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

[摘要] 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 δ 13 C OC ( - 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.

[学科分类] 大气科学