Advances in Organic Geochemistry 1989 Org. Geochem. Vol. 16, Nos 4--6, pp. 749-761, 1990 Printed in Great Britain. All rights reserved

0146-6380/90 $3,00 + 0.00 Copyright © 1990 Pergamon Press plc

Aromatic hydrocarbon composition of the Permian Kupferschiefer in the Lower Rhine Basin, NW Germany

L. SCHWARK and W. PUTTMANN Lehrstuhl fiir Geologic, Geochemie und Lagerst~itten des Erdrls und der Kohle,

RWTH Aachen, Lochnerstr. 4-20, 5100 Aachen, F.R.G.

(Received 10 October 1989; accepted 18 February 1990)

Abstract--The aromatic hydrocarbon composition of 25 samples of Permian Kupfersehiefer from the Lower Rhine Basin of NW Germany has been investigated using GC, GC/MS and direct probe MS analysis. Two narrow-sampled profiles were investigated with respect to compositional changes during Kupferschiefer sedimentation and samples from 13 different wells, covering a depth range from 350 to 1000 m, were included in order to study temperature-sensitive variations. In the Kupferschiefer trimethyl- substituted aryl isoprenoids in the range from Ca to C2~ occur. These compounds presumably are degradation products of isorenieratane, which is also present in the extracts. Within the profile the amount of a preferentially occurring C29-chroman increases from the bottom to the top of the Kupferschiefer by a factor of about 2.5. The chroman distribution observed in the Kupferschiefer points towards euhaline to mesohaline seawater during sedimentation. Moreover, in the investigated profiles a terrestrial source input mainly in the bottom-most part of the shale is reflected by the occurrence of a monoaromatic C24-chrysene derivative, presumably derived from ring-A-degradation of pentacyclic triterpenes.

Key words--aromatic hydrocarbons, chromans, aryl isoprenoids, palaeosalinity, Kupferschiefer

INTRODUCTION

The distribution of aromatic hydrocarbons in crude oils, extracts of sedimentary rocks and coals has been widely investigated for maturation assessment (Radke, 1987). Moreover, the composition of naph- thalenes and phenanthrenes is revealed to be influ- enced by the type of organic matter initially deposited in sediments (Radke et al., 1986; Strachan et al., 1988; Piittmann and Kalkreuth, 1989). The composition of C-ring monoaromatic or triaromatic steriod hydro- carbons, as well as the aromatization stage of steroids used for the investigation of the time/temperature history of sediments (e.g. Mackenzie and McKenzie, 1983) has also been shown to be facies-dependent (Moldowan et al., 1986; Riolo et al., 1986; Curiale and Odermatt, 1989). One of the best investigated groups of specific aromatic biomarkers is the resin- related phenanthrene derivatives (Simoneit et aL, 1986). Moreover, diagenetic products of aromatic carotenoids in crude oils and sediments were shown to indicate the presence of green sulphur bacteria in ancient marine environments (Summons and Powell, 1986, 1987). Oxygen-containing moieties, such as chromans detected in the so-called "aromatic" hydro- carbon fractions of extracts (Brassell and Eglinton, 1986) were suggested to be useful for the assessment of the palaeosalinity of sediments (Sinninghe Damst6 et al., 1987a). Furthermore, the occurrence of aryl isoprenoids in crude oils and sediments from the Michigan and Alberta Basins, Canada, was related to hypersaline environmental conditions (Summons and

Powell, 1987). In carbonate-derived crude oils thio- phenic organo-sulphur compounds (primary benzo- and dibenzothiophenes) were detected as typical con- stituents of aromatic hydrocarbon fractions (Hughes, 1984). Alkyl thiophenes as potential precursors of benzothiophenes have been investigated in detail by Sinninghe Damst6 et al. (1987b). Previous investi- gations of the organic matter in the Kupferschiefer were carried out mainly with emphasis on the satu- rated hydrocarbon fraction for maturation assess- ment (Pfittmann et al., 1989; Piittmann and Eckardt, 1989). Concerning the aromatic hydrocarbons pre- sent in the Kupferschiefer, to date investigations have only been carried out with respect to mono- and triaromatic steroids (P/ittmann et al., 1989).

The present study is aimed at a detailed analysis of the aromatic hydrocarbon composition for the assess- ment of maturation, salinity and biological input of the Kupferschiefer deposited in the shallow lagoon of the Lower Rhine Basin.

