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

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.