Influence of snow surface sublimation on stable isotope and chemical records and on surface energy balance over a Bolivian glacier, Illimani.

Post deposition processes like sublimation of surface snow are on primary importance in cold high latitude glaciers in the tropical Andes. Such a process modifies surface energy balance, isotopic and chemical composition of deposited snow. This is a real problem for interpretation of isotopic and chemical profiles from tropical ice cores. A sublimation experiment has been carried out during May 2001 where a 137 m ice core has been drilled down to the bedrock in June 1999 (Illimani, 17^oS, 68^oW, 6340 m). Variations of surface snow composition (both water stable isotopes and chemistry) have been monitoring over one week, twice a day while measurements of surface enegy balance were made with an automatic weather station. We present here the combination of these results. Main result from energy balance surface study is that sublimation rate is very hight. Chemical and isotopic compositions of surface snow, show also that sublimation probably affects climate signal recorded in the ice. Thus, interpretation of stable isotopes and chemical records from ice cores have to be carefully done under a certain temporal resolution.