Influence of sublimation on stable isotope records recovered from high-altitude glaciers in the tropical Andes

Sublimation dominates the ablation process on cold, high-altitude glaciers in the tropical Andes. Transport of water vapor through the firn and exchan ge with ambient moisture alter the stable isotope composition of the surfac e layers. A sublimation experiment carried out during an ice core drilling campaign on Cerro Tapado (5536 m above sea level, 30 degrees 08 'S, 69 degr ees 55 'W) revealed a strong enrichment in the H-2 and O-18 content in the surface layer. Concerning the deuterium excess, a decrease occurred at dayt ime, 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. L ow surface temperatures at night cause condensation of water vapor in the f irn pores near the surface and thus inhibit penetration of the isotopically enriched surface front into deeper firn layers. Measuring an isotope profi le obtained through detailed sampling between the surface and 38 cm depth p roved this mechanism. The observed modification of the isotopic composition at the surface was quantitatively described by a model, which also reprodu ced the mass loss measured with sublimation pans and calculated from releva nt meteorological data. The results of this study suggest that the influenc e of sublimation on the preserved isotope record of ice cores under compara ble environmental conditions is rather limited. In any case, simultaneous m easurements of delta H-2 and delta O-18 help to identify layers in an ice c ore which might be effected by sublimation. However, since the mass loss du e to sublimation was of the order of 2-4 mm per day during the experiment, important palaeoinformation from an isotope record could be eliminated duri ng extended dry periods.