Biomarker-based quantitative constraints on maximal soil-derived brGDGTs in modern lake sediments

Abstract

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are emerging as a powerful tool for quantifying past terrestrial temperature variations. In lake sediments, mixed lacustrine vs. soil sources of brGDGTs pose a challenge to quantitative application of this paleothermometer, and this effect has not yet been well-constrained. In this work, we analyzed concentrations and distributions of brGDGTs along with two other types of typical lipid biomarkers (isoGDGTs and long-chain n-alkanes) in modern sediments and catchment soils from 26 park lakes across a large climatic gradient in China. BrGDGTs in these lakes may largely represent the lacustrine signal due to the relatively small catchment area and high eutrophic condition. This is confirmed by the combined use of crenarchaeol, its isomer crenarchaeol′, and C33n-alkane as tracers of soil input for brGDGTs. This novel method quantifies the upper limit of soil brGDGT contribution to be less than 10% for most investigated lakes, with an average value of 7% ± 10%. Subsequently, the brGDGT signal of the assumed pure lacustrine origin was constrained by the subtle difference between the original sedimentary brGDGTs and those corrected by the maximum possible soil brGDGT input for those park lakes. A compilation of global freshwater lakes further shows that the correlation between the methylation index of brGDGTs (MBT or MBT′5ME) of their sediments and temperature matches that for the assumed pure lacustrine signals. Hence, soil contribution might be generally minor and may not significantly impact the quantitative application of the brGDGT paleothermometer for worldwide lakes. This study reinforces a foundation for this novel lacustrine paleothermometer to be applied in paleoclimate reconstructions, although factors affecting lacustrine brGDGT distributions within lakes still need to be better constrained.