Phytane

Phytane is the isoprenoid alkane formed when phytol, a chemical substituent of chlorophyll, loses its hydroxyl group. When phytol loses one carbon atom, it yields pristane. Other sources of phytane and pristane have also been proposed than phytol.

Pristane and phytane are common constituents in petroleum and have been used as proxies for depositional redox conditions, as well as for correlating oil and its source rock (i.e. elucidating where oil formed). In environmental studies, pristane and phytane are target compounds for investigating oil spills.

Chemistry
Phytane is a non-polar organic compound that is a clear and colorless liquid at room temperature. It is a head-to-tail linked regular isoprenoid with chemical formula C20H42.

Phytane has many structural isomers. Among them, crocetane is a tail-to-tail linked isoprenoid and often co-elutes with phytane during gas chromatography (GC) due to its structural similarity.

Phytane also has many stereoisomers because of its three stereo carbons, C-6, C-10 and C-14. Whereas pristane has two stereo carbons, C-6 and C-10. Direct measurement of these isomers has not been reported using gas chromatography. The substituent of phytane is phytanyl. Phytanyl groups are frequently found in archaeal membrane lipids of methanogenic and halophilic archaea (e.g., in archaeol). Phytene is the singly unsaturated version of phytane. Phytene is also found as the functional group phytyl in many organic molecules of biological importance such as chlorophyll, tocopherol (vitamin E), and phylloquinone (vitamin K1). Phytene's corresponding alcohol is phytol. Geranylgeranene is the fully unsaturated form of phytane, and its corresponding substituent is geranylgeranyl.

Preservation
In suitable environments, biomolecules like chlorophyll can be transformed and preserved in recognizable forms as biomarkers. Conversion during diagenesis often causes the chemical loss of functional groups like double bonds and hydroxyl groups. Studies suggested that pristane and phytane are formed via diagenesis of phytol under different redox conditions. Pristane can be formed in oxic (oxidizing) conditions by phytol oxidation to phytenic acid, which may then undergo decarboxylation to pristene, before finally being reduced to pristane. In contrast, phytane is likely from reduction and dehydration of phytol (via dihydrophytol or phytene) under relatively anoxic conditions. However, various biotic and abiotic processes may control the diagenesis of chlorophyll and phytol, and the exact reactions are more complicated and not strictly-correlated to redox conditions.

In thermally immature sediments, pristane and phytane has a configuration dominated by 6R,10S stereochemistry (equivalent to 6S, 10R), which is inherited from C-7 and C-11 in phytol. During thermal maturation, isomerization at C-6 and C-10 leads to a mixture of 6R, 10S, 6S, 10S, and 6R, 10R.

Pristane/Phytane ratio
Pristane/phytane (Pr/Ph) is the ratio of abundances of pristane and phytane. It is a proxy for redox conditions in the depositional environments. The Pr/Ph index is based on the assumption that pristane is formed from phytol by an oxidative pathway, while phytane is generated through various reductive pathways. In non-biodegraded crude oil, Pr/Ph less than 0.8 indicates saline to hypersaline conditions associated with evaporite and carbonate deposition, whereas organic-lean terrigenous, fluvial, and deltaic sediments under oxic to suboxic conditions usually generate crude oil with Pr/Ph above 3. Pr/Ph is commonly applied because pristane and phytane are measured easily using gas chromatography.

However, the index should be used with caution, as pristane and phytane may not result from degradation of the same precursor (see *Source*). Also, pristane, but not phytane, can be produced in reducing environments by clay-catalysed degradation of phytol and subsequent reduction. Additionally, during catagenesis, Pr/Ph tends to increase. This variation may be due to preferential release of sulfur-bound phytols from source rocks during early maturation.

Pristane/nC17 and phytane/nC18 ratios
Pristane/n-heptadecane (Pr/nC17) and phytane/n-octadecane (Ph/C18) are sometimes used to correlate oil and its source rock (i.e. to elucidate where oil formed). Oils from rocks deposited under open-ocean conditions showed Pr/nC17< 0.5, while those from inland peat swamp had ratios greater than 1.

The ratios should be used with caution for several reasons. Both Pr/nC17and Ph/nC18 decrease with thermal maturity of petroleum because isoprenoids are less thermally stable than linear alkanes. In contrast, biodegradation increases these ratios because aerobic bacteria generally attack linear alkanes before the isoprenoids. Therefore, biodegraded oil is similar to low-maturity non-degraded oil in the sense of exhibiting low abundance of n-alkanes relative to pristane and phytane.

Biodegradation scale
Pristane and phytane are more resistant to biodegradation than n-alkanes, but less so than steranes and hopanes. The substantial depletion and complete elimination of pristane and phytane correspond to a Biomarker Biodegradation Scale of 3 and 4, respectively.

Carbon isotopes
The carbon isotopic composition of pristane and phytane generally reflects the kinetic isotope fractionation that occurs during photosynthesis. For example, δ13C(PDB) of phytane in marine sediments and oils has been used to reconstruct ancient atmospheric CO2levels, which affects the carbon isotopic fractionation associated with photosynthesis, over the past 500 million years. In this study, partial pressure of CO2 reached more than 1000 ppm at maxima compared to 410 ppm today.

Carbon isotope compositions of pristane and phytane in crude oil can also help to constrain their source. Pristane and phytane from a common precursor should have δ13C values differing by no more than 0.3‰.

Hydrogen isotopes
Hydrogen isotope composition of phytol in marine phytoplankton and algae starts out as highly depleted, with δD (VSMOW) ranging from -360 to -280‰. Thermal maturation preferentially releases light isotopes, causing and pristane and phytane to become progressively heavier with maturation.

Case study: limitation of Pr/Ph as a redox indicator
Inferences from Pr/Ph on the redox potential of source sediments should always be supported by other geochemical and geological data, such as sulfur content or the C35 homohopane index (i.e. the abundance of C35 homohopane relative to that of C31-C35 homohopanes). For example, the Baghewala-1 oil from India has low Pr/Ph (0.9), high sulfur (1.2 wt.%) and high C35 homohopane index, which are consistent with anoxia during deposition of the source rock.

However, drawing conclusion on the oxic state of depositional environments only from Pr/Ph ratio can be misleading because salinity often controls the Pr/Ph in hypersaline environments. In another example, the decrease in Pr/Ph during deposition of the PermianKupferschiefer sequence in Germany is in coincidence with an increase in trimethylated 2-methyl-2-(4,8,12-trimethyltridecyl)chromans, an aromatic compound believed to be markers of salinity. Therefore, this decrease in Pr/Ph should indicate an increase in salinity, instead of an increase in anoxia.