Origin and formation pathways of kerogen-like organic matter in …directory.umm.ac.id/Data Elmu/jurnal/O/Organic... · 2010. 3. 29. · Origin and formation pathways of kerogen-like

Origin and formation pathways of kerogen-likeorganic matter in recent sediments o the Danube delta

(northwestern Black Sea)

Anouk Garcette-Lepecq a,b,*, Sylvie Derenne a, Claude Largeau a,Ioanna Bouloubassi b, Alain Saliot b

aLaboratoire de Chimie Bioorganique et Organique Physique, UMR CNRS 7573, E.N.S.C.P., 11, rue P. et M. Curie,

75231 Paris Cedex 05, FrancebLaboratoire de Physique et Chimie Marines, ESA CNRS 7077, UniversiteÂ P. et M. Curie, 4 place Jussieu, 75252

Paris cedex 05, France

Abstract

The chemical structure, source(s), and formation pathway(s) of kerogen-like organic matter (KL) were investigated

in recent sediments from the northwestern Black Sea, o the Danube delta. Three sections from a sediment core col-lected at the mouth of the Sulina branch of the delta, under an oxic water column, were examined: S0 (0±0.5 cm bsf),S10 (10±13 cm bsf), and S20 (20±25 cm bsf). The bulk geochemical features of these sediments (total organic carbon,organic C/N atomic ratio, d13Corg) were determined. Thereafter, KL was isolated from the samples, as the insolubleresidue obtained after HF/HCl treatment. KL chemical composition was investigated via spectroscopic (FTIR, solidstate 13C and 15N NMR) and pyrolytic (Curie point pyrolysis±gas chromatography±mass spectrometry) methods, andthe morphological features were examined by scanning and transmission electron microscopy. Similar morphological

features and chemical composition were observed for the three KLs and they suggested that the selective preservationof land-plant derived material as well as of resistant aliphatic biomacromolecules (probably derived from cell walls offreshwater microalgae) was the main process involved in KL formation. Besides, some melanoidin-type macro-

molecules (formed via the degradation-recondensation of products mainly derived from proteinaceous material) and/orsome encapsulated proteins also contributed to the KL chemical structure. # 2000 Elsevier Science Ltd. All rightsreserved.

Keywords: Kerogen-like organic matter; Recent sediments; Danube delta; Northwestern Black Sea; Preservation mechanisms; Selec-

tive preservation pathway; Degradation-recondensation

1. Introduction

A wide variety of organic and inorganic substances

are transported from the continent to coastal marineenvironments via river discharge. The fate of riverineorganic material in coastal areas largely depends on the

physico±chemical partition between particulate and dis-solved phases, and how this distribution is modi®ed asfreshwater mixes with saline water (Stumm and Mor-

gan, 1970; Olsen et al., 1982). The rapid ¯occulation ofterrestrially derived humates in seawater has beendemonstrated (Narkis and Rebhun, 1975; Sholkovitz,

1976; Sholkovitz et al., 1978), and a large part of bothriverborne particulate organic matter (POM) and ¯oc-culated colloids is expected to be exported from the

water column to the sediment underlying estuarine sys-tems.The Danube delta is, after the Volga, the second lar-

gest delta in Europe, and 23rd in the world. It lies at the

mouth of a river extending over a distance of 2860 km(exclusive of the delta itself) and drains an area of805,300 km2. Within the delta, two major bifurcations

0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.PI I : S0146-6380(00 )00100-5

Organic Geochemistry 31 (2000) 1663±1683

www.elsevier.nl/locate/orggeochem

* Corresponding author at ®rst address. Fax: +33-1-43-25-

79-75.

E-mail address: [email protected] (A. Garcette-Lepecq)

give rise to three separate tributaries: the Chilia, Sulinaand Saint Gheorghe branches. The Danube collectseuents of eight European countries, and accounts for77% of the total freshwater discharge in the Romanian

coastal shelf, which corresponds to a mean annual dis-charge of ca. 200 km3. Increased anoxia in the north-western Black Sea indicates that anthropogenic

activities in the drainage basin of the Danube raised theinput of riverborne OM and the level of inorganicnutrients, and hence the production of autochthonous

OM, beyond the natural assimilation capacity (Cociasuet al., 1996). The increase in nutrient and OM inputfrom rivers, and from sources such as fertilisers from

agriculture, and industrial and municipal wastewatersalong the shoreline (especially phosphate), is the maincause for the increased eutrophication observed in thesemi-enclosed Black Sea basin during the last decades

(Mee, 1992). Indeed, due to a strong thermohaline stra-ti®cation and the long residence time of water masses,the Black Sea ecosystem is highly sensitive to increased

production of OM (Cociasu et al., 1996). In addition,the mainly wind-driven water circulation along theRomanian coast and the Coriolis force lead to a long

residence time of the riverine waters on the Romanianshelf and the resulting negative eects on the coastalecosystem have been observed (Mee, 1992). Distinct

increases in phytoplankton cell densities, as well as thefrequency of algal blooms, were reported. There was amore than double decrease of benthic biomass in theRomanian shelf area due to suboxia in the bottom water

after the sedimentation of algal blooms (Cociasu et al.,1996).The present study was designed to investigate the

insoluble, HF/HCl resistant, OM isolated from threesections of a recent sediment core collected o theSulina branch of the Danube delta. This organic frac-

tion corresponds to the ``kerogen'' de®ned by Durandand Nicaise (1980) as the fraction of sedimentary OMisolated via organic solvent extraction and HF/HCltreatment, and is therefore referenced as ``kerogen-like''

material (KL) in this paper. KL is characterised by amacromolecular structure with a high resistance to bac-terial, chemical, and diagenetic alterations. Only a few

studies about KL have been reported so far, whilst sucha fraction can account for a substantial part of totalOM in marine sediments (e.g. Eglinton et al., 1994;

Zegouagh et al., 1999). The aim of the present work isto derive information on the mechanism(s) and sour-ce(s) involved in KL formation and preservation in

these Black Sea sediments. To this end, the morpho-logical and chemical features of KLs were examined viaa combination of electron microscopy observationsand spectroscopic and pyrolytic methods, so as to

deconvoluate autochthonous versus allochthonoussources contributing to KLs in such a deltaic environ-ment.

