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. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) areuseful molecular indicators for organic carbon (OC) sources and thepaleoenvironment. Their application in marine environments, however, iscomplicated because of a mixed terrestrial and marine source. Here, weexamined brGDGTs in sediments from the Mariana Trench, the deepest oceanwithout significant terrestrial influence. Our result shows a strongpredominance of hexamethylated 6-methyl brGDGT (IIIa′) (73.40±2.39 % of total brGDGTs) and an absence of 5-methyl brGDGTs, differentfrom previously reported soils and marine sediments that comprised both5-methyl and 6-methyl brGDGTs. This unique feature, combined with highδ13COC (-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 areautochthonous rather than terrestrial products. The compiling of literaturedata shows that the enhanced fractional abundance of hexamethylated 6-methylbrGDGTs is a common phenomenon in continental margins when the marineinfluence was intensified. The cross plot of acyclic hexa- ∕ pentamethylatedbrGDGT ratio and fractional abundance of brGDGT IIIa′ provide a novelapproach to distinguish terrestrial and marine-derived brGDGTs.

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