Soluble ionic and oxygen isotopic compositions of a shallow firn profile, Baishui glacier No. 1, southeastern Tibetan Plateau

摘要

In the summer of 2004, a firn profile, 18.3m long, extending down to glacier ice, was recovered in the accumulation area of the largest glacier, Baishui No. 1, on Yulong mountain, the southernmost glacier-covered area in mainland Eurasia. Multivariate empirical orthogonal function (EOF) and statistical correlation analyses of major-ion data from the profile demonstrate that three distinct types of ionic material contribute to the chemical characteristics of firn in this monsoonal region: material of marine origin, which is transported by the Indian southwest monsoon; crustal materials, which come from local sources; and anthropogenic pollutants, which are produced by industrial and agricultural activities in South Asia. Although the influence of post-depositional processes on the seasonal isotopic and soluble ionic compositions is significant, dust layers in the firn profile are clearly visible. Due to the effects of meltwater percolation, the dust layers generally coincide with latesummer snow surfaces. We therefore use the dust layers, combined with the seasonal variations of electrical conductivity (EC), Ca2+ and Mg2+, to establish a depth/age scale for the firn profile. The reconstructed net accumulation has a significant negative correlation with the temperature at Lijiang, whereas the correlation between the net accumulation and the precipitation amount at Lijiang is weak. Although the d18O time series of the firn profile was modified significantly by meltwater percolation, the correlation between annual mean d18O values and the Indian southwest monsoon index (WSI1) is significant. This result suggests that d18O records from monsoon-influenced temperate glaciers can provide a valuable record of past variations of the Indian southwest monsoon.