EXPERIMENTAL

Geological overview and sample description

In the Upper Permian four major evaporitic sedimentary sequences (Richter-Bernburg, 1955) developed within the Southern Permian Zechstein Basin which extended from the east coast of Great Britain further to the eastern border of Poland. The southern shoreline ran along the London-Brabant, the Rhenish and the Bohemian Massifs. In the area

749

750 L. SCHWARK and W. P'UTTMANN

Fig. I. Tectonic sketch map of the investigated area including the locations of the exploration wells [adopted from Piittmann et al. (1989)].

of the Rhenish Massif two embayments protruded to the south. North-west of the Rhenish Massif the Lower Rhine Basin (Fig. 1) was situated, whereas in the east the Hessian Depression was placed (Ziegler, 1989).

Both areas are characterized by an intensively faulted basement comprising horst and graben struc- tures, the latter filled with clastics and volcanics of the Lower Permian in case of the Hessian Depression. The marine transgression of the Zechstein Sea in- truded from the north into the subsiding Lower Rhine Basin.

The Kupferschiefer was the first sedimentary layer covering the whole basin floor except at local palaeohigh structures (Paul, 1982).

The basinal facies of the Kupferschiefer is char- acterized by a uniform mineralogical composition and a low thickness of about 30-50 cm. A content of organic carbon between 5 and 10% (Jung and Knitzschke, 1976) indicates sedimentation under anoxic conditions. In the marginal facies the Kupfer- schiefer reaches an average thickness of about 2 m, due to a much higher content of carbonates intercalated with argillaceous laminites. The break- down of anoxity is marked by the sudden occurrence of bioturbation in the hanging wall Zechstein Carbonate.

The Lower Rhine Basin resembles the sedimento- logical features characterizing the marginal facies. The Kupferschiefer is developed as a marly shale of about 2 m thickness [see also Teichm/iller (1957) and Wolburg (1957)].

The recent tectonic setting of the Lower Rhine Basin is dominated by NW-SE trending fault systems with throw measures of several tens of meters up to locally more than 300 m (Wolf, 1985). Initial move- ments of low degree occurred during the late Carboniferous and early Zechstein periods. Tectonic activity culminated in Triassic times with com- pressional and uplifting movements. Readjusting of these movements occurred in the Upper Cretaceous/ Lower Tertiary.

In the western part of the Lower Rhine Basin the deep seated basic pluton of the Krefeld High intruded during the Stephanian/early Permian (Niem6ller et al., 1973) causing an area of higher thermal heat flow, as revealed by vitrinite reflectance measurements of the underlying Carboniferous coals (Teichmiiller et al., 1984). Two Kupferschiefer cores derived from well 26 and well 212 (see Fig. 1 for well locations) were analysed to identify facies variations in the Kupferschiefer profile. Samples from different lo- cations within the Lower Rhine Basin were always taken from the bottom-most section of the cores in order to avoid the influence of facies changes on the organic matter composition. The samples investi- gated cover a depth range from about 350 to 1000 m (Table 1).

Analytical methods

The finely ground rock samples were Soxhlet- extracted in pre-extracted thimbles for 24 h using dichloromethane. The obtained extracts were further separated by open column chromatography

Aromatic hydrocarbons in the Kupferschiefer

Table I. Depth and geochemical data of the investigated samples

Depth Co~ s ext./ Sat. Aro. Tri-MTTC Sample (m) (%) gCor B (%) (%) Pri/Phy (ppm)

26/9 561.2 0.9 36.8 8.0 21.6 0.50 ND 26/10 561.4 1.7 42.2 9.8 20.9 0.45 ND 26/11 561.7 2.7 41.7 8.1 19.6 0.53 ND 26/12 561.9 3.4 34.5 8.2 21.7 0.54 ND 26/13 562.2 4.0 33.6 9.3 24.7 0.59 ND 26/14 562.4 2.2 45.4 8.2 24.3 0.59 ND

212/6 754.8 1.8 45.2 11.5 39.8 0.88 ND 212/5 754.9 3.5 59.6 7.0 27.3 0.68 ND 212/4 755.0 4.1 55.9 7.8 32.0 0.85 ND 212/3 755.1 4.3 52.9 9.3 32.9 0.80 ND 212/2 755.2 5.0 63.8 8.2 31.8 0.95 ND 212/1 755.3 3.2 56.3 6.4 33.5 1.07 ND

