Mid-Holocene moisture maximum revealed by pH changes derived from branched tetraethers in loess deposits of the northeastern Tibetan Plateau

Abstract The patterns of Holocene moisture variability inferred from different proxies remain controversial in the northeastern Tibetan Plateau (NETP), which impedes our understanding of the behavior and dynamics of the East Asian Summer Monsoon (EASM). Here we present two pH records spanning the last ~12 kyr from well-dated loess-paleosol sections (YWY14 and SHD09) from the NETP. Based on the modified cyclization ratio index (CBT′) of the branched glycerol dialkyl glycerol tetraethers (brGDGTs) and regional CBT′-pH empirical relationship, the reconstructed pH values from topmost samples are consistent with the instrumental pH values of the surrounding surface soil. Our results illustrate generally similar pH trends from two loess sections characterized by alkaline conditions from 12.0 to 8.5 cal kyr BP, neutral or acidic during the mid-Holocene and alkaline conditions again during the late Holocene. The intervals of lowest CBT′-derived pH values during the mid-Holocene in our records correlate with those of highest tree pollen percentage from adjacent lake sediments and the regionally weakest aeolian activity from records of paleosol development synthesized from the Chinese Loess Plateau (CLP) in northern China, suggesting a moisture maximum during that period. Taken together, we conclude that CBT′-derived pH variability can be potentially used as an indicator of EASM evolution and the wettest climate occurred during the mid-Holocene in the NETP. The changes in summer precipitation during the Holocene on the NETP can be attributed to the interaction between the mid-latitude westerlies and the EASM circulation.