Frontiers of organic geochemistry: some unresolved problems

Advances

in OrganicG e o ~

1989

Org. Geochem. Vol. 16, Nos 1-3, pp. XXI-XXIV,1990

PergamonPress pie. Printed in Great Britain

14TH INTERNATIONAL MEETING ON ORGANIC GEOCHEMISTRY

Paris, France 18-22 September 1989

FRONTIERS OF ORGANIC GEOCHEMISTRY: SOME UNRESOLVED PROBLEMS First Introductory Talk by Guy Ourisson, Member of the Academy of Sciences As Bernard Durand has shown in his introduction, organic geochemistry has come of age. It is an established subdiscipline, even though it is difficult to ascertain of which major discipline. Is it a branch of organic chemistry, or of geology, or of both, or of geochemistry? In fact, who cares? I have come to organic geochemistry as an organic chemist, a professional of the chemistry of the so-called "Natural Products", which have come to mean in fact only moderate molecular weight natural products, excluding proteins, nucleic acids, and other essential polymers. It was unavoidable that the Strasbourg group, started and now run by Pierre Albrecht, be more interested than others in the description of sedimentary organic matter in precise molecular terms, and our major contributions have been in recognizing the structural variety and the ubiquity of hopanoids, or the structure of diasteranes, or the structure of the "extended tricyclic diterpanes" (the tricyclopolyprenanes), or the precise structure of many geoporphyrins. All along this time of work, we have encountered convergence or enjoyed cooperation with our friends in Bristol, Hamburg, Pau, Richmond, Rueil, Delft, Bloomington and elsewhere, and what Strasbourg has ensured is hard structural criteria. But the field could well have advanced also, albeit in other directions, had the earlier concept of structural classes (polycyclic aromatics, branched-chain alkanes, etc.) been the only one retained. This introduction was necessary to explain the somewhat parochial nature of frontiers which I shall explore with you. I have no doubt that there are many more, recognizable better by those who have come to the field with different backgrounds. I shall discuss successively unresolved problems linked with quantitative aspects, with age, with origin, with structural detail and finally with professional aspects. QUALITATIVEASPECTS We and others have tried to assess the quantitative importance of various families of substances in organic geochemistry. The figures used are of course only orders of magnitude, and they tend to be repeated with critical evaluation. I would like for example to quote the following estimations: Total sedimentary organic carbon: Total "living" organic carbon: Average % of soluble matter in sedimentary organic matter: Average % of geohopanoids in soluble sedimentary matter: Total mass of geohopanoids in sedimentary organic matter

1016t

1012t 10% 0.01-1% 101~-10~3t

The message that I want to convey is of course that geohopanoids are extremely important, as their global mass is of the same order of magnitude as the global mass of organic carbon in all living organisms now existing. We must however remain critical, and emphasize at once that the figures on which this conclusion is based are all, in fact, not proper hard data, but estimation. We can no longer count on Egon Degens to refine these figures, and yet they should be constantly brought up-to-data by "globalists". I believe this is undoubtedly a factual, but also a methodological frontier, and one of utmost importance not only for organic geochemistry but for all global sciences: how can one improve the plausibility of such estimations? How can one review critically, on the basis of data engendered by modern geologists, the mass assumed for sediments, the average of organic C content, the global average content, or the total mass, of this or that class of organic constituents, be they geoorganics or pollutants7 Mostly, how can these evaluations be made scientific, i.e. how can one prove XXI

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Introductory Talk by Guy Ourisson

them wrong? This wish for reliable quantitative data is not only the mark of an intemperant curiosity. For instance, and this is the only example I shall give, it would be necessary to have hard data in order to evaluate the relative role of methanogenic archaebacteria, vs that of methylotrophic eubacteria, in the production of organic matter in source rocks, and to build models of the evolution of oils and of recent sediments, but also to appreciate the potential dangers of build-up of atmospheric methane. Some, like Bernard Durand, believe U.S.S.R. geochemists would be in a unique position to undertake these global studies. AGE

