_604______SCIENTIFIC CORRESPONDENCE------N_A_TU_R_E_V_O_L._33-3 -16-JU_N_E_l9_88 /phytane ratio as tstlc of anoxic, and often hypersaline, Sediments containing mature organic environments~.,. The significance of values matter and a few crude oils have been environmental indicator between 0.8 and 2.5 in mature normal checked for the presence of this com­ SIR-The discussion by ten Haven et a!. 1 marine sediments and oils is more difficult pound. In addition to other examples'·• concerning the use of pristane (Pr) to to assess. Whether the origin of pristane is sapropels from an open marine environ­ phytane (Ph) ratio as an environmental ultimately or ' may be ment in the Mediterranean (typical indicator has missed the point of much of irrelevant to the application of the Pr/Ph potential oil-source rocks) are known to the early literature, and misconstrues the ratio as a redox indicator. Tocopherols contain this new compound in concentra­ 10 significance of some recent data. The are claimed to have greater preservation tion three times as high as pristane • problem of error in the measurement of potential than phytol and its derivatives'. Of course, we do not object to using the Pr concentrations caused by coelution The effect of some oxidative degradation Pr/Ph ratio as a correlative tool when of 2,6, 10-trimethyl-7 -(3-methylbutyl)­ of organic matter during deposition would samples of the same origin and maturation dodecane must be kept in perspective. be selectively to preserve tocopherols, level are compared with one another (as in The compound is poorly resolved from Pr giving rise to higher Pr/Ph ratios during oil/source-rock correlation). on most good-quality capillary columns subsequent maturation. The use of Pr/Ph H.L. TEN HAVEN and has been identified in only one very ratios as a correlative tool was originally J. RULLKOTTER unusual and geologically young crude oil based on many empirical observations Institute of and Organic and in few recent sediments'. There is not which have now expanded greatly and are Geochemistry (ICH-5), sufficient evidence that it is present in still valid. Any qualification to the explan­ KFA Jiilich, sufficient concentration to influence sig­ ations for the origins of Pr and Ph do not P. 0. Box 1913, D-5170 Jiilich, FRG nificantly the measurement of Pr concen­ invalidate the fundamental observations. J.W. DELEEUW trations in most ancient sediments and oils. T.G. PowELL J.S. SINNINGHE DAMSTE Bituminous coals and oils originating Division of Continental Geology, Department of Chemistry and from higher plants are known'·' to have Bureau of Mineral Resources Geology Chemical Engineering, high Pr/Ph ratios (5-10) in comparison and Geophysics, Delft University of Technology, with marine oils and sediments ( <1-3). GPO Box 378, Canberra ACT, Australia de Vries van Heystplantsoen 2, Further, Pr/Ph ratios increase from 1-2 in TEN HAVEN ET AL. REPLY- We agree with NL-2628 RZ Delft, The Netherlands brown coals to a maximum of 4--10 in Powell that pristane (Pr) to phytane (Ph) I. ten Haven, H.L., de Leeuw, J.W., Rullkotter, J. & ratios close to unity should be interpreted Sinninghe Damste, J .S. Nature 330, 641-643 (1987). bituminous coals, and significant Pr 2. Volkman J. K. & Maxwell, J .R. in Biological Markers in the generation occurs at the threshold of oil with great caution and that fewer difficult­ Sedimentary Record (ed. Johns. R.B.) 1-42 (Elsevier, generation. These observations were ies arise with extremely high (as in coals) Amsterdam. 1986). 3. Brooks, J.D. eta/. Nature 222, 257-259 ( 1969). rationalized in terms of the preferential or extremely low (as in sediment from 4. Powell, T.G. & McKirdy, D.M. Nature 243,37-39 (1973). formation of phytanic acid by oxidation of hypersaline environments) Pr/Ph ratios. 5. Tissot, B.P. & Welte, D.H. Petroleum Formation and Occurrence 2nd edn (Springer, Berlin 1984). phytol, followed by the conversion of In these latter cases, the Pr/Ph ratios are 6. Boreham, C.J. & Powell, T.G. Org. Geochem. II, 434-449 phytanic acid to Pr during catagenesis'', probably influenced by specific sources of (1987). the organic matter and specific deposi­ 7. Goossens, H., de Leeuw, J.W., Schenck. P.A. & Brassell, and have been completely ignored by ten S.C. Nature 312,440-442 (1984). 