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International Journal of Coal Geology 111 (2013) 5–22

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International Journal of Coal Geology

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Petrological and geochemical composition of lignite from the D field,Kolubara basin (Serbia)

Dragana Životić a,⁎, Ksenija Stojanović b, Ivan Gržetić b, Branimir Jovančićević b, Olga Cvetković c,Aleksandra Šajnović c, Vladimir Simić a, Rajko Stojaković d, Georg Scheeder e

a University of Belgrade, Faculty of Mining and Geology, Djusina 7, 11000 Belgrade, Serbiab University of Belgrade, Faculty of Chemistry, Studentski trg 12-16, 11000 Belgrade, Serbiac Center of Chemistry, IChTM, Studentski trg 12-16, 11000 Belgrade, Serbiad Electric Power Industry of Serbia, EACP MB Kolubara plc, “OPM Baroševac”, 11565 Baroševac, Serbiae Federal Institute for Geosciences and Natural Resources, Steveledge 2, 30655 Hannover, Germany

⁎ Corresponding author. Tel.: +381 11 3219 251; faxE-mail address: [email protected] (D. Životić).

0166-5162/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.coal.2012.10.014

a b s t r a c t
a r t i c l e i n f o
Article history:Received 3 February 2012Received in revised form 10 October 2012Accepted 24 October 2012Available online 14 November 2012

Keywords:Kolubara basinLignitesMaceralsBiomarkersPaleoenvironment

The Upper Miocene lignite from the Main coal seam in the D field, Kolubara basin, is a typical humic coal withhuminite, liptinite and inertinite concentrations of up to 83.7 vol.%, 17.2 vol.% and 15.5 vol.%, respectively. Inthe huminite group, textinite and ulminite are the most abundant macerals with variable amounts ofdensinite and attrinite. Liptodetrinite and sporinite are the most common macerals of the liptinite group,while inertodetrinite is the most abundant maceral of the inertinite group. The mineral matter consists most-ly of clay minerals. The main sources of organic matter were gymnosperms (conifers) and microbial biomass,followed by angiosperms. Based on composition of saturated and aromatic diterpenoids it has been establishedthat coal forming plants belonged to the gymnosperm families Taxodiaceae, Podocarpaceae, Cupressaceae,Araucariaceae, Phyllocladaceae and Pinaceae. Peatification occurred in neutral to slightly acidic, fresh water envi-ronment. Composition and distribution of biomarkers show that diagenetic changes of the organic matter weremainly governed by bacterial activity in a suboxic to oxic environment. Based on distribution of aromaticditerpenoids a novel diagenetic pathway for transformation of abietane-type precursors under suboxic to oxic con-ditions is proposed. Variations in compositions of macerals and biomarkers are in concordance with pronouncedseasonality during Pontian, which caused changes in the water level, redox conditions during peatification, andto some extent vegetation differences in the paleo-plant communities.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The Upper Miocene Kolubara lignite basin is economically one ofthe most important coal basins in Serbia. It is located about 60 kmSSW of Belgrade, and covers an area of almost 600 km2, extendingin the E–W direction up to 55 km, and in the S–N direction up to15 km. This basin is divided into several fields (“A”, “B”, “C”, “D”, “E”,“F”, “G”, “Veliki Crljeni”, “Šopić-Lazarevac”, “Tamnava Istok”, “TamnavaZapad”, “Radljevo”, “Zvizdar” and “Ruklade”; Fig. 1). Lignite is exploitedin the fields “C”, “D”, and “Tamnava Zapad”. The D field which is thefocus of this study is situated in eastern part of the Kolubara basin. Atthe beginning of exploitation it extended over an area of almost 20 km2

and the remaining surface with mineable coal seams is about 6 km2.The northern border is represented by outcroping and erosion of theMain coal seam. The western border of the deposit is a natural extensionto the G field and the southern to the E field. The eastern border is alsomarked by outcroping of the Main coal seam.

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Since 1896, within the Kolubara basin about 883.2 Mt lignite, intotal, has been produced, of which 866.8 Mt in open pit mines and16.3 Mt in underground mines being active until 1974 (http://www.rbkolubara.co.rs). Annually, the Kolubara basin produces about30 Mt lignite, which amounts to 70% of total coal production in Serbia.According to the Geological Survey of the Kolubara basin, the lignite re-sources and reserves are currently estimated at 2811 Mt.Most of the lig-nite produced (90%) is used for electricity generation in thermal powerplants “Nikola Tesla” in Obrenovac and “Kolubara” in Veliki Crljeni, withtotal capacity of 3160 MW(http://www.eps.rs). About 17 billion kWh isannually produced from Kolubara coal, which represents 52% of Serbia'stotal electricity generation.

Geological exploration began at the eastern part of the basin since thelate 19th century. Upper Miocene (Pontian) age of the coal-bearing sed-iments was confirmed in studies carried out by Stevanović (1951) andPantić and Dulić (1993).

The distribution of palynomorph assemblages in the lignite fromthe D field (Pantić andDulić, 1993) suggests that trees and bushes playedan important role in lignite formation. According to this study, the pres-ence of Taxodiaceae–Nyssaceae and Cupressaceae indicates a wet forest

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Fig.

1.Simplified

geolog

ical

map

oftheKolub

araba

sin.

6 D. Životić et al. / International Journal of Coal Geology 111 (2013) 5–22

7D. Životić et al. / International Journal of Coal Geology 111 (2013) 5–22

mire during peat accumulation. Other palynomorph assemblages indi-cate a wetter part of a mire, a mixed mire with Myrica, Alnus, Salix,Graminae sp., and rare water lilies. Dryer parts of the paleomire includespores of Polypodiaceae, Sequoia, Betula, Carya, Fagus, Tilia, Ericaceae,Lauraceae, Luquidambar and Cedrus. Coniferous pollen (Pinus, Abies,Pineace, and Tsuga) indicates the presence of a hilly region in the vi-cinity of the paleomire (Pantić and Dulić, 1993).

Previous petrographical investigations (Ercegovac and Pulejković,1991; Ercegovac et al., 2006) done on the samples from several bore-holes from the Main coal seam, showed that the lignite from theKolubara basin is a typical humic coal with a high huminite content(93.5 vol.%, on a mineral-matter-free basis, mmf) and relatively lowliptinite (3.3 vol.%, mmf) and inertinite contents (3.3 vol.%, mmf).The most abundant macerals are textinite and ulminite with variableamounts of densinite and attrinite. Petrographical study on core sam-ples from seven boreholes (Ercegovac and Pulejković, 1991) showed avariation in maceral composition through the seam profile. The mostabundantmacerals are textinite (6.5–87.0 vol.%,mineral-matter includ-ed) and ulminite (3.5–50.3 vol.%) with variable amounts of attrinite(4.5–35.5 vol.%), densinite (0.5–16.0 vol.%), gelinite (2.5–22.5 vol.%) andcorpohuminite (0.5–7.5 vol.%). Liptinite ranges from 1.0 to 19.0 vol.%,

Fig. 2. General lithostratigraphic column of the Kolub

and inertinite from 0.5 to 13.5 vol.%. Average huminite reflectance ofthe coal seams from the Kolubara basin is 0.30%, thus placing the coal atlignite stage of coalification.

During liquefaction by direct catalytic hydrogenation, lignite fromTamnava field displayed a high degree of conversion (>84%; Aleksić etal., 1997; Vitorović et al., 1996) while on the contrary, lignite from theD field showed a lower degree of conversion (Vitorović et al., 1994).

In the present study, the petrographical and geochemical character-istics of the thickest and most productive D field lignites were investi-gated in detail, in order to evaluate the origin of the organic matterand the characteristics of the depositional environment. In addition, anovel diagenetic pathway for transformation of abietane-type precur-sors under suboxic to oxic conditions is proposed.

2. Geological setting

2.1. Lithological setting of the Kolubara Basin

The area of the Kolubara basin consists of Paleozoic, Mesozoic,Tertiary, and Quaternary rocks (Ercegovac and Pulejković, 1991; Fig. 2).Both, the border and the basement of this basin consist of Devonian

ara basin (after Vučković and Bogdanović, 2010).

image of Fig.�2

Fig.

3.Crosssections

oftheKolub

aralig

nite

basin.


image of Fig.�3


andCarboniferous schist, gneiss, slate and sandstone,Mesozoic carbonateand marly sediments, Tertiary dacite–andesite rocks and pyroclastics, la-custrine, brackish and marine clastic sediments.

2.1.1. PaleozoicPaleozoic rocks consist of Devonian–Carboniferous phyllite with

redeposited blocks and fragments of limestone and Carboniferous ter-rigenous sediments (phyllite, rarely quartz sandstone with lenses oflimestone). In the contact zone with Brajkovac and Bukulja granitePaleozoic rocks are partly metamorphosed, forming hornfels, amphibo-lite, mica schist and gneiss. The Paleozoic rocks occur along the southernborder of the Kolubara basin.

2.1.2. MesozoicMesozoic sedimentation begun in Lower Triassic with clastic rocks–

mica-rich sandstone and shales,while duringMiddle andUpper Triassicbedded to thick-bedded dolomitic limestones and massive limestonesdeposited. Triassic sediments mostly occur in the central and westernpart of the southern rim of the basin.

The Upper Cretaceous rock succession is comprised of flysch(alternation of limestone, marlstone, sandstone and siltstone). Thesesediments outcrop along the southern border of the Kolubara lignitebasin (Fig. 1).

2.1.3. CenozoicVolcanic and pyroclastic rocks of Upper Oligocene and Lower to

Middle Miocene age formed in the SE rim of the Kolubara basin andconsist of phenoandesite, phenodacite, quartz-latite, ignimbrite andquartz-latite tuff.

