Predominance of hexamethylated 6-methyl branched glycerol dialkyl glycerol tetraethers in the Mariana Trench: source and environmental implication

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are useful molecular indicators for organic carbon (OC) sources and the paleoenvironment. Their application in marine environments, however, is complicated because of a mixed terrestrial and marine source. Here, we examined brGDGTs in sediments from the Mariana Trench, the deepest ocean without significant terrestrial influence. Our result shows a strong predominance of hexamethylated 6-methyl brGDGT (IIIa') (73.40 +/- 2.39% of total brGDGTs) and an absence of 5-methyl brGDGTs, different from previously reported soils and marine sediments that comprised both 5-methyl and 6-methyl brGDGTs. This unique feature, combined with high delta(COC)-C-13 (19.82 +/- 0.25 %), low OC=TN ratio (6.72 +/- 0.84), low branched and isoprenoid tetraether (BIT) index (0.03 +/- 0.01), and high acyclic hexa- / pentamethylated brGDGT ratio (7.13 +/- 0.98), support that brGDGTs in the Mariana Trench sediments are autochthonous rather than terrestrial products. The compiling of literature data shows that the enhanced fractional abundance of hexamethylated 6-methyl brGDGTs is a common phenomenon in continental margins when the marine influence was intensified. The cross plot of acyclic hexa- / pentamethylated brGDGT ratio and fractional abundance of brGDGT IIIa' provide a novel approach to distinguish terrestrial and marine-derived brGDGTs.