2. Experimental

2.1. Sediment samples

The sediment core was collected o the Sulina mouthof the Danube delta (N 45�080; E 29�480; water depth 20m; salinity ca. 10; Fig. 1), under an oxic water column,

during the EROS 21 (European River±Ocean Systems)cruise (9 April±3 May 1997). Three core sections wereselected on the basis of their darker colour pointing to

relatively high organic contents, wrapped in pre-combusted aluminium foil, frozen on board and keptfrozen until analysis. The sections correspond to inter-

vals 0±0.5 cm bsf (S0 sample, oxic level), 10±13 cm bsf(S10 sample), and 20±25 cm bsf (S20 sample). The sedi-ments were freeze dried and ground before furthertreatment.

2.2. Total organic carbon (TOC)

TOC contents were determined after decarbonatationby high temperature catalytic oxidation (HTCO) using aShimadzu Solid Sample Module SSM 5000A coupled to

a Shimadzu TOC 5000 analyser.

2.3. KL isolation

About 10 g of dry sediment were treated to isolate the``kerogen-like'' fraction. Lipids were ®rst removed byextraction with CH2Cl2/CH3OH, 2/1, v/v (stirring over-

night at room temperature). The bulk of the mineralconstituents of the lipid-free samples was then elimi-nated via a classical HF/HCl treatment (Durand and

Nicaise, 1980). The organic residue was submittedto extraction with CH2Cl2/CH3OH, 2/1, v/v (stirringovernight at room temperature), and ®nally the inso-

luble organic material was recovered by centrifugationand dried under a nitrogen ¯ow. It is important to note

Fig. 1. Map of the northwestern Black Sea, the Danube delta,

and the sampling site.

1664 A. Garcette-Lepecq et al. / Organic Geochemistry 31 (2000) 1663±1683

that HF/HCl treatment, when applied to recent materi-als, results not only in the elimination of most mineralsbut also in the removal of some insoluble, macro-molecular, organic constituents (e.g. labile protein- and

polysaccharide-derived moieties) due to acid hydrolysis(Durand and Nicaise, 1980). However, some acid-labileorganic constituents could be retained due to incorpora-

tion within the macromolecular network of the KLs.

2.4. Elemental analysis

Elemental composition of the three KLs obtainedafter the above treatments was determined by the ``Ser-

vice Central de Microanalyse'', CNRS, Vernaison.

2.5. Scanning (SEM) and transmission (TEM) electronmicroscopy

KL samples were ®xed by 1% glutaraldehyde incacodylate buer (pH 7.4), post-®xed in 1% OsO4 and

embedded in Araldite. Ultrathin sections were stainedwith uranyl acetate and lead citrate, and TEM observa-tions were performed on a Philips 400 microscope.

For SEM, the ®xed samples were dehydrated usingthe CO2 critical point technique and coated with gold,as previously described by Largeau et al. (1990), prior to

observations on a Jeol 840 microscope.

2.6. Fourier transform infra red spectroscopy (FTIR)

FTIR spectra were recorded on a Bruker IFS 48spectrometer using pellets prepared by grinding 1 wt.%of KL with KBr.

2.7. Solid-state 13C and 15N nuclear magnetic resonancespectroscopy (13C and 15N NMR)

Solid-state 13C NMR spectra were obtained at 100.62MHz for 13C on a Bruker MSL400 spectrometer with arecycle time of 5 s. High power decoupling, cross polar-

isation (contact time 1 ms) and magic angle spinning at4.5 kHz, were used. The spectra were the results of ca.8000 scans. Since all spectra recorded at 4.5 kHz are

similar, only one sample (S20) was analysed at a higherspin frequency (15 kHz), on a Bruker MSL400 with thesame parameters, to remove spinning side bands which

make interpretation of some resonances dicult in thespectra at 4.5 kHz.A Bruker DMX 400 operating at 40.56 MHz was

used for acquiring the solid-state CP/MAS 15N NMRspectrum of one of the three samples (S20), applying acontact time of 1 ms and a recycle time of 150 ms. Ca. 2million scans were accumulated at a magic angle spin-

ning speed of 5.5 kHz. The chemical shift is referencedto the nitromethane scale (= 0 ppm) and was adjustedwith 15N labeled glycine (ÿ347.6 ppm).

2.8. Curie point pyrolysis±gas chromatography±massspectrometry (CuPy±GC/MS)

KL aliquots (ca. 3 mg) were loaded in tubular ferro-

magnetic wires with a Curie temperature of 650�C andpyrolysis was performed by inductive heating of thesample-bearing wires to their Curie temperature in 0.15

s (10 s hold time). A Curie point high frequency gen-erator was used to produce the magnetic ®eld and thepyrolysis unit (Fisher 0316M) was directly coupled to

GC±MS (a Hewlett Packard HP-5890 gas chromato-graph and a Hewlett Packard HP-5889A mass spectro-meter, electron energy 70 eV, ion source temperature

250�C, scanning from 40 to 650 a.m.u., 0.7 scan/s).Separation of the products was achieved by a 30 mfused silica capillary column coated with chemically-bound Restek RTX-5MS (0.25 mm i.d., ®lm thickness

0.50 mm). Helium was used as carrier gas. The tempera-ture of the GC oven was programmed from 50 to 300�Cat a rate of 4�C/min, after a ®rst stage at 50�C for 10min to allow a better separation of the most volatilepyrolysis products. The wires were weighed after pyr-olysis, and the weight loss calculated. Compounds were

identi®ed based on their mass spectra, GC retentiontimes, comparisons with library mass spectra, and withpublished mass spectra (Danielson and Rogers, 1978;

van de Meent et al., 1980a,b; van der Kaaden et al.,1983; Saiz-Jimenez and de Leeuw, 1984; Tsuge andMatsubara, 1985; Ralph and Hat®eld, 1991).