1 Haven et a!. • Clearly, high Pr/Ph ratios tional environments, where the redox 8. Didyk, B.M. eta/. Nature 212,216-222 (1978). (;:,:3) are indicative of an input from condition is only one of the factors. 9. Robson,J.N. & Rowland, S.J. Nature324, 561-563 (1986). 10. Smith. D.J. Thesis, Univ. Bristol (1984). terrestrial organic matter that has a high The main point of our paper1 concerns probability of being exposed to oxidation the generalizations and oversimplifica­ before or during deposition. But as the tions implied in the use of the Pr/Ph ratio Radiocarbon dating depositional conditions and source inputs as an indicator of the level of oxygen at the SIR-Radtke proposes electron-spin­ control the distribution of Pr and Ph pre­ site of organic matter deposition into resonance analysis as a means of identifying cursors, it is inappropriate for Pr/Ph ratios sediments. In much recent work (mainly shell and coral samples that are unsuitable 1 to be used as a redox indicator in low­ that dealing with sediments containing for radiocarbon dating • An alternative is 1 maturity samples without consideration immature organic matter), aPr/Ph ratio of to date samples by 'first-order assay' , of the precursors and the maturity level. unity has become a kind of magic number which provides reliable ages for material Most marine, organic-rich sediments to differentiate between palaeo-oxygen less than 10,000 years old and, in so doing, and oils have a narrow range of Pr/Ph levels, commonly with reference to Didyk distinguishes between Holocene and 5 1 ratios (0.8-2.5) , and with increasing eta!.'. We made several new arguments earlier samples. Each analysis (excluding maturity there can be a gradual increase in based on novel geochemical findings to labour) costs about £0.50; the requisite Pr/Ph ratio within this range. This has emphasize the restricted use of the Pr/Ph liquid scintillation counters are commoner been associated in one case' with other ratio as a palaeoenvironmental indicator. than electron-spin-resonance spectro­ evidence of oxidation of organic matter Note that the intermediates in the diagen­ meters in universities and, of course, during deposition. The proposal that Pr/ etic scheme of Didyk et a!.", who studies radiocarbon laboratories. Ph ratios cannot reflect the redox con­ the conversion of phytol in a few recent C. VITA-FINZI ditions of the final resting place of organic sediments, have not been verified in many University College London, matter1 is a question of semantics because cases. Moreover, it is becoming obvious Gower Street, London WC1E6BT, UK all sediments ultimately become reducing that many precursors of both pristane and I. Radtke, U. Nature 333, 307-303 (1988). during burial; it is the depositional history phytane have to be taken into account'·'. 2. Vita-Finzi, C. Earth planet. Sci. Lett. 65,389-392 (1983). that is the critical factor in determining the The selective preservation of these pre­ extent of oxidation of organic matter, not cursors will be different in different sedi­ Scientific Correspondence the final resting place. mentary environments. In other words, Scientific Correspondence is intended Pr/Ph ratio cannot be used as a palaeo­ the ultimate Pr/Ph ratio is determined by a to provide a forum in which readers environmental indicator at low maturity large variety of known and unknown pre­ may raise points of a scientific charac­ level' and this is implicit in the early cursors, their selective preservation and ter. They need not arise out of anything literature''. At higher maturation levels, their pathways of . published in Nature. In any case, pri­ high Pr/Ph ratios (> 3.0) are reliable The coelution of pristane with 2,6,10- ority will be given to letters of less than indicators of input of terrestrial organic trimethyl-7-(3-methylbutyl)-dodecane on 500 words and five references. D matter' and very low values are character- apolar stationary phase is a new result.