Identification of novel 7-methyl and cyclopentanyl branched glycerol dialkyl glycerol tetraethers in lake sediments. (December 2016)

• Record Type: Journal Article
• Authors: Ding, Su; Schwab, Valérie F.; Ueberschaar, Nico; Roth, Vanessa-Nina; Lange, Markus; Xu, Yunping; Gleixner, Gerd; Pohnert, Georg
• Is Part Of: Organic geochemistry. Volume 102(2017:Jan.)
• Page Start: 52
• Page End: 58
• Publication Date: 2016-12
• Subjects: 7-Methyl brGDGTs -- Biomarker -- Lake sediments -- Membrane lipids -- Liquid chromatography–mass spectrometry; Organic geochemistry -- Periodicals; Biogeochemistry -- Periodicals; Géochimie organique -- Périodiques; 553.205
• DOI: 10.1016/j.orggeochem.2016.09.009
• Languages: English
• ISSNs: 0146-6380

Abstract

Highlights: New 7-methyl branched glycerol dialkyl glycerol tetraethers (brGDGTs) are described. New cyclopentane-moiety containing compounds are also described. These novel brGDGTs were extracted from lake sediments. Consideration of the new brGDGTs may improve brGDGT-derived proxies. Abstract: Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids that are widely used as valuable paleoenvironmental proxies. The recently discovered 6-methyl brGDGTs improved the accuracy of the proxies for temperature "methylation branched tetraethers (MBT)" and soil pH "cyclization branched tetraethers (CBT)". However, the calibration uncertainties are still substantial for brGDGT-derived proxies (e.g., 5 °C for MBT′5ME ). Here we report a series of novel 7-methyl brGDGT isomers that co-eluted with the known 5- and 6-methyl brGDGTs in commonly applied normal phase high performance liquid chromatography (HPLC). Using an optimized HPLC gradient the novel 7-methyl brGDGTs could be structurally characterized and quantified. Their mean relative abundance was in the range of 6% of the total brGDGTs in Chinese and Cameroon lake sediments. The 7-methyl brGDGT IIa7 correlates with sediment pH ( R 2 = 0.44, root-mean-square error = 0.26 pH unit), a result that motivates the re-analysis of brGDGTs in soils and sediments to further reassess brGDGT-based proxies and to determine the source of 7-methyl brGDGTs. In addition to the 7-methyl brGDGTs, we identified two novel pentamethylated brGDGTs based on the mass spectra of its ether-cleaved hydrocarbon products.