19/11 388.2 3.4 39.2 9.9 24.9 0.60 2502 132/7 384.4 3.7 31.0 11.7 40.8 1.09 2530 76/10 488.3 3.3 20.1 8.5 30.2 0.57 2072 20/10 538.8 5.1 54.6 8.3 29.6 0.66 ND

126/23 568.9 3.3 51.2 8.6 39.1 1.04 ND 123/6 611.4 5.9 21.1 9.4 34.4 0.83 4467 70/12 631.9 3.7 47.5 7.8 40.6 1.00 2003 94/8 725.7 2.5 47.3 5.9 28.9 0.70 1739 41/6 749.2 5.0 44.1 7.0 34.0 0.77 1902

127/7 750.1 3.0 40.6 9.0 42.4 1.03 2000 116/8 801.5 5.7 27.0 10.5 35.3 0.74 2742 136/20 802.9 3.6 47.1 9.5 37.3 0.70 1883 124/12 1012.8 2.8 46.4 8.5 39.1 1.05 2115

Abbreviations: Ext. = extract yield in mg/g CorB; Sat. = saturated hydrocarbons; Aro. = aromatic hydrocarbons with Ni- and V=O-porphyrins included; Pri/Phy = ratio of pristane vs phytane; Tri-MTTC = amount of trimethylated chroman in ppm of aromatic hydrocarbon fraction.

751

using pre-washed silica gel (Macherey & Nagel, 70-230 mesh, 50 x 1 cm). Elution of saturated hydro- carbons was carried out using n-hexane. Aromatic hydrocarbons were eluted with dichloromethane and polar heterocompounds with methanol. 40 ml of each solvent were used.

Separation of Ni- and V=O-porphyrins was achieved by low-pressure column chromatography with a constant flow rate of 5ml solvent/min. Silica gel (Merck, 230-400 mesh, 20 x 1.25 cm) was used as the stationary phase. After removal of saturated and low molecular weight (MW) aromatic hydrocarbons with a solvent mixture of hexane/ dichloromethane (98:2, v/v, 30ml) the mixture was changed to hexane/dichloromethane (75:25, v/v, 20 ml) for the elution of Ni-porphyrins. V = O - porphyrin fractions were eluted with 20 ml of pure dichloromethane.

UV/vis quantification of the Ni- and V = O - porphyrins was carried out using a Perkin-Eimer spectral photometer (PE 550S). A measuring range of 750-350 nm was scanned at a rate of 120 nm/min. The length of the measuring cell was 1 cm and dichloromethane was used as a blank. For quan- titation of Ni- and V=O-porphyrins extinction coefficients of 22,000 and 17,600, respectively (Chicarelli, 1985), were used.

A Leco WR 12 carbon determinator was applied for the quantitation of total organic carbon (TOC) contents. Carbonates were removed from the samples by treatment with hydrochloric acid prior to analysis. GC of aromatic and saturated hydrocarbon fractions was carried out using a Carlo Erba 5160 gas chro- matograph equipped with a 25 m fused silica capillary

column coated with SE 54 CB. The oven temperature was raised from 80 to 300°C at a rate of 4°C/min followed by an isothermal period of 20 min. Hydro- gen was used as the carrier gas. Absolute con- centrations normalized to organic carbon content, were determined by adding 1,1'-binaphthyl internal standard prior to GC analysis. GC/MS facilities consisted of a Varian 3700 gas chromatograph coupled to a Finnigan MAT 8200 mass spectrometer. The GC conditions were the same as applied for GC analyses with the exception that helium was used as the carrier gas. The mass spectrometer was operated in the EI-mode at 70eV. Data acquisition and processing were performed using the INCOS data system.

RESULTS AND DISCUSSION

In the area investigated, the Kupferschiefer pro- vides a TOC-content of 3-6% at the bottom decreas- ing to about 1% at the top (Table 1). The extract yields amount to 20 to more than 60 mg extract/g organic carbon. The concentrations of aromatic hydrocarbons (porphyrins included) vary between about 20 and 40% and the saturated hydrocarbons usually make up 6-12% of the total extractable organic matter (Table 1).

