Upper intermediate layer-derived brGDGTs revealed from surface sediments along an elevation transect of a seamount

Abstract

Different from the initial thought of uniquely soil microbial production, more and more evidence supports in-situ generation of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in marine environments. However, the key output position of brGDGTs, i.e., in sediments or at a certain depth of water column, is still controversial. In this study, brGDGTs occurring as intact polar lipids (IPL) and core lipids (CL), respectively, are analyzed to address this issue in surface sediments along an elevation transect of a seamount with water depths from 733 m to 3097 m in the South China Sea (SCS). These sedimentary brGDGTs are characterized by high percent abundances of IIIa’ (> 20%) and Ia (> 20%) and high ∑IIIa/∑IIa ratios (> 0.92), and are distinct from global soil data, hence indicating their marine origin. A detailed comparison is made for the brGDGTs-derived parameters of #Ringstetra and MBT’5ME between IPL and CL in each sample at different water depths, showing that the IPLs-CLs offsets of the two parameters vary in correlation with water depth. This finding suggests that sedimentary brGDGTs are derived mainly from the water column, and most likely at intermediate water depth of about 300–700 m.