Molecular Organie Geochemistry

P. A. Schenck, J. W. de Leeuw

Delft University of Technology Department of Chemistry and Chemical Engineering Organic Geochemistry Unit Delft, The Netherlands

Introduction

Organic geochemistry can be characterized as a field of science which studies the "fate" of organic compounds in sediments. In its earlier days research efforts were limited - from sheer lack of other possibilities - to the determination ofbulk char­acteristics as for instance contents of organic carbon, nitrogen, hydrogen and per­centage of organic matter extractable with organic solvents.

Development of chromatographie methods in general and of gas chromatogra­phy and gas chromatography/mass spectrometry more specifically have stimulated organic geochemical work enormously over the past two decades. As a result stud­ies have become possible on the molecular level thus leading to a much better in­sight into the processes occurring in sediments and involving organic compounds. Consequently much more has become known about the relation between organic compounds found in recent or ancient sediments and crude oils on the one hand and their precursors in living nature on the other hand.

This knowledge may be of help in environmental studies since it gradually be­comes better known which specific compounds and what amounts can be expected as natural background and/or input contrary to what has to be considered as result ofhuman activities [1]. The present chapter does not attempt to cover the complete fie1d of organic geochemistry. On a limited number of c1asses of compounds it will be demonstrated what detailed insight can be obtained in several cases with our present day analytical techniques. Because of the importance of hydrocarbons from crude oils as pollutants in the environment, several groups of hydrocarbons and their possible natural precursors have been chosen to illustrate the develop­ment of organic geochemical knowledge.

H.-J. Bolle et al., The Natural Environment and the Biogeochemical Cycles© Springer-Verlag Berlin Heidelberg 1982

112 P.A. Schenck and J. W. de Leeuw

Normal Alkanes

The occurrence of n-alkanes in organisms, sediments and crude oils has been inves­tigated extensively [1, 2]. These studies are facilitated by the fact that gas chromato­graphie and mass spectrometric methods make their quantitation relatively simple [3].

It has been known for a long time that crude oils contain very differing amounts of n-alkanes, varying form dominant in certain fractions to undetectable amounts [2, 4]. In paraffinic crudes significant differences in distribution may occur, al­though in the molecular weight range over n-C25 the relative amounts are always decreasing with increasing chain length.

The study of n-alkanes in extracts from recent and ancient sediments has at­tracted much interest since the finding of recent hydrocarbons in recent sediments by P. V. Smith [5] and even more after Stevens, Bray and Evans [6] showed that the distribution ofn-alkanes in recent sediments is significantly different from that in crude oils. The consequence of this latter finding is that oil accumulations are not the result of mere concentration of hydrocarbons already present. In other words, there must be an oil genereation process. A relatively great number of recent sediments investigated are characterized by a preferent occurrence ofthe odd num­bered n-alkanes with chain lengths greater than 15 carbon atoms. Tbis odd pre­dominance is most characteristic in the range n-C25 - n-C33 . The odd predomi­nance is often expressed by means of the carbon preference index (C.P.I.) defined as [7]:

C P I =!{C25 +C27+C29+C31 +C33 C25 +C27+C29+C31 +C33 } ... 2 C24+C26+C28+C30+C32 + C26+C28+C30+C32+C34

In many cases use is made of the R 29 value [8J, defined as R 29 = C 2 X C~9 , 28+ 30

because the n-C29 is often the most predominating n-alkane and because the calculation in the latter case is much simpler.

Numerous studies have revealed that an odd predominance in tbis range is caused by the presence ofn-alkanes belonging to higher terrestrial plants [9,10] in which they are biosynthesized [11]. Most of the recent sediments investigated have an input of terrestrial material brought into them by rivers. Marine organisms on the other hand are known for their preference for one specific n-alkane, mostly n-C15, n-C17 or n-C19 [12]. Consequently, recent sediments containing only marine organic matter do not show the above mentioned odd predominance in the high molecular weight range. The odd predominance gradually disappears [2] in sedi­ments that have undergone the influence ofhigher temperatures because n-alkanes without any preference are formed from organie material in the sediments. Tbis process is known as maturation. The n-alkanes originally present are diluted by those formed during diagenesis ofthe organic matter. This brings the C.P.I. value in crude oils down to about l.0-l.3. In some cases, when sediments have not been exposed to higher temperatures in the earth crust, the original n-alkane distribu­tion pattern can still be recognised. A relevant example is given by Knoche and Ourisson [13] for the n-alkanes isolated from arecent and a fossil (50 x 106 yrs old)

Molecular Organic Geochemistry 113

plant (Equisetum) in which the odd numbered n-alkanes C23 , C25 , C27 , C29 and C31 dominate in a comparable way.