The gas chromatogram of an aromatic hydro- carbon fraction (sample 212/1) representing the Kupferschiefer at a relatively high maturation stage shows low intensities of C2- and C3-naphthalene isomers (marked 11 and 12 in Fig. 2, structures shown in the Appendix) and phenanthrenes (Fig. 2). Tetra- hydronaphthalenes (1, 4) and alkylbenzenes (2, 3) are

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Table 2. List of compounds assigned in the gas chromatogram presented in Fig. 2. Peak identification was carried out on the basis of published mass spectra and comparison of retention behaviour

1 I, 1,6-Trimethyl-(1,2,3,4-tetrahydro)naphthalene 2 Ci3-aryl isoprenoid 3 Ci4-aryl isoprenoid 4 1,1,5,6-Tetramethyl-(l ,2,3,4-tetrahydro)naphthalene 5 1,2,5,6-Tetramethylnaphthalene 6 Unknown aryl isoprenoid (?) 7 Hydrogenated monoaromatic chrysene derivative 8 5,7,8-Trimethyl-2-methyl-2-(4,8,12-trimethyltridecyl)chroman 9 Unknown

I0 C32- to C35-Hexacyclic benzohopanes l i Dimethylnaphthalenes 12 Trimethylnaphthalenes 13 Monoaromatic steroid hydrocarbons 14 Triaromatic steroid hydrocarbons STD 1,1'-Dinaphthyl (internal standard)

the dominant constituents of the aromatic hydro- carbons in the low MW region. In the higher MW range a methylated monoaromatic chrysene deriva- tive (7) and a trimethylated chroman (8) are the dominant compounds in the more mature samples. An unknown compound (9) and monoaromatic steroids (13) as well as benzohopanes (10) contribute to the aromatic hydrocarbon fraction of sample 212/1, only in relatively low amounts. These compounds appear in significant intensities only in samples of lower maturation. The geochemical significance of the compounds detected is discussed below.

Aryl isoprenoids

The presence of a series of C~0- to C30-trimethy- lated benzenes with isoprenoidal side chain was first noted in a Siberian crude oil of Silurian age (Ostroukhov et al., 1982). The methyl substi- tution pattern at the aromatic ring was confirmed by synthesis of standards, coelution and GC/MS analysis for the C~3 and the Cts compounds. The authors assume the aryl isoprenoids to derive from naturally occurring carotenoids by degradation reactions. Summons and Powell (1986, 1987) detected C~0- to C3~-aryl isoprenoids in high abundances in palaeozoic reef hosted oils and sediments from the Michigan and Ontario Basins, Canada. They proved the structure of the C~4 member within this series of homologues to possess a 2,3,6-methyl substitution pattern. Therefore, renieratene, iso- renieratene and okenone were postulated as possible biological precursors. Because of the absence of any isoprenoids >Cal, Summons and Powell (1986, 1987) suggested the diaromatic isorenieratene or renieratene compounds as preferential precursor mol- ecules. However, it has to be considered, that the generation of a C3~-aryl isoprenoid from renieratane-like compounds would require an ~- cleavage next to the aromatic moiety, which is highly unfavourable. The diaryl isoprenoids are reported to be present in green sulphur bacteria and are known to occur in sponges or their symbiotic bacteria, respectively (Liaaen-Jensen, 1978; Hamasaki et al., 1973). According to Summons and Powell (1986,

in the Kupferschiefer 753

1987) aryl isoprenoids occur in sediments deposited under euhaline to hypersaline conditions in the water column.

In the Kupferschiefer samples investigated here, several aryl isoprenoids are present in the El0 to C2~ range. The recognition of the compounds is achieved by the mass chromatogram of the m / z 133 ion. A mass chromatogram of the aromatic hydrocarbon fraction in sample 127/7 is shown in Fig. 3. Aryl isoprenoids maximize at Cl4 (3) and rapidly decrease to higher MW. The obvious lack of the C~2 and C~7 components results from the presence of an irregular isoprenoid side chain, as mentioned before (Ostroukhov et al., 1982). A gap in the occurrence of aryl isoprenoids ranging from the C22 to the C30 member is followed by the appearance of a com- ponent with an m/z 133 fragment (9) in the higher MW range. The MW of this compound (M ÷ = 426) is not in agreement with that of the C3x-aryl iso- prenoid (M ÷ = 428) detected by Summons and Pow- ell (1986, 1987). A structural assignment of this compound (9) cannot be provided to date.