The Kolubara basin was formed in the Pannonian Basin System inshallow lacustrine, delta plain and fluvial environments. During thelate Miocene it became increasingly widespread as Lake Pannon(e.g. Magyar et al., 1999) filled with coal-bearing sediments in the

Fig. 4. Block-diagram of contemporary relief of the Kolubara basin with position of major fauified by Kezović, 2010); 1. Peštan fault; 2. Barič-Šljivovica fault; 3. Radljevo fault; 4. Dren faufault.

central part of Serbia. Neogene of the Kolubara basin consists of thefollowing units (Kezović, 2011):

1. Lower Miocene comprises fresh water marlstone, tuffite, andclaystone;

2. Badenian (Middle Miocene) consisting of marine sand, looselybounded conglomerate, freshwater sand, clay and gravel;

3. Badenian–Sarmatian (Middle Miocene) consists of sand, clay andgravel;

4. Sarmatian (Middle Miocene) comprises brackish clayey–marly andsandy sediments and limestone;

5. Pannonian (Upper Miocene) contains caspi-brackish sand, sandyclay, marly clay, silt, rarely gravel and marlstone;

6. Pontian (Upper Miocene) consists of fresh water clastic sediments,with three coal seams: seam III or Lower coal seam (Fig. 3), seam IIor Main coal seam, and seam I or Upper coal seam, having averagethickness of 7 m, 25 m, and 11 m, respectively (Kezović, 2011). Thetotal thickness of the Pontian series is between 250 and 320 m;

7. Quaternary formed of fluvial gravel, sand, clay and sandy–clayeysediments.

In the Kolubara coal basin all the three coal seams are hosted(Lower, Main and Upper; Fig. 3). They formed in the southern partof the basin, while towards the north the Upper coal seam outcropsand the Lower coal seam either outcrops or merges coal seam togetherwith the Main coal seam. The southern and northern parts of theKolubara coal basin are separated by the Medoševac fault zone (Fig. 4).

The Lower coal seam formed locally in the southern part of thebasin, mostly in F and Šopić-Lazarevac fields, although the extent ofthis seam is not completely explored. The seam thickness is variableup to 20 m.

The Main coal seam occurs across the entire Kolubara coal basin. Inthe southern part of the basin it is more or less a uniform coal seam,but towards the north it splits into several coal layers. Thickness variesgenerally from a few to 50 m (including interbeddedwaste rock), but is

lt zones, intra-basin and surrounding neotectonic units (after Đoković et al., 1988, mod-lt; 5. Obrenovac fault; 6. Dubrava fault; 7. Ćelije fault; 8. Vrelo fault; and 9. Medoševac

image of Fig.�4


locally up to 110 m (in the Zeoke area). Someparts of that coal seam areeroded. TheMain coal seamstill hosts themajority of coal resources andreserves within the Kolubara basin.

The Upper coal seamhas been explored in the entire southern part oftheKolubara basin, from the A field in the SE, to the Šopić-Lazarevac fieldin the central southern part. It is a relatively homogenous coal seam, notmuch disturbed by subsequent faulting, but occasionally eroded byKolubara and Peštan rivers. The thickness is variable, up to 25 m. In theB and C fields, as well as in a very small part of the D field, one or twodiatomite layers formed between Main and Upper seams. The majorityof the reserves are already exploited.

The area of the D field is comprised of Paleozoic schist, Tertiarydacite and Pannonian sandy–clayey sediments. The base of the Maincoal seam consists of clay, quartz sand and clayey sand. Usual thicknessof theMain coal seamvaries from a few to 50 m (including interbeddedwaste rock), locally up to 75 m. The overburden consists of Miocenesandy clay, sand and Quaternary gravel and sand.

2.2. Paleoclimate and paleontological characteristics

Recent paleoclimate investigations based on analysis of 14megaflorascomprise 14 to 42 different fossil taxa from 12 locations indicate relative-ly uniform, warm and humid climate at the territory of Serbia duringwhole Miocene (Ivanov et al., 2011; Utescher et al., 2007). Major vegeta-tion changes occurred in the Upper Miocene. This period is characterizedby relatively diverse climatic conditions, which were directed by globalclimatic changes and probably complicated by regional paleogeographicreorganizations and tectonic processes. The vegetation shows a decreas-ing trend in abundance of paleotropic and thermophilous elements,reduction of macrothermic elements, and disappearance of evergreenlaurel forests. Together with these changes is a corresponding increasein the role of arctotertiary species in plant communities, and they becamedominants in mesophytic forests. Slight cooling and some drying duringUpper Miocene, followed by fluctuations of paleoclimate parameterswhich display cycling changes of humid/dryer andwarmer/cooler condi-tions were observed.

The coal-bearing strata cover almost complete by the Kolubara basin.It is of Upper Pontian age (uppermost Miocene according to Paratethyssubdivision of theMiocene; Rögl, 1996), based on the following paleonto-logical material (Pantić and Dulić, 1993):

• Paleoflora: Taxodium, Nyssa, Polypodiaceae, Larix, Pinus silvestris,P. haploxylon, Picea, Abies, Fagus, Corylus, Salix, Sequoia, Myrica,Magnoliae, Glyptostrobus;

• Mollusca: Budmania sp., B. histiophora Brus., Limnocardium pensliiFuchs., L. Zagrabiensis; Congeria rhomboidea Hoern.;

• Ostracodae: Candona alta (Zal.), C. labiata Sok, C. hastata Krst., Cyproidestriangulata Krst.

2.3. Tectonic setting of the Kolubara basin

The most important fault is (Đoković et al., 1988; Kezović, 2010):Radljevo fault (WSW–NNE direction; Fig. 4), separating the central fromthe southern parts of the basin.

Tectonic features of Neogene sediments are relatively uniform inthemajor part of the basin; coal seams dip at low angles to the northernand central parts of the basin. Only along the southern border of the SEpart of the basin, coal seams are characterized by a synform, due to in-tense post-sedimentary faulting, causing occasional coal erosion in theSE part of the basin.

3. Samples and analytical methods

Fourteen lignite samples were collected from core Dg-29/03 fromthe D field, Kolubara basin (Fig. 1), representing different parts of theMain coal seam. The sampling interval was determined on the basis of

lithological changes. The reconstructed column, with relative samplingpositions, is given in Fig. 5. The maceral and geochemical analyses wereperformed on all fourteen lignite samples.

The macroscopic description of the coal lithotypes followed the no-menclature adopted by the ICCP (1993 in Taylor et al., 1998).

3.1. Petrographic analysis

Formaceral analyses, the lignite sampleswere crushed to amaximumparticle size of 1 mm, mounted in epoxy resin and polished. The maceralanalyseswere performed on a Leitz DMLPmicroscope inmonochromaticand UV light illumination on 500 points. The maceral description used inthis study follows the terminology developed by the InternationalCommittee for Coal and Organic Petrology for huminite (Sykorovaet al., 2005), liptinite (Taylor et al., 1998) and inertinite (ICCP, 2001)nomenclature.

The reflectance measurements were performed under a monochro-matic light of 546 nm using a Leitz DMRX microscope and an opticalstandards having a reflectance of 0.899% and 1.699% in oil, followingthe procedures outlined by Taylor et al. (1998). The rank was deter-mined by measuring the random reflectance on ulminite B.

3.2. Organic geochemical analysis

Part of the samples was ground to b150 μm and analyzed on a VarioEL III CHNS/O Elemental Analyzer, Elementar Analysensysteme GmbH.Elemental analysis was carried out to determine the contents of sulfurand organic carbon (Corg). Organic carbon content was determinedafter removal of carbonateswith diluted hydrochloric acid (1:3, v/v). An-alytical moisture determination followed SRPS B.H8.390/1987 standard(1987) and total moisture determination followed SRPS B.H8.338/1986standard (1986). Ash yield determination followed ISO 1171 (1997).Calorific value measurements were performed on IKA-Calorimeter adia-batic C400, followed standard procedure SRPS B.H8.318/1972 (1972).

The soluble organic matter (bitumen) was extracted from pulverizedlignite samples (b150 μm) using a Dionex ASE apparatus with amixtureof isohexane and acetone (1:1, v:v) at a temperature of 80 °C and a pres-sure of 8 MPa. The asphaltenes were precipitated with petroleum-etherand the remainder (maltenes) was separated into three fractions usingcolumn chromatography over silica gel and aluminum oxide. The satu-rated hydrocarbon fraction was eluted with isohexane, the aromatic hy-drocarbons with dichloromethane and the NSO fractions (polar fraction,which contains nitrogen, sulfur, and oxygen compounds) with mixtureof dichloromethane and methanol (1:1, v:v).

Saturated and aromatic fractions isolated from the bitumen were an-alyzed by gas chromatography–mass spectrometry (GC–MS). A gas chro-matograph Agilent 7890A GC (H5-MS capillary column, 30 m×0.25 mm,He carrier gas 1.5 cm3/min, FID) coupled to a Agilent 5975Cmass selec-tive detector (70 eV) was used. The column was heated from 80 to310 °C, at a rate of 2 °C/min, and the final temperature of 310 °C wasmaintained for additional 25 min. The individual peaks were identifiedby comparison with literature data (Killops et al., 1995, 2003; Otto andSimoneit, 2002; Peters et al., 2005; Philp, 1985; Stout, 1992; Tuo and Li,2005; Wakeham et al., 1980) and on the basis of the total mass spectra(library: NIST5a). Biomarker parameters were calculated from GC–MSchromatogram peak areas (software GCMS Data Analysis).

4. Results and discussion

4.1. Lithotypes and ash yield

The macroscopic examination of the core (Fig. 5) shows that thefloor of the Main coal seam consists of sand and clay, and the roof ofsand. Xylite-rich coal predominated in the central, upper and lowerpart of the Main seam. Stratified matrix coal (Taylor et al., 1998) pre-dominates in the bottom part of the Main seam. Small fragments of

Fig. 5.Macropetrographic profile of the D field lignite and vertical distribution of the maceral groups, gelification index, tissue preservation index, ash content, total sulfur content, organic carbon content, the relative ratio of diterpenoids tosum of di- and triterpenoids in saturated fraction, the relative ratio of diterpenoids to sum of di- and triterpenoids in aromatic fraction, relative content of alka-2-ones in aromatic fraction, relative ratio of 1-Ar ring/(1+2+3-Ar rings)diterpenoids and 1-Ar ring/(1+3+4-Ar rings) triterpenoids, relative ratio of n-alkanes and hopanoids. 11

D.Životić

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image of Fig.�5


charcoal were found in the central and lower part of the seam.Mineral-rich coal is present in thin layers near the roof, upper andlower parts of seam.