The characteristic distribution of n-alkanes in recent sediments offers possi­bilities for discriminating between fossil and recent alkanes. It should be kept in mi nd that the fossil alkanes can be either anthropogenie or brought into the sedi­ment by natural influences (seeps, erosion). The choise between these two ex­planations has to be made on other grounds than the geochemical considerations mentioned here, like e.g. the actual presence of natural oil seeps in the area under consideration. A disturbing factor when using n-alkane distributions for determin­ing anthropogenie origin and even a specific origin of pollution (e.g. emde oil spills) is the fact that n-alkanes are relatively easily consumed by microorganisms, causing changes in composition after the emde oil has entered the environment [14]. Although n-alkane distribution patterns may be of great he1p in finding sources of pollution, other data and considerations (e.g. period of time in the en­vironment, analytical control of possible sources) have to be taken into account.

Acyclic Isoprenoid Hydrocarbons

The observation ofthe acyclic isoprenoid hydrocarbons pristane (2,6,1O,14-tetra­methyl pentadecane) and phytane (2,6,10, 14-tetramethyl hexadecane) in sediments and emde oils [15, 16] has focussed interest on the group of acyclic polyisoprenoids. They can be considered as significant examples of biochemical fossils because of their stmctural relationship to naturally occurring compounds containing the same carbon skeleton. A direct relation between phytane and phytol has been suggested early [17], phytane thus being an indicator for the original presence of photosyn­thetic organisms. Phytol is li be ra ted from chlorophyll in a very early stage of de­gradation; pristane and phytane might weIl be formed from it via aseries of com­plex conversions [18, 19]. Stimulation experiments in the laboratory have given proof for several reactions mentioned in the scheme by Didyk et al. [20].

The interrelation between phytol and the isoprenoid hydrocarbons Cl9 and C20 has been shown by studying the stereochemistry of the corresponding chiral centra in phytol and in the isoprenoid hydrocarbons respective1y [21-24]. It could proved to be the same in recent sediments [24]. It has been shown, however, from both simulation experiments [25] and from results of analyses of extracts from sed­iments of increasing "maturity" [26] that epimerization at the chiral centra in the isoprenoid hydrocarbons occurs with increasing degree of diagenesis.

Apart from the above-mentioned isoprenoids pristane and phytane aseries of isoprenoid hydrocarbons of lower molecular weight have also been found [27]. They are often considered as being formed from phytol by exclusive cleavage of carbon bonds of the main chain. This mere cleavage of the main chain's carbon bonds could provide an explanation for the fact that the Cl7-isoprenoid (2,6,10-trimethyl tetradecane) as well as the C12 (2,6-dimethyl decane) are often virtually absent in extracts from sediments or emdes or only present in minor amounts [28].

On the other hand, lower molecular weight isoprenoid hydrocarbons like far­nesane (2,6, lO-trimethyl dodecane) could also be derived from corresponding alco­hols like farnesol. I t is interesting in this respect that in two emde oils from the Gulf

114 P.A. Schenck and J. W. de Leeuw

Coast and Syria all four famesane stereoisomers occur in equivalent amounts [29]. This may point indeed to farnesol as aprecursor although phytol as precursor can­not be precluded in case of epimerization at the chiral centra.