In order to investigate the influence of facies changes on the aryl isoprenoid distribution, quanti- tation of the Cl3- and Cl4-aryl isoprenoids has been carried out for six samples of well 212 (Table 3). Results show that the absolute amounts of the aryl isoprenoids significantly decrease towards the top of the horizon to approx. 30% of the concen- tration in the bottom section. Based on geological evidence (Teichm/iller, 1957; Wolburg, 1957) it is well-established that during Kupferschiefer sedi- mentation the salinity of the water column in- creased. Therefore, the decrease of aryl isoprenoids towards the top might be a result of increasing salinity of the Kupferschiefer sea and the occurrence of these compounds appears to be related rather to a euhaline (30--40 permil) to mesohaline (< 30 permil) environment than to hypersaline (>>40 permil) con- ditions. This is consistent with the absence of aryl isoprenoids with irregular linked tail-to-tail iso- prenoidal side chain in various samples investigated from hypersaline environments (Sinninghe Damst6 et al., 1987a; ten Haven et al., 1985, 1988; Connan et al., 1986).

Diaryl isoprenoids

Aryl isoprenoids have been assumed to originate from isorenieratane-like compounds (15), though these diaromatic precursors could not be detected in the above-mentioned Canadian oils and sediments which are enriched in aryl isoprenoids (Summons and Powell, 1987). However, diaromatic carotenoids were detected by Schaefle et aL (1977) in immature Jurassic shales of the Paris Basin. On the other hand, no short chain aryl isoprenoids have been reported from these immature shales. This might suggest that in the Michigan and Alberta Basin crude oils the caro- tenoids have been converted to aryl isoprenoids almost completely, whereas in the Jurassic shales

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508 1808 158fl 2080 2500 3888 9: I 8 I 8:20 27:30 3(;: 48 45:58 55: GO

Fig. 3. Mass chromatogram of m/z 133 obtained from an aromatic hydrocarbon fraction (sample 127/7) by GC/MS analysis for recognition of aryl isoprenoids. Peaks are assigned either by the carbon number of the aryl isoprenoids or by the compound numbers listed in Fig. 2 and Table 2. Peaks marked with an

asterisk belong to an unknown series (see the text).

.SC~ TIME

maturity was too low to generate aryl isoprenoids by carotenoid breakdown.

In the Kupferschiefer samples investigated here, isorenieratane (15a) or its positional isomers (renie- ratane 15b, perhydrorenierapurpurin 15e) have been detected as a broadened peak of insufficient resol- ution in the G C / M S acquisition. The peak shape points to decomposit ion reactions of the molecule during GC analysis. However, the mass spectrum [Fig. 4(a)] obtained allows us to identify the com- pound tentatively as isorenieratane, with m/z 133,

134 as the most abundant ions and m/z 546 as the molecular ion. For this reason, an alternative analytical approach has been attempted in order to recognize the high M W aromatic hydrocarbons. Direct probe MS, a technique commonly applied to the analysis of metal porphyrins in crude oils and sedimentary extracts proved to be useful for the detection of high M W aromatics. An example of direct probe MS is shown in Fig. 5. Due to the higher temperature gradient (2°C/s) in comparison to GC analysis, a distinctive peak separation can not be

Table 3. Concentration (#g/g Co,g) of individual compounds in the Kupferschiefer profiles of well 212

% Ksf Sample thck. CI3-Ar . I CI4-Ar . I Tr i -MTTC MA-CD

212/6 81 3.7 11.9 103.1 10.9 212/5 69 7.1 19.1 105.2 14.6 212/4 56 8.5 17.5 84.6 14.3 212/3 36 9.9 22.3 77.4 20.9 212/2 17 16.9 4].9 49.4 37.3 212/1 5 13.4 33.4 51.8 70.4

26/9 70 ND ND 70.2 8.7 29/10 61 ND ND 73.2 12.8 26/11 40 ND ND 41.7 10.6 26/12 28 ND ND 21.3 12.3 26/12 14 ND ND 19.4 18.7 26/13 0 ND ND 5.0 81.5

Abbreviations: % Ksf thck. = position of core section relative to basis of the horizon in % of total Kupferschiefer thickness; C~3-Ar.I=C~3-aryl isoprenoid; CI4-Ar.I = Ct4-aryl isoprenoid; Tri-MTTC = trimethylated chroman; MA-CD = monoaromatic chrysene derivative.

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Aromatic hydrocarbons in the Kupferschiefer

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units relative to (a).

expected. However, the results verify that during sample evaporation in the ion source of the mass spectrometer two peak maxima are generated in the

reconstructed ion current (Fig. 5). The first peak (A) is mainly composed of monoaromatic steroids and the second one (B) of high MW aromatics.

