The 6-methyl branched tetraethers significantly affect the performance of the methylation index (MBT′) in soils from an altitudinal transect at Mount Shennongjia

Abstract

The methylation index (MBT or MBT′) of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils along altitudinal transects of some mountains, e.g. Mount (Mt.) Shennongjia and Mt. Gongga, exhibited no correlation with altitude or mean annual air temperature (MAT) alone. However, a significant correlation between MBT′, cyclization ratio (CBT) of brGDGTs and MAT for each transect could still be found. To investigate the reason, we reanalyzed the brGDGTs in soils from Mt. Shennongjia using an improved liquid chromatography method and examined whether the newly described 6-methyl brGDGTs were the main cause of the large scatter in the correlation between MBT′ and MAT. In contrast to MBT′, the MBT′5ME, based on 5-methyl brGDGTs alone, correlated significantly with MAT (R2 0.89, p<0.001) but had no relationship with soil pH. However, MBT′6ME, based only on 6-methyl brGDGTs, was strongly dependent on soil pH. This suggests that pH dependent 6-methyl brGDGTs introduce large scatter in the correlation between MBT′ and MAT, which is likely to mechanistically explain the poor relationship between MBT′ and MAT, yet good correlation between MBT′, pH and MAT. A series of indices derived from 6-methyl brGDGTs, including IRIIIa′ and IRIIa′ (isomer ratio of structures IIIa′ and IIa′, respectively), correlated with soil pH and can be used as new paleo pH indicators to supplement the classic CBT proxy. The tetramethylated brGDGTs with 0–1 cyclopentyl rings (Ia and Ib) were found to comprise unknown isomers, whose abundances also appear to increase with soil pH. A further optimized liquid chromatography method is needed to quantify these new isomers more accurately and to determine the environmental controls on them.