The impact of precipitation on the distributions of branched tetraethers in alkaline soils

Abstract

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) have shown promise for quantitative reconstructions of temperature and soil pH. However, several confounding factors (e.g., precipitation) have been suggested to influence the brGDGT distributions in various soils, and it is difficult to tease apart the brGDGTs’ dependencies on individual environmental parameters. Here, we present the first report of the brGDGT distributions in 48 surface soils along a pronounced mean annual precipitation (MAP) gradient from northern Iran and compare these with an updated global dataset. Our results indicate that: (i) the distribution pattern of soil brGDGTs in northern Iran is dominated by non-cyclopentane-containing compounds, which is characterized by a higher fractional abundance of Ia and IIa′ and lower abundances of IIa and IIIa compared to those in adjacent regions (i.e., Xinjiang and Tajikistan), owing to the relatively higher soil pH and temperatures; (ii) the cyclopentane-containing brGDGT Ib and IIb′ show positive correlations with MAP, and 6-methyl brGDGT IIa′ and IIIa′ are negatively related to MAP in alkaline soils from both northern Iran and other regions. This differs from the pH dependency of these brGDGTs in global acid soils; (iii) the Cyclisation of Branched Tetraethers ratios (CBT5ME and CBT6ME) are negatively correlated with MAP in northern Iran, which can be potentially used for regional MAP reconstructions and a series of CBT5ME- and CBT6ME-MAP calibrations and multiple linear regression-based MAP calibrations are proposed. Notably, we assume that the correlation of MAP with 5-methyl brGDGTs (e.g., IIa) in northern Iran partly accounts for the inaccurate temperature estimation derived from the methylation index (MBT′5ME).