From accumulation to discharge: modification of stable isotopes during glacial and post-glacial processes

The isotopes of the water molecule (tritium, deuterium and oxygen-18) are c onsidered to be ideal tracers of the water cycle, an isobalance of accumula tion and discharge from glaciated areas is therefore challenging. In this a rticle we review data from the Alps concerning both the isotopic informatio n of accumulated snow and ice and the relevant information of glacial disch arge on a local to regional scale. In the Alps the seasonal distribution of accumulated snow is governed by small-scale meteorology and exposition and distribution of cold and temperate glaciers. From comparison with direct p recipitation measurements it follows that uncontrolled erosive loss of part s of the seasonal (mainly winter) snow cover is more important for input-ou tput considerations of glacial discharge, reconstruction of mean accumulati on rates and palaeotemperatures by means of isotopes, than occasional melti ng or even rainfall events. The order of magnitude of the latter is control led by laboratory and field experiments. Sublimation and evaporation turn o ut to be of minor influence if only the absolute values and not the deuteri um excess (the relationship of both stable isotopes, deuterium and oxygen-1 8) is considered. Nevertheless, during warming periods a pronounced increas e of deuterium excess caused by intensive melting and evaporation may be us ed as an additional climate indicator. The different isotopic signature of the seasonal snow cover, old glacier ice and groundwater stored in the glac ier bed enables a quantitative separation of the individual water component s discharging at a given time of the day or the season. Although several de tailed isotope studies are reported, a complete isobalance from glaciated a reas based on an interdisciplinary approach, including hydrology and glacio logy, is still missing. The hydroclimatic imprint of the discharge from glaciated areas remains eve n on a regional scale. Snowmelt shifts the more negative isotope values fro m winter precipitation to the summer months. Interannual climatic variabili ty and changes in seasonal distribution of precipitation (for instance redu ced winter precipitation during the late 1980s) are recorded by the stable isotopes in precipitation and discharge as well indicating that the modific ation of the isotopic information during glacial and post-glacial processes does not override climatic trends. Copyright (C) 2000 John Wiley & Sons, L td.