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

•The 5- and 6-methyl brGDGTs in soils from an altitudinal transect were investigated.•The 6-methyl brGDGTs in the soils were pH-dependent.•A new liquid chromatography method was used to separate 5- and 6-methyl isomers.•Two new isomers of tetramethylated brGDGTs were found.•Global calibrations of IRIIIa′ and IRIIa′ for soil pH reconstruction are proposed.