Dominant environmental variables affecting brGMGT-derived temperature proxies differ in lake surface sediments and sediment cores

Branched glycerol monoalkyl glycerol tetraethers (brGMGTs), a suite of orphan membrane lipids structurally similar to branched glycerol dialkyl glycerol tetraethers (brGDGTs), have been proposed as promising tools for quantitative paleotemperature reconstruction in lakes. However, the applicability of brGMGT-based temperature proxies and the influence of salinity and dissolved oxygen (DO) concentration on brGMGT distributions remain largely unexplored, limiting their use as reliable and quantitative paleoclimate proxies in sediment cores. To address these uncertainties, we examined brGMGT concentrations and distributions in surface sediments from West China lakes with varying temperatures and salinities, as well as transects in Lake Yangzonghai featured by a wide gradient of DO concentration. Our results show that brGMGTs are abundant and primarily autochthonous in lake surface sediments, with minimal contributions from surrounding soils. Temperature was found to be the primary factor influencing brGMGT distributions. The brGMGT-based proxy brGMGTII, newly proposed in this study, shows significant correlations with mean annual air temperature (MAAT) in surface sediments from both West China and East African lakes (R2 = 0.90 and 0.91, respectively; both p < 0.01), underscoring its potential as paleotemperature indicator. BrGMGTII calibrations for temperature reconstructions were established for these regions, yielding RMSE (Root mean squared error) values of 2.8 °C and 2.6 °C, respectively. Additionally, high salinity appears to reduce the methylation degree of brGMGTs, whereas low DO concentration may increase the relative abundance of brGMGTs vs. brGDGTs, though DO shows a complex effect on brGMGTI and brGMGTII. Further analysis of brGMGTs in a Holocene sediment core from Lake Yangzonghai in China reveals that these brGMGT-derived temperature proxies inferred unrealistic temperature trends and absolute values. This is likely due to (1) non-temperature factors, particularly DO concentration, which may exert a stronger influence on brGMGT distributions in sediment core than temperature itself; and (2) the unique mechanism by which brGMGTs respond to temperature, i.e., shifts in bacterial communities between high- and low-temperature brGMGT producers rather than a homeoviscous adaptation of membrane lipids.