The ash yield (on dry basis) of the coal samples from the D field(Kolubara basin) ranges between 6.67 wt.% in the xylite-rich coal to40.45 wt.% in the mineral-rich coal from the upper part of the Mainseam (Table 1). Generally, the mineral-rich and matrix coal displayshigher ash yield than the xylite-rich coal. The very high ash yieldand its frequent vertical variation in the Main coal seam (Fig. 5) areindicative of periodic flooding of the paleomire during deposition.

4.2. Maceral analysis

The results of the huminite reflectance measurements show thatthe coal from the Main coal seam of the D field can be classified as lig-nite, with a mean random huminite reflectance of 0.30±0.03 (ISO11760, 2005; Table 2).

Results of maceral analysis are reported with mineral-matter in-cluded. Huminite is the prevailing maceral group in the Main seam(51.9–83.7 vol.%; Table 2). The most abundant macerals are textinite(5.7–57.0 vol.%; Fig. 6d) and ulminite (8.2–45.8 vol.%; Fig. 6d) withvariable amounts of densinite (6.1–33.7 vol.%; Fig. 6a) and attrinite(1.5–24.4 vol.%). Telohuminite is strongly impregnated by resinous-likesubstances, while detrohuminite occurs as groundmass surround-ing liptinite or inertinite macerals. In some cases, detrohuminiteis interbeddedwith clayminerals. Contents of gelinite and corpohuminite(Fig. 6b) are low (b5 vol.%). Gelinite appears in many cases interbeddedwith ulminite or densinite, sometimes as thick layers. Corpohuminite isdisseminated throughout ulminite and textinite, sometimes in densinite,mainly as phlobaphinite of globular or tabular shape.

Liptinite ranges from 3.8 to 17.2 vol.% with liptodetrinite(1.5–7.2 vol.%) and sporinite (0.9–4.7 vol.%) being the most abundantmacerals. Suberinite (Fig. 6b), cutinite and resinite are present in vari-able amounts in all samples. Most of the sporinite have thin walls(tenuisporinite), while sporinitewith thickwall (crassisporinite) occursin traces. Resinite mostly occurs as cell filling or isolated small globularbodies. They are associated with telohuminite as single bodies, com-monly as impregnation in telohuminite and less with detrohuminite.Liptodetrinite is usually associated with detrohuminite. Suberiniteusually appears as cell wall tissue associated with phlobaphinite.Cutinite usually occurs as the thin walled (tenuicutinite) variety. Thethick walled (crassicutinite) variety was rarely observed. Alginite andfluorinite were observed in few samples (Table 2).

The percentages of inertinite, mainly inertodetrinite (Fig. 6a),fusinite (Fig. 6c), funginite, semifusinite and macrinite range from

Table 1Total moisture, calorific value, ash yield, sulfur and organic carbon contents, and values of

Sample Depth (m) Wartot

(wt.%)Wan

(wt.%)Qmaf

g

(MJ/kg)Qmaf

n

(MJ/kg)Adb

(wt.%)Sdb

tot

(wt.%

Dg-29/03_1 115.60–115.90 40.38 6.92 12.84 11.34 40.45 0.91Dg-29/03_2 116.52–116.90 43.44 7.67 13.30 11.71 33.81 1.18Dg-29/03_3 119.50–120.75 49.47 8.52 11.65 10.03 18.50 0.26Dg-29/03_4 121.00–121.90 53.22 7.99 10.42 8.86 6.69 0.28Dg-29/03_5 123.16–123.70 51.25 8.08 12.57 10.90 13.21 0.86Dg-29/03_6 123.70–127.00 48.36 8.33 11.65 10.08 21.60 0.47Dg-29/03_7 130.30–133.10 52.52 9.27 11.73 10.15 9.09 0.00Dg-29/03_8 133.10–133.70 52.05 8.62 11.79 10.15 10.66 0.43Dg-29/03_9 133.70–136.00 53.23 8.29 11.09 9.43 6.67 0.36Dg-29/03_10 138.20–140.40 51.56 8.29 11.88 10.26 12.21 0.73Dg-29/03_11 143.80–146.40 48.86 8.18 12.52 10.94 20.21 1.01Dg-29/03_12 146.40–147.00 51.42 9.01 12.09 10.50 12.67 1.58Dg-29/03_13 148.90–149.90 46.01 7.20 13.16 11.60 27.65 0.60Dg-29/03_14 153.20–154.40 42.65 6.79 14.08 12.58 35.52 0.66Minimum 40.38 6.79 10.42 8.86 6.67 0.00Maximum 53.23 9.27 14.08 12.58 40.45 1.58

Wartot — Total moisture content; Wan — Analytical moisture content;Qmaf

g — Gross calorific valudry basis; Sdb

tot — Total sulfur content, dry basis; and Cdborg — organic carbon content, dry bas

1.5 to 15.5 vol.% in the Main coal seam. Funginite is especially abun-dant in sample Dg-29/03_7 (3.7 vol.%). Inertodetrinite is dissemi-nated throughout the coal samples. Funginite, including single andmulti-celled fungal spores and sclerotia, occurs as single bodies oras colonies. The pores are usually filled with mineral matter, rarelywith resinite. Fusinite mostly occurs in thick bands. The pores areusually empty but sometimes they are filled with mineral matter.Macrinite appears in all samples.

The content of mineral matter varies between 4.4 and 34.1 vol.%,which is consistent with the ash yield (Fig. 5; Table 2). Clays arethe most abundant minerals, while carbonates, pyrite and themineral-bituminous groundmass, impregnation of microscopic tosubmicroscopic grains of minerals (clays/carbonates/quartz) withdispersed bituminite (Teichmüller, 1989) are less abundant. The increaseinmineral content is often related tomore intense degradation of organ-ic matter and/or contribution of clastic material. As expected, significantpositive linear correlation (correlation coefficient, r=0.91; Fig. 7a)between mineral matter content (Table 2) and ash yield (Table 1) isobserved.

The Tissue Preservation Index (TPI; Diessel, 1986 modified byErcegovac and Pulejković, 1991), taken as the ratio between struc-tured and unstructured macerals of the huminite and inertinitegroup, ranges from 0.80 to 6.30 (Table 2). As expected, coals withwell-preserved plant tissues (textinite and ulminite) display high TPIvalues. Variations of TPI with depth (Fig. 5) could reflect, to some extent,differences in the type of peat-forming plant communities. The domina-tion of structuredmacerals in almost all samples (TPI>1; Table 2) couldimply significant contribution of gymnosperm species, which are moreresistant to degradation in comparison to angiosperms. On the otherhand, it was suggested that tissue preservation depends mostly on thewater level, the acidity/alkalinity and the climatic conditions duringpeat accumulation, rather than on the botanical properties of the vege-tation (Dehmer, 1995). According to this study, lowering the waterlevel, i.e. causing more oxic conditions, results in more extensive tissuedegradation.

The Gelification Index (GI; Diessel, 1986 modified by Ercegovacand Pulejković, 1991), expressed as the ratio of gelified (ulminite,densinite, gelinite and corpohuminite) to non-gelified (textinite andattrinite) macerals, may indicate the relative humidity in the mire.It is controlled by wet/dry conditions because gelification requiresthe continuous presence of water (Dehmer, 1989; Diessel, 1986;Kolcon and Sachsenhofer, 1999). Lignite from the D field displays GIvalues ranging between 0.3 and 4.7 (Table 2).

Slight cooling and some drying during Upper Miocene, followed byfluctuations of paleoclimate parameters which display cycling change

group organic geochemical parameters of the D field lignite (Kolubara basin).

)Cdb

org

(wt.%)Extract yield(mg/g Corg)

Asphaltenes(wt.%)

Alkanes(wt.%)

Aromatics(wt.%)

NSO compounds(wt.%)

36.5 35.62 49.90 4.94 6.12 39.0539.8 72.62 51.35 1.76 4.30 42.5948.9 54.35 52.15 5.97 5.23 36.6555.7 39.35 52.62 3.11 3.98 40.2955.1 129.10 62.10 5.19 3.40 29.3247.2 58.57 47.33 5.18 4.66 42.8356.4 54.90 53.64 2.36 4.52 39.4856.0 55.92 54.05 2.48 4.42 39.0560.0 62.93 58.41 2.15 3.75 35.6855.6 59.95 54.17 2.42 4.82 38.5848.7 40.16 54.15 2.66 4.57 38.6354.7 40.18 61.40 4.11 3.67 30.8244.6 44.09 53.25 3.15 5.11 38.4940.2 45.46 55.55 2.46 4.75 37.2536.5 35.62 47.33 1.76 3.40 29.3260.0 129.10 62.10 5.97 6.12 42.83

e, moist, ash-free basis;Qmafg — Net calorific value, moist, ash-free basis; Adb — Ash yield,

is.

Table 2Maceral composition (vol.%), huminite reflectance and petrographic indices of the D field lignite, Kolubara basin.

