Identification and characterisation of a novel mono-unsaturated highly branched isoprenoid (HBI) alkene in ancient Arctic sediments

The occurrence of a novel C25 highly branched isoprenoid (HBI) mono-unsaturated hydrocarbon in Arctic marine sediments from the Fram Strait is described. The structure was determined following synthesis from a closely related diene and NMR spectroscopic analysis. The position of the double bond was confirmed by oxidation (RuO4) and comparison of the mass spectrum of the resulting C19 ketone with that of an authentic sample synthesised previously. The same C19 ketone was also present following oxidation of the Fram Strait sediment extracts. The source of the novel HBI monoene biomarker is not known, but is believed to be certain marine diatoms on the basis of structural similarities with other HBIs from such sources. It does not, however, appear to be biosynthesised by Arctic sea ice diatoms, unlike its regio isomer, IP25, an established proxy for Arctic sea ice. Alternatively, it may represent a diagenetic product of the sea ice diatom biomarker IP25; this seems unlikely, however, on the basis of their respective structures. Since the new HBI monoene and IP25 possess similar gas chromatographic and mass spectral properties, but probably have contrasting sources, we suggest that future paleo sea ice reconstruction studies based on the occurrence of IP25 should pay further attention to the rigorous assignment of this biomarker in marine sediment extracts. Some suggestions for achieving this, based on analytical purification and mass spectrometric measurements, are provided.