GEOLOGIJA 48/1, 83–95, Ljubljana 2005 Stable isotope geochemistry of different lithotypes of the Velenje lignite (Slovenia) Geokemija stabilnih izotopov razli~nih litotipov velenjskega lignita Tja{a KANDU^1, Milo{ MARKI^2 & Jo‘e PEZDI^3 1Jo`ef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia, tjasa.kanduc@ijs.si 2Geological Survey of Slovenia, Dimi~eva 14, SI-1000 Ljubljana, Slovenia, milos.markic@geo-zs.si 3Department of Geology, Faculty of Natural Sciences and Engineering, University of Ljubljana, A{ker~eva 12, SI-1000 Ljubljana, joze.pezdic@ntfgeo.uni-lj.si Key words: carbon isotopes, nitrogen isotopes, Velenje lignite, microbial activity Klju~ne besede: ogljikovi izotopi, du{ikovi izotopi, velenjski lignit, mikrobna aktivnost, Abstract The stable isotope composition of carbon and nitrogen in low-rank coals is often used as a supplementary method to assess coal-forming processes in different paleoenviron-ments. In this study, carbon and nitrogen isotopes were investigated in different macroscopic varieties i.e. lithotypes, of the Velenje lignite. ?13C and ?15N values were determined in 47 samples. The quantity and petrographical variability of the samples is considerably higher than in previous studies. ?15N characterization of the Velenje lignite is presented in this paper for the first time. It was found that ?13C and ?15N values of different lignite lithotypes were influenced by original isotopic heterogeneity of the source plant ingredients, and by biogeochemical processes (gelification, mineralization of organic matter) at the early stage of biomass accumulation and its early diagenesis. Povzetek V premogih nizke stopnje karbonizacije uporabljamo stabilne izotope ogljika in du{ika kot dopolnilno metodo pri dolo~evanju procesov nastanka premogov v razli~nih paleooko-ljih. V na{i raziskavi smo dolo~ili stabilne izotope ogljika in du{ika v 47 vzorcih razli~nih litotipov velenjskega lignita. Preiskali smo znatno ve~je {tevilo vzorcev kot v predhodnih raziskavah in prvi~ doslej smo dolo~ili ?15N vrednosti. Ugotovili smo, da na vrednosti ?13C in ?15N razli~nih litotipov lignita vplivajo tako izotopske zna~ilnosti prvotnih rastlinskih komponent kot biogeokemi~ni procesi (gelifikacija, mineralizacija organske snovi) v zgodnji fazi akumulacije biomase in zgodnji diagenezi. Introduction The Velenje lignite is palinologically characterized by the predominance of gymno-sperms over angiosperms ([ercelj, 1985/ 86, H e m leben et al., 2000). Recent investigations of biomarkers in the Velenje lignite (B e c h t e l et al., 2003) indicate that gelifi-cation of plant tissues might be governed by the activity of anaerobic rather than by aerobic bacteria. They also indicate that biogeo-chemical decomposition of plant tissues by bacteria affected ?13C values of the lignite. The isotopic composition of carbon in low– rank coals may be used to reconstruct changes in the global carbon cycle, as well as climatic changes (Lücke et al., 1999, Arens et al., 2000). Plants use atmospheric CO2 to produce 84 Tja{a Kandu~, Milo{ Marki~ & Jo‘e Pezdi~ carbohydrate (CH2O) during photosynthesis by three pathways: the C3 or Calvin pathway, the C4 or Hatch–Slack pathway, and CAM cycle photosynthesis. The amount of fractionation depends on the pathway followed. In all cases, photosynthetic uptake of CO2 is accompanied by significant depletion in 13C. Organic matter produced from atmospheric CO2 (?13C ? -7 ‰) by land plants using the C3 pathway consequently has an average ?13C of -27 ‰ (VPDB), and by the C4 pathway of about -14 ‰ (O’Lary , 1988). It is difficult to determine the specific process governing ?13C of bulk organic matter since different environmental factors such as atmospheric CO2, the type of plant material, bacterial degradation and temperature of formation can influence the ?13C value of coal (Duponey et al., 1993, Schleser , 1995). Organic nitrogen in sedimentary organic matter represents the transfer of nitrogen from