Branched tetraether lipids in Chinese soils: Evaluating the fidelity of MBT/CBT proxies as paleoenvironmental proxies

The ubiquitous occurrence of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils has allowed development of new proxies for reconstruction of past climate and environment. The methylation and cyclization degrees of brGDGTs, expressed as MBT and CBT, respectively, are reported to be mainly controlled by mean annual air temperature (MAAT) and soil pH. However, the brGDGT-derived temperatures and soil pH scatter widely when data from different environmental conditions are considered. In this study, we collected over 300 soil samples from China, which are representative of humid (Xishuangbanna, Guangzhou, and Shanghai), semi-arid (Dongying) and semi-arid/arid (Lanzhou, Tibetan Plateau) regions. Collectively we have the most extensive dataset that broadly characterizes the distribution of brGDGTs according to climate zones in China. The overall data demonstrate that the MBT/CBT derived temperatures better match the measured MAATs in humid and non-alkaline regions than those from regions of low MAP (< 400 mm/yr) and above neutral soil pH (> 7.0-7.5). Similarly, CBT describes soil pH much better in humid and non-alkaline soils than in semi-arid/arid and alkaline soils; the semi-arid/arid and alkaline soils tend to show a positive correlation between soil pH and CBT, which contradicts that in the humid and non-alkaline soils. While soil pH, MAAT and mean annual precipitation (MAP) are dominating factors controlling the brGDGT distribution across all climate zones, conductivity, total organic carbon and total nitrogen, as well as soil water content can also play an important role locally. Removing brGDGT-II resulted in a revised CBT index that provides more accurate estimation of pH, especially in semi-arid/arid and alkaline soils. The overall Chinese dataset demonstrates that continental air temperature derived from brGDGT-proxies can vastly deviate from real measurements and should be used with extreme caution in paleo-climate or -environment studies.