Another view on the origin of the isoprenoid hydrocarbons is presented by their occurrence as such in recent organisms. Blumer and his collaborators showed as early as the mid-sixties that phytol can be converted into pristane by copepods (or bacteria living in their intestinal system [30, 31]. Consequently pristane can thus be introduced into recent sediments as such. Unsaturated hydrocarbons with the same carbon skeleton could be isolated too [32]. It is noteworthy that no phytane could be found. Recent work has made it clear that many more isoprenoid hydro­carbons occur as such in the so called Archaebacteria; the whole range of CIS-C30

could be shown in several species, most of these isoprenoids of the regular head­to-tail type [33]. Since part of the methane producing bacteria belong to these Ar­chaebacteria it may very weIl be that at least part of the isoprenoid hydrocarbons found in recent or ancient sediments and in crude oils have been introduced as such and have not been derived via diagenetic pathways from phytol or comparable iso­prenoid alcohols. In this context it should be emphasized that the Cl7-isoprenoid hydrocarbon has been observed in some Archaebacteria, viz. in several thermoaci­dophilic species [33]. Considering what has been said above on the cleavage ofthe main carbon chain of phytol it could be speculated that this Cl 7-isoprenoid hydro­carbon enters the sediment as such and is not the product of diagenetic reactions of phytol.

In addition to the free isoprenoids several species of Archaebacteria contain among others dialkylether glycerides with phytanyl groups as alkyl moieties [34, 35]. These ethers might undergo diagenetic reactions leading to the formation of isoprenoid hydrocarbons. The stereoehemistry of these phytanyl groups is the same as that of phytol as far as the two corresponding chiral centra are concemed [36]; determination of the stereochemistry at these centra thus does not point un­ambiguously to an origin from phytol.

The isoprenoids up to C20 mentioned hitherto all belong to the so called "reg­ular", i.e. "head-to-tail" type. Many "irregular" ones of both the "head-to-head" and "tail-to-tail" type are found with more than 20 carbon atoms. An example of this group is squalane (2,6,10,15, 19,23-hexamethyl tetracosane) found in sediments and erude oils [37]. It eontains a "tail-to-tail" eombination; it is almost eertainly related to squalene, widely occurring in living nature.

Isoprenoid struetures with "head-to-head" bound isoprene units are also known in living nature: De Rosa et al. [38] found di-(biphytanyl) diglycerol tetra­ether lipids in some thermoacidophilic Archaebacteria with w-wl-diphytanyl struc­tures. This finding is the more interesting sinee this same type of biphytanyl strue­tures is present in some kerogens, the insoluble part of organie matter in sediments [39,40].

In eonclusion it ean be stated that at the present stage of our knowledge iso­prenoid hydrocarbons in (ancient) sediments and crude oils are not necessarily mainly derived from phytol from chlorophyll. On the eontrary, it might very weIl be that a eontribution from Archaebacteria plays a mueh more important role than previously antieipated; the hydroearbons ean be introdueed into the sediment ei­ther as such and/or derived from the bacterial membranes (Fig.l).

Molecular Organic Geochemistry

Fig.1

Chlorophyll

! Phytol

C-O~ t-o~ I C-OR

~/ Isoprenoid hydrocarbons

l~p~~1d / ~ Hydrocarbons y-O~-Y in Archaebacteria C-O~-y

I C -OR RO -C

Steroids

Occurrence and Diagenesis

115

Steroids are tetracyclic isoprenoid compounds which occur widespread in nature. Almost all eukaryotic organisms contain free and bound steroids as lipid com­ponents. The steroids present in eukaryotic organisms have specific distribution patterns as the result ofbiosynthesis and/or dietary uptake. Only a few prokaryotic organisms contain steroids. The steroids biosynthesized by these prokaryotic or­ganisms are often of a different structural type (e.g. 4,4-dimethyl substitution) when compared with those biosynthesized by eukaryotic organisms [41]. Numer­ous detailed investigations of steroid components oceurring in different types of sediments ranging from very recent to very aneient have enabled organie geo­ehemists to unravel the major ehemieal pathways operating in sediments shortly after deposition of fresh organic matter and during maturation of sediments.