3. Results and discussion

3.1. Bulk geochemical features of the untreatedsediments

TOC contents, C/N atomic ratios and d13Corg valuesof the three sediment samples are reported in Table 1.TOC content varies between 1.34 and 1.61%. Pre-

vious studies on marine surface sediments revealed a

wide range of OM contents. Most surface sedimentsfrom deltaic or estuarine systems exhibit TOC valuesaround 0.1±1% in temperate, subtropical and equatorial

climates (e.g. van Vleet et al., 1984; Kennicutt II et al.,1987; Bigot et al., 1989; Bouloubassi and Saliot, 1993),although higher values (1±5% range) were recorded for

the Rhone delta (Bouloubassi et al., 1997), the Ebrodelta (Grimalt and AlbaigeÂ s, 1990), the MackenzieRiver delta (Yunker et al., 1993) and the Lena River

delta (Zegouagh et al., 1996). Intermediate TOC con-tents are, therefore, observed in the present study.The stable carbon isotopic composition (d13Corg) of

OM produced by photosynthetic organisms principally

re¯ects the dynamics of carbon assimilation and theisotopic composition of the inorganic carbon source(e.g. Hayes, 1993). The dierence in d13Corg between

A. Garcette-Lepecq et al. / Organic Geochemistry 31 (2000) 1663±1683 1665

OM produced by land plants and marine algae has suc-cessfully been used to trace the sources and distribution

of OM in coastal ocean sediments (e.g. Prahl et al.,1994). Average values of ÿ24 to ÿ23% have beenreported for Black Sea suspended particles dominatedby marine phytoplankton (Calvert and Fontugne, 1987;

Fry et al., 1991; Arthur et al., 1994), whereas the ter-restrial end-member has a d13Corg value of ÿ27 %, onaverage (Arthur and Dean, 1998). In contrast to marine

algae, freshwater algae have d13Corg values typicallyindistinguishable from those of OM from vascular landplants, i.e. between ÿ30 and ÿ25% (e.g. Benson et al.,1991; Tenzer et al., 1997). The stable carbon isotopicratios of bulk OM in the three studied sediments, fromÿ26.3 to ÿ25.9%, point to a strong contribution fromland plants and/or freshwater algae. Indeed, surfaceparticles collected near the rivermouth during the sameperiod exhibited d13Corg values identical to those foundin the present study (Saliot et al., in press).

Organic carbon to nitrogen atomic ratios (C/N) alsohave been widely used to distinguish between algal andland-plant sources of sedimentary OM (Premuzic et al.,

1982; Ishiwatari and Uzaki, 1987; Meyers, 1994; Prahlet al., 1994; Silliman et al., 1996). Algae typically haveatomic C/N ratios between 4 and 10, whereas vascular

land plants have C/N ratios greater than 20 (Meyers,1997). This dierence arises from the protein richness ofalgal OM and is largely preserved in sedimentary OM(Jasper and Gagosian, 1990; Meyers, 1994). Riverine

and estuarine sediments typically contain a large con-tribution of OM derived from land plants and thusshow elevated atomic C/N ratios, in the 13±35 range

(Kennicutt II et al., 1987). Interestingly, our samplesshow relatively low C/N ratios with values in the 9±10range, that characterise OM of mainly algal origin.

Diagenetic modi®cations cannot explain these lowvalues since early diagenesis rather results in an increaseof C/N ratios due to the preferential loss of labile N-

bearing components. Lowering of C/N ratios also hasbeen observed in soils (e.g. Sollins et al., 1984) and inocean sediments (e.g. MuÈ ller, 1977), where it re¯ects theabsorption of ammonia derived from OM decomposition,

accompanied by remineralisation and release of carbondioxide (Meyers, 1997; Bouloubassi et al., 1998). Inaddition, the C/N ratios of OM in ®ne-sized sediments

generally are lower than those of coarse sediments (Keilet al., 1994; Prahl et al., 1994). Fine sediment fractionscontain a smaller proportion of intact land-plant debristhan clay minerals. The latter have both large surface

areas and negative surface charges and therefore e-ciently adsorb ammonia. However, these artifactuallydepressed C/N ratios (

the resistant carbon retained after this treatmentaccounts for ca. 50 wt.% of the total initial carbon forthe three samples (Table 2). The slightly more pro-nounced decrease observed for the surface sample S0 is

in agreement with the presence of intact (or slightlymodi®ed) biomacromolecules such as proteins andpolysaccharides, which are acid labile. Indeed, the latter

compounds are known to be partly retained in surfacesamples and to be rapidly degraded with depth in theupper layers of sediments (Hedges and Oades, 1997).

The organic sulphur to carbon atomic ratio is rela-tively low (Sorg/C � 0.02), indicating a low contributionof organic sulphur compounds (OSC) in the three KLs.

The hydrogen to carbon atomic ratio (H/C � 1.04)indicates a substantial contribution of aromatic and/orunsaturated structures. Elemental analysis also revealedrelatively high levels of nitrogen (ca. 2.5%) and iron (ca.

3%) in the three KLs. In a H/C ratio vs. N/C ratio dia-gram, the KL representative point falls close to OM insoils and terrestrial humic acids, thus pointing to a

mainly continental origin.X-ray analyses revealed that the residual inorganic

fraction chie¯y corresponds to ¯uorides, neo-formed

during the HF/HCl treatment and not eliminated after-wards, which account for the substantial ash content ofthe KLs (ca. 30%).

3.3. KL morphological features

Observations of the three KLs by SEM (Fig. 2)

showed that they are mainly composed of shapelessaggregates of various sizes ranging from ca. 8�10 up toca. 30�40 mm (Fig. 2a and b). Most of these aggregateshave a granular appearance (Fig. 2b and d), while a fewof them exhibit a ``gel-like'' appearance (Fig. 2c). Anumber of pollen grains and/or land-plant spores (Fig.

2e and f), and of ligneous debris (Fig. 2g) were recog-nised, pointing to some contribution of land plants. Afew framboids of pyrite, a mineral known to resist toHF/HCl attack, were also observed (Fig. 2h).

TEM observations (Fig. 3) revealed that the threeKLs are largely composed of various types of lamellarstructures with dierent thicknesses. The thinner ones

(ca. 20±40 nm thick; Fig. 3a) represent a relativelyminor fraction and are tightly associated into bundles.Similar lamellar structures, termed ultralaminae, have

been recently observed by TEM in a number of kero-gens from source rocks and oil shales (Largeau et al.,1990), so far considered as amorphous based on light

microscopy observations. Ultralaminae have beenshown to derive from the selective preservation of thethin outer walls of some microalgae (Largeau et al.,1990; Derenne et al., 1991, 1992a,b; Gillaizeau et al.,

1996), which are composed of highly resistant bioma-cromolecules termed algaenans (Tegelaar et al., 1989).Thicker structures (ca. 50±500 nm thick) resembling

preserved cell walls of dierent shapes and thicknessespredominate in the three KLs (Fig. 3b±f). Based on theirabundance these various lamellar structures likely cor-respond to the shapeless aggregates observed by SEM.

