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

Xiao, Wenjie; Wang, Yasong; Liu, Yongsheng; Zhang, Xi; Shi, Linlin; Xu, Yunping

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 (IIIainline-formula) (inline-formula73.40±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 inline-formulaδ13COC (inline-formula M4inlinescrollmathml - normal 19.82 ± normal 0.25 70pt10ptsvg-formulamathimg8a12ac96970bdd4833d7364f64d697b5 bg-17-2135-2020-ie00001.svg70pt10ptbg-17-2135-2020-ie00001.png  %), low inline-formulaOC∕TN ratio (inline-formula6.72±0.84), low branched and isoprenoid tetraether (BIT) index (inline-formula0.03±0.01), and high acyclic hexa- inline-formula∕ pentamethylated brGDGT ratio (inline-formula7.13±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- inline-formula∕ pentamethylated brGDGT ratio and fractional abundance of brGDGT IIIainline-formula provide a novel approach to distinguish terrestrial and marine-derived brGDGTs.

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Xiao, Wenjie / Wang, Yasong / Liu, Yongsheng / et al: Predominance of hexamethylated 6-methyl branched glycerol dialkyl glycerol tetraethers in the Mariana Trench: source and environmental implication. 2020. Copernicus Publications.

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