is derived from plateau monsoon circulation (Murakami, 1976). The region generally receives limited precipitation during the winter, although the occasional influx of westerly disturbances in the winter can result in large snowfall events (Seko and Takahashi, 199 1). Snow samples were collected from a 2m deep snowpit at 5950 m.

Fresh snow samples were collected at 5700 m on Kangwure Glacier from two separate snowfall events. The first snowfall, on the evening of September 10 and early morning of September 11, deposited approximately 35-40 cm of snow. The second, on the evening of September H, deposited 3-4 cm of snow, mostly in the form of graupel. Fresh snow samples were collected in the morning following the event.

Air temperature and atmospheric pressure were measured hourly by a Grant (trademark) automatic weather station at 5900 m on Kangwure Glacier (V. Aizen, pers. comm.) (Fig.2). The temperature record reveals a shift to cooler nighttime temperatures during the afternoon on September 11th. The ambient air temperature during the September 10/11 snowfall event was approximately 10C warmer than during the late evening event on September 11. Note also that atmospheric pressure dropped rapidly at midday on Sept. 11.

Analysis of the d180 and maJor ion concentrations of the snow reveal distinct differences for the two snowfall events (Table1). Precipitation from the September 10/11 event shows very low ion burdens and relatively greater depletion of 180 while recipitation from the September 11 event shows very high ion burdens and less negative d180 values.

The meteorological and snow chemistry data indicate that the two snowfall events originated from two very different air masses, and that the d180 values vary inversely with surface air temperature. The low d180 values associated with the warm, moist air on' September 10/11 is typical of monsoonal air masses (Wushiki et al., 1977). The data suggest that the isotopic composition of precipitation during this snowfall event was controlled by the amount of rain falling over the Indian subcontinent as the air mass traveled from the Bay of Bengal to the Himalaya. This interpretation is supported by the very low ion concentrations, which are typical for monsoon derived precipitation and result from the washout of ions as the airmass travels inland (Wake and Mayewski, 1993; Wake et al., 1993). In contrast, the September I I event derived from a colder, low pressure air mass, characteristic of low pressure systems brought into the region with the westerly jet stream (Barry, 198 1). The relatively high ion concentrations (especially calcium) also suggests a westerly source, as vast and regions lie in the western regions of the Tibetan Plateau and are a large source of desert dust rich in calcium.

Snow samples covering an entire years annual accumulation were collected in late summer/early autumn from Ngozumpa Glacier, Qlang Yong Glacier and Mt. Geladaindong. This allowed for collection of the most recent summer monsoon snow accumulation, as well as the previous winters snow accumulation. Snow samples were collected over a continuous vertical section at 5 or 10 cm intervals, resulting in approximately 15 to 25 samples per annual layer.

The physical stratigraphy, d180, and calcium depth profiles, along with a delineation of seasonal layers, are shown in (Fig.3). As all the pits were sampled at the end of the summer monsoon period, the uppen-nost snow represents accumulation from monsoonal air masses. In the Khumbu region, two dust layers are commonly deposited in each annual layer in snow accumulation zones - one during the autumn and one during the spring (Miller et al., 1965). The winter layer from Ngozumpa Glacier is readily identified lying between two dirty horizons. At Qiang Yong Glacier, the transition from summer to the previous winters snow is marked by a change to wet, soft granular snow (warmed during the spring before the onset of the summer monsoon), a change in 8180 to less negative values, and an increase in calcium concentrations. At Mt. Geladaindong, dust layers are deposited during the spring when dust storms in western China transport large quantities of dust eastward over the Tibetan Plateau (Middleton et al., 1986).

Although there are differences from site to site, the d180 profiles from all three locations show the greatest depletion of 180 in snow layers which accumulated during the summer monsoon season. This relationship between d180 values in monsoon derived precipitation indicates that in the eastern Himalaya and the southern/central region of the Tibetan Plateau the stable isoto ic composition of precipitation is controlled by the amount effect, and that this seasonal d18O signal is well preserved in the accumulation zones of high elevation glaciers in the region.

The fresh snow data from Xixabangma reveal the inverse relationship between d180 values in precipitation and surface air temperature. The chemistry and meteorological data suggest that the amount effect is the controlling factor on 180 values in summer monsoon precipitation. The annual records from snow accumulation zones in the Himalaya and the southern/central region of the Tibetan Plateau indicate that the amount effect, which controls the seasonal signals in the stable isotopic composition of precipitation at lower elevations in the Himalayas (Wushiki, 1977) and in New Delhi (Rozanski et al., 1993), is clearly represented and preserved in the annual layers accumulating on high elevation glaciers.

