The Branched GDGT Isomer Ratio Refines Lacustrine Paleotemperature Estimates

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane-spanning lipids synthesized by bacteria in numerous substrates. The degree of methylation of the five methyl brGDGTs in both soils and lake sediments, described by the MBT′_5Me index, is empirically related to surface atmospheric temperature. This relationship in lakes is generally assumed to reflect lake surface temperatures captured by brGDGT production in the water column and exported to lake sediments, and the MBT′_5Me index has been applied to brGDGTs in lake sediment successions to reconstruct changes in temperature through time. We analyzed the relationship between MBT′_5Me and the isomerization of brGDGTs (IR_6Me) in globally distributed surficial lake sediments and demonstrated that the relationship, and calibrations, of MBT′_5Me and temperature in middle and high latitude lakes are sensitive to incompletely understood factors related to IR_6Me. IR_6Me does not appear to track a non-thermal influence of brGDGT methylation in tropical lakes, but this could change as the data set is expanded. We address ongoing challenges in the application of the MBT′_5Me paleothermometer in middle and high latitude lakes with new MBT′_5Me-temperature calibrations based on grouping lakes by IR_6Me. We demonstrate how IR_6Me can distinguish samples with a significant non-thermal influence on MBT′_5Me by targeting anomalously warm temperatures during the Last Glacial Maximum from newly analyzed piston and gravity core samples from Lake Baikal, Russia.