I am sure many of you have like myself experienced some frustration during discussions following one of your own lectures on the topic, when a question revealed that at least some of your auditors have misunderstood you, and believe that you have described molecular markers of primitive life, maybe even that you have some information on prebiotic markers. We have been led by experience to believe, quite to the contrary, that all the complex organic structures uncovered in sediments originate from precursors which must still be produced by some living organisms. For instance, even through "extended", C31/3~, hopanoids were not known in living organisms when we first discovered their ubiquity in sediments, we had no doubt that they would have to be found, and that they were not the molecular signature of "primitive" species. Of course, another attitude could have prevailed: when Pierre Albrecht first isolated isoarborinol from the Messel shale, our first conclusion had been to consider it as the marker of fossil tropical plants, as a paleaoecological marker, which it probably is not. Similarly, when Walter Michaelis first isolated the C~0 bisphytane, even though we had no idea that it could originate from Archaebacteria, we never entertained the idea that it might have been a metabolite of some disappeared bacterial species. We have progressively learnt that geochemical molecules come from precursors that are still available in some strains of living organisms. We shall come back to that point in a moment. Remains therefore virgin a frontier: that of the very, very old sediments. Our very limited experience in Strasbourg is that the amounts isolated from sediments, 1.5 Byr old for instance, are so tiny that they elicit the urge for a repeat, with more stringent precautions against contamination. I would welcome a critical, really critical, appraisal of all the work that has been carded out on samples older than, say 5000 Myr; have all precautions been taken against manipulative contamination, from the selection of the rock in situ to the collection of samples, to their preservation, their study? Are the blanks really blank? And, most important, remains the question of the nature of the formation; any natural contamination from younger, overlying sediments, firmly excluded? Can one really hope to build a satisfactory model of diffusion of organic matter into compact rocks over billions of years? If the answer to these questions can be given and is positive, then, but only then, will it be of the utmost importance to try to organize, maybe through an international collaborative programme, a renewed study of the most ancient sediments available, focussing our attention not only on the substances of known structures, like phytane or pristane, but also on unexpected ones. The analytical methods have progressed so much, both for separations and for structural elucidation, that novel results can be expected confidently. Maybe this could be undertaken by the "Precambrian Group" of l U G led by Manfred Schlidowski. At the other end of the spectrum lie extremely young sediments. It is not rather amazing that so little has been done to provide a molecular understanding of the mechanisms of soil formation?

ORIGIN

For some sedimentary organic molecules, or even for some families we are in a paradoxical situation, for the resolution of which no logical course of action can exist other than to wait and to make aware of the problems to as many scientists as possible, which is what I am now doing. This is the case for the "extended tricyclic diterpanes", tricyclic derivatives of a regular polyterpane chain, i.e. one built from head-to-tail linked prenyl units. No derivative of this skeleton is known in plants beyond C25, whereas the sedimentary representatives go higher; up to C3o, we thought, up to C35 now be believed the Braz/lian group of Francisco Radler. It takes little expertise to reach the conclusion that sedimentary tricyclopolyterpanes must derive from the corresponding tricyclopolyterpenoids, and to deduce, from the knowledge of basic facts in biosynthesis, that acceptable precursors would be tricylohexa or heptaprenols. These would be perfectly orthodox natural products, were it not that they have never been isolated from living sources. We have deduced from the consideration of possible courses of molecular evolution that these tricyclopolyprenols are probably essential membrane constituents of some "primitive" bacteria, but they are still orphan lipids, whose predecessors are not known, and whose sources cannot be searched in a rational

Introductory Talk by Guy Ourisson

XXIII

way, and we have shown that synthetic tricyclohexaprenol does indeed improve the impermeability of membranes (tricycloheptaprenol would certainly too). Another similar case is that of cyclobisphytane, isolated by Bertrand Chappe along with phytane and bisphytane, both of probable methanogenic, archaebacterial origin. There again, it does not require much expertise to postulate that cyclobisphytane will also, some time, be found to be derived from still unknown phospholipids, probably coming from still recognized and therefore unstudied Archaebacteria, and there again, no rational search can be launched. Problems of origin also occur when a multiple choice remains open. This is still the case for steranes and diasteranes, which could be derived either from some of the few procariotes containing sterols, or from eucariotes. Methylotrophic bacteria like Methylococcus accumulate in their membranes 4~-methylsterols (along with hopanoids) but so do (without hopanoids) Dinoflagellates. The abundance of 4~-methyl steranes in some sediments is therefore of ambiguous significance concerning their origin. Fortunately, this type of ambiguity can now probably be lifted in many cases by the study of isotope contents run in an examplary collaboration between John Hayes and Pierre Albrecht. Development of this method of study will certainly bring many rewards; the D/H and ~3C/12Cratios introduce a large information content into even the least informative structures, like that of methane, and has already shown geohopanoids to be of multiple bacterial origins. STRUCTURES

