Microbial lipid records of highly alkaline deposits and enhanced aridity associated with significant uplift of the Tibetan Plateau in the Late Miocene

Saline alkaline sediments and soils are widespread in arid and semiarid regions, but their occurrence in ancient dry periods remains unknown due to the lack of a suitable proxy. On the basis of investigations of modern Chinese soils with a wide pH range of 3.5-9.1, we suggest that the microbial lipid ratio R-i/b, i.e., the abundance ratio of archaeal isoprenoid GDGTs (glycerol dialkyl glycerol tetraethers) to bacterial branched GDGTs, indicates the presence of drought-induced alkaline deposits in terrestrial settings. The R-i/b is invariant in modern soils with pH < 7.5 and when the local mean annual precipitation >600 mm, but it increases sharply at higher pH values and lower mean annual precipitation (< 600 mm). In contrast, the CBT index (the cyclization ratio of branched GDGTs), which has been proposed to reflect environmental pH in other contexts, appears to be relatively stable in the highly alkaline Chinese soils from semiarid and arid regions investigated. We further explore the R-i/b ratio in a fluviolacustrine section in the Zhada basin of the southwestern Tibetan Plateau, covering the time period 9.2-2.6 m.y. ago. The R-i/b ratio remains relatively stable in most intervals but exhibits maxima in some horizons, indicative of the occurrence of severe drought and alkaline deposits in the basin catchment. These occur in fluvial sediments deposited 9 m.y. ago, a critical time with respect to the intensification of the East Asian and Indian monsoons, and the significant uplift of the plateau that has previously been associated with enhanced aridity in Central Asia.