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Identification of novel 7-methyl and cyclopentanyl branched glycerol dialkyl glycerol tetraethers in lake sediments
- Ding, Su, Schwab, Valérie F., Ueberschaar, Nico, Roth, Vanessa-Nina, Lange, Markus, Xu, Yunping, Gleixner, Gerd, Pohnert, Georg
- Organic geochemistry 2016 v.102 pp. 52-58
- glycerol, high performance liquid chromatography, isomers, lakes, lipids, methylation, paleoecology, sediments, soil pH, temperature, uncertainty, Cameroon
- 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 (R2=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.