Recent climatic significance of chemical signals in a shallow firn core from an alpine glacier in the South-Asia monsoon region

There are 19 sub-tropical temperate glaciers on Mount Yulong, the southernmost currently glacier-covered area in Eurasia, controlled by the south-western monsoon climate. In the summer of 1999, a firn core, 10.10 m long, extending down to glacier ice, was recovered in the accumulation area of the largest glacier, Baishui No. 1. Periodic variations of climatic signals above 7.8 m depth were apparent, and net accumulation of four years was identified by the annual oscillations of isotopic and ionic composition. The boundaries of annual accumulation were confirmed by higher values of electrical conductivity and pH, and by dirty refreezing ice layers at the levels of summer surfaces. Calculated mean annual net accumulation from 1994/95 to 1997/98 was about 900 mm water equivalent. The amplitude of isotopic variations in the profile decreased with increasing depth, and isotopic homogenization occurred below 7.8 m as a result of meltwater percolation. Variations of δ18O above 7.8 m showed an approximate correlation with the winter climatic trend at Lijiang station, 25 km away. Concentrations of Ca2+ and Mg2+ were much higher than those of Na+ and K+, indicating that the air masses for precipitation were mainly from a continental source, and that the core material accumulated during the winter period. The close correspondence of Cl- and Na+ indicated their common origin. The decreasing trend of Na+/Cl- ratios with increasing depth further reflects a progressive homogenization process caused by meltwater percolation. Concentrations of SO42- and NO3- in the core are quite low. The mean annual net accumulation in the core and the estimated ablation indicate that the average annual precipitation above the glacier's equilibrium line is 2400-3100 mm, but this needs to be confirmed by long-term observation of mass balance. © 2002 Elsevier Science Ltd. All rights reserved.