Ligneous debris (Fig. 3g), pollen grains and/or land-plant spores (Fig. 3h), as well as a nanoscopicallyamorphous fraction (Fig. 3e) were also observed by

TEM.The combination of SEM and TEM observations

thus allowed to conclude that the selective preservation

of resistant cell walls (probably of microalgal origin)and of terrestrial material played an important role inthe formation of the kerogen-like material of the three

samples.

3.4. Spectroscopic studies

Because the three samples exhibit similar spectro-scopic features, only the spectra of S20 are discussedhere in detail.

The FTIR spectrum of S20 (Fig. 4) is characterisedby an intense absorption centred at 3400 cmÿ1 (wideband in the 3600±3000 cmÿ1 range) corresponding toOH and/or NH groups. Carbonyl and/or carboxylfunctions are responsible for the relatively weakerabsorption at 1710 cmÿ1. An intense band in the 1685±1585 cmÿ1 range comprises two contributions: thestronger one, centred at 1655 cmÿ1, corresponds to car-bonyl groups in primary amide functions, while ole®nicand aromatic unsaturations lead to a wide absorption

occurring as a shoulder of the former band. Theoccurrence of aromatic groups is also suggested byabsorptions at 790 and 750 cmÿ1, although the bandsin the aromatic C±H out-of-plane deformation region(900-700 cmÿ1) are overlapped by relatively strongabsorptions caused by residual minerals at wavelengths

below 800 cmÿ1. A weak absorption centred at 1540cmÿ1 is assigned to amide N±H bonds. Several absorp-tions are observed between 1000 and 1200 cmÿ1 whichcan be due to C±O bonds (ethers and alcohols) and/or

minerals. A wide band in the 1300±1200 cmÿ1 range(maximum at 1230 cmÿ1) is also noted. The latter maycorrespond to C±N bonds in aromatic amine functions.

Relatively weak absorptions in the 3000±2800 cmÿ1

range re¯ect the contribution of alkyl groups. Theintensity ratio of the 1450 cmÿ1 (CH3 and CH2 asym-metric bending) and 1380 cmÿ1 (CH3 symmetric bend-ing) absorptions shows that CH3 groups are relativelyabundant, pointing to a high degree of branching. A

very weak absorption at 420 cmÿ1 re¯ects the presenceof a low amount of pyrite, in agreement with SEMobservations.The solid-state 13C NMR spectrum of S20 KL (15

kHz; Fig. 5) is dominated by a peak centred at 30 ppmcorresponding to aliphatic carbons without hetero-substituents (10±40 ppm range). This peak is relatively


wide indicating a substantial level of branching. The

latter feature, which is in agreement with FTIR data, isalso indicated by the occurrence of a shoulder at 15 ppmdue to CH3 groups. Several peaks re¯ect the occurrence

of heteroelements in the KLs. The peak at 74 ppm cor-responds to carbons bearing O or N atoms, includingcarbons from carbohydrates. The presence of the latter

compounds is con®rmed by a weak signal at 105 ppmdue to anomeric carbons (acetal groups). Carbons a tothose occurring at 74 ppm are responsible for the peakat 55 ppm. However, the latter peak can also include

carbons in methoxyl groups and/or carbons bearing theamino group in amino acids. The broad resonancecentred at 130 ppm with a high relative intensity corre-

sponds to aromatic and ole®nic carbons. A weak signal

at 150 ppm is assigned to phenols. A narrow peakcentred at 175 ppm corresponds to ester and/or amidefunctions.

The solid-state 15N NMR spectrum of S20 KL (Fig. 6)is dominated by a peak at ÿ258 ppm assigned to amides.The shoulder between ÿ200 and ÿ230 ppm indicates theoccurrence of both N-alkyl-substituted pyrroles: indolesor carbazoles (ÿ200 ppm), and unsubstituted pyrroles(ÿ230 ppm) (Derenne et al., 1993). A weak signal atÿ344 ppm may be due to amine functions.The combination of FTIR and solid-state 13C and

15N NMR data showed that oxygenated and nitrogen-ous functions, together with aromatic and ole®nic

Fig. 2. Scanning electron microscopy of the three KLs.


unsaturations, are predominant features for the threeKLs. These bulk chemical observations were completedby pyrolytic studies to derive further information aboutthe molecular structure and origin(s) of KLs.

3.5. Pyrolytic studies

CuPy-GC/MS of the KLs isolated from samples S0,S10, and S20 resulted in a substantial weight loss (ca. 20

Fig. 3. Transmission electron microscopy of the three KLs. (a) Thin lamellar structures similar to ultralaminae, (b) preserved cell

walls packed together into bundles, (c) ``organised'' multicellular structures, (d) tangles of cell walls, isolated structures forming a

closed (e) or an open (f) line, (g) ligneous debris, (h) pollen grain or land-plant spore. a, Amorphous.


wt.%). The ¯ash pyrolysates generated from the three

KLs are highly complex and show similar compositions.Their total ion current (TIC) traces (Fig. 7) are char-acterised by (i) an abundant presence of phenols andmethoxyphenols, (ii) the presence of a complex mixture

of alkylbenzenes, naphthalenes, indenes, indoles, pyr-roles, pyridines and aromatic nitriles, with a low degreeof alkylation (44 carbons) for the alkylated homo-logues of each family, (iii) a substantial contribution ofnormal alkanes and alk-1-enes, (iv) a virtual lack ofsulphur compounds, (v) a low relative abundance of C6±

C18 saturated fatty acids. The distributions of these dif-ferent types of pyrolysis products are described in thefollowing paragraphs and their origin and signi®cance

are discussed.

3.5.1. n-Alkanes/n-alk-1-enesThe mass chromatogram of m/z 57 (Fig. 8) shows an

homologous series of n-alkanes ranging from C9 to C33(maximum C13), with no signi®cant odd- or even-car-bon-number dominance. A series of n-alk-1-enes with

similar features was also observed. n-Alk-1-ene/n-alkanedoublets are abundant in the pyrolysates of the highly

aliphatic, resistant, biopolymers present in some higherplant tissues such as cutans (in cuticles), or suberans (inperiderm tissues), as well as in tegmens (inner seed

coats) (Nip et al., 1986a, b; van Bergen et al., 1993,1994). Owing to the presence of such highly resistantcomponents, these tissues and inner seed coats canretain their morphological features upon fossilisation.