Sample Dg-29/031

Dg-29/032

Dg-29/033

Dg-29/034

Dg-29/035

Dg-29/036

Dg-29/037

Dg-29/038

Dg-29/039

Dg-29/0310

Dg-29/0311

Dg-29/0312

Dg-29/0313

Dg-29/0314

Min Max AM

Maceral composition (vol.%)Textinite 15.3 12.7 37.0 57.0 19.0 34.2 20.7 26.2 14.3 17.7 8.7 10.7 13.9 5.7 5.7 57.0 13.8Ulminite 8.2 13.8 12.7 12.6 12.0 20.8 35.2 36.1 32.4 45.8 22.2 38.1 18.8 42.6 8.2 45.8 12.8(Total Telohuminite) 23.5 26.5 49.7 69.6 31.0 55.0 55.9 62.3 46.7 63.5 30.9 48.8 32.7 48.3 23.5 69.6 14.8Attrinite 19.0 14.7 11.0 4.9 24.4 4.2 5.8 6.5 8.1 5.2 5.6 1.5 8.5 1.8 1.5 24.4 6.6Densinite 10.6 9.7 13.5 6.1 15.5 15.5 11.2 11.5 24.0 8.8 33.7 14.8 30.4 19.1 6.1 33.7 8.2(Total Detrohuminite) 29.6 24.4 24.5 11.0 39.9 19.7 17.0 18.0 32.1 14.0 39.3 16.3 38.9 20.9 11.0 39.9 9.8Gelinite 0.1 0.7 0.9 0.4 1.1 0.3 0.9 1.3 1.0 2.0 1.2 3.7 1.9 1.0 0.1 3.7 0.9Corpohuminite 1.4 0.3 1.6 2.7 2.3 2.2 2.2 1.7 1.7 3.5 3.3 4.4 3.8 2.5 0.3 4.4 1.1(Total Gelohuminite) 1.5 1.0 2.5 3.1 3.4 2.5 3.1 3.0 2.7 5.5 4.5 8.1 5.7 3.5 1.0 8.1 1.9Total Huminite 54.6 51.9 76.7 83.7 74.3 77.2 76.0 83.3 81.5 83.0 74.7 73.2 77.3 72.7 51.9 83.7 74.3Sporinite 1.9 3.3 4.1 1.1 2.8 1.5 2.6 1.3 4.7 2.5 1.7 0.9 1.6 1.5 0.9 4.7 1.1Cutinite 0.3 1.9 0.4 0.2 0.5 0.3 0.6 0.4 0.4 0.4 0.3 0.4 0.2 0.2 0.2 1.9 0.4Resinite 0.6 0.4 0.2 0.4 1.4 0.7 0.2 0.7 0.2 0.7 0.2 0.2 0.3 0.5 0.2 1.4 0.3Suberinite 0.9 2.2 0.5 0.6 1.4 0.3 1.1 2.0 1.1 0.7 0.3 0.4 0.2 0.5 0.2 2.2 0.6Alginite 0.9 2.1 0.4 0.2 0.2 0.2 2.1 0.6Fluorinite 0.1 0.2 0.1 0.2 0.1Liptodetrinite 4.1 7.2 3.1 1.5 3.9 2.0 2.8 2.8 2.7 2.8 3.4 2.4 1.9 1.5 1.5 7.2 1.4Total Liptinite 8.7 17.2 8.3 3.8 10.4 4.8 7.3 7.2 9.3 7.1 6.3 4.3 4.2 4.2 3.8 17.2 3.5Fusinite 0.3 0.7 0.7 2.3 0.5 1.2 0.2 0.2 0.6 0.7 3.3 0.2 0.7 0.2 3.3 0.9Semifusinite 0.1 0.2 0.6 0.4 0.2 0.6 0.4 0.2 0.2 1.4 3.0 0.5 1.2 0.1 3.0 0.8Macrinite 0.1 0.9 0.4 0.6 0.4 0.2 0.9 0.5 0.6 0.4 1.9 1.5 1.2 1.4 0.1 1.9 0.5Funginite 0.8 0.6 0.9 0.6 0.4 0.5 3.7 0.7 0.8 0.6 0.3 1.3 0.7 1.0 0.3 3.7 0.8Inertodetrinite 1.4 2.3 1.5 4.0 1.8 0.6 3.7 0.6 2.6 1.6 2.2 6.4 2.8 2.0 0.6 6.4 1.5Total Inertinite 2.6 4.6 3.7 8.1 3.5 1.5 10.1 2.4 4.4 3.4 6.5 15.5 5.4 6.3 1.5 15.5 3.7Clay 30.5 21.1 5.0 2.1 3.7 13.2 3.4 4.3 3.4 4.7 7.9 2.7 7.8 9.2 2.1 30.5 8.1Pyrite 1.8 2.1 0.7 0.4 2.6 0.3 0.7 1.2 0.6 0.7 1.2 3.0 1.0 2.0 0.3 3.0 0.9Carbonates 0.5 0.1 0.9 0.2 0.5 0.8 0.4 0.7 0.2 0.4 1.0 0.5 0.9 0.5 0.1 1.0 0.3Mineral-bituminousgroundmass

2.2 3.6 1.3 4.6 1.0 1.7 0.2 0.4 0.5 2.2 0.4 0.2 4.6 1.4

Other minerals 1.3 0.8 1.1 0.4 0.4 1.2 0.4 0.7 0.2 0.2 0.2 0.4 3.4 5.1 0.2 5.1 1.4Total Mineral 34.1 26.3 11.3 4.4 11.8 16.5 6.6 7.1 4.8 6.5 12.5 7.0 13.1 16.8 4.4 34.1 8.5Average huminitereflectance (%)

0.29±0.02

0.29±0.02

0.30±0.02

0.30±0.03

0.31±0.03

0.30±0.02

0.30±0.02

0.31±0.02

0.30±0.03

0.30±0.02

0.30±0.02

0.31±0.03

0.30±0.02

0.31±0.03

0.29±0.02

0.31±0.03

0.30±0.03

TPIa 0.8 1.1 2.0 6.3 0.8 2.8 3.2 3.3 1.4 4.1 0.9 2.8 0.9 2.3 0.8 6.3 1.6GIb 0.6 0.8 0.6 0.3 0.7 1.0 1.4 1.4 2.2 2.3 2.9 2.2 2.0 4.7 0.3 4.7 1.2

Min — minimum value; Max — maximum value; and AM — arithmetic mean value.a TPI=(textinite+ulminite+corpohuminite+fusinite+semifusinite)/(gelinite+macrinite+detrohuminite), by Diessel (1986) and modified by Ercegovac and Pulejković (1991).b GI=(ulminite+densinite+gelinite+corpohuminite)/(textinite+attrinite+inertinite), by Diessel (1986) and modified by Ercegovac and Pulejković (1991).

13D.Životić

etal./

InternationalJournalofCoalGeology

111(2013)

5–22

Fig. 6. Photomicrographs of typical macerals from the D field lignite; a) Densinite (D), Inertodetrinite (Id); b) Corpohuminite (Ch), Su (Suberinite); c) Fusinite (F); and d) Textinite(T) and Ulminite (U).


of humid/dryer and warmer/cooler conditions (Ivanov et al., 2011;Utescher et al., 2007) caused at least to some extent vegetation differ-ences in the paleo-plant communities. Pronounced seasonality alsocaused the changes in the water table, which resulted in changes ofredox potential (Eh) during peatification. Based on generally higher GIvalues in the lower part of the seam (depth interval 133–153 m; Fig. 5;

Fig. 7. Correlations mineral matter content vs. ash yield (a) and mineral matter contentvs. organic carbon content (b).

Table 2), it could be assumed that during peatification water columnlevel was higher in the lower part of the seam. Moreover, higher GIvalues in the lower part of the Main seam could imply pronounced mi-crobial activity.

4.3. Group organic geochemical parameters

Total moisture content is typical for lignite and ranges from40.38 wt.% to 53.23 wt.% (Table 1). The gross and net calorific value(moist, ash-free basis) of the lignite samples ranges from 10.42 to14.08 MJ/kg and from 8.86 to 12.58 MJ/kg, respectively (Table 1),which is expected for a coal of this rank. The highest calorific valueis observed in the xylite-rich coal from the central part of the Mainseam.

Organic carbon contents (Corgdb ) vary between 36.5% and 60.0%(Table 1). As expected significant negative linear correlation betweenCorgdb and mineral matter content (r=−0.91) is observed (Fig. 7b).

This indicates that the differences in Corgdb contents of the lignite sam-

ples are mainly controlled by varying amounts of mineral matter.The total sulfur content in the D field coal is low and ranges from

0.00 wt.% to 1.58 wt.% (Table 1). It is well known that the sulfur con-tent is influenced by the pH of the peat and by the sulfate content ofthe peat waters (Bechtel et al., 2004; Casagrande, 1987). The verticalvariation of the sulfur content in the Main coal seam (Fig. 5) suggests avariation of pH values in peat during the deposition of organic matter.

The yield of the soluble organic matter (bitumen) is high and rangesfrom 35.62 to 129.10 mg/g Corg (Table 1). It has been attributed to thehigh proportion of biogenic and diagenetic compounds. Several otherstudies have also reported high bitumen yields for immature coals fromall around the world (Avramidis and Zelilidis, 2007; Bechtel et al., 2005;Vu et al., 2009). In the studied samples, the contents of saturated

image of Fig.�6

image of Fig.�7


hydrocarbons (1.76–5.97 wt.%) and aromatics (3.40–6.12 wt.%) are low,while the contents of asphaltenes (47.33–62.10 wt.%) and NSO com-pounds (29.32–42.83 wt.%) are high, as expected for immature terrestrialorganic material (Table 1).

4.4. Molecular composition of the organic matter

4.4.1. General characteristicsThe main constituents of the saturated fraction in the lignite sam-

ples are diterpenoids, hopanoids and n-alkanes. Non-aromatic, non-hopanoid triterpenoids and steroids were identified in low amounts(Fig. 8).

The main components in the aromatic fractions of D field ligniteare diterpenoids and non-hopanoid triterpenoids. Other constituentsof the aromatic fraction are long-chain acyclic alkan-2-ones,sesquiterpenoids, aromatized hopanoids and monoaromatic steroids(Fig. 9). Perylene and β-tocopherol were observed in most samples.

Domination of diterpenoids and hopanoids in the saturated frac-tion indicates that the main sources of organic matter (OM) weregymnosperms (conifers) and microbial biomass, whereas relativelyhigh content of non-hopanoid triterpenoids in aromatic fraction im-plies the contribution of angiosperms to the lignite organic matter(Figs. 8 and 9).

4.4.2. n-Alkanes and isoprenoidsn-Alkanes are abundant in the total ion current (TIC) of saturated

fraction (Fig. 8). On the basis of mass fragmentogram, m/z 71(Fig. 10a), n-alkanes are identified in the range of C21 to C33. Then-alkane patterns of the lignite samples are dominated by long-chain homologues (C27–C31) with a maximum at n-C29 and a markedodd over even predominance, indicating a significant contribution ofepicuticular waxes. The values of the CPI (carbon preference index)and OEP (odd-to-even-predominance) 2 range between 3.33–9.68and 2.93–6.45, respectively (Table 3) are in accordance with the lowrank of the lignite.

