Changes in ionic and oxygen isotopic composition of the snow-firn pack at Baishui Glacier No. 1, southeastern Tibetan Plateau

To properly interpret climatic and environmental information recorded in ice cores retrieved in the temperate-glacier region, the post-depositional processes, especially the meltwater percolation, need to be well understood. From late April to mid-July 2006, two successive snow pits, site P1 located at 4,750 m a.s.l. in the ablation zone and site P2 at 4,900 m a.s.l. in the accumulation area at Baishui Glacier No. 1, Mt. Yulong, southeastern Tibetan Plateau, were excavated. Changes in soluble ions (K+, Na+, Ca2+, Mg2+, Cl-, NO3 (-) and SO4 (2-)) and stable oxygen isotopes (delta O-18) in the snow pits were investigated. Before the melting period, soluble ions and delta O-18 exhibit pronounced seasonal variations, with high values in the upper snow-firn layer (accumulated in winter/spring season) and relatively low values in the lower firn layer (deposited in summer season), reflecting seasonal shift in the Indian summer monsoon and winter westerlies over the region. During the melting period, most ions in the upper snow-firn layer were leached with different efficiencies. Although no significant changes in ion concentrations in the lower firn layer were observed, the impurities in the upper snow-firn layer could be eluted down into the lower firn layer. However, the pre-melting isotopic stratigraphy remained relatively unchanged during the observation period, indicating that the influence of meltwater percolation on stable isotopes is less significant than that on soluble ions. This understanding is crucial for the interpretation of ice-core records in the monsoon temperate-glacier region.