TEM observations on the three KLs revealed the abun-dant presence of relatively thick ``lamellar'' structureswith dierent types of organisation (Fig. 3). A part of

these structures might be related to some of the aboveresistant materials and correspond to selectively pre-served structures derived from higher plants. In addi-

tion, SEM observations showed the presence of someplant spores and/or pollen grains (Fig. 2e and f). It iswell documented that pollen and spore walls are com-posed of highly resistant materials, the so-called spor-

opollenins, which comprise aliphatic moieties and arealso selectively preserved in the fossil record. Accord-ingly, a signi®cant part of the n-alkanes and n-alk-1-enes

observed in KL pyrolysates can be derived from land-plant materials.However, straight-chain hydrocarbons have also been

described in the pyrolysates of lacustrine and marinematerials (Fukushima and Ishiwatari, 1988; Fukushimaet al., 1989). These hydrocarbons are likely derived from

the algaenans which occur in the outer walls of numer-ous freshwater and marine microalgae (Largeau et al.,1984, 1986; Goth et al., 1988; Derenne et al., 1992b,1995; de Leeuw et al., 1995; Gelin et al., 1999). The

chemical structure of a number of algaenans was shownto be based on a network of long polymethylenic chains(Largeau et al., 1986; Derenne et al., 1991, 1992 a,b;

Fig. 5. Solid-state 13C NMR spectrum of S20 KL (at 15 kHz).

Fig. 4. Fourier transform infrared spectrum of S20 KL (similar

spectroscopic features were observed for the two other KLs).

Fig. 6. Solid-state 15N NMR spectrum of S20 KL (similar

spectroscopic features were observed for the two other KLs).


Gelin et al., 1996, 1997), which generate large amountsof n-alkanes and n-alk-1-enes (up to C35) upon pyr-olysis. Moreover, owing to their extremely high resis-

tance to chemical and microbial degradation (Tegelaaret al., 1989) such macromolecular constituents can sur-vive almost unaected during diagenesis. However,

depending upon the thickness of the wall the initialmorphology of the cell may be retained upon fossilisa-tion, which leads to microfossils as in Torbanite (Lar-geau et al., 1984, 1986) or in Messel Oil Shale (Goth et

al., 1988), or may be partly altered as in ultralaminae(Derenne et al., 1991, 1992a). Such lamellar structureswere observed by TEM in the KLs (Fig. 3) and they are

probably derived from algaenan-composed thin outerwalls of microalgae. However, the lack of predominanceof the n-alk-1-ene/n-alkane doublets in the KL pyr-

olysates points to a moderate contribution of selectivelypreserved, resistant, aliphatic macromolecules. The lat-ter are probably derived both from terrestrial plants and

from freshwater microalgae (in agreement with thed13Corg and C/N values).

3.5.2. Isoprenoid hydrocarbons

Prist-1-ene and trace amounts of prist-2-ene wereidenti®ed in KL pyrolysates (Fig. 7 and 8). Several ori-gins were previously put forward for these ubiquitous

pyrolysis products. The phytyl side chain of chlorophylla was proposed as a precursor of pristene upon pyr-olysis (Ishiwatari et al., 1991, 1993). However, Goossens

et al. (1984) demonstrated that the pyrolysis of toco-pheryl moieties produces pristene through an intramo-lecular rearrangement, in much higher yields than

chlorophyll a. Tocopherols are known to be relativelyabundant in the chloroplasts of most photosyntheticmicroorganisms. Prist-1-ene and prist-2-ene in pyr-olysates of fossil leaf material (Nip et al., 1986a; Logan

et al., 1993) and tegmens of fossil water plants (vanBergen et al., 1994) were considered as being derivedfrom tocopherol. The absence of phytadienes, which

have been shown also to originate from chlorophyll-bound phytol (van de Meent et al., 1980a,b; Ishiwatari etal., 1991), suggests that chlorophyll awas probably not the

precursor of pristenes inKL pyrolysates. Indeed the phytylchain is esteri®ed to the cyclic tetrapyrrole moiety ofchlorophyll a, which renders it less stable than tocopheryl

units upon hydrolysis during HF/HCl treatment.

3.5.3. Organic sulphur compounds (OSC)The three KLs are characterised by a very low Sorg/C

atomic ratio and a virtual lack of organic sulphur com-pounds (OSC) in the three pyrolysates. Accordingly, theso-called ``natural sulphurisation'' pathway (Sinninghe

Fig. 7. Total Ion Current (TIC) trace of the 650�C Curie-point pyrolysate of S20 KL showing the main series. *, n-Alkane/n-alk-1-ene doublets.


DamsteÂ et al., 1986, 1989; Schmid et al., 1987; Adam et

al., 1993) only played a negligible role in their forma-tion. The presence of pyrite (Fig. 2h) shows that somebacterial sulphate-reducing activity took place in the

sediment. Nevertheless, this activity was probably low,and there was enough active iron to scavenge thereduced sulphur species.

3.5.4. PhenolsThe KL pyrolysates are characterised by large

amounts of phenolic compounds, including a large

number of methoxyphenols (Fig. 7 and Table 4). Phenoland its alkylated homologues were identi®ed by selectiveion detection (m/z=94+107+108+121+122; Fig. 9).

This series is dominated by phenol, 3- and 4-methyl-phenols, and 4-ethylphenol, which also correspond torelatively important peaks in the TIC traces (Fig. 7).

The abundance of the alkylated isomers decreases withthe number of alkyl carbons, the amount of C3+homologues being quasi negligible.

Alkylphenols, associated to short-chain (C1±C3)methoxy-substituted alkylphenols are typical pyrolysisproducts of unaltered or weakly altered lignin (e.g.Martin et al., 1979; Obst, 1983; Saiz-Jimenez and de

Leeuw, 1986a; Hatcher and Spiker, 1988), thus re¯ect-ing a terrigenous input. Catechols (1,2-benzenediols) areoften assigned to demethylated units formed from

microbial degradation of lignin (Hatcher et al., 1988;

Stout et al., 1988). Therefore, the recognition of a widerange of lignin monomer units (Table 4) with intactmethoxyl groups and the minor amount of catechols

indicate that weakly altered lignins were signi®cant pre-cursors of the phenols in KL pyrolysates. This terrige-nous contribution as evidenced by the pyrolytic study is

consistent with SEM observations showing selectivelypreserved ligneous debris (Fig. 2g), as well as with therelatively 13Corg-depleted isotopic composition (ca.ÿ26%0).

