Localized environmental control on the distribution of brGDGTs in Chinese soils: Implication for paleo-pH reconstruction

Abstract:

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are unique bacterial lipids that commonly occur in soil and peat bog. The methylation and cyclization degrees of brGDGTs, expressed as MBT and CBT, respectively, are mainly controlled by mean annual air temperature (MAAT) and soil pH. However, the brGDGT-derived temperatures scatter widely compared to actual MAATs in both regional and global calibrations. In this study, we collected 297 soil samples from diverse regions of China (Lanzhou, Guangzhou, Shanghai, Yunnan, Dongying and Tibetan Plateau) in order to identify environmental variables that control the distribution of brGDGTs locally. The results show that MBT correlated positively with MAAT under the global calibration framework; however, large variation in MBT occurred for a given MAAT for individual regions. When confined within a specific region, MBT index positively correlated with conductivity in Dongying soils and negatively with pH in Yunnan and Guangzhou soils. Removing GDGT-II from CBT calculation resulted in a revised CBT index that provides more accurate estimation of pH (R²=0.79 vs 0.67), especially in alkaline soils. In addition, the combination of MBT and revised CBT also improves the estimation of soil pH (R² =0.79) than the original CBT index alone (R²=0.67). Our results demonstrate that brGDGTs-derived proxies may be more reliable for estimating paleo-soil pH than paleo-continental temperature.