TEX86 as a potential proxy of lake water pH in the Tibetan Plateau

Abstract Water pH represents one of the most important indicators of chemical and biological changes in lacustrine systems. However, although water pH is also a major influence on inland water bodies and related ecosystems, it is not well studied in the lakes of the Tibetan Plateau (TP). The applicability of diatom assemblages, an effective proxy of variations in water pH in freshwater lakes, is greatly limited in the TP because the alkaline environment in most of the lakes is unfavorable for the preservation of diatom frustules. Isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs) are a series of specific membrane lipids biosynthesized by archaea, which appear to be a promising method for reflecting lake water pH variations. In this study, we analyzed the distribution of iGDGTs in the surface sediments of 34 alpine TP lakes and combined the data with published iGDGTs results from across the TP, in order to examine the effect of various environmental factors on iGDGT cyclization. The results show that water pH appears to be the most important factor affecting the degree of cyclization of iGDGTs in high-elevation alkaline lakes. This also suggests that the novel temperature proxy, TEX86, is also a promising water pH proxy for alpine lakes in the region. Despite a moderate correlation is observed between TEX86 and water pH, caution is needed in the interpretation of the TEX86 records, since other factors, such as temperature and the dissolved oxygen level, may also have an influence. Nevertheless, TEX86 appears to be a promising indicator of water pH for the alkaline lakes of the TP, and it could be a potentially useful tool for reconstructing pH in alkaline lakes in other areas, after regional calibrations.