Distributions of 5- and 6-methyl branched glycerol dialkyl glycerol tetraethers (brGDGTs) in East African lake sediment: Effects of temperature, pH, and new lacustrine paleotemperature calibrations

The distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils, peats, and lake sedimentshas been shown to correlate with mean annual air temperature (MAAT) and has provided valuablenew climate reconstructions. Here we use an improved chromatographic method to quantify the fractionalabundances of 5- and 6-methyl isomers in surface sediments from 65 East African lakes spanningtemperatures 1.6–26.8 °C, and investigate the relationships between these fractional abundances andtemperature, lake pH, and other environmental variables. We find that temperature exerts a strong controlon brGDGT distributions, including the relative abundances of 5- and 6-methyl isomers, whereasother environmental variables, including lake pH, are weakly correlated to the fractional abundancesof the brGDGTs. The distributions of brGDGTs in our lake sediments differ from those of soils and peats,leading to temperature offsets if soil- and peat-based brGDGT temperature calibrations are applied. Wedevelop new calibrations for MAAT for use in lake sediment based upon the MBT05Me and Index 1 ratios,as well as a multivariate regression of brGDGT fractional abundances on temperature using stepwiseforward selection. We obtain root mean square errors (RMSE) between ? 2.1 and 2.5 °C for these calibrations,highlighting the potential for brGDGTs to provide precise temperature reconstructions using lakesediment cores. Calibrations for lake pH perform more poorly, likely due to weak correlations betweenpH and brGDGT distributions in East African lakes. These results indicate that quantification of 5- and6-methyl isomers separately in lake sediment can improve paleoclimatic reconstructions