the atmosphere to the lower crust. ?15N of bulk organic matter in sediments depends on ?15N of atmospheric nitrogen, the source of the organic matter and on biogeochemical processes such as ammonification, nitrification and denitrification. However, the fate of organic nitrogen and denitrogenation during early and late stages of diagenesis is still poorly understood (Ader et al, 1998). ?15N can be used to distinguish between either prevailingly algal or land–plant sources of the host organic matter. It was shown (Peterson & Howarth, 1987) that the nitrogen reservoir difference is preserved in the isotopic contents of organic matter produced by plankton (?15N of + 8.6 ‰) and C3 plants (?15N of + 0.4 ‰) in an estuarine ecosystem. In comparison to previous isotopic studies of the Velenje lignite (Pezdi~ et al., 1998; B e c h t e l et al., 2003) the present one embraces spatially much more widely distributed samples, both vertically and laterally. Since data on ?15N for world–wide coals are quite scarce, ?15N for the Velenje lignite were determined for the first time. Sampling of Figure 1. Geological map of the Velenje lignite basin (simplified after B r e z i g a r et. al., 1987). Stable isotope geochemistry of different lithotypes of the Velenje lignite 85 the lignite was carried out on the basis of its macropetrographic heterogeneity described in terms of lithotypes. Correlation between the lithotypes and their isotopic composition, both resulting from specific vegetal precursors and specific peat/lignite-forming processes, was the main target of this study. Geological setting and lignite formation The Velenje basin is situated in the NE part of Slovenia (Fig. 1). It is located between the WNW–ESE trending [o{tanj fault and the E–W trending Smrekovec fault, which represents a segment of the dextral strike-slip Periadriatic lineament. The Periadriatic lineament was formed due to collision of the Adriatic and the European continental plates 35 Ma ago (Fodor et al., 1998). In the Oligocene, the heavier oceanic plate sank into the mantle. Due to partial melting and fracturing, tonalite intrusions and andesitic extrusions with corresponding volcanicla-stics were generated and deposited in the wider area. Pre-Oligocene rocks of the Velenje area consist mainly of Triassic limestones and dolomites, whereas the Oligo-Mi-ocene non-volcanic sediments consist of differently lithified marls, silts, clays and sands. In Pliocene times, tectonic activity gradually ceased. Sedimentary filling of the intermontane depression started with terrigenous coarse clastics (Fig. 2) of the predominantly high energy fluvial environment. Due to the energy decrease of the depositio-nal environment, accumulation of coarse clastics was gradually replaced by finer cla-stics. As a final result of this tecto-sedimen-tary process, a peat-forming marshy environment was established. Decimetre to metre thick beds of mineral-rich lignite and clay-stones represent the record of the initial marshy sedimentation. Due to a further decrease in inorganic input, more and more clean peat was accumulated giving rise to what we know at present as the Velenje lignite seam. Taking into account the maximum lignite seam thickness of 160 m (Brez-igar , 1985/86), an estimated 2:1 peat-to-lignite compaction ratio and 1 mm peat growth per year (Taylor et al., 1998), it can be roughly concluded that the time of peat accumulation lasted for about 300,000 ye- ars. Considerable ash contents (15–40 weight % on a dry basis; increasing with depth) and the petrographic characteristics of the lignite indicate that the original peat-forming environment was in general topogeno-us, with alternations between the following vegetation subenvironments (based on pe-trographic indices): wet forest swamp, dry forest swamp, bush moor, and fen (Marki~ & Sachsenhofer, 1997). As known from palinological investigations ([ercelj , 1985/ 86, H e m l e b e n et al., 2000) Taxodium, Sequoia, Osmunda and Tsuga floristic species were the most important contributors to the forest and bush vegetation. Sulphur contents (Stot, dry basis,) in the Velenje lignite mostly vary between 2 and 3 wt. % (Marki~ & Sachsenhofer, 1997). In the case of freshwater peats, such high S contents are known for Ca-rich environments characterized by neutral or even alkaline pH values (up to Figure 2. Typical stratigraphic column of the Velenje basin sedimentary fill (after B r e z i g a r 1985/86). 