Ubiquitous production of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in global marine environments: a new source indicator for brGDGTs

Presumed source specificity of branched and isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) led to the development of several biomarker proxies for biogeochemical cycle and paleoenvironment. However, recent studies reveal that brGDGTs are also produced in aquatic environments besides soils and peat. Here we examined three cores from the Bohai Sea and found distinct difference in brGDGT compositions varying with the distance from the Yellow River mouth. We thus proposed an abundance ratio of hexamethylated to pentamethylated brGDGT (IIIa / IIa) to evaluate brGDGT sources. The compiling of globally distributed 1354 marine sediments and 589 soils shows that the IIIa / IIa ratio is generally < 0.59 for soils, 0.59–0.92 and > 0.92 for marine sediments with and without significant terrestrial inputs, respectively. Such disparity confirms the existence of two sources of brGDGTs, a terrestrial origin with lower IIIa / IIa and a marine origin with higher IIIa / IIa, likely due to different pH influence. The application of the IIIa / IIa ratio to the East Siberian Arctic Shelf proves it a sensitive source indicator for brGDGTs, which is helpful for accurate estimation of organic carbon source and paleoclimates in marine settings.