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Fig. 5. Direct probe MS analysis of an aromatic hydro- carbon fraction (sample 41/6). Peak A in the total ion current corresponds to low MW and monoaromatic steroid hydrocarbons. The displayed m/z 532 and 546 fragments are produced by diaryl isoprenoids for peak B and Ni-por- phyrins (C~2-DPEP and C3a-DPEP-like) for peak C. Note the separation of aromatic hydrocarbons and prophyrins even at the high temperature gradient (2°C/s) applied for

the analysis.

Renieratane and related compounds (15) contribute significantly to this peak as indicated by the mass chromatogram of m/z 546 (Fig. 5). A second com- pound (16), also present in peak B, is characterized by a molecular ion of m/z 532 (55% intensity of base peak) and a base peak of m/z 133 [Fig. 4(b)]. This component has also been detected by GC/MS analysis. The mass spectral data allow a tentative structural assignment [see Fig. 4(b)]. The biological origin of this particular compound is unknown. However, this diaromatic isoprenoid might serve as a potential precursor for the phytanylbenzene (17) recently reported by Sinninghe Damst6 et al. (1988). Even with the relatively high temperature gradient of 2°C/s applied in direct probe analysis, a separation of the high MW aromatics from the porphyrins (18) is achieved. The Cs3-porphyrin (Ni-DPEP-like) comprises the second eluting peak in the mass chromatogram of m/z 546 (Fig. 5). By analogy, the related C32-porphyrin (Ni-DPEP) is detected in the higher scan range of the mass chromatogram of m/z 532.

The m/z 133 fragmentogram additionally shows a series of unknown compounds whose molecular ions

do not correspond with those of the alkylbenzenes. The MW is reduced by 8 mass units respectively, so that four compounds with MW of 266, 280, 294 and 336 could be detected (marked with an asterisk in Fig. 3). The most prominent compound is the one with M + =m/z 280 which is assigned 6 in Figs 2 and 3. The structures of the compounds remain unknown. A correlation in the occurrence of these compounds with isoprenoidal alkylbenzenes or the above-mentioned tetraterpenoid (15) was not noticed.

Ni- and V~O-porphyrins

The organic material of the Kupferschiefer deposited in the Lower Rhine Basin is enriched in porphyrins with contents up to 3700/~g/g Corg (Eckardt, 1989). The content of Ni- + V--O- porphyrins (18) reaches about 4000/~g/g Corg in the bottom sample of well 212 (Table 4). A decrease in the content of Ni-porphyrins is accompanied by an increase of V~----O-porphyrins towards the top of the Kupferschiefer (Fig. 6). This results in a decrease of the Ni-/(Ni + V~---O)-porphyrin ratio from 0.93 to 0.80 (Fig. 6). This ratio has previously been shown to vary between 0.7 and 0.92 (Eckardt, 1989) in the Kupferschiefer profile of well 35 (about 2 km east of well 212, see Fig. 1).

The strong dominance of Ni- over V~-----O- porphyrins in Lower Toarcian shales of south Germany has recently been ascribed to partly suboxic conditions during sedimentation, and a decrease from about 1.0 to 0.1 in the ratio of Ni-/(Ni + V~-----O)- porphyrins was interpreted as a shift to more reduc- ing conditions (Moldowan et al., 1986). A similar trend has been observed when measuring the absolute contents of Ni and V by ICP analyses for Pennsyl- vanian black shales (Wenger and Baker, 1986).

These observations could not be confirmed in the Kupferschiefer of the Lower Rhine Basin, as all other geochemical and geological evidence does not point to deposition under oxic conditions. The ratio of Ni- vs V--Q-porphyrins has been applied to characterize the depositional palaeoenvironment in Brazilian marginal basins by Mello et al. (1988). Within the large variety of sediment samples investi- gated there, those defined as marine highly anoxic with a dominance of siliciclastic lithology revealed Ni/(Ni + V~------O) ratios between 0.8-1.0, whereas in

Table 4. Concentration (#g/g Corg ) of Ni- and V~-----O-porphyrin species as determined by UV/vis analyses in the Kupferschiefer

profile of well 212

% Ksf Ni/ Sample thck. Ni V~-------O Ni + V~-----O Ni + V~----'O

212/6 81 2049 312 2361 0.87 212/5 69 2057 508 2565 0.80 212/4 56 1767 445 2212 0.80 212/3 36 1963 321 2284 0.86 212/2 17 2861 320 3181 0.90 212/1 5 3752 201 3953 0.93

Abbreviations: % Ksf thck. = see Table 3; Ni = nickelporphyrins; V~-------O = vanadylporphyrins.