86 Tja{a Kandu~, Milo{ Marki~ & Jo‘e Pezdi~ 8.6) (T a y l o r et al., 1998). Calcite replacement and incrustations of vegetal remnants, as well as increased CaO contents in the low-ash Velenje lignite samples (recent investigations) provide support to this thesis. It is well known that alkaline environments promote a high bacterial activity, crucial for reduction of sulphates to sulphides (formation of marcasite/pyrite), as well as for biochemical degradation (gelification) of organic matter. Variability in vegetal precursors and co-alification processes is reflected in the petrography of the lignite. It is characterized by different ratios of xylitic fragments of very different dimensions compared to fine detrital matrix. Fusinitic particles can be mostly observed as incrustations over xyli-tic lumps. Mineral-rich varieties of fine de-trital lignite occur especially at the bottom of the seam. Quite common are xylitic fragments replaced by epigenetic calcite. Such forms occur in separate layers, but are particularly frequent in the close vicinity of faults and fractures (Vrabec, personal communication). Fine detrital lignite is gelified to different degrees. If nongelified, it is dark brown, whereas it is black or even brittle, if strongly gelified. Gelification increases toward the top of the seam (Marki~ & Sach-senhofer , 1997). However, it is interesting that gelified xylites occur in the lower part of the seam, whereas they are almost absent in the upper part. Peat accumulation, the thickest in the basin centre and pinching out toward its periphery, was terminated due to its deeper submersion below the water surface. Vegetation development was replaced by a lacustrine sedimentation of marls, clays and silts, sporadically interrupted by fluvial sedimentation of sands and gravels (Brezigar , 1985/86). Accommodation space for the post-peat sedimentation was mainly governed by the syn-sedimentary activity of the [o{tanj and Smrekovec faults. The total Pliocene to Holocene sedimentary fill of the basin reaches more than 1000 metres. Dislocations of the lignite seam (up to 70 m) and of the Pliocene to Plio-Quaternary (Villafranchian) hanging-wall strata with the greatest effects along the dextral strike-slip [o{tanj fault and obliquely related ac- Figure 3. Map of sampling locations (heights relative to the sea level are cited in parentheses); borehole j.v. 3123-7 coincides with boreholes D70 (1-4), L45 (2), 115 (1-4). Stable isotope geochemistry of different lithotypes of the Velenje lignite 87 companying faults, indicate a strong tectonic activity of the area in its very young geological history (Brezigar, 1985/86; Brezigar et al., 1987; Vrabec, 1999). Experimental Lignite samples, representing different lithotypes, were taken from underground roadways ahead of the working faces. The lateral (an area of ca 2 x 2.5 km2) and vertical (from -128.7 to +110.0 m height a.s.l.) distribution of the samples is plotted on the mining-plan map in Fig. 3. In the W part of the mine-workings, at the locality of a gas outburst in February 2003 (long-wall lignite excavation at the level of -90), numerous samples (18) were taken from the JV 3123-7, JV 115(1-4), L45(2) and D70(1-4) boreholes, which were drilled immediately after the above mentioned event. On a given scale of Fig. 3, all these boreholes coincide. A macroscopic description of the lignite samples in terms of lithotypes was carried out following the criteria of M a r k i ~ at al. (2001) (Fig. 