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

• Main Authors: W. Xiao, Y. Wang, Y. Liu, X. Zhang, L. Shi, Y. Xu
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-04-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/17/2135/2020/bg-17-2135-2020.pdf

<p>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<span class="inline-formula">^′</span>) (<span class="inline-formula">73.40±2.39</span>&thinsp;% 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 <span class="inline-formula"><i>δ</i>¹³C_OC</span> (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">19.82</mn><mo>±</mo><mn mathvariant="normal">0.25</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="70pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="8a12ac96970bdd4833d7364f64d697b5"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-17-2135-2020-ie00001.svg" width="70pt" height="10pt" src="bg-17-2135-2020-ie00001.png"/></svg:svg></span></span>&thinsp;%), low <span class="inline-formula">OC∕TN</span> ratio (<span class="inline-formula">6.72±0.84</span>), low branched and isoprenoid tetraether (BIT) index (<span class="inline-formula">0.03±0.01</span>), and high acyclic hexa-&thinsp;<span class="inline-formula">∕</span>&thinsp;pentamethylated brGDGT ratio (<span class="inline-formula">7.13±0.98</span>), 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-&thinsp;<span class="inline-formula">∕</span>&thinsp;pentamethylated brGDGT ratio and fractional abundance of brGDGT IIIa<span class="inline-formula">^′</span> provide a novel approach to distinguish terrestrial and marine-derived brGDGTs.</p>