Radiometric chronology of Changme-Khangpu glacier, Sikkim

Abstract

The ³²Si concentration in a sample of surface ice from the snout of Changme-Khangpu glacier is 0.36 disintegrations per minute/tonne compared to the fall-out value of 0.7 d.p.m./tonne. If this decrease is assumed to be solely due to decay of ³²Si, an age of c. 100 years is estimated for the surface ice of the snout, leading to an average flow velocity of c. 40 m/year for the past century. A vertical profile of ²¹⁰Pb in a core taken at an altitude of 5040 m shows two horizons where this isotope is enriched, one between 3 and 4 m and another between I I and 12 m, indicating that the primary concentration of ^2l0Pb can change by physicochemical processes like adsorption on dust. None the less, a longitudinal profile along the glacier shows a systematic decrease of ^2l0Pb activity with decreasing altitude, the surface ice of the snout giving a value of 0.2 d.p.m./I, corresponding to an age of 100 years which is concordant with the ³²Si age. This surface flow-rate of the glacier is much larger than the average contemporary flow-rate (c. 13 m/year). The difference can be understood of the past history of advance and recession of the glacier as revealed by the geomorphic evidence.