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The Branched GDGT Isomer Ratio Refines Lacustrine Paleotemperature Estimates
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The Branched GDGT Isomer Ratio Refines Lacustrine Paleotemperature Estimates
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Journal Article
The Branched GDGT Isomer Ratio Refines Lacustrine Paleotemperature Estimates
2025
Overview
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 (IR6Me) 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 IR6Me. IR6Me 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 IR6Me. We demonstrate how IR6Me 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. Plain Language Summary Branched glycerol dialkyl glycerol tetraethers are fats used by bacteria to build their cell walls. Bacteria build their cell walls with different kinds of brGDGTs in response to ambient temperature. BrGDGTs are often preserved in lake sediments, making them a useful tool for reconstructing past climate. While working on samples from Lake Baikal, Russia, we noticed unexpectedly warm temperatures during the last ice age estimated from brGDGTs. These warm temperatures coincided with unusually high relative amounts of 6‐methyl brGDGTs. This observation spurred the analysis of a large data set of published globally distributed lake brGDGT data. We found that samples from middle and high latitude lakes with relatively more 6‐methyl brGDGTs tended to have higher than expected brGDGT‐estimated temperatures. We use our findings to refine the equations that relate brGDGT distributions to surface air temperature in middle and high latitude lake sediments. Key Points A non‐thermal effect on brGDGT paleotemperature estimates in lakes is identified by the isomer ratio IR6Me The calibration of MBT′5Me to temperature in middle and high latitude lakes is strengthened if samples with IR6Me > 0.4 are excluded IR6Me does not appear to identify non‐thermal effects on tropical lake samples, but this could change as the data set is expanded
