Dissolved oxygen concentrations influence the distribution of isoGDGTs in a deep Lake Fuxian in China: Implications for the use of TEX86 in stratified lakes

Abstract

The TEX86 (tetraether index of 86 carbon atoms) temperature proxy has been widely applied to marine settings. The application of TEX86 in lakes, however, is more complicated, as many other factors, such as dissolved oxygen (DO), also appear to influence the lacustrine TEX86 signal. In order to better understand how DO content affects TEX86, we analyzed an annual cycle of isoGDGT distributions and abundances in the water column (0–100 m) of Lake Fuxian, in southwest China. It appears that isoGDGTs in suspended particulate matter (SPM) of the water column are mainly produced by aquatic Group I.1a Thaumarchaeota. The peak concentration of total isoGDGTs occurs mainly in June and December, which is probably driven by release of nutrients during phytoplankton blooms with a lag of about one month. About 95% of isoGDGTs occurred in the bottom water layer (hypolimnion) of this thermally stratified lake, indicating that Thaumarchaeota prefer to live in suboxic water conditions in Lake Fuxian. IsoGDGT-2 is the most sensitive component to monthly variations in DO contents. This relationship leads to a similar negative correlation between TEX86 and DO. When TEX86 values, especially at the hypolimnion, are lower than anticipated, this is caused by %isoGDGT-2 falling below 45%. Conversely, a warm bias of TEX86 arising from a %isoGDGT-2 greater than 45% appears to be associated with lower DO contents. Therefore, this threshold of %isoGDGT-2 could be used in future paleoclimate reconstructions (of Lake Fuxian, and possibly similar lakes), especially since Thaumarchaeota typically reside at suboxic water layers.