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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-391
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-391
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 11 Oct 2019

Submitted as: research article | 11 Oct 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

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

Wenjie Xiao, Yasong Wang, Yongsheng Liu, Xi Zhang, Linlin Shi, and Yunping Xu Wenjie Xiao et al.
  • Shanghai Engineering Research Center of Hadal Science & Technology, College of Marine Sciences, Shanghai Ocean University, Shanghai 201306, China

Abstract. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are useful molecular indicators for organic carbon (OC) source and paleoenvironment. Their application in marine environments, however, is complicated because of the mixed terrestrial and marine contributions to brGDGTs. Here, we employ two dimensional (2D) ultrahigh-performance liquid chromatography-mass spectrometry (UHPLC-MS) to analyze brGDGTs in sediments from the Challenger Deep, Mariana Trench, the deepest ocean in the absent of terrestrial influence. The unique feature is the absence of 5-methyl brGDGTs, and the strong predominance of hexamethylated 6-methyl brGDGT (IIIa') (73.4 ±2.4 % of total brGDGTs). The brGDGTs-reconstructed pH is 8.22 ± 0.07, close to seawater pH. This, combined with characteristics of δ13C (− 19.82 ± 0.25 %), OC / TN ratio (6.72 ± 0.84), branched and isoprenoid tetraether (BIT) index (0.03 ± 0.01) and the acyclic hexa-/pentamethylated brGDGTs ratio (7.13 ± 0.98), strongly suggest that brGDGTs are of autochthonous products from benthic bacteria or planktonic bacteria. The compiling of literature data reveals that enhanced fractional abundance of hexamethylated 6-methyl brGDGTs is common in diverse continental margins when the marine influence became intensified. This may reflect an adaption of brGDGTs-producing bacteria to weak alkaline seawater and low ambient temperature. Based on the global dataset, the cross plot of acyclic hexa-/pentamethylated brGDGTs ratio and fractional abundance of brGDGT-IIIa' is an effective approach to distinguish the terrestrial vs. marine provenance of brGDGTs.

Wenjie Xiao et al.
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Short summary
Hadal zone (6–11 km water depth) is the least explored habitat on Earth. We studied microbial branched glycerol dialkyl glycerol tetraethers (brGDGT) in Challenger Deep, Mariana Trench. One unique feature is the strong predominance of 6-methyl brGDGT. This character reflects an adaption of brGDGT-producing bacteria to alkaline seawater and low temperature. BrGDGT, combined with elemental and isotopic data, suggest autochthonous product for brGDGT. A new method was proposed for brGDGT source.
Hadal zone (6–11 km water depth) is the least explored habitat on Earth. We studied microbial...
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