Molecule of the Month
Pristane - By Paul Farrimond
Pristane (Pr) is an acyclic isoprenoid with 19 carbon atoms, visible in gas chromatograms eluting immediately after the C17 n-alkane. Its relatively early elution is due to the increased volatility imparted by the large number of methyl branches on the chain. Like its C20 homologue, phytane (Ph), it can be derived during sediment diagenesis and burial from the phytol side chain of chlorophyll, and consequently is both widespread and abundant in source rocks and their oils.
The ratio of pristane/phytane (Pr/Ph) was popularised as an oxicity/anoxia indicator by an important paper by Didyk et al. (1978; extending earlier work by Brooks et al., 1969), but many additional biological sources of both pristane and phytane mean that the ratio is often more sensitive as an input marker. In the Jurassic source rocks of the North Sea and surrounding areas, higher Pr/Ph ratios are commonly used to indicate relatively higher inputs of terrestrial plant material, a signal that is preserved in oils and can help identify the contributions from Kimmeridge Clay, the Heather formation and Middle Jurassic coals (and their regional equivalents).
References:
Brooks J.D., Gould K. & Smith J.W. (1969). Isoprenoid hydrocarbons in coal and petroleum. Nature 222, 257-259.
Didyk B.M., Simoneit B.R.T., Brassell S.C. & Eglinton G. (1978). Organic geochemical indicators of palaeoenvironmental conditions of sedimentation. Nature 272, 216-222.