Identification and carbon isotope composition of a novel branched GDGT isomer in lake sediments: evidence for lacustrine branched GDGT production

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids that occur ubiquitously in soils and lacustrine sediments and have great potential as proxy indicators for paleotemperature and pH reconstructions. Initially, brGDGTs in lakes were thought to originate from soils of the watershed. The composition of the lacustrine brGDGT pool, however, often differs substantially from that in catchment soils, complicating the application of the brGDGT paleothermometer to lake sediments. This suggests that terrigenous brGDGT signals in lacustrine sedimentary archives may be affected by aquatic in situ production. In sediments of a Swiss mountain lake, we detected a novel hexamethylated brGDGT, which elutes between the known 5- and 6-methyl brGDGT isomers during HPLC-MS analysis. This novel isomer accounted for 8.5% of the total brGDGTs. Most remarkably, this brGDGT was not detected in soils collected from the catchment of the lake, providing circumstantial evidence for an in situ brGDGT source in the lake's water column or sediments. Isolation of the compound by preparative HPLC and subsequent GC-MS analysis of the alkyl chains revealed that the novel brGDGT comprises two structural isomers. One possesses a 5,13,16- and a 6,13,16-trimethyloctacosanyl moiety and constitutes 84% of the new brGDGT; the second contains a 13,16-dimethyloctacosanyl and a 5,13,16,23-tetramethyloctacosanyl moiety. The δ13C values of both the alkyl chains derived from the novel brGDGT (−46‰) and all other major brGDGTs (−43‰ to −44‰) were significantly lower than those of brGDGT-derived alkanes in catchment soils (−27‰ to −28‰) further attesting to in situ production of brGDGTs in the studied lake.