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Aromatic hydrocarbons in the Kupferschiefer 757

I i

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Fig. 6. Variation of Ni- and V~---O-porphyrins (ppm of total extract) in the Kupferschiefer profile of well 212. The ratio of Ni-/(Ni + V=O)-porphyrins, used to distinguish between reducing and oxidizing environments (Mold•wan et al., 1986), is not applicable to the Kupferschiefer of the Lower Rhine Basin.

the more calcareous dominated sediments values could decrease from 0.9 to 0.3. The values reported by Mello et al. (1988) are in good agreement with those noted in the Kupferschiefer of the Lower Rhine Basin, indicating that this parameter might be useful for palaeoenvironmental characterization, though further investigations are necessary.

Chromans

A series of re•no- to trimethylated 2-methyl-2- trimethyltridecylchromans (MTTC) has been ident- ified in a variety of sediments and oils ranging from Permian to Pleistocene age (Sinninghe Damst6 et al., 1987a). Unambiguous identification of the structures of these compounds was elucidated by synthesis of standards and following coinjection.

Out of this series a trimethylated MTTC (8, C29 Hs00) occurs in the Kupferschiefer samples in signifi- cant amounts and is identified by comparison of the mass spectrum with published data (Sinninghe Damst6 et al., 1987a). The spectrum shows an M + of m / z 414 and a base peak of m/z 149. Trace amounts of a dimethylated MTTC (m/z 135, 175, 400) could be detected, but monomethylated MTTC are not detectable in the Kupferschiefer samples.

This feature is of major importance because it was shown that lower substituted MTTC are indicative of hypersaline depositional environments. Ratios of tri- vs monomethylated MTTC <2 have been mentioned to be indicative of hypersalinity. Strong dominance of trimethyl-MTTC is thought to be an indicator of non-hypersaline environmental con- ditions (Sinninghe Damst6 et al., 1987a).

The fact that monomethyl-MTTC are not detect- able and dimethyl-MTTC appear in trace amounts only leads to the conclusion that the Kupferschiefer

sea had no hypersaline character. Nevertheless, it has to be noted that the Kupferschiefer is the basal member of the evaporitic Zechstein 1 sequence. Therefore, it has to be assumed that the salinity increases towards the hanging wall of the Kupfer- schiefer. This assumption is clearly reflected by the results from quantitation of trimethyI-MTTC in the Kupferschiefer profiles of wells 212 and 26 (see Table 3). The absolute amounts of the compounds increase by a factor of about 2.5 from the bottom to the top of the horizon. Figure 7 displays the steady

6( u

40 ~

2O

[]

n

[]

[]

o

4 o

[]

[ ]

• WeLL 26 •WELL 212

[ ] O

O []

i i i +

20 60 I00 20 60 I00 F.g/gCo,g /~g/gCo~g

t r i - MTTC MA - CD

Fig. 7. Absolute amounts of the trimethylated C29-chroman (tri-MTTC) and the methylatcd and hydrogenated monoaromatic chrysene derivative (MA-CD) in wells 26 and 212. The change in concentration reflects the decrease of terrestrial input and its concomitant replacement by marine biomass production during Kupferschiefer sedimentation.

758 L. SCHWARK and W. PI~TTMANN

increase in the trimethyl-MTTC content to the top of Kupferschiefer. This behaviour is clearly parallel with respect to two different wells. In 10 samples derived from different locations the Kupferschiefer revealed relatively constant intensities of trimethyl- MTTC in the bottom section (see Table 1), which is evaluated as an indicator for constant salinity in the Lower Rhine Basin at the onset of Kupferschiefer sedimentation.

The increase in palaeosalinity during Kupfer- schiefer sedimentation is additionally reflected by a decrease in the pristane/phytane ratios (see Table 1) over the above-mentioned well profiles. A close relationship between the pristane/phytane ratio and palaeosalinity has been reported previously (ten Haven et al., 1985). The very low pristane/phytane ratios of < 0.1-0.2, indicative of hypersaline environ- ments, are not reached in the Kupferschiefer of the Lower Rhine Basin. This again confirms the above- mentioned conclusions drawn from the trimethyl- MTTC distribution. High abundances of organic sulphur compounds, predominance of even over odd number alkanes and hopane distributions maximiz- ing at C34~C35 have also been introduced as indicators of hypersalinity (ten Haven et al., 1985, 1988). None of these characteristic features are detectable in the biomarker distribution of the Kupferschiefer (Piittmann and Eckardt, 1989; Pfittmann et al., 1989).