The amount effect on d180 values in precipitation in the Himalayas and the southern/central region of the Tibetan Plateau is related to the amount of precipitation falling over the Indian subcontinent as air masses travel inland from the Bay of Bengal. Longer records of d180 in precipitation in the region influenced by the summer monsoon, available through the recovery and analysis of ice cores from high elevation glaciers, should be collected in order to investigate the relationship between the amount of monsoonal precipitation over India and Nepal over the last 100 years, and the stable isotope signal preserved in snow accumulation zones. An improved understanding of this relationship should provide us with the capability of using the ice core record to reconstruct summer monsoon precipitation over the Indian subcontinent for hundreds to thousands of years before present.

We thank Paul Mayewski for his support of this research, and the Lanzhou Institute of Glaciology and Geocryology, China and the Nepalese Department of Hydrology and Meteorology for their help during the field programs. Vladamir Aizen kindly provided meteorological data from the Kangwure glacier. This research was supported by the US National Science Foundation.

Barry, R.G. (198 1) Mountain Weather and Climate. Methuen, New York.

Dansgaard, W. (1964). Stable isotopes in precipitation. Tellus 16, 436-468.

Dansgaard, W. and 10 others (1993) Evidence for general instability of past climate from a 250-kyr ice-core record. Nature 364, 218-220.

Grootes, P. M., M. Stuiver, L.G. Thompson, and E. Mosley-Thompson (1989) Oxygen Isotope Changes in Tropical Ice, Quelccaya, Peru. Journal of Geophysical Research 94,1187-1194.

Inoue, H. (1976) Climate of Khumbu Himal. SEPPY0 (Journal of the Japanese Society of snow and ice) 38 (Special Issue), 66-73.

Jouzel, J., C. Lorius, J.R. Petit, C. Genthon, N.I. Barkov, V.M. Kotlyakov and V.M. Petrov(1987). Vostok ice core: a continuous isotope temperature record over the last climatic cycle (160,000 years). Nature 329, 403-407.

Mayewski, P. A., W. B. Lyons and N. Ahmad (1983) Chemical composition of a high altitude fresh snowfall in the Ladakh Himalayas. Geoph * vs. Res. Lett. 10, 105-108.

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. 30,66-76.

Middleton, N., A. Goudle and G. Wells (1986) The frequency and source areas of dust storms. In Aeolian Geomorphology, W. Nickling (ed.) Allen Unwin, Boston, 237-259.

Miller, M.M., J.S. Leventhal, and W.F. Libby (1965) Tritium in Mt. Everest ice-annual glacier accumulation and climatology at great equatorial altitudes. J. Geophys. Res. 70. 3885-3888.

Murakami, T. (1987) Effects of the Tibetan Plateau. In C.P. Chang and T.N. Krishnamurti (eds.) Monsoon Meteorology, Oxford University Press, New York. p. 235-270.

Rozanski, K., L. Araguds-Araguds and R. Gonfiantini (1993) Isotopic patterns in modem global precipitation. In P.K. Swart, K.C. Lohman, J. McKenzie and S. Savin (eds.) Climate Change in Continental Isotopic Records - Geophysical Monograph 78, American Geophysical Union, Washington, D.C., p. 1-36.

Rozanski, K., L. Araguds-Araguds and R. Gonfiantini (1992) Relation between long-term trends of 180 isotope composition of precipitation and climate. Science 258, 981-985.

Seko, K. and S. Takahashi (1991) Characteristics of winter precipitation and its effect on glaciers in the Nepal Himalaya. Bulletin of Glacier Research 9, 9-16.

Wake, C. P. and Mayewski, P. A. (1993). The spatial variation of Asian dust and marine aerosol contributions to glaciochemical signals in central Asia. In G. Young (Ed.), International Symposium on Snow and Glacier Hydrology. Kathmandu. IAHS Pub. No. 218,385-402

Wake, C. P., Mayewski, P. A., Xie, Z., Wang, P., and Li, Z. (1993). Regional variation of monsoon and desert dust signals recorded in Asian glaciers. Geophys. Res. Lett. 20, 1411-1414.

Wushiki, H. (1977) Deuterium content in the Himalayan precipitation at Khumbu District, observed in 1974/1975. SEPPY0 (Journal of the Japanese Society of snow and ice) 39 (Special Issue), 50-56.

Wushiki, H. (1981) Some characteristics of stable isotope content in Himalayan waters. In Liu, D.S. (ed.) Geological and Ecological Studies of the Qinghai-Xizang Plateau, Vol. 2. Gordon and Breach Science Publishers Inc., New York, 1671-1676

Yao, T., L. Thompson, K. Jiao, E. Mosley-Thompson and Z. Yang (1995) Recent warming as recorded in the Qinghai-Tibetan cryosphere. Annal. Glaciol. 21, 196-200.