Geochemical Transformations of Sedimentary Sulfur - American

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Chapter 17

The Distribution of Free Organic Sulfur Compounds in Sediments from the Nordlinger Ries, Southern Germany 1

2

Assem O. Barakat and Jürgen Rullkötter 1

Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426, 21321 Alexandria, Egypt Institute of Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University of Oldenburg, P.O. Box 2503, D-26111 Oldenburg, Germany

2

Black shales in the Miocene crater lake of the Nördlinger Ries (southern Germany) were deposited under largely stagnant conditions in slightly saline, sulfate-rich water in an arid climate. Black shale layers in the sedimentary sequence contain more than 10% organic carbon and up to 5% total sulfur, of which a significant portion is bound to organic matter. The low-molecular-weight organic sulfur compounds in the extractable organic matter consist of thiolanes and thiophenes with a phytane carbon skeleton, structural isomers of C 2,5dialkylthiophenes, C /C /C 2,6-dialkylthianes and 2,5-dialkylthiolanes as well as sulfurized steroids. Structures were assigned to these constituents based on gas chromatographic retention times, mass spectrometric fragmentation patterns and analysis of Raney nickel desulfurisation products. There are distinct differences between the distributions of unsulfurized carbon skeletons and carbon skeletons of organosulfur compounds in the extracts, indicating selective interaction of functionalized lipids with inorganic sulfur species during early diagenesis. However, not all of the potentially reactive species are found in the sulfur fraction. For example, ketocarboxylic acids represent up to 50% of some black shale bitumens, and yet no sulfur analogs have been detected. 33

33

37

38

The Nordlinger Ries is a circular Miocene sedimentary basin 20 km wide located about 100 km NW of Munich in southern Germany (Figure 1). It was formed by meterorite impact into the late Jurassic carbonates of the Schwâbische Alb mountain range about 15 Ma ago (1-2). The extruded material formed a rim around the crater which prevented freshwater supply except by rainfall. Erosion of gypsum deposits at the crater rim supplied sulfate into the lacustrine environment. The climate in southern Germany was semi-arid during the Miocene (3). The basement of the crater 0097-6156/95/0612-0311$12.25/0 © 1995 American Chemical Society Vairavamurthy et al.; Geochemical Transformations of Sedimentary Sulfur ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Vairavamurthy et al.; Geochemical Transformations of Sedimentary Sulfur ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Figure 1. Location map of Nôrdlinger Ries crater in southern Germany (after 3).


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17. BARAKAT & RULLKOTTER

Free Organic Sulfur Compounds in Sediments 313

consists of suevite (meteorite impact brecciae) containing high-pressure modifications of quartz. Post-impact sediments were penetrated by research well Nordlingen-1973 near the center of the basin and described in detail by Jankowski (3). They consist of a "basal unit" overlying the suevite, a 140 m thick laminate sequence, which contains two particularly distinct series of highly bituminous black shales, 60 m of marl and a clay layer at the top. Figure 2 shows a stratigraphie correlation of commercial wells NR-10 and NR-30 with the research borehole Nordlingen 1973 (e.g., 1-3). Deposition of the entire laminite series extended over a period of 0.3 to 3 Ma (3). In a previous study, the organic matter in a variety of black shale samples was characterized using bulk organic geochemical and organic pétrographie parameters as well as by the analysis of the aliphatic and aromatic hydrocarbon fractions (4). Several of the sediments - those selected for this study - contained close to or more than 10% organic carbon and were rich in sulfur (Table I). High hydrogen indices were consistent with a predominance of algal and bacterial remains in the sediments. Total bitumen contents were in the range of 80 - 135 mg/g TOC. Microscopically, the organic matter consisted of a mixture of small alginite macérais and homogeneous groundmass (4). The aromatic hydrocarbon fraction previously was found to actually contain organic sulfur compounds as dominant constituents (4). This study extends the earlier investigation of organic sulfur compounds in the Nordlinger Ries black shales by the assignment of distinct chemical structures to a great number of previously undefined constituents.