Lignin pyrolysis products are characterised by theoccurrence (or predominance) of monomer units withvery speci®c substitution patterns (e.g. Martin et al.,

1979; Obst, 1983). Gymnosperms (non-¯owering plants)and angiosperms (¯owering plants) synthesise dierenttypes of lignin, which allows for the identi®cation of the

general vascular plant sources of sedimentary lignin-derived material (Hedges and Mann, 1979). The pre-dominance of guaiacyl (vanillyl) over syringyl phenols

(G vs. S) among KL pyrolysis products is consistentwith the predominance of gymnosperms in the water-shed vegetation of the Danube delta.Alkylphenols with a similar distribution as in the

present study have already been observed in pyrolysatesof soil OM, marine sediments, and suspended and dis-solved marine OM (Saiz-Jimenez and de Leeuw, 1986b;

Fig. 8. Mass chromatogram (m/z 57) of the 650�C Curie-point pyrolysate of S20 KL showing alkane distribution. , n-Alkanes; *,branched alkanes; �, C16 fatty acid; 28, methoxyphenol.


Sicre et al., 1994; PeulveÂ et al., 1996a,b; van Heemst et

al., 1993, 1997; Zegouagh et al., 1999). In some of theseprevious studies (Sicre et al., 1994; PeulveÂ et al.,1996a,b) the distribution is relatively invariant with

depth, thus indicating that this series of alkylphenols is

probably derived from complex resistant macro-molecules, corresponding to melanoidin-type compo-nents. Melanoidins originate from random

Table 4

Phenolic compounds identi®ed in KL pyrolysates

Peak numbera Main mass fragments Lignin parameter

1 94,66 Phenol ±

2 122, 121, 77 (4-Methoxytoluene) ±

3 108, 107, 79 2-Methylphenol (o-cresol) ±

4 107, 108, 77 4-Methylphenol (p-cresol) ±

5 107, 108, 79 3-Methylphenol (m-cresol) ±

6 109, 124, 81 Guaiacol G

7 107, 122, 77 2,6-Dimethylphenol ±

8 2-Ethylphenol ±

9 107, 122, 121 2,4-Dimethylphenol+2,5-dimethylphenol ±

10 107, 122 4-Ethylphenol ±

11 110, 64 1,2-Benzenediol (catechol) ? ±

12 123, 138, 95 2-Methoxy-4-methylphenol (4-methylguaiacol) G

13 3-Ethylphenol+3,5-dimethylphenol ±

14 2,3-Dimethylphenol ±

15 3,4-Dimethylphenol ±

16 Trimethylphenol ? ±

17 120, 91 4-Vinylphenol ±

18 121, 136 4-Ethyl-2-methylphenol ? ±

19 121, 136 Ethyl-methylphenol ? ±

20 Propylphenol ? ±

21 137, 152 2-Methoxy-4-ethylphenol (4-ethylguaiacol) ±

22 150, 135, 77, 107 2-Methoxy-4-vinylphenol (4-vinylguaiacol) G

23 154, 139 2,6-Dimethoxyphenol (syringol) S

24 164, 77, 103, 149 2-Methoxy-4-(2-propenyl)-phenol (eugenol) G

25 137, 166 2-Methoxy-4-propylphenol (4-propylguaiacol) G

26 164, 77, 103, 149 2-Methoxy-4-(1-propenyl)-phenol

(cis soeugenol)

G

27 168, 153, 125 2,6-Dimethoxy-4-methylphenol

(4-methylsyringol)

S

28 164, 77, 103, 149 2-Methoxy-4-(1-propenyl)-phenol

(trans isoeugenol)

G

29 147, 162, 91 1-(4-Hydroxy-3-methoxyphenyl)-propyne ? G

30 151, 166 1-(4-Hydroxy-3-methoxyphenyl)-ethanone

(acetovanillone)

G

31 167, 182 4-Ethyl-2,6-dimethoxyphenol (4-ethylsyringol) S

32 137, 180 1-(4-Hydroxy-3-methoxyphenyl)-2-propanone

(guaiacylacetone)

G

33 180, 165, 137 2,6-Dimethoxy-4-vinylphenol (4-vinylsyringol) S

34 151, 180 1-(4-Hydroxy-3-methoxyphenyl)-1-propanone

(propiovanillone)

G

35 194, 119 4-Allyl-2,6-dimethoxyphenol (4-allyl-syringol) S

36 194, 119 cis 2,6-Dimethoxy-4-propenylphenol S

37 167, 210 trans 2,6-Dimethoxy-4-propenylphenol S

38 181, 196 1-(4-Hydroxy-3,5-dimethoxyphenyl)-ethanone

(acetosyringone)

S

39 167, 210 1-(4-Hydroxy-3,5-dimethoxyphenyl)-2-propanone

(syringylacetone)

S

40 181, 210 1-(4-Hydroxy-3,5-dimethoxyphenyl)-1-propanone

(propiosyringone)

S

a See Figures.


condensations of altered moieties derived from proteinsand polysaccharides, and provide OM preservation viathe so-called degradation-recondensation pathway.Melanoidins are characterised by relatively high oxygen

contents, and their infra red spectra exhibit intenseabsorptions due to OH and CO groups (Rubinsztainet al., 1984, 1986a,b). Their general IR features, as well

as the solid-state 13C and 15N NMR spectra obtainedfor some synthetic, amino acid-rich, melanoidins (Ikanet al., 1986; Benzing-Purdie et al., 1983), show impor-

tant similarities with those of the KLs. Van Heemst etal. (1997) also prepared synthetic melanoidins frommixtures of a tyrosine-containing protein and carbohy-drate components, leading to similar distributions of

alkylphenols upon pyrolysis as observed in the presentstudy. These observations therefore point to melanoidincontribution to the three KLs. In addition, some con-

tribution of proteinaceous material which would havesurvived diagenesis and HF/HCl hydrolysis cannot beexcluded. Indeed, alkylphenols are also generated upon

pyrolysis of tyrosine-containing proteins or derivedmaterials (Tsuge and Matsubara, 1985), and as recentlyshown (Knicker and Hatcher, 1997; Zang et al., in

press), proteins can be protected from diagenetic andchemical alterations via encapsulation in macro-molecular organic substances. The presence of melanoi-dins and/or of such protected proteins is supported by

the predominance of amides in the solid-state 15N NMRspectra of the three KLs (Fig. 6), as well as by theirrelatively low C/N atomic ratios.

