Absence of a water depth signal in archaeal tetraether lipids from surface lake sediments in areas with intensive human activity: A case study from Daihai Lake, Inner Mongolia

Abstract

Indices based on isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) and hydroxylated GDGTs (OH-GDGTs), such as %Cren, Cren/Cren', and %OH-GDGTs, have been widely used as lake-level proxies in lake sediment cores. However, the interpretation of these indices is potentially complex and more research is needed to determine the contexts in which they can reliably be used for paleohydrological reconstruction. We analyzed the isoGDGTs and OH-GDGTs of multiple sample types (watershed surface soils, lake surface sediments, and sediments from a 70-cm sediment core) from Daihai Lake. Our results show that while the widely-used lake level proxies are not correlated with water depth for the surface sediments, this correlation exists in the samples from the sediment core over the past 60 years. We propose that archaeal GDGTs of modern surface sediments in Daihai Lake may be modified by intensive human activities, including eutrophication and potential terrestrial soil input, which prevents their use for lake-level reconstruction. Data from three other lakes (Qinghai, Lugu, and Yangzong) confirm that lake level proxies based on surface sediments are applicable for Qinghai Lake and Lugu Lake (both of which are in regions with relatively weak human activity), but not for Yangzong Lake (in a region with intensive human activity). Interestingly, the transition zone between Qinghai Lake/Lugu Lake and Daihai Lake/Yangzong Lake corresponds to the Hu Line, which divides China into two parts with contrasting human population densities. We propose that future research should consider these findings when using surface lake sediments for paleoenvironmental calibration in areas with intensive human activity, and that sediment cores may sometimes be more reliable for modern validation studies. Although more studies are needed to verify our conclusions, our findings contribute to the application of archaeal GDGTs to lake-level reconstruction, and they are potentially also significant for the validation of other proxies.