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 considered to be ideal tracers of the water cycle, an isobalance of accumulation and discharge from glaciated areas is therefore challenging. In this article we review data from the Alps concerning both the isotopic information of accumulated snow and ice and the relevant information of glacial discharge 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 precipitation measurements it follows that uncontrolled erosive loss of parts of the seasonal (mainly winter) snow cover is more important for input-output considerations of glacial discharge, reconstruction of mean accumulation rates and palaeotemperatures by means of isotopes, than occasional melting or even rainfall events. The order of magnitude of the latter is controlled by laboratory and field experiments. Sublimation and evaporation turn out to be of minor influence if only the absolute values and not the deuterium excess (the relationship of both stable isotopes, deuterium and oxygen-18) is considered. Nevertheless, during warming periods a pronounced increase of deuterium excess caused by intensive melting and evaporation may be used as an additional climate indicator. The different isotopic signature of the seasonal snow cover, old glacier ice and groundwater stored in the glacier bed enables a quantitative separation of the individual water components discharging at a given time of the day or the season. Although several detailed isotope studies are reported, a complete isobalance from glaciated areas based on an interdisciplinary approach, including hydrology and glaciology, is still missing. The hydroclimatic imprint of the discharge from glaciated areas remains even on a regional scale. Snowmelt shifts the more negative isotope values from winter precipitation to the summer months. Interannual climatic variability and changes in seasonal distribution of precipitation (for instance reduced winter precipitation during the late 1980s) are recorded by the stable isotopes in precipitation and discharge as well indicating that the modification of the isotopic information during glacial and post-glacial processes does not override climatic trends. Copyright (C) 2000 John Wiley & Sons, Ltd.