The above observations suggest the signi®cant occur-rence of another type of precursor, in addition to ligninsand lignin-derived components, for the alkylphenols ofKL pyrolysates: tyrosine-derived moieties preserved

from degradation by incorporation in melanoidin-typemacromolecules and/or by encapsulation within themacromolecular network of the KLs.

3.5.5. AlkylbenzenesToluene, an ubiquitous pyrolysis product of sedi-

mentary OM, represents a relatively important con-tribution to the TIC traces of the three KL pyrolysates(Fig. 7). Several isomers of alkylbenzenes with C2- toC16-chains were identi®ed in mass chromatogram

m/z=91+92+105+106+119+120 (Fig. 10), but therelative abundance of the latter series is low andstrongly decreases with the number of alkyl carbons.

Some of these compounds (e.g. n-alkylbenzenes witha C5- to C16-chain, or n-alkyl-methylbenzenes with a C6-to C11-chain) are probably formed during pyrolysis by

secondary reaction via cyclisation/aromatisation pro-cesses (Hartgers et al., 1994a). This is con®rmed by theortho-position observed for the latter series. Never-

theless, some isomers with a low alkyl carbon number(i.e. in the C1±C4 range) might be primary pyrolysisproducts formed by �-cleavage of substituted aromaticrings linked to the macromolecular structure via an

alkyl chain (Hartgers et al., 1994b).Toluene and C2-alkylbenzenes can be produced upon

pyrolysis of phenylalanine-containing proteins or

Fig. 9. Mass chromatogram (m/z 94+107+108+121+122) of the 650�C Curie-point pyrolysate of S20 KL showing alkylphenoldistribution. * ethyl-methylphenols, & trimethylphenol, and 12, 21, 22 methoxyphenols, 17 vinylphenol.


protein derivatives (Tsuge and Matsubara, 1985; Stan-kiewicz et al., 1998). The former source cannot be ruledout for the three KLs since, as already stressed, some

proteins may have survived diagenesis and HF/HClhydrolysis owing to encapsulation in the macro-molecular structure of the KLs. Similar distributions of

alkylbenzenes with C1- to C3-chains were recentlyobserved in pyrolysates of particulate OM from thenorthwestern Mediterranean Sea (PeulveÂ et al., 1996a)

and of refractory OM isolated from sediments of thenorthwestern African upwelling system (Zegouagh etal., 1999). In the above studies, these alkylbenzenes havebeen considered as originating from phenylalanine-con-

taining moieties derived from altered proteins, andincorporated within melanoidin-type structures. Thissuggests, as in the case of alkylphenols, that the degra-

dation-recondensation preservation mechanism also hasplayed a role in the formation of the three KLs.Alkylbenzenes, sometimes including long chain com-

pounds, and probably derived from algaenans, weredetected as major pyrolysis products in a few previousstudies on kerogens (e.g. Hors®eld et al., 1992; Sin-

ninghe DamsteÂ et al., 1993). Such compounds were alsoobserved in the pyrolysis products of resistant bioma-cromolecular material isolated from marine microalgae(Derenne et al., 1996; Kokinos et al., 1998). TEM

observations of the three KLs showed the presence ofnumerous cell walls, probably containing resistantalgaenans or algaenan-like, biopolymers, based on their

excellent morphological preservation. Consequently,resistant macromolecular material from microalgal cellwalls might be an additional source of some of the

alkylbenzenes detected in the KL pyrolysates.The presence of 1,2,3,4-tetra-methylbenzene (TMB)

in the pyrolysates of various kerogens was ascribed to

speci®c aromatic carotenoids of Chlorobiaceae (Hart-gers et al., 1994b,c; Guthrie, 1996; Clegg et al., 1997).Chlorobiaceae are anaerobic, photosynthetic, green sul-

phur bacteria, and their presence is an indicator ofanoxia in the photic zone. Since the water column isoxic in the zone where the studied sediment core wascollected, such a bacterial origin can be ruled out. The

occurrence of 1,2,3,4-TMB in pyrolysates of the resis-tant material isolated from several marine algae (Hoefset al., 1995) also suggests an algal origin, which would

be consistent with TEM observations in the presentcase.

3.5.6. Other aromatic hydrocarbonsNaphthalenes and indenes were identi®ed by selective

ion detection at m/z=128+142+156+170 (Fig. 11)

and m/z=115+116+129+130 (Fig. 12), respectively.Naphthalene and indene are the dominant compoundsof both series, but their contribution to the KL pyr-olysates is minor. The distribution pattern of C1- to C3-

alkylated isomers shows decreasing relative abundancewith the number of alkyl carbons. Such distributions ofnaphthalenes and indenes are comparable to those

Fig. 10. Mass chromatogram (m/z 91+92+105+106+119+120) of the 650�C Curie-point pyrolysate of S20 KL showing alkylben-zene distribution. *, n-Alkylbenzenes.


observed in previous studies of marine, particulate and

sedimentary, OM (PeulveÂ et al., 1996a; Zegouagh et al.,1999). However, the sources and precursors of bothfamilies are unknown at present.

3.5.7. Nitrogen-containing compounds

Pyrroles, indoles, pyridines, and aromatic nitrileswere identi®ed in KL pyrolysates. These series wereidenti®ed in mass chromatograms m/z=103+117+

Fig. 11. Mass chromatogram (m/z 128+142+156+170) of the 650�C Curie-point pyrolysate of S20 KL showing alkylnaphthalenedistribution.

Fig. 12. Mass chromatogram (m/z 115+116+129+130) of the 650�C Curie-point pyrolysate of S20 KL showing alkylindene dis-tribution.