Experimental Methods Sediment samples were supplied by BEB Erdgas und Erdôl GmbH (Hannover), and are from different depths (Figure 2) of commercial wells NR-10 and NR-30 drilled in the Nordlinger Ries in the early 1980s. General information and bulk data of the samples are compiled in Table I. Sample preparation, extraction, liquid chromatographic separation and gas chromatographic analysis of the aromatic hydrocarbonfractionswere described previously (4). The "aromatic hydrocarbon" fractions were desulfurized using Raney nickel in ethanol. Approximately 5 mg of thefractionwas dissolved in absolute ethanol (2 ml) and mixed with 0.5 g of a suspension of Raney Ni (Merck, Darmstadt, Germany) in ethanol. The mixture was stirred and refluxed under N2 for 2 h. The desulfurized products were purified by flash chromatography over silica gel (0.4 χ 2 cm) using CH2CI2 as eluent (10 ml), dried, concentrated, and further hydrogenated with Pt02 in acetic acid at room temperature for 2h. GC-MS analyses were performed with a Carlo Erba Fractovap 4160 gas chromatograph equipped with a fused silica column (25 m χ 0.25 mm i.d.) coated with CP-Sil-5 silicone and coupled to a VG 7070E mass spectrometer operating at 70 eV. Helium was used as carrier gas, and the temperature programmedfrom110


314

GEOCHEMICAL TRANSFORMATIONS OF SEDIMENTARY SULFUR

NR-30


NR-10

100-

200H

3001

Figure 2. Stratigraphie cross section of boreholes through the Nordlinger Ries sediments showing origin of black shale samples used for this study. Vairavamurthy et al.; Geochemical Transformations of Sedimentary Sulfur ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

17. BARAKAT & RULLKOTTER


to 310°C at 3°C/min with an initial hold time of 4 min and afinalhold time of 20 min. The magneticfieldof the mass spectrometer was scanned over a mass range of mlζ 45 - 900 at a rate of 2.5 s/scan. Data were acquired, stored and processed using a Kratos DS 90 data system.

Table I. General information and bulk data of black shale samples used in this study Sample C rg Stot Hydrogen Index Extract yield Aromatic fraction (% of extract) (%) (mg hc/g Core) (mg/g Core) (%) 2.2 NR-10 132.4 12.3 1.7 470 (151.5 m) 4.7 81.5 NR-10 14.4 3.0 863 (250.0 m) 7.0 NR-30 70.8 10.3 3.5 626 (215.1 m) 6.5 NR-30 61.7 8.4 4.3 480 (222.9 m)


0

Results and Discussion Figure 3 shows gas chromatograms of the "aromatic hydrocarbon" fractions of three black shale extracts. The traces are dominated by a variety of organic sulfur compound classes (A through F). The differences among the samples demonstrate that organic matter accumulation and transformation during early diagenesis was quite variable for the different black shale layers both in time and laterally within the former crater lake. This is not surprising, since fluctuations in the supply of freshwater to this restricted environment may, for example, have caused significant salinity changes and thus ecosystem changes in the lake over relatively short periods. Type A. Alkylated thiophenes and thiolanes with an acyclic carbon skeleton are the single most abundant components in each fraction (Figure 3). Figure 4 shows the carbon number distributions and absolute concentrations of alkyl thiophenes with unbranched carbon skeletons but different alkyl substitution patterns on the thiophene ring (or different positions of the thiophene ring within the carbon chain) for one sample each from wells NR-10 and NR-30 (both from second black shale event). Differences are obvious both in concentration and distribution, again illustrating lateral organic facies changes. In addition, the concentrations of the C20 compounds with a phytane skeleton which are most abundant in both samples and in each isomer class are shown in Figure 4. Isoprenoid thiophene homologs were found as well but at much lower concentrations and, therefore, are not shown. Table II summarizes the concentration data for C20 isoprenoid thiophenes and thiolanes. The two shallower samples in each well are enriched in thiophenes whereas the other two samples show a dominance of thiolanes. Interestingly, the isoprenoid thiophene ratio (ITR) introduced by Sinninghe Damsté et al. (5) and de Leeuw and Sinninghe Damsté (6) to determine paleosalinity, varies with this difference in thiolane and thiophene concentrations. Because thiolanes and thiophenes are thought to be diagenetically related, both compound types should be