Figure 2 shows a simplified overview of the fate of naturally occurring sterols after burial in sediments. The sterols present in organisms are free or bound (e.g. as steryl ester, sulfate ester, ete.). The bound sterols have to be hydrolyzed to the free ones before they ean undergo bioehemieal transformation reaetions in the top layers of the sediment [42]. Free sterols (I) are transformed by two different path­ways. Direet dehydration results in the formation of,d 3,5 -steradienes (11) and other isomerie steradienes, whenever the substrate possesses the eommon ,d5-double bond [43]. Naturally occurring 5ocH-stanols are probably dehydrated to ,d2-5ocH­sterenes (111). Another bioehemieal pathway involves the transformation of ,d5_ sterols to either 5ocH- or 5ßH-stanols via several intermediates such as ,d4-stenones and saturated stanones [44, 45]. 5ßH-Stanols are only generated in this way by anaerobic microorganism [42]. The stanols are eonverted to the corresponding ,d2_ sterenes (III) and to the ,d4_ and ,d5-sterenes (IV). The ,d4_ and ,d5-sterenes mayaiso be the result of isomerization of initially formed ,d2-sterenes [46]. All these trans­formation reaetions take plaee in the very early stages of diagenesis and are mainly of microbiological nature as incubation studies have shown. Starting from these early stage diagenesis produets (11,111, IV) several pathways during further matu-

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Molecular Organic Geochemistry 117

ration can occur. Part ofthe sterenes and steradienes oftype II, III, and IV are hy­drogenated to 5a- or 5ß-steranes (V) with preservation of the natural stereochem­istry at positions 14, 17, and 20 (14aH, 17aH, 20R). Upon increasing maturation isomerization reactions occur resulting in more thermostable steranes, such as SaH - and 5ßH -steranes with the 14ßH - and 17 ßH -configuration and both the 20R and 20S stereoisomers (X) [47]. Another part of the steradienes of type II may undergo A-ring aromatization with either loss or a shift of the original angular methyl group at C 1D (XI). Upon further maturation the aromatization can spread out from the A-ring via the B-ring to the C-ring, ultimately resulting in a suite of completeley aromatized steroids of type XII.

Apart from saturation the ,14_ and ,1 5-styrenes (IV) undergo backbone rear­rangements resulting in the so called diasterenes of type VI with 20R and 20S ste­reoisomers [46]. These diasterenes are either hydrogenated to isomerie mixture of diasteranes (VIII) or they are aromatized starting from ring C (VII) to completely aromatized steroids of type IX with loss or shift of the original angular methyl­groups at C 1D and C13 [48, 49].

In summary we can say that in ancient sediments or oils the original sterols are reflected by isomerie mixtures of saturated steranes of types VIII and X and aromatized steroids of types XII and IX. The relative distribution of these four groups of steroid hydrocarbons is a result of all kind of sediment parameters, such as anoxie vs oxic deposition, the mineral and eIay content, the water content and the thermal gradient.

Especially the nature ofthe inorganic matrix probably plays an important role in the ultimate ratio of regular steranes and back bone rearranged steranes. It has been shown that so called superacid sites, present in eIay minerals such as kaolinite and montmorillonite, catalyze the backbone rearrangement transformations [50].

Steroids as Biological Markers

As shown above, sterols undergo a variety of diagenetic reactions. However, de­tailed structural elucidation of the sterol derived components present in sediments still gives eIues to the original environment of deposition. Especially the structure and stereochemistry of the several side chains and their distribution pattern enable us to reconstruct to some extent the original environment. In general it can be stated that marine organisms, especially algae, contain complex mixtures of sterols with a great variety in side chain structures. Higher plants on the contrary exhibit simple sterol patterns and the sterol side chain structures are generally limited [44]. In marine algae the number of carbon atoms of the steroidal side chain varies from 2 to 11, in higher plants form 8 to 10 only. Within the overlapping Cs to C 1D side chain series discrimination of an origin from either algae or higher plants is pos­sible to some extent due to specific structures and the stereochemistry. Dinoflagel­lates and to some extent coelenterates and diatoms for instance biosynthesize sterols with 23,24-diMe substitution [51-53], while higher plants exeIusively biosynthesize sterols with side chains having alkyl substituants (Me or Et) at C24.

Moreover the stereochemistry of the alkyl substituents at C24 differs between higher plants and algae [41]. Among steroids obtained from sediments aseries of

118 P.A. Schenck and J. W. de Leeuw

4-methyl substituted steroid hydrocarbons is often encountered. As far as it is known only dinoflagellates and a limited number of methanotrophic bacteria are capable of 4-methylsterol biosynthesis. The occurrence of 4-methyl steranes in combination with typical algal side chain structures such as 23,24-diMe substitu­tion in sediments reflects therefore the original presence of dinoflagellates [54]. By means of advanced capillary gas chromatography it is possible to separate 24R and 24S isomers [55]. Application ofthis method to mixtures of steranes obtained from sediments will enhance the value of steroid hydrocarbons as molecular fossils.