Mid-chain n-alkanes (n-C21–C25), originated from vascular plants,microalgae, cyanobacteria, Sphagnum spp. and aquatic macrophytes(Ficken et al., 2000; Matsumoto et al., 1990; Nott et al., 2000), arepresent in notably lower amount in comparison to long-chain ho-mologues (Fig. 10a). The predominance of odd over even carbon-numbered n-alkanes in the mid-range n-alkanes (OEP 1; Table 3)

Fig. 8. TIC (total ion current) of saturated fraction typical for investigated samples. Peak assigD2— 4β(H)-19-Norisopimarane; D3— Pimaradiene; D4— 4α(H)-18-Norisopimarane; D5 —

D9 — Pimarane; D10 — 16β(H)-Phyllocladane; D11 — 16α(H)-Phyllocladane; D12 — 1T2 — Des-A-olean-12-ene; T3 — Des-A-lupane; ββ, βα and αβ designate configuration

suggests a microbial origin, consistent with high content of hopanoidbiomarkers (Fig. 8). Moreover, relatively high abundance of C23 andC25 n-alkane homologues implies input of aquatic macrophytes tothe organic matter, consistent with previous palynological investiga-tion (Pantić and Dulić, 1993).

Short chain n-alkanes (bn-C20) are found mostly in algae and mi-croorganisms. Short chain n-alkanes are identified in very low quan-tities only in the samples containing alginite according to the resultsof the maceral analyses (Table 2).

Isoprenoids pristane (Pr) and phytane (Ph) are identified as tracesin three samples only. Therefore, Pr/Ph ratio could not be calculated.Mature organic matter contains significant amount of isoprenoids,while immature organic matter is usually characterized by very lowconcentrations of pristane and phytane (Dzou et al., 1995; Hugheset al., 1995; Vu et al., 2009). This is probably related to the fact thatisoprenoid precursors were incorporated into the macromolecularmatrix either by esterification (Brooks et al., 1978) or by naturalsulfurization (Brassell et al., 1986) during the early diagenesis. The re-lease of saturated isoprenoids from kerogen macromolecular matrixoccurs only after the organic matter increases due the thermal matu-ration (Vu et al., 2009). As shown previously, the total sulfur contentsin the lignite samples are generally low (b1.58%; Table 1), implyingthat even if the depositional conditions for the investigated coalswere sufficiently reducing for sulfate reduction to occur, naturalsulfurization of phytol and its derivatives would be somewhat limitedfor the samples investigated here. Therefore, it is assumed that phytoland its derivatives (e.g., phytenic acid, formed from phytol under oxicdepositional conditions) were incorporated into the kerogen (organicmatter insoluble in conventional organic and inorganic solvents)structure during the earliest stages of diagenesis by esterification(Brooks et al., 1978). The second pathway implies pristane formationfrom the isoprenoidal side chain of tocopherol, which is often foundin relatively high concentrations in higher plants (Goossens et al.,1984; ten Haven et al., 1987) and phytane formation from bacteriallipids (Volkman and Maxwell, 1986). The presence of β-tocopherol inlignite extracts (Fig. 9) and absence or traces amounts of isoprenoidsare consistent with low degree of maturity.

4.4.3. Steroids and hopanoidsThe analysis of the aliphatic fraction reveals extremely low con-

tents of steroids (Fig. 8). Steroid biomarkers consist predominantly

nments: n-Alkanes are labeled according to their carbon number; D1— Isopimaradiene;C20 Diterpane; D6— Norpimarane; D7 — Isopimarane; D8 — Fichtelite (norabietane);6α(H)-Kaurane; D13 — Abieta-8,11,13-trien-7-one; T1 — Des-A-olean-13(18)-ene;s at C17 and C21 in hopanes, (R) designates configuration at C22 in hopanes.

image of Fig.�8

Fig. 9. TIC (total ion current) of aromatic fraction typical for investigated samples. Peak assignments: 1— Dihydro-ar-curcumene; 2— Cuparene; 3— 1,2,3,4-Tetrahydronaphthalene;4— Calamenene; 5— Cadina-1(10),6,8 triene; 6— 5,6,7,8 Tetrahydrocadalene; 7— Cadalene; 8— Isocadalene, 9— Norabietadiene; 10— 6,10,14-Trimethylpentadecan-2-one; 11— 19-Norabieta-8,11,13-triene; 12 — Norabietatetraene; 13 — Hibane; 14 — 18-Norabieta-6,8,11,13-tetraene; 15 — 18-Norabieta-8,11,13-triene; 16 — Dehydroabietane; 17 — 1,2,3,4-Tetrahydroretene; 18 — 2-Methyl-1-(4′-methylpentyl)-6-i-propyl-naphthalene; 19 — Simonellite; 20 — Totarane; 21 — Sempervirane; 22 — Retene; 23 — Diaromatic diterpenoid;24 — Retene isomer; 25 — 6,7-Dehydroferruginol; 26 — Ferruginol; 27 — 2-Methylretene; 28 — 3,4,7,12a-Tetramethyl-1,2,3,4,4a,11,12,12a-octahydrochrysene; 29 — 3,3,7-Trimethyl-1,2,3,4-tetrahydrochrysene; 30 — Perylene; 31 — C27 Alkan-2-one; 32 — 24,25-Dinoroleana-1,3,5(10),12,14-pentaene; 33— 24,25-Dinoroleana-1,3,5(10),12-tetraene; 34 — D-ringmonoaromatic hopane; 35 — β-Tocopherol; 36 — 24,25-Dinorursa-1,3,5(10),12-tetraene; 37 — 24,25-Dinorlupa-1,3,5(10)-triene; 38 — Norlanosta(eupha)hexaene; 39 — C29 Alkan-2-one;40 — Isomer of 2,2,4a,9-Tetramethyl-1,2,3,4,4a,5,6,14b-octahydropicene; 41 — 1,2,4a,9-Tetramethyl-1,2,3,4,4a, 5,6,14b-octahydropicene; 42 — 2,2,4a,9-Tetramethyl-1,2,3,4,4a,5,6,14b-octahydropicene; 43—Mixture of oleana-dien-3-one and unknown compound; 44— 4-Methyl, 24-ethyl, 19-norcholesta-1,3,5(10)-triene; 45— 7-Methyl, 3′-ethyl, 1,2-cyclopentanochrysene;46 — C31 Alkan-2-one; 47 — 1,2,9-Trimethyl-1,2,3,4-tetrahydropicene; 48 — 2,2,9-Trimethyl-1,2,3,4-tetrahydropicene; 49 — 1,2,9-Trimethyl-1,2-dihydropicene; 50 — C33 Alkan-2-one;and 51— C35 Alkan-2-one.

Fig. 10. GC–MS mass chromatograms of n-alkanes, m/z 71 (a), sterenes, m/z 215 (b), hopanoids, m/z 191 (c) and alkan-2-ones, m/z 58 (d) typical for investigated samples6,10,14-TMPD-2-one–6,10,14-Trimethylpentadecan-2-one; for other peak assignments, see Fig. 8 legend.


image of Fig.�9

image of Fig.�10

Table 3Parameters calculated from distribution and abundance of n-alkanes.

Sample CPItotala CPI25–31b OEP 1c OEP 2d

Dg-29/03_1 3.87 3.71 2.01 2.93Dg-29/03_2 5.45 4.88 2.16 3.24Dg-29/03_3 6.02 5.50 2.09 3.67Dg-29/03_4 9.68 8.85 2.27 5.96Dg-29/03_5 6.79 6.35 2.06 3.74Dg-29/03_6 6.15 6.36 1.82 4.52Dg-29/03_7 9.31 10.22 2.21 6.45Dg-29/03_8 7.14 6.63 1.89 4.47Dg-29/03_9 7.47 6.70 2.09 4.59Dg-29/03_10 5.09 4.37 1.50 3.31Dg-29/03_11 5.17 5.31 1.57 4.85Dg-29/03_12 4.62 4.82 1.71 4.42Dg-29/03_13 3.33 3.40 1.91 2.45Dg-29/03_14 3.36 3.49 1.78 3.47

a CPItotal — carbon preference index determined for full distribution of n-alkanes C23–C33,CPItotal=1/2[Σodd(n-C23−n-C33)/Σeven(n-C22−n-C32)+Σodd(n-C23−n-C33)/Σeven(n-C24−n-C34)] (Bray and Evans, 1961).

b CPI25–31=1/2[(n-C25+2n-C27+2n-C29+n-C31)/(n-C26+n-C28+n-C30)] (Marynowskiand Zatoń, 2010).

c OEP 1=1/4[(n-C21+6n-C23+n-C25)/(n-C22+n-C24)] (Scalan and Smith, 1970).d OEP 2=1/4[(n-C25+6 n-C27+n-C29)/(n-C26+n-C28)] (Scalan and Smith, 1970).


of C29Δ4-,Δ2- andΔ5-sterenes. C28Δ4-,Δ2- andΔ5-sterenes are identi-fied in trace amounts in few samples, whereas C27 homologues areabsent. The marked predominance of C29 sterenes (Fig. 10b; Table 4)clearly indicates peat formation from terrigenous plants. Kolubara lig-nite has low steroids/hopanoids ratio (b0.23; Table 4), which impliesa bacteria-influenced facies and suggests the role of microorganismsin degradation of plant tissue.

Hopanoids are important constituents of the Kolubara lignitesamples, representing the most abundant compounds in TIC of thesaturated fraction in samples Dg-29/03_7, Dg-29/03_8, Dg-29/03_10,Dg-29/06_11 and Dg-29/03_13 (Fig. 8).

Themost probable biological precursors of hopanoid biomarkers arebacteriohopanetetrol and 3-desoxyhopanes (Ourisson et al., 1979;

Table 4Parameters calculated from distribution and abundance of steroid and hopanoidbiomarkers.