Vairavamurthy et al.; Geochemical Transformations of Sedimentary Sulfur ACS Symposium Series; American Chemical Society: Washington, DC, 1995. 20

40 Retention time

= 440...482

(min)

60

Figure 3. Gas chromatograms of aromatic hydrocarbon fractions showing different types of organic sulfur compounds. Capital letters refer to the following figures and discussion (modified from 4). Note: In this liquid chromatographic separation, thiolanes partly coeluted with thiophenes. For quantitation, these compound types were separated completely in a subsequent step.

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17. BARAKAT & RULLKÔTTER



included into this ratio in order to avoid diagenetic perturbation effects. Consequently, ITTR (isoprenoid thiophene plus thiolane ratio) values were calculated (Table II). Although there is indeed a change in the absolute values compared to the ITR, the relative relationships among the samples are not changed significantly. This means that despite the observed general variations of the thiophene distributions (Figure 4), the isomer ratio of the isoprenoidal organic sulfur compounds does not change much between thiolanes and thiophenes.

Table II. Concentrations of C20 thiophenes and thiolanes and salinitity indicators ITR (6; thiophenes only) and ITTR (including thiolanes) C20 Isoprenoid Thiolanes Sample C20 Isoprenoid Thiolshenes ITTR** c' ITR* b' a' a b c y/g extract */g extract μι 23 NR-10 (151.5 m) 13 71 23 215 1.6 372 562 NR-10 (250.0 m) 74 2 133 28 28 NR-30 (215.9 m) 47 25 51 246 41 1681 1.6 1255 NR-30 (222.9 m) 72 738 0.6 29 17 *ITR = (b + c)/a ** = φ + c + b' + c')/(a + a') I T T R

The diagenetic relationship, on the other hand, does not exhibit a straightforward depth (age, temperature) relationship in terms of conversion of the supposedly less stable thiolanes into the corresponding thiophenes, because the deeper (older) samplesfromthe Nordlinger Ries contain more thiolanes. According to our results, salinity may have varied considerably even during the deposition of a single black shale unit, which may have lasted less than 10,000 years. This conclusion is consistent, however, with the general variability of the organic sulfur compound distributions, if this is considered a facies indicator. Type B. Steroids with an extended carbon skeleton and a thiophene moiety are the second group of compounds. Figure 5a shows distribution patterns based on mass fragmentogramsfromGC-MS analysis for several homologous series found in the black shale from 151.5 m depth of well NR-10. Mass spectral data indicate that the steroids contain a carbon chain extension at C-3 ranging from two to six carbon atoms, and the thiophene sulfur atom is attached to C-2. Dominant fragmentation then occurs by benzylic cleavage throughringA leading to key ions at m/z 110+14n (n=0-4) (compare spectrum in Figure 6, top). Two main series of thiophene steroids were detected, one of them corresponds to the regular 4-desmethyl steroids, while the other one - based on the relative


320


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Figure 4. Concentrations and carbon number distributions of «-alkyl and isoprenoid thiophenes in Nordlinger Ries black shales. For structural type identification see Table II.


17.

BARAKAT & RULLKÔTTER


retention time and the fragmentation pattern - carries a 4a-methyl group (hatched peaks in Fig. 5a). Besides the main series of thiophene steroids, three Δ thiophene steroids, with closely related mass spectra were also found in the sample, although in relatively low concentrations (components marked with asterisks in Fig. 5a). This interpretation is supported by the sterane distribution after Raney Ni desulfurization which contains a series of 3-ethyl- to 3-hexylcholestanes together with some 4methyl analogs (Figure 5b; Figure 6, bottom; 7-8). Within the series with identical carbon number extension at C-3, pseudohomologous series are due to alkylation at C-24 and isomerization possibly at C-5 as indicated by differences in B-ring fragmentation quite analogous to that observed for regular steranes. For a given alkylation at C-3, pseudohomologous series are due to alkylation at C-24 and isomerization, possibly at C-5. The fact that 23,24-dimethylsteroids were also found in this group of thiophenes (Figure 5, middle) shows that steroids with extended carbon chains at C-3 are widely distributed among the principal carbon skeletons encountered in biological systems and the geosphere. Since C-3 extended steroids have not been found in natural product surveys, the structural diversity of the steroid thiophenes may indicate that the carbon extension at C-3 is of diagenetic rather than of (planktonic) biogenic origin. Dahl et al. (7) noted that the length of the carbon chain (' )' î" ι