When crude oils or ancient sediments become available for microbial attack several phenomena have been observed as far as the steranes are concerned. Mod­erate biodegradation hardly affects the steranes, but heavy biodegradation results in destruction of regular steranes and survival of the diasteranes (20R better than 20S) [56].

Due to the rather detailed knowledge of steroid diagenesis and due to the tax­onomic specificity of the sterol biosynthesis, it can be stated in conclusion, that a detailed study of sedimentary steroid hydrocarbons enables us to estimate the de­gree of maturation of a sediment and to partly reconstruct the paleo-environment.

Triterpenoids

Occurrence and Diagenesis

Introduction

The triterpenoids are widespread both in the biosphere and the geosphere [57, 58]. A relatively large number of organic geochemical investigations (especially those by the Organic Geochemistry Unit ofthe Louis Pasteur University at Strasbourg, France) have resulted in a detailed knowledge of the pathways by which several classes ofnaturally occurring triterpenoids are converted during early and late dia­genesis. Since these diagenetic pathways - which determine the fate oftriterpenoids - are highly dependent on structural entities present or absent in the starting com­pounds it is nescessary to chemically classify the naturally occurring triterpenoids in several groups. Figure 3 schematically shows the several triterpenoid classes which will be described separately further on in this paragraph.

For geochemical reasons the triterpenoids can be divided firstly into triter­penoids having the hopanoid skeleton and those with other skeletons. The non-ho­panoid triterpenoids occur widespread in higher plants and very often possess an oxygen function at the C3 position (either alcohol or ketone). These non-ho­panoids do not have representatives with more than 30 carbon atoms, in other words no extended homologues occcur. The hopanoids can be divided into two subgroups: the 3-oxy hopanoids and the 3-desoxy hopanoids.

The 3-oxy hopanoids occur in a few families ofhigher plants. Finally the 3-des­oxi hopanoids can be divided into the so called non-extended and extended ones. The non-extended 3-desoxy hopanoids do have carbon skeletons with a

Molecular Organic Geochemistry

Triterpenoids

A hoponoids non-hoponoids (3-ox Yinon-extended)

(moinly higher plonts)

e.g.; s~eleton: 21

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3-oxy-hoponoids 3-desoxy-hoponOids (few fomilles of higher plonts) e.g. :

/. ,~[ O~ extended

119

non-extended

(ferns,lichens,some microorgonisms) e.g.;

(several groups of microorganismsl

~ Fig.3

maximum of 30 carbon atoms. They mainly occur in ferns, lichens and some micro­organisms. It should be noted that especially ferns do also contain other series of triterpenoids, such as fernene, adianene and filicene derivatives.

The extended 3-desoxy hopanoids exclusively occur in certain groups of micro­organisms and are characterized by an extended side chain of 8 carbon atoms at position 21 often substituted with 4 or 5 hydroxygroups [58]. After this classifica­tion of the naturally occurring triterpenoids in terms of structural differences and related occurrences in organisms an overview is given of the several diagenetic pathways and the subsequent resulting suite of geochemical products for each de­fined class of triterpenoids.

3-0xy-Triterpenoids (N on-Hopanoids)

The triterpenoids belonging to this dass which occur alm ost exdusively in higher plants are converted in the first stages of diagenesis. Two major diagenetical path­ways can be discriminated (Fig.4; ß-amyrin as an example).

120 P. A. Schenck and J. W. de Leeuw

Dicgenesis of 3-cxy-triterpenoids (non-hopcnoids)

?? ) )crom.

