Global calibration of a novel, branched GDGT-based soil pH proxy

Abstract

Recently, 6-methyl branched glycerol dialkyl glycerol tetraethers (brGDGTs) were separated from 5-methyl brGDGTs, which are used in brGDGT-based proxies. Here we analyzed brGDGTs in 27 soil samples along the 400mm isoline of mean annual precipitation in China by using tandem 2D liquid chromatography. The fractional abundance of 6-methyl brGDGTs showed a positive correlation with soil pH, while that of 5-methyl brGDGTs decreased with increasing soil pH. The abundance ratio of 6-/5-methyl brGDGTs, namely the isomerization of branched tetraethers (IBT), was calculated. The correlation of IBT with pH (pH=6.33−1.28×IBT; R2 0.89; root mean squared error, RMSE, 0.24) was much stronger than that of the traditionally used cyclization index of branched tetraethers (CBT) with pH (R2 0.52; RMSE 0.49) and comparable with that of CBT′ with pH (R2 0.88; RMSE 0.25). Compiling all available data from 319 soil samples resulted in a global calibration: pH=6.53−1.55×IBT (R2 0.72; RMSE 0.65), which has a better correlation than the CBT5ME-pH proxy (R2 0.63; RMSE 0.78), but a weaker correlation than the CBT′-pH proxy (R2 0.85; RMSE 0.52). Our result suggests that the IBT is a promising indicator for soil pH, particularly in cases when some compounds in the CBT′ index cannot be determined.