Assessment of branched glycerol monoalkyl glycerol tetraether (brGMGT)-based paleothermometry in the 250,000-year sediment record of Lake Chala, equatorial East Africa

Branched glycerol monoalkyl glycerol tetraethers (brGMGTs), a relatively understudied group of bacterial membrane lipids structurally similar to branched glycerol dialkyl glycerol tetraethers (brGDGTs), appear to be strongly influenced by temperature in terrestrial settings. In surficial bottom sediments of East African lakes, the abundance of brGMGTs relative to the sum of brGMGTs and brGDGTs (%brGMGT) and brGMGT distribution are strongly related to local mean annual air temperature (MAAT), stimulating development of new paleothermometers. However, applications of these methods to lake-sediment records are currently lacking. Here we investigate brGMGT concentrations and distributions in 916 samples throughout the 250,000-year (250-kyr) sediment sequence from Lake Chala, a presently fresh and permanently stratified (meromictic) tropical crater lake. All seven previously identified brGMGTs occur abundantly, reflected in a relatively high average %brGMGT of 19%. BrGMGTs and brGDGTs concentrations throughout the sequence are strongly correlated (R = 0.83, p $<$ 0.001), suggesting that their producers and/or associated ecological niches substantially overlap. Clear distinction can be made between brGMGTs produced predominantly in the bottom sediments (H1034a and H1034c) versus the anoxic lower water column (H1020a-c and H1034b). Although a 17-month monitoring study of Lake Chala suggested brGMGTs are primarily produced in the sediments, down-core data assign greater importance to aquatic production than previously estimated. Instead of reflecting temperature, %brGMGT variations showed greatest similarity to GDGT proxies reflecting lake depth and/or mixing regime. BrGMGT-based temperature models produce ambiguous reconstructions, showing little similarity to known global temperature trends or the brGDGT-based mean summer temperature (MST) reconstruction from the same sediments.