Lass of A_ring (via ~l crom. HOf

:n:

Fig.4

It is thought that microorganisms play an important role in these transforma­tion reactions, although it cannot be precluded that the initialloss of the original A-ring (pathway 11) occurs via a photochemical conversion [59]. The first products via pathway I are the AB or ABC-ring aromatized compounds depending on the starting product. Possible intermediates with one aromatized ring have not been found. Obviously, during the aromatization several methylgroups are removed [60, 61]. During further diagenesis the aromatization continues via intermediates with ABC-aromatic rings to the fully aromatized compounds. Similar end products are known with more or less methylgroups [61]. Following pathway 11 the A-ring is lost, possibly via the intermediate shown in Fig.4 which points to microbial activ­ity. The resulting products possess an aromatic B-ring. Ultimately, due to further aromatization of the C-, D-, and E-ring, fully aromatized tetracyclic end products are generated [61, 62]. Due to the original precursor molecule and due to slightly different diagenetic pathways the methyl substitution pattern varies to some ex-

Molecular Organic Geochemistry 121

Diogenesis of 3-desoxy hoponoids

Fig.5

teht. It should be emphasized that these partly and/or fully aromatized compounds are already present in very recent sediments, indicating that these diagenetic path­ways occur under very mild conditions and are probably in part of microbial na­ture.

It seems surprising that fully saturated triterpenoid compounds of this type are less frequently encountered in both recent and ancient sediments [59], since other cyclic compounds possessing a hydroxylgroup in the A-ring such as sterols, are partly reflected in sediments as saturated hydrocarbons, e.g. steranes.

3-Desoxy Hopanoids (Non-Extended)

In ferns, lichens and several micro-organisms non-extended 3-desoxy hopanoids occur sometimes together with other classes of triterpenoids. Figure 5 shows the several diagenetical pathways by which the 3-desoxy hopanoids, such as diplop­terol are converted. During early diagenesis important intermediates are gener­ated, e.g. the C27-ketone and C30-olefins [63]. It is believed that the intermediate ketones initiate the aromatization of ring D upon further diagenesis [60]. Starting

122 P.A. Schenck and J.W. de Leeuw

Dlageneslc; of C3S-hopanepolyols

#~H t early stage dia genesis

t

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+ n=O-6

Fig.6

from the D-ring aromatized eompound, rings C and Bare aromatized respeetively. The fuHy aromatized eompounds are produeed upon further diagenesis [62, 64].

Another part ofthe C2rketone-intermediate is eonverted via several intermedi­ates to the eorresponding hydroearbon with the thermostable 17IXH -eonfiguration [63]. The initiaHy formed C30-o1efins undergo hydrogenation resulting in isomerie C30-hopanes with both the 17ßH, 21IXH and 171XH, 21ßH eonfigurations [63].

Extended Hopanoids

The C35-hopanepolyols, whieh are exdusively eneountered in a vast number of baeteria and eyanobaeteria undergo rapid ehanges during very early diagenesis (Fig. 6). A suite of intermediate eompounds with C27 , C29-C35 earbon atoms and

Molecular Organic Geochemistry 123

3-oxy-hopanoids

Fig.7 ~ ~ ···· .. fOH ,

~ o ..... , hydroxyhopanone

several functional groups have been isolated from very recent sediments [65]. Upon further diagenesis these compounds are converted via two major diagenetical path­ways which parallel the pathways described for the non-extended 3-desoxy-ho­panoids. Hence, the intermediates partly react to compounds with an aromatic D­ring and subsequently to the ABCD-ring aromatized products via compounds with CD- and BCD aromatized rings respectively. On the other hand the initial diage­netic compounds are reduced to the corresponding hydrocarbons with the 17 ßH, 21ßH configuration. These hydrocarbons isomerize in a later stage of diagenesis to hydrocarbons with the 17aH-configuration (the C27 compound) and to hydro­carbons with the 17aH, 21ßH and 17ßH, 2laH configurations (the C29-C35 series) [63]. The latter two series of compounds occur as 22R and 22S isomers after con­siderable diagenesis. The recent finding of 17aH-hopanoids in a contemporary un­polluted mud suggests that 17aH-hopanoids can also be obtained by microbial processes and not only via an acid catalyzed isomerization in older sediments [65]. In a Balthic sea sediment a number of 3-methyl hopanoid hydrocarbons are shown to be present. They are of bacterial origin also, since it is known that Acetobacer xylinum and Acetobacter rancens biosynthesize hopanepolyoles with a 3-methyl­group [66].

3-0xy Hopanoids

These triterpenoids (Fig.7) occur in the resins or saponins of a few higher plant families. As yet hardly anything is known ab out the geological fate of these com­pounds, although one might speculate that major diagenetical pathways are com­parable with those described above for the 3-desoxy hopanoids and the non-ho­panoid 3-oxy triterpenoids.

