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.

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

Biogeosciences ◽

10.5194/bg-17-2135-2020 ◽

2020 ◽

Vol 17 (7) ◽

pp. 2135-2148 ◽

Cited By ~ 1

Author(s):

Wenjie Xiao ◽

Yasong Wang ◽

Yongsheng Liu ◽

Xi Zhang ◽

Linlin Shi ◽

...

Keyword(s):

Continental Margins ◽

Environmental Implication ◽

Mariana Trench ◽

Novel Approach ◽

Fractional Abundance ◽

Marine Source ◽

Marine Influence ◽

Molecular Indicators ◽

Trench Sediments ◽

Glycerol Dialkyl Glycerol Tetraethers

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.

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Composition of Sedimentary Organic Matter across the Laptev Sea Shelf: Evidences from Rock-Eval Parameters and Molecular Indicators

Water ◽

10.3390/w12123511 ◽

2020 ◽

Vol 12 (12) ◽

pp. 3511

Author(s):

Elena Gershelis ◽

Andrey Grinko ◽

Irina Oberemok ◽

Elizaveta Klevantseva ◽

Natalina Poltavskaya ◽

...

Keyword(s):

Organic Matter ◽

Surface Sediments ◽

Continental Margins ◽

Laptev Sea ◽

Lena River ◽

Nearshore Sediments ◽

Rock Eval ◽

Molecular Indicators ◽

Sedimentation Regime ◽

The Laptev Sea

Global warming in high latitudes causes destabilization of vulnerable permafrost deposits followed by massive thaw-release of organic carbon. Permafrost-derived carbon may be buried in the nearshore sediments, transported towards the deeper basins or degraded into the greenhouse gases, potentially initiating a positive feedback to climate change. In the present study, we aim to identify the sources, distribution and degradation state of organic matter (OM) stored in the surface sediments of the Laptev Sea (LS), which receives a large input of terrestrial carbon from both Lena River discharge and intense coastal erosion. We applied a suite of geochemical indicators including the Rock Eval parameters, traditionally used for the matured OM characterization, and terrestrial lipid biomarkers. In addition, we analyzed a comprehensive grain size data in order to assess hydrodynamic sedimentation regime across the LS shelf. Rock-Eval (RE) data characterize LS sedimentary OM with generally low hydrogen index (100–200 mg HC/g TOC) and oxygen index (200 and 300 CO2/g TOC) both increasing off to the continental slope. According to Tpeak values, there is a clear regional distinction between two groups (369–401 °C for the inner and mid shelf; 451–464 °C for the outer shelf). We suggest that permafrost-derived OM is traced across the shallow and mid depths with high Tpeak and slightly elevated HI values if compared to other Arctic continental margins. Molecular-based degradation indicators show a trend to more degraded terrestrial OC with increasing distance from the coast corroborating with RE results. However, we observed much less variation of the degradation markers down to the deeper sampling horizons, which supports the notion that the most active OM degradation in LS land-shelf system takes part during the cross-shelf transport, not while getting buried deeper.

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