brGDGT paleothermometer MBT’5ME as a novel diagnostic tool to detect thermal stratification in lakes

The brGDGT-based ratio MBT’5ME has been widely used in soil (1) and lake (2) systems globally to estimate changes in mean annual temperature over time. Despite the development of numerous lake-specific calibrations (3, 4), a substantial residual error (2.0-2.7 ℃) remains in temperature calibrations, suggesting the potential influence of confounding factors other than temperature on the paleothermometer MBT’5ME. Investigating changes in brGDGT distribution over both shorter (seasonal water column) and longer (last 14 ka) time scales within a single lake system has provided significant novel insights for the application of MBT’5ME in lacustrine systems.Analyzing a dataset of lake sediment samples (n= 95) spanning three distinct climate periods: the Glacial interstadial, Younger Dryas, and Holocene, variations in brGDGTs with time within a prealpine lake sediment core offered valuable insights into the lake's environmental history and the interdependencies of brGDGTs. Within the Holocene,  the higher fractional abundance of brGDGT Ia is interpreted as driven by stronger thermal water column stratification, especially in the early Holocene (11.7-7.4 cal. ka BP) . Furthermore, the provenance and environmental drivers of the sedimentary brGDGTs were identified by comparing sediment samples to the lake's seasonal particulate matter (SPM) samples (n= 20). Although hypolimnion brGDGT isomer ratio (IR) was characterized as a function of dissolved oxygen in the lake’s SPM, only a muted influence from seasonal or permanent lake water anoxia was observed on the sedimentary brGDGTs isomer ratio (IR), and the IR is instead proposed as a secondary proxy for temperature.While Lake Rot’s epilimnion showed a dependency between MBT’5ME and temperature (and lack of hypolimnion’s MBT’5ME dependency on temperature), the concentration changes and interdependencies of brGDGTs during the oligotrophic phase of the lake mimicked the epilimnion conditions. This suggests that coring at the oxic depth of (seasonally) anoxic lakes can prove successful for using MBT’5ME as a lake surface paleothermometer. The presence of brGDGT Ia, commonly associated with thermally stratified surface summer water, reinforced this observation, and suggests that the temperature proxy is primarily linked to thermal stratification rather than single influence of mean annual temperature. These findings have substantial implications for paleoclimate studies. It is evident that the development of seasonal stratification plays a crucial role in influencing MBT'5ME values. Consequently, MBT’5ME mainly reflects summer warmings in Lake Rot during early Holocene and colder summer temperatures for late Holocene. Finally, with minimum impact from lake’s XRF-based anoxia history, brGDGTs isomer ratio (IR), shows a strong potential to be employed as a mean annual temperature proxy.1: Crampton-Flood et al., (2020). Geochimica et Cosmochimica Acta, 268, 142-1592: Stefanescu et al., (2021). Organic Geochemistry, 152, 1041743: Martínez-Sosa et al., (2021). Geochimica et Cosmochimica Acta, 305, 87-1054: Raberg et al., (2021). Biogeosciences, 18(12), 3579-3603