4). The following lithotypes were distinguished (with number of samples): xylite (9), poorly gelified fine-detrital lignite (16), moderately gelified fine-detrital lignite (9), strongly gelified fine-detrital lignite (11), fusinite and semidegradofusinitic lignite (2) (Tab 1). In the later two cases, fusinite represents an encrustation over xyli-te, whereas the semidegradofusinitic (semi-fusinitic) designation derives from the microscopic inspection. Heterogeneous lithotypes characterized by different ratios between fine detrital matrix and xylitic fragments (lithotypes coded by values from 4 to 9 in Fig. 4) were not analysed as whole samples. In such cases (samples B93, B97 and B111 in Tab. 1) only their fine detrital matrix was analysed. As possible lignite precursors, some representative parts of recent plants (trunk of a Figure 4. Right: Lithotype classification for the Velenje lignite based on different ratios (vol. %) between lithotype components; X - xylite, fD - fine detrite, XD – xylo-detrite, G – gelite (from Marki~ et al., 2001). Left: characteristic micropetrographic components from textinite (xylinite) to detrinite at different magnifications (50 to 200 ×; optic, and SEM - scaning electron microscopy). 88 Tja{a Kandu~, Milo{ Marki~ & Jo‘e Pezdi~ Table 1. Macroscopic description and ?15N and ?13C values of the analyzed Velenje lignite samples Num. Sample Macroscopic description S^Cvpue (%o) S^Njuj. (%o) 1 B91 2 B92 3 B93 4 B94 5 B95 6 B96 7 B97 8 B98 9 B99 10 B100 11 B102 12 B103 13 B104 14 B105 15 B106 16 B107 17 B108 18 B109 19 B110 20 B111 21 B112 22 B113 23 B116 24 B117 25 B118 26 B119 27 B120 28 B121 29 B122 30 JV 3123-7 31 JV 3123-7 32 JV 3123-7 33 JV 3123-7 34 JV 3123-7 35 JV 3123-7 36 JV 3123-7 37 JV 3123-7 38 JV 3123-7 39 JV 115(-90)1 40 JV 115(-90)3 41 JV 115(-90)2 42 JV 115(-90)4 43 L 45 (2) 44 D 70 (2) 45 D 70 (1) 46 D 70 (3) 47 D 70 (4) poorly gelified detrital lignite lamellar xylite strongly gelified fine detrital lignite with xylite inclus. moderately gelified black fine detrital lignite poorly gelified detrital lignite moderately gelified detrital lignite strongly gelified fine detrital lignite with xylite inclus. xylite greyish black xylite strongly gelified detrital lignite strongly gelified fine detrital lignite (mineral-rich) xylite + fusinite moderately gelified fine detrital lignite moderately gelified fine detrital lignite brown xylite poorly gelified fine detrital lignite poorly gelified fine detrital lignite xylite poorly gelified black detrital lignite poorly gelified detrital lignite with xylite inclusions poorly gelified black fine detrital lignite strongly gelified fine detrital lignite strongly gelified black detrital lignite poorly gelified detrital lignite poorly gelified detrital lignite moderately gelified black fine detrital lignite moderately gelified fine detrital lignite poorly gelified black fine detrital lignite poorly gelified blackish - brown fine detrital lignite xylite, 0.3 m poorly gelified detrital lignite, 0.5 m semidegradofusinitic lignite,1.8 m xylite, 2.5 m xylite, 7.1 m moderately gelified detrital lignite, 8.3 m poorly gelified detrital lignite, 9.8 m strongly gelified detrital lignite, 12.3 m xylite, 14 m poorly gelified detrital lignite strongly gelificated detrital lignite poorly gelified detrital lignite moderately gelified detrital lignite strongly gelified detrital lignite, 2.5 m strongly gelified detrital lignite, 7.8m strongly gelified detrital lignite, 2.5 m poorly gelified detrital lignite, 13.5 m moderately gelified detrital lignite, 11.5 m -26.8 4.0 -26.5 4.4 -27.6 2.5 -27.7 2.9 -27.1 2.8 -27.8 3.0 -27.4 2.0 -26.6 3.2 -27.2 2.5 -28.7 2.0 -28.2 2.0 -24.7 4.0 -27.9 3.8 -27.8 3.3 -25.6 3.5 -26.9 4.2 -27.4 4.2 -25.3 2.1 -27.1 4.0 -26.8 2.6 -26.8 3.1 -28.0 2.4 -28.0 2.2 -26.8 3.5 -27.2 3.6 -27.8 3.1 -28.0 4.0 -26.3 2.8 -26.7 3.3 -24.2 2.4 -27.3 3.3 -25.2 4.6 -23.0 2.0 -23.6 3.9 -28.1 3.2 -27.4 3.3 -28.0 1.8 -27.5 3.9 -27.0 3.4 -27.6 2.6 -27.2 3.7 -27.2 2.4 -27.5 2.6 -28.2 2.3 -28.5 2.7 -27.5 3.0 -28.0 2.9 conifer, conifer needles, grass, and bushes) on the present surface of the Velenje basin were collected as additional 4 samples (Tab. 2). The bulk of the sampled lignite material represents the humic organic matter, except for two samples of fusinite and semidegra-dofusinitic lignite that represent inertinitic Table 2. 