Abstract. Sublimation dominates the ablation process on cold, high-altitude glaciers in the tropical Andes. Transport of water vapor through the firn and exchange with ambient moisture alter the stable isotope composition of the surface layers. A sublimation experiment carried out during an ice core drilling campaign on Cerro Tapado (5536 m above sea level, 30ø08'S, 69ø55'W) revealed a strong enrichment in the 2H and •80 content in the surface layer. Concerning the deuterium excess, a decrease occurred at daytime, while during the night, the values remained comparatively constant. At daytime the sublimation is enhanced due to the higher moisture deficit of the ambient air accompanied by relatively high firn surface temperatures. Low surface temperatures at night cause condensation of water vapor in the firn pores near the surface and thus inhibit penetration of the isotopically enriched surface front into deeper firn layers. Measuring an isotope profile obtained through detailed sampling between the surface and 38 cm depth proved this mechanism. The observed modification of the isotopic composition at the surface was quantitatively described by a model, which also reproduced the mass loss measured with sublimation pans and calculated from relevant meteorological data. The results of this study suggest that the influence of sublimation on the preserved isotope record of ice cores under comparable environmental conditions is rather limited. In any case, simultaneous measurements of 82H and 8•80 help to identify layers in an ice core which might be effected by sublimation. However, since the mass loss due to sublimation was of the order of 2-4 mm per day during the experiment, important palaeoinformation from an isotope record could be eliminated during extended dry periods.

In order to quantify deep-water exchange in the Caspian Sea, the world's largest inland water body, water samples were analyzed for the transient tracers H, He, He, CFC-11, CFC-12 and atmospheric noble gases. Measurements of temperature, salinity (calculated from conductivity for the ionic composition of Caspian Sea water), and dissolved oxygen were employed to investigate the processes responsible for deep-water renewal. The Caspian Sea consists of two deep basins, the southern and central basins, separated by a sill, and a shallow northern basin. The deep water (below 200 m) accounts for almost 60% of the total water mass. Below 200 m the concentrations of H and He are much lower in the southern basin than at the same depths in the central basin, but this is not the case for either of the CFCs. However, apparent water ages calculated from H}He and from CFC-12 concentrations are the same for the deep water of the southern and central basins, and yield deep-water exchange rates of approximately 7% per year for each of the two basins. This implies volume #uxes across the 200-m level of about 2220 km yr\ within the southern basin and 770 km yr\ within the central basin. Based on the apparent water ages, the oxygen depletion in the deep water is estimated to be about 0.35 mg l\ yr\. The processes responsible for deep-water exchange have not yet been identi"ed conclusively. However, vertical temperature and salinity gradients observed during two expeditions, in September 1995 and 1996, suggest that within the southern and central basins large-scale convection cannot be triggered by seasonal cooling alone, but requires the surface water to be cold/saline or to contain high suspended sediment loads. In the central basin the increase in salinity occurring during ice formation in early winter is possibly su$cient to cause 0967-0637/00/$ -see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 -0 6 3 7 ( 9 9 ) 0 0 0 6 6 -7First publ. in : Deep Sea Research / 1, 47 (2000) convection. In late summer, the horizontal transport of water from the upper 170 m of the central basin into the southern basin results in mixing down to 400 m. In winter this process might result in convection down to the maximum depth. Alternatively, the data are also consistent with the hypothesis that rare events cause large-scale convection down to the maximum depth in the southern and in the central basin simultaneously, followed by slight mixing that mainly a!ects only the top 500 m. According to apparent water ages from below 500 m, the last such major mixing event could have occurred in 1976 shortly before the water level of the Caspian Sea began to rise.

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The link between long term changes in the isotopic composition of precipitation and surface air temperature at a given location is of exceptional importance for paleoclimatic studies, as ahs been demonstrated by many recent publications based on the isotope records from polar ice cores. By means of direct comparison with instrumental data, this paper evaluates the potential of the deuterium and oxygen-18 records from two continental glaciers for monitoring climatic trends. The isotopic data presented characterize climatically contrasted enviroments. The records from the Swiss glacier show distinct seasonal variations. Oxygen-18 is fairly well correlated with the instrumental record of atmospheric temperature; the seasonal differences in deuterium excess reflect nearness to the oceanic moisture source. By contrast, the isotope data from the Mongolian site show poor correlation with atmospheric temperature. The seasonal variations in deuterium excess, with higher values during summer time, indicate that precipitation largely originates from re-evaporated continental moisture sources. In both cases however, the correlation with temperature is significantly improved by the elimination of values derived from years where major changes in seasonal distribution and/or snow loss obviously have occurred, thereby distoring the isotopic ratios for that particular year. Depending on the site selected for study, the stable isotope composition of ice cores should therefore be viewed not only as a proxy for atmospheric temperature, but also as an additional hydrometeorological parameter and source indicator for atmospheric moisture.