Sample % C29a Ster/Hopb

C30Hop-17(21)-ene/C30αβ-Hopane

C30ββ/C30(ββ+αβ)c

n-Alkanes/Hopanoidsd

Dg-29/03_1 90.81 0.2254 9.68 0.66 1.33Dg-29/03_2 100.00 0.0032 0.41 0.81 0.47Dg-29/03_3 87.59 0.0052 0.00 0.74 0.42Dg-29/03_4 N.D. 0.0000 0.00 0.65 0.27Dg-29/03_5 100.00 0.0067 0.30 0.77 0.73Dg-29/03_6 100.00 0.0029 0.16 0.74 0.34Dg-29/03_7 N.D.e N.D. 0.00 0.72 0.30Dg-29/03_8 N.D. N.D. 0.00 0.67 0.09Dg-29/03_9 100.00 0.0008 0.00 0.76 0.22Dg-29/03_10 94.78 0.0031 0.65 0.84 0.09Dg-29/03_11 100.00 0.0028 0.14 0.77 0.16Dg-29/03_12 95.49 0.0044 0.20 0.78 0.13Dg-29/03_13 100.00 0.0002 0.00 0.67 0.09Dg-29/03_14 91.96 0.0081 0.24 0.80 0.08

a %C29=100×C29(Δ2+Δ4+Δ5)-Sterenes/Σ(C27–C29)(Δ2+Δ4+Δ5)-Sterenes.b Ster/Hop=[Σ(C27–C29)(Δ2+Δ4+Δ5)-Sterenes] / [Σ(C29–C32)17α(H)21β(H)-+

Σ(C29–C31)17β(H)21α(H)-+Σ(C29–C31)17β(H)21β(H)-+C2717α(H)-+C2717β(H)-Hopanes+Σ(C29–C31)Hop-17(21)-enes+C27-Hop-17(21)-ene+C27-hop-13(18)-ene](Peters et al., 2005).

c C30ββ/C30(ββ+αβ)=C3017β(H)21β(H)-Hopane/(C3017β(H)21β(H)-Hopane+C3017α(H)21β(H)-Hopane) (Mackenzie et al., 1981); Stereneswere quantified frommass chromatogramm/z 215, hopenes and hopanes were quan-tified from mass chromatogram m/z 191.

d n-Alkanes/Hopanoids— sum of n-alkanes divided by the sum of hopanoids quan-tified in the TIC (Olivella et al., 2006).

e N.D. — not determined due to the absence of sterenes.

Rohmer et al., 1992), identified in prokaryotic organisms (i.e. bacteria)and fungi. Chaffee et al. (1986) emphasized that for hopanes withfewer than 30 carbon atoms additional sources, such as ferns, lichensand mosses, are also possible.

The low n-alkanes/hopanoids ratio (Olivella et al., 2006), calculated asthe sum of n-alkanes divided by the sum of hopanoids quantified in theTIC (Table 4), denotes a significant contribution of bacterial input, partic-ularly for samples from lower parts of the seam. Identification of funginitein all samples (Table 2) and presence of ferns, confirmed by palynologicaldata (Pantić and Dulić, 1993), imply that hopanoid biomarkers in D fieldlignite could have also originated from fungi and ferns. However, on thebasis of mass chromatogram, m/z 243, fernenes were not identified inthe lignite extracts, with exception of one sample, which containedtrace amounts of these biomarkers. A negative correlation (r=−0.73;logarithmic curve; Fig. 11) was observed between the n-alkanes/hopanoids ratio (Table 4) and the gelification index (Table 2). The resultssuggest that gelification of plant tissue is related to increased microbialactivity.

Based on mass chromatogram, m/z 191 of the saturated fraction(Fig. 10c) the hopane composition in all samples is characterized bythe presence of 17α(H)21β(H), 17β(H)21α(H) and 17β(H)21β(H)compounds with 27–31 carbon atoms, with exception of C28 ho-mologues. Other hopanoid type constituents of saturated fractionare C27hop-13(18)-ene, C27hop-17(21)-ene, C30hop-17(21)-ene andC3217α(H)21β(H)22(R)-hopane. Several samples contain C31hop-17(21)-ene, C27hopan-21-one and C30hop-17(21)-en-20-one. Thepresence of these hopanoid ketones clearly reflects the suboxic tooxic, slightly acidic conditions prevailing during peatification (Burhanet al., 2002; Duan and Ma, 2001).

C3117α(H)21β(H)22(R)-Hopane dominates by far the hopane dis-tribution, being the most abundant compound in the saturated fractionof the samples Dg-29/03_7–Dg-29/03_11 and Dg-29/03_13 (Figs. 8 and10c). Prominent C31αβ(R) hopane is often reported in low rank coals(Stefanova et al., 2005a; Vu et al., 2009). According to van Dorselaer etal. (1975) high amount of C31αβ(R) hopane implies complex reactionsin acidic environments under oxic conditions. Killops et al. (1998)suggested that the decarboxylation of 31,32-bishomohopanoic acid,which is generally abundant in peats and soils (Quirk et al., 1984;Ries-Kautt and Albert, 1989), could result in the formation of C3117α(H)21β(H)-hopane in immature coals. Thiel et al. (2003) reported the pres-ence of 22R-bishomohopanoic acid in the “geological” 17α(H)21β(H)configuration in livingmicrobial mats in the Black Sea, supporting the as-sumption that the C31 αβ-hopane can be derived directly from microor-ganisms by decarboxylation. A direct bacterial origin is also possible forthe C29- and C31 17β(H)21α(H)-hopanes (Vu et al., 2009).

The contents of hopanes are considered to reflect the level of thedegradation of organic matter by aerobic bacteria, whereas for theC30hop-17(21)-ene a microbial origin from anaerobic (iron-reducing)bacteria is assumed, as suggested by Wolff et al. (1992). Extremely lowC30hop-17(21)-ene/C30 hopane ratio, with exception of sample Dg-29/03_1 (Table 4), predominance of C31αβ(R)-hopane and the presence

Fig. 11. Correlation between n-alkanes/hopanoids ratio and gelification index (GI).

image of Fig.�11


of hopanoid ketones (Fig. 10c) confirm the peatification in a suboxic tooxic environment for the lignite samples.

Domination of ββ-isomers in range C27 to C30 over αβ-hopanesand the presence of unsaturated hopenes are consistent with an im-mature stage of the organic matter. The ratio of 17β(H)21β(H) to(17β(H)21β(H)+17α(H)21β(H)) C30-hopanes is within the limitsestablished for lignite (0.5–0.7; Mackenzie et al., 1981) or even higher(Table 4).

The identification of D-ring monoaromatic hopane, 7-methyl,3′-ethyl, 1,2-cyclopentanochrysene and 4-methyl, 24-ethyl, 19-norcholesta-1,3,5(10)-triene (Philp, 1985) in the aromatic fraction ofall samples (Fig. 9) suggest partial aromatization of hopanoids andsteroids during diagenesis, which are consistent with pronouncedmicrobial activity and peatification in a suboxic to oxic environment.

4.4.4. Alkan-2-onesAcyclic alkan-2-ones are presented in all the samples, with exception

of sample Dg-29/03_14. n-Alkan-2-one homologues contain 31 and 33C-atoms are prominent even in the TIC of aromatic fraction (Fig. 9).Based on mass fragmentogram, m/z 58, n-alkan-2-ones are identifiedin the range of C23 to C35 (Fig. 10d). Similar to the n-alkane distribution,the n-alkan-2-one patterns of the lignite samples are dominated by oddlong-chain homologues, C29–C33. Although the distributions of then-alkanes and the n-alkanones in the samples are generally similar, amain difference could be seen. In almost all of the samples, n-alkaneshave maximum at C29, followed by C31, whereas the most abundantn-alkan-2-one is C31 followed by C33 or C29 (Fig. 10a, d). The results indi-cate that n-alkan-2-ones probably have few sources: a) direct contribu-tion of ketones from higher plant waxes, which consist of C23–C33homologues with a significant odd C number predominance andCmax at C29 or C31; b) microbially mediated β-oxidation of the corre-sponding n-alkanes in the sediments or prior to incorporation intothe sediments; c) oxidative decarboxylation of n-fatty acids and oxi-dized alcohols, as well as elongation of a suitable fatty acid precursorand subsequent decarboxylation, yielding longer-chain alkanones withpronounced odd C predominance and major constituents of C27, C29and C31 (Tuo and Li, 2005 and references therein). Recent investigations(Ortiz et al., 2011) also showed that microbial degradation can be amajor contributor to the n-alkan-2-one distribution in sediments, ratherthan a direct input of ketones from plants.

In addition to n-alkan-2-ones, 6,10,14-trimethylpentadecan-2-one(6,10,14-TMPD-2-one), an isoprenoidal ketone was also found in mostof the samples studied (Fig. 10d). 6,10,14-TMPD-2-one is quite commonin nature, occurring widely in sediments (de Leeuw et al., 1977; Ikan etal., 1973) and in particulate organic matter collected from the watercolumn (Rontani et al., 1992). Since pristane and phytane were absentin most of the samples, it could be argued that the main pathways of6,10,14-TMPD-2-one formation imply bacterial degradation (Brooksand Maxwell, 1974; Brooks et al., 1978) and photosensitized oxidation(Rontani and Giral, 1990) of free phytol and/or photodegradation ofchlorophyll-a (Rontani et al., 1991).

Therefore, alkan-2-one distribution in lignite extracts from the Mainseam confirms that diagenetic changes of the organicmatterweremainlygovernedby bacterial activity in a suboxic to oxic environment, consistentwith conclusions derived from interpretation of hopanoid biomarkersand pronounced aromatization of angiosperm derived triterpenoids(see Section 4.4.5.).

4.4.5. Sesquiterpenoids, diterpenoids, and triterpenoids with non-hopanoidskeleton

In all samples, aromatic sesquiterpenoids occur at low quantities(Fig. 9). Cadalene predominates over isocadalene, calamenene and5,6,7,8-tetrahydro-cadalene. Other sesquiterpenoid constituents of thelignite extracts are cuparene, cadina-1(10),6,8-triene and 1,2,3,4-tetrahydronaphthalene, whereas dihydro-ar-curcumene was identifiedonly in three samples. Sesquiterpenoids are used as markers for higher

land plants because they occur in their resinous material. However,sesquiterpenoid biomarkers are often not useful for a precise determi-nation of the precursor plant community (Otto and Simoneit, 2002;Otto et al., 1997; van Aarssen et al., 1990), with exception of cuparene.The presence of cuparene in lignite from the D field (Fig. 9) clearly indi-cates contribution of conifers family Cupressaceae, and family generaCupressus, Thuja and Juniperus as precursors to OM (Haberer et al.,2006; Otto and Wilde, 2001), which is consistent with palynologicaldata.