Monoaromat ic chrysene derivative

In the aromatic hydrocarbon fraction of the Kupferschiefer samples investigated, a C24-tetracyclic

100-

monoaromatic hydrocarbon compound of proposed structure 7 occurs. It is thought to be closely related to a C23 methylated and hydrogenated chrysene de- rivative, the structure of which has been identified by Spyckerelle (1975). The mass spectrum of the C24 monoaromatic hydrocarbon (Fig. 7) shows the same triplet of fragment ions at m / z 145, 157, 171 as has been reported for the C23 component which reveals a molecular ion of m / z 310 and a base peak of M + - 15.

This tetracyclic monoaromatic compound is thought to be a diagenetic product of pentacyclic triterpenes of the friedelane or taraxerane type. The mechanism of ring-A-degradation is supposed to be either produced by microbial attack (Spyckerelle, 1975) or photochemically induced (Corbet et al., 1980). Up till now methylated tetracyclic monoaro- matics and higher aromatized analogues originating from the same precursors have been reported from terrestrial and, to a minor extent, lacustrine sediments (Spyckerelle et al., 1977; Corbet and Albrecht, 1983; Schmitter et al., 1981).

Consequently, the occurrence of the methylated and hydrogenated C24-chrysene derivative in the organic matter of the Kupferschiefer is suggested to be a geochemical indicator for terrestrial input.

Such a terrestrial input into the Kupferschiefer sea of the Lower Rhine Basin is well-established by the micropetrographical identification of bisacchate pollen (Wolf et al., 1989). The contribution of terres- trial derived organic material into the Kupferschiefer sea could not be detected by the investigation

:389

C

50' 145 I~7

tTl

?

g5 324

• , . , . , . , . ? . . . . . . . . . . , . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . , .

5e 15$ 2~ m / z

Fig. 8. Mass spectrum and proposed structure of the methylated and hydrogenated monoaromatic C24 chrysene derivative (MA-CD).

Aromatic hydrocarbons in the Kupferschiefer 759

of saturated hydrocarbon fractions (see Pfittmann et al., 1989; Piittmann and Eckardt, 1989). The obvious drastic decrease in the monoaromatic chrysene de- rivative towards the top of Kupferschiefer (Fig. 7, Table 3) is caused by the diminishing input of terres- trial organic matter into the Lower Rhine Basin in the wake of the marine transgression.

The absence of higher aromatized analogues ident- ified in various sedimentary records could be caused by the relatively low maturity of the Kupferschiefer. Photochemical degradation of ring-A might have been favoured by the palaeogeographical position of the Southern Permian Basin, only approx. 10 ° north of the equator (Glennie, 1984), providing inten- sive irradiation by solar rays.

CONCLUSIONS

The analysis of the aromatic hydrocarbon distri- bution in the Kupferschiefer of the Lower Rhine Basin provides information about the palaeosalinity of the Kupferschiefer sea and about facies changes during sedimentation.

- -Aryl isoprenoids present in the Kupferschiefer are presumably derived from isorenieratane, which was tentatively identified in low concentration.

- - A trimethylated chroman dominates by far over its mono- and dimethylated counterparts, arguing for an euhaline to mesohaline environment in the Kupferschiefer sea. Increasing absolute amounts of the compound towards the top of the shale reflect an increase in palaeosalinity.

- - A monoaromatic C24-chrysene derivative indicates an input of terrestrial derived organic matter primarily in the bottom part of the sedimentary profile. This source diminished when marine con- ditions followed the Zechstein transgression and marine biomass production dominated.

Acknowledgements--Support of this study by Professor Dr M. Wolf (RWTH Aachen) is gratefully acknowledged. Thanks are also due to N. Nettekoven (Claytex Consulting, Inden, F.R.G.) for providing UV/vis facilities and to Dr C. B. Eckardt (University of Bristol) for assistance with the porphyrin analysis.

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Appendix opposite

Aromatic hydrocarbons in the Kupferschiefer

APPENDIX

1 2 3

761

7

R

R=H, CH 3, C2H5, C3H7

10

(CH 3) (CH3) 2

(CH 3) (CH 3)

11 12 13 14

15

H3) 2

(CH3) 2 15

X : C21 H42 N: N l o r V-O

various alkyl ~ubstituents on ringa possible