Triterpenoids as Biological and Maturation Markers

A detailed analysis of triterpenoid compounds in a sediment may reveal to some extent the environment of deposition. a) The presence of extended saturated andjor aromatized hopanoid hydrocarbons

reveals the original presence of bacteria andjor cyanobacteria. b) The presence of 3-methyl-hopanoid hydrocarbons points to the original occur­

rence of sepcific bacteria, like Acetobacter xylinum. c) The presence of pentacyclic and tetracyclic aromatic compounds with well de­

fined structures points to the original presence of higher plant waxes and hence to a terrestrial input.

124 P. A. Schenck and J. W. de Leeuw

Diagenesis of di terpenoids

Fig.8

The state of maturation of the sediment can be estimated by detailed analyses of the sedimentary triterpenoids. a) Functionalized triterpenoids only occur in very recent sediments. b) The configurations of the Cl?' C21 and C22 positions in the hopanoid hydro­

carbons reflect the state of relative maturity of the sediments. It should be no ted that the occurrence ofpartly or fully aromatized triterpenoid

hydrocarbons is not restricted to ancient sediments. Certain types already occur in very recent sediments.

Diterpenoids

Over the past few years a number of partly and fully aromatized tricyc1ic com­pounds (Fig.8) with specific alkylsubstitution patterns have been encountered in young sediments [61, 67, 68]. The cooccurrence ofthese hydrocarbons with similar functionalized tricyclic compounds provides evidence for the diagenetic pathways of diterpenoids, especially for those with the abietin skeleton. Abietic acid, a com­pound relatively abundant in higher plant resins, especially from conifers, is thought to be the most important precursor of these sedimentary hydrocarbons. Via a number offunctionalized intermediates [67] ring C is aromatized and the car­boxylgroup is removed (pathway B).

Molecular Organic Geochemistry 125

Abietatriene may result from the reduetion ofthe earbonyl group of abietie acid (pathway A). But it also oeeurs in nature [69]. Further aromatization ultimately results in the formation of the fully aromatized eompound retene.

These aromatie hydroearbons, when found in sediments are exeellent biologieal markers of resinous higher plants. Sinee these diterpenoid derived aromatie eom­pounds are already present in very young sediments and soils we must assurne that the transformations are mainly mierobial in nature [70].

Polycyclic Aromatic Hydrocarbons

Within the class of aromatie hydrocarbons, much attention has been paid in recent years to the polycyclic aromatic hydrocarbons. Many polycyclic aromatic hydro­carbons (PAH) are known earcinogens (e.g. benzo(a)pyrene, benz(a)anthracene, methylchrysenes). The oeeurrence of polycyclic aromatic hydrocarbons in geologi­cal sampies in the environment has therefore increased the search for these com­pounds in different environments.

Examples of relatively simple polycyclic aromatic hydrocarbon mixtures in geological settings are benzopyrene in asbestos [71-73]; fluoranthene and picene in mercury ores [74], perylene in recent sediments [75-78] and pyrene and fluoren­thene in mangane se nodules [79].

It is known that in ancient sediments and erude oils very complex mixtures of polyeyclie aromatic compounds occur [2]. Extensive search for this type of com­pounds has shown that they occur widespread in soils and recent aquatic sedi­ments. Blumer and coworkers analysed many recent marine sediments [80-82]; the same type of sediments were also analysed by other investigators [67, 83, 84]. Giger and Schaffner [85, 86] and Wakeharn et al. [61, 87, 88] concentrated on lacustrine sediments.

In erude oils the alkylhomologues within aseries are more abundant than the non-substituted parent aromatic moleeule [81, 82]. In recent sediments it is the par­ent moleeule whieh is the predominant one; this may offer an opportunity to decide upon anthropogenie pollution or natural oecurrenee.

E.g. Blumer [81] mentions a case in whieh a specific PAH-series (the Cn H2n - 18

series) was shown to be present in an oil spill at West Falmouth (Massachusetts); its characteristics eould be found in the marsh sediment, even five years after the spill. Other series (Cn H2n - 22 and Cn H2n - 24) did not occur in the oil, but were shown in the sediment.

