Abstract
Abstract. 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 δ13COC (-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.
Highlights
Glycerol dialkyl glycerol tetraethers (GDGTs), including isoprenoidal GDGTs and branched GDGTs, are widely distributed biomarkers in terrestrial and marine settings (De Rosa and Gambacorta, 1988; Sinninghe Damsté et al, 2000). iGDGTs containing an isoprenoid carbon skeleton are predominantly synthesized by archaea belonging to the phylum Thaumarchaeota (Sinninghe Damsté et al, 2002; Schouten et al, 2008; Knappy et al, 2011; Zeng et al, 2019)
A survey for global soils reveals that the cyclization of branched tetraethers (CBT) correlates with soil pH, while the methylation of branched tetraethers (MBT) is dependent on mean annual air temperature (MAT) and to a lesser extent on soil pH (Weijers et al, 2007b; De Jonge et al, 2014a), leading to the development of branched GDGTs (brGDGTs)-based MBT–CBT proxies for pH and MAT
Similar to the North Sea Basin, the proportion of GDGT IIIa in the Kara Sea showed a significant correlation with δ13COC (R2 = 0.34; p < 0.001) and the branched and isoprenoid tetraether (BIT) index (R2 = 0.50; p < 0.001) in a 303 cm sediment core covering at least the past 13.3 × 103 years (Fig. 7d, e, f) (De Jonge et al, 2016). These results suggest that brGDGTs synthesized by marine organisms comprise higher fractional abundance of hexamethylated 6-methyl brGDGTs
Summary
Glycerol dialkyl glycerol tetraethers (GDGTs), including isoprenoidal GDGTs (iGDGTs) and branched GDGTs (brGDGTs), are widely distributed biomarkers in terrestrial and marine settings (De Rosa and Gambacorta, 1988; Sinninghe Damsté et al, 2000). iGDGTs containing an isoprenoid carbon skeleton are predominantly synthesized by archaea belonging to the phylum Thaumarchaeota (Sinninghe Damsté et al, 2002; Schouten et al, 2008; Knappy et al, 2011; Zeng et al, 2019). Unlike iGDGTs, brGDGTs consisting of four to six methyl groups and zero to two cyclopentane moieties are synthesized by some bacteria including, but not limited to, Acidobacteria (Sinninghe Damsté et al, 2011). These bacteria are able to alter the degree of methylation and cyclization of brGDGTs with changing ambient environmental conditions (Weijers et al, 2007b). BrGDGTs and iGDGTs were thought to be biomarkers for terrestrial ( soil) and marine organic matter, respectively.
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