Diterpenoids are main constituents of both the saturated and aro-matic fraction indicating significant contribution of gymnosperms tothe precursor OM. 16α(H)-Phyllocladane and pimarane are by fardominant in the saturated fractions. Other diterpenoid type constituentsof saturated fraction are isopimarane, norpimarane, norisopimarane,16α(H)-kaurane, 16β(H)-phyllocladane, norabietane, isopimaradiene,pimaradiene and abieta-8,11,13-trien-7-one (Fig. 8). A high amount of16α(H)-phyllocladane indicates that the lignite forming plants inpart belonged to the conifer families Taxodiaceae, Podocarpaceae,Cupressaceae, Araucariaceae, Sciadopityaceae and Phyllocladaceae,while the high abundance of pimarane suggests Pinaceae, Taxodiaceaeand Cupressaceae (Otto and Wilde, 2001; Otto et al., 1997; Stefanovaet al., 2002, 2005b). The higher proportion of pimarane than 16α(H)-phyllocladane in several samples, particularly Dg-29/03_4 and Dg-29/03_6 (Table 5) could imply higher impact of Pinaceae to the precursorbiomass, especially as their abundance was confirmed by previous pal-ynological investigation (Pantić and Dulić, 1993).

The aromatic diterpenoids consist of norabieta-6,8,11,13-tetraenes, norabieta-8,11,13-trienes, 2-methyl-1-(4′-methylpentyl),6-isopropylnaphthalene, dehydroabietane, simonellite, retene,sempervirane, totarane, hibaene, ferruginol, 6,7-dehydroferruginol,tetrahydroretene and 2-methylretene, with simonellite anddehydroabietane predominating in all of the samples (Fig. 9).

Almost all of the aromatic diterpenoids are non-specific conifermarkers, because they are the diagenetic products of a great varietyof abietane-type precursors that are common constituents of all coni-fers except Phyllocladaceae (Otto and Simoneit, 2001; Otto et al.,1997; Stefanova et al., 2005b). On the other hand, the presence offerruginol, totarane and hibaene in the aromatic fraction of the Dfield lig-nite extracts (Fig. 9) clearly indicates the contribution of Cupressaceae,Taxodiaceae, Podocarpaceae and Araucariaceae to precursor biomass(Otto and Wilde, 2001). This is consistent with the observation derivedfrom analysis of saturated biomarkers.

Abietic acid and dehydroabietane are probably the biological pre-cursors of the saturated and aromatic abietanes. These compounds,however, are not the only source of abietane type diterpenoids inthe geosphere, as numerous other abietane products are present inmodern resins (Wakeham et al., 1980). For example, the origin ofdehydroabietane, simonellite and retene probably partly associateswith phenolic abietanes such as ferruginol, sugiol and hinokiol, wide-spread inmodern species of the families Taxodiaceae, Cupressaceae andPodocarpaceae (Otto and Wilde, 2001). Pimarane- and phyllocladane-derivatives are also known as possible precursors for aromaticabietane-type diterpenoids (Alexander et al., 1987; Wakeham et al.,1980). Transformations of pimaranes and phyllocladanes, which leadto the formation of dehydroabietane and other aromatic diterpenoids,are favored under acidic conditions and catalyzed by clay mineralsand, therefore, could be expected to be generated during Kolubaralignite formation.

Under alkaline anaerobic conditions reductive processes, i.e. de-carboxylation and hydrogenation of resin acids are dominant. The de-carboxylation of the abietic acid followed by reduction leads to thesaturated norabietane and abietane. Abietane has not been identifiedin the lignite extracts, whereas norabietane is present in relatively lowquantity. Therefore, as mentioned previously (abundant telohuminitemacerals and C31αβ(R)-hopane, and presence of hopanoid ketonesand alkan-2-ones), it is suggested that the dominant alteration pathway


was the oxidation of abietic acid to dehydroabietic acid. Its decarbox-ylation generates norabieta-8,11,13-triene, which is identified in allsamples. Further aromatization leads to 1,2,3,4-tetrahydroreteneand finally to retene, which is observed in all lignite extracts(Figs. 9 and 12). The presence of abieta-8,11,13-trien-7-one (Fig. 8)in all samples suggests another possible degradation pathway ofdehydroabietic acid under oxic conditions. It considers in the first step,as proposed by Otto and Simoneit (2002), oxidation of dehydroabieticacid to 7-oxo-dehydroabietic acid, followed by decarboxylation gener-ating abieta-8,11,13-trien-7-one or norabieta-8,11,13-trien-7-one. Fur-ther alteration of abieta-8,11,13-trien-7-one generates simonellite,which is the most abundant aromatic diterpenoid in all samples(Figs. 9 and 12). Possible products formed by degradation of norabieta-8,11,13-trien-7-one are norabieta-8,11,13-triene, norabieta-6,8,11,13-tetraene, tetrahydroretene and finally retene. Simonellite and2-methyl, 1-(4′-methylpentyl), 6-isopropylnaphthalene, could alsooriginate from direct natural product, dehydroabietane (Fig. 12).These three compounds are observed in the aromatic fraction of allsamples in relatively high proportion (Fig. 9). Another possible pathwayfor the formation of abieta-8,11,13-trien-7-one could be oxidation ofdehydroabietane (Fig. 12).

The nonhopanoid triterpenoids in saturated fraction consist ofolean-12-ene, des-A-olean-enes, des-A-urs-enes and des-A-lupane.These compounds are presented in relatively low amounts (Fig. 8).Marked domination of des-A-ring degraded compounds is observed,and only in four samples non-degraded oleanenes are identified. Deg-radation of triterpenoids' A-ring implies intense microbial activity.

Although the nonhopanoid triterpenoids represent a minor compo-nent of saturated fraction, these compounds are abundant in aromaticfraction of coal extracts (Fig. 9). This result shows that angiosperms alsocontributed to the organic matter. Considerably higher abundance ofaromatized in comparison to non-aromatized angiosperm triterpenoidsindicates intense aromatization of triterpenoids during diagenesis. The

Table 5Parameters calculated from distribution and abundance of diterpenoids and triterpenoids, a

Sample Pimarane/16α(H)-Phyllocladane Di/(Di+Tri) sata Di/(Di+Tri) arom

Dg-29/03_1 1.51 0.9591 0.34Dg-29/03_2 0.13 0.9743 0.45Dg-29/03_3 0.14 0.9977 0.47Dg-29/03_4 4.44 1.0000 0.41Dg-29/03_5 0.03 1.0000 0.83Dg-29/03_6 2.67 0.9997 0.68Dg-29/03_7 0.44 1.0000 N.D.Dg-29/03_8 1.05 1.0000 0.55Dg-29/03_9 0.78 0.9978 0.67Dg-29/03_10 1.05 0.9760 0.52Dg-29/03_11 0.08 0.9693 0.65Dg-29/03_12 0.03 0.9970 0.52Dg-29/03_13 1.23 1.0000 0.76Dg-29/03_14 0.14 0.9749 0.70

a Di/(Di+Tri)sat=ΣDiterpenoids/(ΣDiterpenoids+ΣTriterpenoids), calculated from thPhyllocladane+Pimarane+Isopimarane+Norpimarane+Norisopimarane+16α(H)-Kaurane+(Olean-12-ene+Olean-13(18)-ene+Des-A-olean-12-ene+Des-A-olean-13(18)-ene+Des-A-ol

b Di/(Di+Tri)arom=ΣAromatic diterpenoids/(ΣAromatic diterpenoids+ΣAromatic triterpenoΣAromatic diterpenoids=(18-Norabieta-6,8,11,13-tetraene+19-Norabieta-8,11,13-triene+1Dehydroabietane+Simonellite+Retene+Sempervirane+Totarane+Hibaene+Ferruginotriterpenoids=(24,25-Dinoroleana-1,3,5(10),12-tetraene+24,25-Dinoroleana-1,3,5(10),12,14Pentamethyldecahydrochrysene+3,4,7,12a-Tetramethyl-1,2,3,4,4a,11,12,12a-octahydrochrysenetetrahydrochrysene+3,3,7-Trimethyl-1,2,3,4-tetrahydrochrysene+1,2,4a,9-Tetramethyl-1,2,3,4,1,2,9-Trimethyl-1,2,3,4-tetrahydropicene+2,2,9-Trimethyl-1,2,3,4-tetrahydropicene+1c 1-Ar ring diterpenoids=(18-Norabieta-6,8,11,13-tetraene+19-Norabieta-8,11,13-triene

fraction, 2-Ar ring diterpenoids=(2-Methyl, 1-(4′-methylpentyl), 6-isopropylnaphthalene+diterpenoids=(Retene+2-Methylretene), calculated from the TIC of aromatic fraction (Habe

d 1-Ar ring triterpenoids=(24,25-Dinoroleana-1,3,5(10),12-tetraene+24,25-Dinorolean1,3,5(10)-triene), calculated from the TIC of aromatic fraction, 3-Ar ring triterpenoids=(1,2,4a,9octahydropicene), calculated from the TIC of aromatic fraction, 4-Ar ring triterpenoids=(1,21,2,9-Trimethyl-1,2-dihydropicene), calculated from the TIC of aromatic fraction.e Alkan-2-ones (%) — relative content of alkan-2-ones in the aromatic fraction.

same observation was also reported by Kalkreuth et al. (1998) andNakamura et al. (2010), which showed that aliphatic angiosperm-derived triterpenoids are more easily altered to aromatic derivativesthan gymnosperm-derived diterpenoids, resulting in the selectiveloss of analogous aliphatic compounds. Pronounced aromatizationof triterpenoids is consistent with intense microbial activity duringpeatification in a suboxic to oxic environment.Moreover, aromatizationmay also be mediated by clay-catalytic processes (Rubinstein et al.,1975; Sieskind et al., 1979).