The remarkable similarity in composition of the PAH's over a wide range of depositional environments, the predominance of the unsubstituted "parent mole­cule" of aseries and the extended alkylhomology for PAH shown in recent marine sediments lead Y oungblood and Blumer [82] to the conclusion that they cannot have been formed biochemically.

Hase and Hites [89] conclude from aseries of experiments that bacteria do not produee polycyclic aromatie hydrocarbons but rather bioaccumulate them.

Thermal PAH formation can oceur over a wide range of temperatures: high temperatures favour the unsubstituted aromatics whereas lower temperatures pre­serve a greater degree of alkylation. These considerations explain the highly alkyl­ated PAH assemblages in erude oils and point to expect even more highly alkylated

126 P. A. Schenck and J. W. de Leeuw

mixtures for this type of compounds when formed by abiotic transformations dur­ing early diagenesis of organie matter. The composition of the mixtures found sug­gest a pyrolytic origin at intermediate temperatures, which Y oungblood and Blu­mer [82] think to be forest and prairie fires, the products ofwhich are widely spread by aeolian transport.

Wakeharn et al. [61,88] have concentrated on the search for PAH in lake sed­iments in rather highly industrialized and very populated areas [Lake Washington (USA) and three lakes in Switzerland]. Surface sediment layers in these lakes are strongly enriched in PAH - up to 40 times - compared to deeper layers, deposited in pre-industrial revolution sediments. This led the authors to coneIude that most of the PAH are of anthropogenie origin. Based upon detailed comparison of data obtained for sediment sampies, street dust, weathered asphalt, tire particles and automobile exhaust they coneIude that urban runoff containing street dust parti­eIes is possibly the major present-day source for the PAH in the lakes investigated. The authors' data suggest that asphalt particles in the street dusts may be an ex­tremely important contribution to the PAH content of the lake sediments. On the contrary a limited number of PAH are apparently of natural, early diagenetic or­igin [61]. This group of compounds consists of perylene, an extended series of phenanthrene homologues, retene and pimanthrene derived from diterpenes (abietic acid and primarie acid respectively), aseries oftetra- and pentacyeIic PAH derived from pentacyeIic 3-oxy triterpenoids (e.g. of the amyrin type) and penta­cyeIic PAH derived from pentacyclic 3-desoxy triterpenoids, mostly of the hopane type.

The presence of perylene has been mentioned by many authors as a predomi­nant PAH in recent freshwater [87, 90] and in marine sediments [67, 75, 77, 91]. Perylene is present though not abundant, in soils and unaltered river sediments [67, 87] and in plankton [85, 87]. Terrestrially derived biogenic perylenequinone pig­ments are known in nature and are therefore suggested as possible percursors. The recent findings of perylene in Namibian Shelf sediments [78] contradicts a terres­trial origin since the input of terrestrial material into these sediments is thought to be negligible.

Wakeharn et al. [61] coneIude that early diagenesis can be an important process responsible for the formation of the special PAH mentioned before in recent lake sediments. The authors state that - based upon the abundance of this suite of nat­ural P AH in very young lake sediments - the transformation reactions of natural compounds into these PAH must be rapid and probably mediated by micro-or­ganisms. The last hypothesis remains untested, however, up till now.

The possible formation ofP AH from natural precursors as indicated above and mentioned in the previous sections on steroids, triterpenoids and diterpenoids has to be taken into account when interpretating data on P AH regarding their or­igin. Only detailed analyses and careful comparison of results will open possibilities for deciding on an origin either from natural sources or due to human activities.

Epilogue

The foregoing paragraphs have illustrated the growing insight into the relationship between organic compounds occurring in nature and those found in sediments and

Molecular Organic Geochemistry 127

erude oils. Speeific distributions and elueidation of detailed struetural features as e.g. stereoehemistry open possibilities for diseriminating between reeent and fossil eompounds.

Inereasing information on the chemotaxonomy of many organisms has given a mueh better basis for the interpretation of data obtained from analyses of sed­iments.

With inereasingly better methods for detailed analyses more results ean be foreseen for the years to eome. This will not only give better insights into relation­ships between fossil and reeent eomponents, but also open horizons for diserimi­nating between natural and anthropogenie origin.

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