815N and 813C values of recent plant parts collected around the Velenje basin Type of plant parts S^Cvfdb (%o) S^N^j. (%o) trunk -28.0 -3.3 conifer needles -27.0 -3.6 grass -31.1 -3.7 bush -25.0 -2.3 organic matter. The only lipoid-rich material in the study is represented by the recent conifer needles. After macroscopic description, the samples were ground in an agate crusher to powder for isotopic analyses. The isotopic composition of carbon and nitrogen in lignite samples (Tab. 1), as well as in recent plant material (Tab. 2), was determined using an Europa 20–20 continuous flow IRMS ANCA–SL preparation module. 10 mg of homogenized sample was weighed in a tin capsule for nitrogen and 1 mg for carbon analysis. Samples for carbon analysis were pre-treated with 1 molar HCl to Stable isotope geochemistry of different lithotypes of the Velenje lignite 89 remove carbonates. The sample residues were washed in distilled water, dried and homogenized. The isotopic composition of nitrogen and carbon was determined after combustion of the capsules in a hot furnace (temperature 1000°C). Generated products were reduced in a Cu tube (600°C), where excess O2 was absorbed. H2O was trapped on a drying column composed of MgClO4. Gases were separated on a chromatographic column and ionized. NBS 22 (oil) and IAEA N -1 (ammonium sulfate) reference materials were used to relate the analytical results to the VPDB and AIR standards as follows: where: Rsample - ratio 13C/12C in sample (15N/14N for ?15N in sample) RRM - ratio 13C/12C in reference material (15N/14N for ?15N in reference material) Sample reproducibility for carbon and nitrogen was ± 0.2 ‰. Results and discussion In general, the initial peat-forming depo-sitional environment of the studied coal is characterized by its peat–forming plant communities, the ratios between the amount of organic versus inorganic material sedimentation, nutrient supply, pH value, bacterial activity, sulphur supply, temperature and the redox potential (aerobic, anaerobic). After peat accumulation, early diagenesis of organic matter begins with biochemical processes, which last up to the stage of the sub-bituminous coal rank (Stach et al., 1982; p.38), or according to some authors even up to the stage of the bituminous coal rank (B u -stin et al., 1983; p.21). The biochemical stage of coalification is continued by the physi-cochemical stage of coalification, which is not of a special interest in this paper since we are dealing with a typical ortho-lignite. As described in Bustin et al. (1983; after Tissot & Welte, 1978), biochemical co-alification is characterized by the following two stages: “firstly, plant material is mecha- Figure 5. Dendrogram illustrating the results of cluster analysis of ?13C and ?15N values of lignite. The association of samples at a low value of the distance coefficient means a high similarity between the respective entities. nically disintegrated and depolymerized into aromatic, phenolic, and carboxylic groups with the aid of micro-organisms, and secondly, the humic polymers undergo random repolymerisation and polycondensati-on of the molecular types. With progressive diagenesis, the humic acids (formed during depolymerization) lose their acid groups and turn into humins, which are insoluble in alkalies. The process of humification is followed by gelification, a physico-colloidal process by which the humic matter passes through a soft plastic gel stage and takes on 90 Tja{a Kandu~, Milo{ Marki~ & Jo‘e Pezdi~ 5 " ^ 4 ¦ MINERALIZATION ¦ ¦ D 3 ¦ 2 ¦ 1 ¦ OF ORGANIC MATTER - = 0~n