Soil pH Dominates the Distributions of Both 5‐ and 6‐Methyl Branched Tetraethers in Arid Regions

Abstract

AbstractBranched glycerol dialkyl glycerol tetraether (brGDGT)‐based indices are increasingly used for quantitative mean annual air temperature (MAAT) and soil pH reconstructions. However, large scatter in the relationship between the methylation index of 5‐methyl brGDGTs (MBT′5ME) and MAAT still exists in arid and/or cold regions, and specific reasons for that offset remain unclear. Here, we present the brGDGT distributions from 18 soil samples along an altitudinal transect spanning a large gradient in MAAT at Mount (Mt.) Tienshan in arid central Asia, which we compare with previously published data from 256 globally distributed soils. Our results indicate that a dramatic increase of brGDGT concentrations occurs at 2,500 m above sea level for the soils from Mt. Tienshan, and a different pattern of brGDGT distributions in response to environmental changes appears below and above this elevation. Soil pH is observed to control the distributions of both 5‐ and 6‐methyl brGDGTs in the soils from Mt. Tienshan and other arid regions. Importantly, we find that the fractional abundances of the 5‐methyl isomers of pentamethylated and hexamethylated brGDGTs (IIa, IIIa) show a distinct negative correlation with soil pH, and the noncyclopentane‐containing tetra‐methylated brGDGT (Ia) is not related to MAAT in arid soils, which can explain the remaining temperature offset of reconstructed MAAT based on the MBT′5ME index in arid regions. In addition, our data suggest that either the brGDGT‐producing community differs or brGDGT‐producers respond differently to temperature in arid relative to moist soils.