131+144 (alkylindoles and aromatic nitriles; Fig. 13),m/z=67+81+95 (pyrroles), and m/z=79+93+107(pyridines). In each family the unsubstituted compoundpredominates, whereas C1 alkylated isomers are minor

compounds and C2 homologues occur in negligibleamounts.Alkylated pyrroles can be derived from chlorophyll

pigments. But, in the present case, such an origin cannotbe considered since their distribution pattern diersfrom that reported for alkylpyrroles present in pyr-

olysates of porphyrin derivatives (Sinninghe DamsteÂ etal., 1992).All the above mentioned classes of N-containing

compounds can be ascribed to pyrolysis products ofproteins or protein-derived units. For instance, indolesand pyrroles are often associated with tryptophan andproline moieties, respectively (Danielson and Rogers,

1978; Tsuge and Matsubara, 1985). As already dis-cussed, some proteins may have been protected viaencapsulation in the three KLs. Similar distributions of

alkylindoles and alkylpyrroles were also observed inprevious studies of dissolved OM from Paci®c Ocean,suspended OM from the Rhone delta, sediment trap

material from the northwestern Mediterranean Sea, andmarine sediments from the northwestern Africanupwelling system (van Heemst et al., 1993; Sicre et al.,

1994; PeulveÂ et al., 1996a; Zegouagh et al., 1999) andconsidered to be related to melanoidin-type structures.Accordingly, as already concluded for alkylphenols and

alkylbenzenes, these N-containing pyrolysis productsmay be related to melanoidins comprising degradedproteinaceous material, and/or to proteins encapsulatedin the most refractory part of the KL structure.

3.5.8. Polysaccharide pyrolysis productsPolysaccharides are ubiquitous compounds in the

marine environment, occurring as constituents of cellwalls, storage material, constituents of marine snow,and mucus (de Leeuw and Largeau, 1993).

Pyrolysis of polysaccharides and polysaccharide-derived components produces furans and furan-deriva-tives (Helleur et al., 1985a,b). Dimethylfuran, 2-fur-

aldehyde, 5-methyl-2-furaldehyde and 2-furanmethanolwere identi®ed in the KL pyrolysates in very lowamounts, 2-furaldehyde being the major furanic frag-ment. The presence of polysaccharidic moieties in the

three KLs is con®rmed by the signals at 74 and 105 ppmin their solid-state 13C NMR spectra. Furfurals andmethylfurfurals have also been observed by Boon et al.

(1986) in the pyrolysates of sedimentary OM from theestuary of the Rhine river, and cellulose was consideredas a potential source for these pyrolysis products. Ther-

mal degradation of cellulose results in a wide variety ofproducts including furans and pyrans (van der Kaadenet al., 1983; Pouwels et al., 1989). These compounds are

generally detected in minor amounts, whereas highyields of levoglucosan were obtained from dierent cel-luloses (20±60 wt.%) (Sha®zadeh et al., 1979). Since (i)

Fig. 13. Mass chromatogram (m/z 103+117+131+144) of the 650�C Curie-point pyrolysate of S20 KL showing alkylindole andaromatic nitrile distributions. 28, methoxyphenol.


most of these compounds, and in particular anhy-drosugars like levoglucosan, were not detected in KLpyrolysates, and (ii) the bulk of the polysaccharides that

occurred in the sediments was most likely eliminated bythe HF/HCl attack applied for eliminating the mineralmatrix, the presence of furanic components in KL pyr-

olysates can be attributed to degraded ligno-cellulosiccomplexes of higher plants and polysaccharide-derivedmoieties in melanoidin-type components.

3.5.9. Fatty acidsA minor series of straight-chain saturated fatty acids

(C6-C18) was detected in KL pyrolysates (chromatogram

of m/z=60+73; Fig. 14). The C16 homologue is pre-dominant and is observed in a signi®cant relative abun-dance in the TIC traces of KL pyrolysates. This series is

dominated by even-carbon-numbered homologues.Considering the moderate level of alteration of the acidmoieties (indicated by this even predominance), it is

likely that weakly altered saturated fatty acids wereincorporated early within melanoidin-type structures, aspreviously observed for sediments from the north-

western African upwelling system (Zegouagh et al.,1999). Indeed, Larter and Douglas (1980) reported onthe participation of some lipids in the condensationreactions occurring during melanoidin formation. The

absence of branched acids among KL pyrolysis pro-ducts points to a negligible contribution from bacteriallipids in the condensation steps.

4. Conclusions

The main conclusions of this study, carried out via a

combination of microscopic, spectroscopic, isotopic,and pyrolytic methods, on the KL isolated from threesections of a recent sediment core o the Danube delta,

are summarised below:

(i) The kerogen-like material accounts for a sub-

stantial fraction of total OM in the three samples(ca. 50%).

(ii) Similar morphological features and chemicalcomposition are observed for the KL isolated

from these sediments.(iii) The main source of the KLs is an admixture of

land-plant and freshwater microalgal material,

whereas the contribution of bacterial materialwas low.

(iv) The selective preservation of terrigenous material

(including ligneous debris and pollen and plantspores) as well as of resistant aliphatic bioma-cromolecules (probably derived from cell walls

of microalgae) was the main process for KL for-mation.

(v) The KLs comprise substantial amounts of protei-naceous moieties, for which two preservation

pathways can be considered: incorporation inmelanoidin-type structures formed, via the degra-dation-recondensation pathway, from degraded

Fig. 14. Mass chromatogram (m/z 60+73) of the 650�C Curie-point pyrolysate of S20 KL showing fatty acid distribution.


proteinaceous and polysaccharidic materials,and/or encapsulation of proteins within themacromolecular network of the three KLs.

(vi) The role of natural sulphurisation was negligible

in KL formation.

Acknowledgements

The authors wish to thank Dr. H. Knicker (T.U.

MuÈ nchen) for the solid-state 15N NMR spectrum, andP. Florian (CRPHT, CNRS, OrleÂ ans) for the solid-state13C NMR spectrum. SEM observations were performed

in the CIME (UniversiteÂ Pierre et Marie Curie). Dr. J.M. Martin, Co-ordinator of the EROS 21 Project, C.Lancelot, Chief Scientist during the EROS 21 cruises,and the crew of the Research Vessel ``Professor Vodya-

nitsky'' are gratefully acknowledged. This work wascarried out in the framework of the EROS 21 (EuropeanRiver-Ocean System) project. We kindly acknowledge

the support of the European Commission under the 4thEnvironment Programme (ENV-4CT 96-0286).Anonymous referees are sincerely thanked for their

constructive reviews of the manuscript.

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