Assessing the reliability of H-brGDGTs as terrestrial paleothermometers in Plio-Pleistocene fluvio-lacustrine deposits from Northern China

Abstract

Branched glycerol monoalkyl glycerol tetraethers (brGMGTs, or H-brGDGTs), lipids that occur widely in anoxic sediments of peatlands, lakes, and oceans, have been proposed as new tools for paleotemperature reconstructions in lakes and peatlands. However, their reliability has not been widely assessed, particularly in sediment cores. Here, we report the H-brGDGT distributions in the Xiong'an (XA) 3 sediment core from North China, spanning 5.5 million years (Ma), to evaluate their reliability as a terrestrial paleothermometer. Seven major H-brGDGTs are consistently abundant throughout the sediment core. Their isomerization index (IR_H) values, characteristic of river and lake environments, suggest H-brGDGTs were predominantly produced in situ with negligible soil input. Their abundances and distributions are strongly related to a widely used paleothermometer, MBT′_5ME (methylation index of branched tetraethers for 5-methylated branched GDGTs), likely indicating their temperature dependence. An H-brGDGT-based temperature proxy, H-MBT′, which includes only the H-1020c and H-1034b isomers, was found to track temperature in both published modern peat/lake H-brGDGT data set and the XA3 sediment core. H-MBT′ reconstructed temperature trend is consistent with the brGDGT temperature record in the sediment core as well as global climate signals, revealing stable warmth during 5.5–2.6 Ma, abrupt cooling at ∼2.6 Ma coinciding with the onset of Northern Hemisphere Glaciation (NHG), and oscillating decline during 2.2–0 Ma. Our results validate H-brGDGTs as a potential paleothermometer in aquatic systems but highlight the need for the selection of proper H-brGDGT-based temperature proxies and environment-specific calibrations to refine accuracy.