Land Plant Markers
Assessment of the contribution from land plant organic matter to a source rock is important as it is likely to be closely related to the depositional environment of the source rock, and also the nature of the hydrocarbons generated (as land-plant organic matter is largely gas-prone). Several molecular characteristics can be used in either the source rock or generated oils to provide some indication of land plant input; in some cases, the biomarkers are highly specific.
Long-Chain n-Alkanes
Although some n-alkanes are biosynthesised directly in nature (e.g. leaf waxes of higher plants), most of the n-alkanes found in geological samples are derived from the diagenetic defunctionalisation of precursor lipids such as n-alcohols and n-acids. Long-chain n-alkanes (C25-35) with an odd carbon number predominance are indicative of terrestrial higher plant input, whilst short chain components (especially C15 and C17) are generally algal or bacterial in origin. An indication of the relative abundance of terrestrial and algal-derived n-alkanes in a sample may be obtained from a ratio such as n-C31/n-C17 (or similar). (1997) have demonstrated a relationship between n-alkane composition and depositional environment, distinguishing between coastal (long-chain compounds dominant) and pelagic settings for a suite of modern sediments.
A gas chromatogram showing the saturated hydrocarbon distribution of an immature source rock with a noticeable higher plant input - recorded in the presence of long-chain n-alkanes with an odd carbon number predominance (labelled with their carbon number).
Steranes
The carbon number distribution of steranes can also contain some general indication for the contribution of land plant organic matter, as many of the precursor sterols in land plants contain 29 carbon atoms. Consequently, a high relative abundance of C29 steranes is often ascribed to source rocks with significant land plant input, and is a common feature of coals and deltaic source rocks (, 1994).
It should be noted, however, that many plants can synthesise C27 and C28 sterols, and that C29 sterols can be produced by algae. For example, source rocks pre-dating the evolution of land plants in the Silurian still contain C29 steranes, and in some cases (e.g. the Precambrian-Lower Cambrian Huqf sourced oils of Oman; , 2000) these can actually be dominant.
An m/z 217 mass chromatogram showing the sterane and diasterane distribution of a coal sample, dominated by C29 steranes and diasteranes (labelled and shaded).
Triterpanes
Oleananes (18α and 18β isomers) are used as markers for higher plant input, although they are derived from only some angiosperms (flowering plants). Because angiosperms only became prominent in the Late Cretaceous to Tertiary, source rocks of Jurassic age and older, and their associated oils, do not contain oleananes. They are often abundant in deltaic samples of Cretaceous or younger age (e.g. in Nigeria, New Zealand and Southeast Asia).
The relative importance of oleanane is commonly measured using the Oleanane Index (, 1994), or a variant of it:
OI = 100 * (18α + 18β-oleananes) / (18α + 18β-oleananes + 30αβ hopane)
However, oleananes are not quantitative markers for land plant input, having been found in oils with only minor terrestrial organic matter (, 1997; , 1998). Furthermore, oleananes are not the only diagenetic products from oleanene precursors; aromatics may also be formed, the relative significance of which depend upon marine influence (, 1997). Contact between the sediment and seawater during early diagenesis apparently promotes the formation of oleananes.
Other higher plant triterpanes can also be found, principally based on the lupane and ursane structures, which are also indicators of angiosperm land plant contribution.
Bicadinanes
Bicadinanes are derived from the biopolymer polycadinene which is found in 'dammar' resins of angiosperms, particularly the Dipterocarpaceae family (, 1992). As such, they are more specific markers than the oleananes, and are often observed not to covary (, 1993). They are found in oils but not in immature source rocks, although they are generated from their biopolymer precursor in advance of oil generation.
Diterpanes
Many C19 and C20 diterpanes have been related to an origin from the resins of higher plants (see , 1994 for a review). The six principal families of diterpanes (labdane, abietane, pimarane, beyeran, kaurane and phyllocladane) are common or widespread in gymnosperms (mainly conifers), but most are also found in the angiosperms (flowering plants).
Aromatic Hydrocarbons
Although many aromatic hydrocarbons have structures that cannot be related directly to biological precursor, some, including cadalene, are useful markers for higher plants in general, whilst some others (e.g. retene and simonellite) may be more specific conifer markers (, 2005).
Higher plant aromatics.
Some of these compounds have been used in the Higher Plant Index parameter (HPI; , 1996):
HPI = (retene + cadalene + iHMN) / 1,3,6,7-tetramethylnaphthalene
Where iHMN = 1-isohexyl-2-methylnaphthalene, one of a series of higher plant markers of uncertain specific source(s). The denominator 1,3,6,7-tetramethylnaphthalene is from bacteria rather than higher plants.
References
Koshi Yamamoto, , Takashi Okada, , Yukio Takayanagi, & Koichi Mimura, (1997). Normal paraffins in shales as an indicator of depositional environment. In: Geochimica et Cosmochimica Acta vol. 61 pp. 4403-4410.
Moldowan, J.M., Dahl, J., Huizinga, B.J., Fago, F.J., Hickey, L.J., Peakman, T.M. & Taylor, D.W. (1994). The molecular fossil record of oleanane and its relation to angiosperms. In: Science vol. 265 pp. 768-771.
Murray, A.P., Sosrowidjojo, I.B., Alexander, R., Kagi, R.I., Norgate, C.M. & Summons, R.E. (1997). Oleananes in oils and sediments: Evidence of marine influence during early diagenesis?. In: Geochimica et Cosmochimica Acta vol. 61, Elsevier pp. 1261-1276.
Pearson, M.J. & Alam, M. (1993). Bicadinanes and other terrestrial terpenoids in immature Oligocene sedimentary rocks and a related oil from the Surma Basin, NE. Bangladesh. In: Organic Geochemistry vol. 20 pp. 539-555.
Peters, K.E., Walters, C.C. & Moldowan, J.M. (2005). The biomarker guide., Cambridge University Press p. 1155 ISBN: 0 521 83763 4.
Philp, R.P. (1994). Geochemical characteristics of oils derived predominantly from terrigenous source materials. In: Coal and Coal-bearing Strata as Oil-prone Source Rocks? Geol. Soc. Special Publication No. 77 , Geological Society pp. 71-92 ISBN: 0-903317-99-0.
Terken, J.M.J. & Frewin, N.L. (2000). The Dhahaban petroleum system of Oman. In: American Association of Petroleum Geologists Bulletin vol. 84 pp. 523-544.
Van Aarssen, B.G.K., Alexander, R. & Kagi, R.I. (1996). The origin of Barrow Sub-basin crude oils: a geochemical correlation using land-plant derived biomarkers. In: The Australian Petroleum Production and Exploration Journal vol. 36 pp. 465-476.
Van Aarssen, B.G.K., Hessels, J.K.C., Abbink, O.A. & de Leeuw, J.W. (1992). The occurrence of polycyclic sesqui-, tri-, and oligoterpenoids derived from a resinous polymeric cadinene in crude oils from Southeast Asia. In: Geochimica et Cosmochimica Acta vol. 56 pp. 1231-1246.
Waseda, A. & Nishita, H. (1998). Geochemical characteristics of terrigenous- and marine-sourced oils in Hokkaido, Japan. In: Organic Geochemistry vol. 28 pp. 27-41.
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