With proper mastery of the modern analytical techniques, it should now be possible to obtain a plausible structure for each constituent of the soluble fraction of any sediment: gas or liquid chromatography, coupled with Fourier transform N M R or IR, or with mass spectrometry, are powerful enough tools for any new component to be assigned a hypothetical structure. Synthesis of samples of the postulated structure requires other sills, but these are also available, and comparison of the synthetic samples with the sedimentary ones, while rarely giving an absolute proof of identity, can be done so fully as to leave no reasonable doubt. This part of the structural studies does not qualify therefore as a frontier, despite its obvious difficulty in individual cases. Unsolved problems lie however in the study of the major part of the sedimentary organic matter: kerogen, which may form up to 9/10ths of the fossil organic matter, and even asphaltenes and resins; these heterogeneous polymers cannot be studied by the same techniques, and are still largely unknown structurally. I should emphasize at once that, for an organic chemist like myself, the various 'models" of kerogens which have been proposed, like those of humic or fluvic acides, have little significance even if they display some bulk analogies with the sedimentary materials. I see no compelling reason for believing that superficial bulk analogies imply a structural analogy. Even more, I believe any chemist interested in precise molecular structures must have an instinctive distrust for terms which refer to operational definition, but which have a semantic analogy with structural names. Take for instance "kerogen"; this defines what is left after all the inorganic matrix and all the constituents soluble in organic solvents have been removed, an operational definition which must be further narrowed down by precise procedures; there is absolutely no reason why "kerogen" would have a "structure". Each new sample should be studied, and a typology should be established--and has been, as is well known, on the basis of bulk properties like C/H ratios, or N content. There is in fact a very unsatisfactory situation as regards the study of insoluble polymeric materials, especially if they are heterogeneous. Most physical methods of study fail; mass spectrometry of course, N M R also, except in its magic angle version, which is not as useful to disentable structures. IR or Raman spectroscopy are not especially revealing either. The remains to use the chemical methods, and these imply partial degradation to achieve solubilization; "Corpora non agunt nisi soluta". This is precisely the philosophy of the work which has begun to give some indications on the structural elements present in some kerogens, in some research groups. The principle of these studies is to use a selective reaction cleaving cleanly one type of function, and to obtain identifiable fragments. The difficulty lies in the choice of the selective reactions; no reagent works well with a polymeric substrate, and one cannot be sure to have achieved a reasonable degree of attack without harsh conditions, which are prohibited by the requirement for selectivity. As Pierre Albrecht and his group have shown, this is possible with boron tribromide, a selective reagent cleaving ethers (but also esters, etc.), or with ruthenium oxide, the choice reagent to destroy aromatic rings without losing track of their presence, as they leave one additional carbon in the form of a carboxylic acid, as a souvenir in the small molecules dissected from the insoluble matrix. In Poitiers, Ambles has been using crown ether doped potassium hydroxide; we had tried to use hyperefficient solvent mixtures, controlled pyrolysis is another open way (but its interpretations are perilous), etc. More similar selective reagents may be applied alternatively and repeatedly to test whether the polymeric matrix "hides" some potentially reactive groups in its midst, as these are of course necessary before a material balance can be obtained. And then, once such a methodology is developed, it will be possible to apply it to many different samples, to check the degree of generality of the results.

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Introductory Talk by Guy Ourisson CONCLUSION

The tremendous problems solved in Organic Geochemistry during the last twenty years should not leave us with the wrong impression that this science is now ripe and concluded. Significant problems remain to be solved, in a very large number of directions. In most cases, these problems become accessible only once novel techniques can be used; this is the case of the isotopic studies mentioned above, but also of the structural study of solids. In other cases, all that remains to be done is to wait and keep, for instance hoping that somebody will find tricyclopolyprenols in some bacteria. Still in other cases, what has to be done is rather to sit down and think of new ways of looking at obstacles. I believe this is what is needed to introduce a fundamental breakthrough in problems of a global nature. In any case, much remains to be done! I would like to conclude by two remarks of a less scientific nature, but maybe no less important. First, to repeat, after many of you on both sides of the Atlantic, that something must be done to introduce some organized teaching at a high level. Personally, I believe this should be organized in only a few places, maybe only one in Europe, and rely heavily on initial training in chemistry or in geology: some compensatory courses in the field not yet covered, and a few courses giving the precepts of organic geochemistry (or of geochemistry?), could certainly be organized easily. Previous failures at obtaining support should not prevent us from trying again. And lastly, I would like to preach for a better visibility of the successes obtained in private companies, in the real use of organic geochemical data as a help in petroleum exploration. I have been very pleased for instance to meet during this summer a member of the staff of a French Company working in South America, who told me that they have been very successful in their exploration thanks to the complementary of organic geochemical data and of geophysics. This success remains unknown outside a very small group of people in that particular company. How could you (how could we) keep track of the positive case of this type, to build up a corporate image for the field? I leave it to you to answer.