The following aromatic tetra- and pentacyclic triterpenoidsoccur in the aromatic hydrocarbon fractions: ring-A-monoaromatictriterpenoids (24,25-dinoroleana-1,3,5(10),12-tetraene, dinoroleana-1,3,5(10),12,14-pentaene, 24,25-dinorursa-1,3,5(10),12-tetraene,24,25-dinorlupa-1,3,5(10)-triene), pentamethyldecahydrochrysene,tetramethyloctahydrochrysenes, trimethyltetrahydrochrysenes,tetramethyloctahydropicenes, trimethyltetrahydropicenes,dimethyltetrahydropicene and triaromatic des-A-lupane (Fig. 9). In allsamples pentacyclic triterpenoids are notably more abundant thantetracyclic chrysene derivatives. This result shows that the main path-way of aromatization was progressive aromatization (Stout, 1992)and explains the low abundance of pentacyclic triterpenoids in the sat-urated fraction. The most prominent peaks are ring-A-monoaromatictriterpenoids, which confirms its relatively high stability observed alsoin earlier investigations (Stefanova et al., 2005a; Stout, 1992).

Due to enhanced aromatization of angiosperm derived triterpenoids,the ratio of diterpenoids to sum of di- and terprenoids in saturated frac-tion, Di/(Di+Tri)sat (Bechtel et al., 2002, 2003) shows extremely highand uniform values (above 0.95) indicating that conifers nearly ex-clusively contributed to lignite formation (Fig. 5; Table 5). Therefore,in order to estimate the contribution of gymnosperm and angiospermvegetation in the ancient peat bogs we have used the ratio of diterpenoidand angiosperm-derived triterpenoid aromatic biomarkers (Di/(Di+Tri)arom; Haberer et al., 2006; Nakamura et al., 2010; Fig. 5; Table 5).

nd relative content of alkan-2-ones in the aromatic fraction.

b 1-Ar ring/(1+2+3-Ar rings)diterpenoidsc

1-Ar ring/(1+3+4-Ar rings)triterpenoidsd

Alkan-2-ones(%)e

0.41 0.90 0.990.34 0.59 6.130.38 0.33 9.860.36 0.41 8.980.27 0.45 4.220.32 0.41 7.25N.D. N.D. N.D.0.43 0.35 1.450.31 0.29 4.410.31 0.43 4.950.46 0.43 4.260.38 0.52 6.140.66 0.59 0.410.26 0.51 0.00

e TIC of saturated fraction (Bechtel et al., 2002, 2003), ΣDiterpenoids=(16α(H)-16β(H)-Phyllocladane+Norabietane+Isopimaradiene+Pimaradiene), ΣTriterpenoids=ean-18-ene+Des-A-urs-13(18)-ene+Des-A-urs-12-ene+Des-A-lupane).ids), calculated from the TIC of aromatic fraction (Haberer et al., 2006;Nakamura et al., 2010),8-Norabieta-8,11,13-triene+2-Methyl, 1-(4′-methylpentyl), 6-isopropylnaphthalene+l+6,7-Dehydroferruginol+2-Methylretene+1,2,3,4-Tetrahydroretene), ΣAromatic-pentaene+24,25-Dinorursa-1,3,5(10),12-tetraene+24,25-Dinorlupa-1,3,5(10)-triene++3,3,7,12a-Tetramethyl-1,2,3,4,4a,11,12,12a-octahydrochrysene+3,4,7-Trimethyl-1,2,3,4-4a,5,6,14b-octahydropicene+2,2,4a,9-Tetramethyl-1,2,3,4,4a,5,6,14b-octahydropicene+,2,9-Trimethyl-1,2-dihydropicene+Triaromatic des-A-lupane).+18-Norabieta-8,11,13-triene+Dehydroabietane), calculated from the TIC of aromaticSimonellite+Tetrahydroretene), calculated from the TIC of aromatic fraction, 3-Ar ringrer et al., 2006).a-1,3,5(10),12,14-pentaene+24,25-Dinorursa-1,3,5(10),12-tetraene+24,25-Dinorlupa--Tetramethyl-1,2,3,4,4a,5,6,14b-octahydropicene+2,2,4a,9-Tetramethyl-1,2,3,4,4a,5,6,14b-,9-Trimethyl-1,2,3,4-tetrahydropicene+2,2,9-Trimethyl-1,2,3,4-tetrahydropicene+

Fig. 12. Proposed schematic pathway for oxidative diagenetic transformation of abietic acid and dehydroabietane. (Few steps are adopted from Otto and Simoneit, 2001, 2002;Stefanova et al., 2005b). Compounds with an asterisk (*) were detected in coals from the D field, Kolubara basin; compounds in boxes represent natural product precursors.


The lower values of this ratio in upper part of the seam indicate in-creasing proportion of angiosperms within the peat-forming vegeta-tion (Table 5). Variations in Di/(Di+Tri)arom with depth (Fig. 5;Table 5) could be attributed to the pronounced seasonality, whichcaused changes of water column level, redox potential (Eh) in thepaleoenvironment and differences in vegetation of the paleo-plantcommunities, consistent with conclusions derived from GI ratio. Posi-tive correlation of Di/(Di+Tri)arom ratio (Table 5) with GI (Table 1)(r=0.65 for linear and 0.73 for logarithmic curve; Fig. 13, sampleDg-29/03_5 was excluded from the diagram) suggests that loweringthe water column level, i.e. more oxidative conditions were more suit-able for angiosperms.

The 1-Ar ring/(1+2+3-Ar rings) diterpenoids ratio, which is usedfor an assessment of the degree of aromatization (Haberer et al., 2006)has a relatively uniform value in the lignite samples, with exception ofone sample. Here, no relationship between increasing diagenetic alter-ation and increasing depth is detectable (Fig. 5; Table 5); which is notbe unexpected for a sedimentary section of only 150 m. Corresponding1-Ar ring/(1+3+4-Ar rings) triterpenoids ratio (according to pathwayof progressive aromatization), proposed in this study, show also relativeuniform values (Fig. 5; Table 5). Considering low degree of the OM

Fig. 13. Correlation between Di/(Di+Tri)arom ratio and gelification index (GI).

maturity changes of 1-Ar ring/(1+2+3-Ar rings) diterpenoids and1-Ar ring/(1+3+4-Ar rings) triterpenoids ratios with depth probablyimply changes in Eh potential of the depositional environment and/orbacterial activity, rather than changes due to thermal alteration. Inaddition, the influence of clay-catalytic processes on aromatizationcannot be ruled out. Negative correlation observed between 1-Arring/(1+2+3-Ar rings) diterpenoids and 1-Ar ring/(1+3+4-Arrings) triterpenoids, and relative proportion of alkan-2-ones in aromatic

Fig. 14. Correlations between 1-Ar ring/(1+2+3-Ar rings) diterpenoids (a) and 1-Arring/(1+3+4-Ar rings) triterpenoids (b) ratios with relative proportion of alkan-2-onesin aromatic fraction (%).

image of Fig.�13

image of Fig.�14

image of Fig.�12


fraction, suggests that aromatization processes are preferably attributedto microbial activity under suboxic to oxic conditions (Fig. 14).

5. Conclusions

The petrographic analysis of lignite samples collected from theMainseam, D field of Kolubara basin shows that huminite is the prevailingmaceral group, with a relatively high content of liptinite and inertinite.The most abundant macerals of the huminite group are: textinite,ulminite, densinite and attrinite, and of the liptinite group: liptodetriniteand sporinite. Inertodetrinite, fusinite, funginite and semifusinite are themost common macerals of the inertinite group. Clay minerals predomi-nate, while pyrite and carbonates are less abundant.

Main sources of organic matter were gymnosperms (conifers) andmicrobial biomass followed by angiosperms. Based on composition ofsaturated and aromatic diterpenoids it has been established that coalforming plants belonged to the gymnosperm families Taxodiaceae,Podocarpaceae, Cupressaceae, Araucariaceae, Phyllocladaceae andPinaceae.

The results of petrological and organic geochemical examinationssuggest that the lignite from the D field of the Kolubara basinformed in a slightly acid to neutral deposition environment. Abun-dant hopanoids, predominance of C31αβ(R)-hopane with extremelylow C30hop-17(21)-ene/C30hopane ratio and the presence of hopanoidketones and alkan-2-ones in the lignite extracts indicate that diageneticchanges of the organic matter were mainly governed by bacterial activ-ity in a suboxic to oxic environment, which caused an intense progres-sive aromatization of angiosperm-derived triterpenoids and partialaromatization of hopanoids and steroids during diagenesis. Based onthe distribution of aromatic diterpenoids a novel diagenetic pathwayfor transformation of abietane-type precursors under suboxic to oxicconditions is proposed.

The vertical variations of ash yield, sulfur content, maceral com-position, TPI and GI ratios, as well as the ratio of diterpenoid andangiosperm-derived triterpenoid aromatic biomarkers of lignite, pointto the changes in pH values andwater level in themire during the accu-mulation of organicmatter. Pronounced seasonality caused the changesin thewater table, which resulted in changes of the redox potential (Eh)during peatification as well as vegetation differentiations in the paleo-plant communities.

Group organic geochemical parameters, distribution of n-alkanes,domination of ββ-isomers in the range of C27 to C30 over αβ-hopanesand the presence of unsaturated hopenes are consistent with an imma-ture stage of the organicmatter (phase of intense diagenetic processes).

Acknowledgments

This work was financed by the Ministry of Education and Scienceof the Republic of Serbia (Projects Nos. 176006 and 176016), whichis gratefully acknowledged. We are also grateful to Prof. Dr. DeolindaFlores, Dr. Hamed Sanei, Prof. Dr.WolfgangKalkreuth and an anonymousreviewer whose helpful suggestions and comments greatly benefitedthis paper. Authors also gratefully acknowledge the MontanuniversitätLeoben, Department Angewandte Geowissenschaften und Geophysik(Austria) for the technical support.

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