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Vairavamurthy et al.; Geochemical Transformations of Sedimentary Sulfur ACS Symposium Series; American Chemical Society: Washington, DC, 1995. 1

124

Figure 6. Mass spectra of sulfur-bearing steroid derivatives in Nordlinger Ries black shales (top and middle) and of a corresponding saturated hydrocarbon after desulfurization with Raney nickel (bottom). Structures are based on mass spectral interpretation and com-parison with distribution patterns of 3-alkanoic steroids also found in these samples.

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1800

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328


The C33 thiolane compounds appear to be mainly comprised of 2-hexyl-5tricosylthiolane and 2-pentyl-tetracosylthiolane, but other alkylation patterns (position of thiolane ring along the carbon chain) also occur, although slightly less pronounced than with the thianes. In addition, stereoisomers at the alkylation site also occur. The origin of the precursor compound(s) is not known, although a corresponding group of thiophenic organosulfur compounds has been described earlier (e.g., 12). The C37 and C38 compounds are also complex isomer mixtures, but no terminal thianes and thiolanes were found. The sulfur compounds are likely to have been formedfromlong-chain alkenones - or the corresponding long-chain alkadienes and alkatrienes - characteristic of certain Prymnesiophyte algae (cf. 13-14). The absence of terminal sulfur-bearingringsindicates that thiane and thiolane formation by reaction with reduced inorganic sulfur species occurred exclusively via the double bonds and not through the keto group. This does not exclude, however, that the isolated doubled bonds - like the keto group (Jl) - in a competitive reaction formed intermolecular sulfur bonds as well. Abundant C and C OSC with w-alkane carbon skeletons have been noted before in the upper Cretaceous Jurf ed Darawish oil shale (Jordan; 12,16), a Miocene marlfromthe Northern Appennines (Italy; Π), the phosphatic unit of the Lower Maastrichtian Timahdit oil shale (Morocco; 18), and the Rozel Point oil (Utah; 19). Of these, only the source rock of the Rozel Point oil is related to a lacustrine depositional environment which obviously was much more saline than the Nordlinger Ries crater lake. 3 7

3 8

Types Ε and F. Two groups of organic sulfur compounds are represented by mass spectra in Figure 8, but no structural assessment was possible. Desulfurisation did not yield corresponding high-molecular-weight hydrocarbons. This suggests sulfur compounds consisting of smaller units. The mass spectral isotope pattern of the molecular ions indicate that the compounds contain one or at most two sulfur atoms. The presence of an m/z 280 ion in the mass spectrum of the MW 538 compound may indicate the presence of a C20 (phytanyl?) hydrocarbon unit.

Summary and Conclusions Organic sulfur compound distributions in Nordlinger Ries sediments are highly variable. Drastic differences occur between the two black shale events as well as within a single event. Short-term fluctuations of depositional conditions including the paleoecosystem are likely, considering the unique geological and geographical situation of the Miocene crater lake. Calculation of ITTR ratios (including thiolanes)fromthe isomer distributions of isoprenoid organic sulfur compounds yields the same gross trend as the ITR ratios, but different absolute values. We identified a new series of thiophene steroids based on C-3 alkylated skeletons. The carbon number distribution (alkylation up to CO) suggests C-C-addition of a hexose unit (probably diagenetic) similar to extension of hopanes in the sidechain. The variation of structural types including that of 23,24-methylsteroids in this series favors the diagenetic formation hypothesis.



17. BARAKAT & RULLKÔTTER


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Geochemical Transformations of Sedimentary Sulfur - American

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