Effect of water depth on the TEX86 proxy in volcanic lakes of northeastern China

摘要

The TEX(86 )index, based on the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs), has been widely applied for reconstructing past sea surface temperature (SST). However, its successful application in lacustrine environments has been limited to certain large lakes in which aquatic Thaumarchaeota are thought be the dominant iGDGT producers. In small lakes, multiple factors, such as iGDGTs from soil and non-thaumarchaeotal sources, could complicate the use of TEX86 as a temperature proxy (Powers et al., 2010). Here we investigate iGDGTs from surface sediments and suspended particulate matter (SPM) in a series of small volcanic lakes (areas: similar to 0.1-8.2 km(2)) from three main volcanic fields of northeastern China. The iGDGT distribution patterns from surface sediments of these lakes differ significantly from those of surrounding soils, global marine sediments, as well as cultured Group I.1 a and Group I.1b Thaumarchaeota, suggesting a substantial contribution of autochthonous non-thaumarchaeotal sources. Interestingly, however, the correlation between TEX86 values in the SPM samples and in situ water temperatures remain strong and display temperature sensitivity similar to previously published linear calibrations from global marine and lake sediments. Our results suggest that the in situ TEX86-temperature relationship is little affected by the non-thaumarchaeotal iGDGT sources in these lakes. However, the TEX86-inferred temperatures from surface sediments show increasingly colder offsets relative to air temperatures with increasing lake depth, which can be explained by the observed increase in iGDGTs production and decrease in water temperatures in water column as depth increases. Our results thus suggest that TEX86 values in the sediments of these volcanic lakes are affected by both temperature and lake depth, and when temperature can be constrained independently or remains constant over a given period of time, have potential application