Palaeoceanography of the early Zechstein Sea during Kupferschiefer deposition in the Lower Rhine Basin (Germany): A reappraisal from stable isotope and organic geochemical investigations

ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 136 (1997) 331-358

Palaeoceanography of the early Zechstein Sea during Kupferschiefer deposition in the Lower Rhine Basin (Germany): A reappraisal from stable isotope and organic geochemical investigations A. Bechtel

a,,,

W. Ptittmann b

Mineralogisch-Petrologisches Institut der Universittit Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany b Johann Wolfgang Goethe- Universitgit, Institutfiir Mineralogie/Umweltanalytik, Georg- Voigt-Strafle 14, 60054 Frankfurt a.M., Germany Received 5 December 1996; accepted 13 June 1997

Abstract

In the study area of the Lower Rhine Basin the Permian Kupferschiefer was deposited in a shallow water environment. Water exchange from this marginal basin with the Zechstein Sea was restricted by palaeohighs. The burial depth of the shale did not exceed 1000 m during the depositional history of most parts of the basin. Maturation of the organic matter was governed only by the geothermal gradient not exceeding 68°C/km during the Late Carboniferous. Due to minor thermal alteration of organic matter, diagenetic effects on the biomarker composition and the light stable isotope ratios (C, H, O, N) of organic matter and carbonates are of minor importance. In the present study, isotopic and organic geochemical data are interpreted with respect to palaeoceanographic aspects. The data provide information about salinity variations, the nature of organisms living in the water column and the importance of bacterial activity in the sediment during deposition. From the degree of methylation of 2-methyl2-trimethyl-tridecylchromans (MTTC) an euhaline to mesohaline (30-40%0) sea water salinity during Kupferschiefer sedimentation can be inferred. The high abundance of biomarkers derived from green/purple sulphur bacteria suggests HzS saturation of the bottom waters and maximum water depths below 100 m in the basin because these organisms live near the boundary between the photic zone and the anoxic (euxinic) bottom water at a depth of 10-30 m. Primary production in the upper water column was dominated by photosynthetic cyanobacteria or green algae. In the sediment, sulphate reduction occurred due to the availability of abundant sulphate and organic detritus from the overlying water column. Furthermore, methanogenesis was active mostly during early Kupferschiefer deposition. This is reflected by the light carbon isotopic composition of organic matter originating from recycling of CO2 generated by methaneoxidizing bacteria in the water column. Saccate pollen are the only morphologically preserved organic matter in the sediment. © 1997 Elsevier Science B.V.

Keywords." carbon cycle; nitrogen cycle; methane oxidation; methanogenesis; palaeosalinity; saccate pollen

* Corresponding author. 0031-0182/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0031-0182(97)00104-1

332

A. Bechtel, II'] Pt?ttmamt

Paha%~eoL,rap/ll'. Pa/aeoc/imato/og.!',

1. Introduction Stable isotopic compositions of organic matter and carbonates are widely used for the reconstruction of sedimentary environment. ['or the assessment of palaeoclimates and for palaeoceanographic studies of ancient marine ecosystems (Craig, 1965: Shackleton and Opdyke, 1973: Savin. 1977: Schidlowski, 1982: Anderson and Arthur, 1983). However, the interpretation of variations in the stable isotope record are often complicated by the effect of Eh, pH, salinity, temperature and pCO 2 on the ~513C and ~180 values, particularly for pre-Cenozoic sediments (Veizer. 1983: Longinelli, 1996). Apart t¥om facies-dependent variations, the temperature history and post-depositional events during and after diagenesis can substantially influence the isotopic composition of organic matter and carbonates (Degens and Epstein, 1962: Clayton and Bostick, 1986: Bechtel and Pt~ttmann, 1991 ). The results of recent studies of organic-rich sedimentary successions indicate that combined isotopic and organic geochemical analyses can be used to overcome these difficulties (Hayes et al., 1987: Popp et al., 1989: Macko et al., 1991: Westerhausen et al., 1993). The molecular composition of organic matter can be used to study the thermal history of sedimentary basins, the type and origin of organic matter, as well as sedimentary facies and palaeosalinity (Albrecht et al., 1976: Patience et al., 1978; Mackenzie et al., 1980: Gilbert et al., 1985: Radke, 1987). The Permian Kupferschiefer of central Europe has been investigated intensively during the last few decades (Wedepohl, 1964: Marowsky, 1969: Ptittmann et al., 1988, Pattmann et al., 1989: Bechtel and Hoernes, 1993: Wodzicki and Piestrzynski, 1994). In most of these studies, the results have been interpreted primarily with respect to base metal accumulation processes. Information about maturation differences and alteration of organic matter of the Kupferschiefer from the Lower Rhine Basin has been obtained from hydrocarbon geochemistry, kerogen composition, vitrinite reflectance and isotopic composition of organic carbon (Ptittmann and Eckardt, 1989: Pfittmann et al., 1989; Bechtel and Ptittmann. 1992). The

Pahleoeco/o~,[l' 130 ~ /997) 331 35~'

results of these studies enables a distinction to be made between altered (either by enhanced temperature or by interaction with mineralizing fluids) and unaltered samples, in which maturation of organic matter was governed only by the geothermal gradient. In the study area, the rank of the organic matter in Kupferschiefer is low enough to enable biomarker investigations of facies variations during deposition (Wolf et al., 1989: Schwark and Piittmann, 1990). However, the results from combined isotopic and organic geochemical analyses have so far not been interpreted with respect to palaeoceanographic aspects. The present study integrates new stable isotope data from organic matter (C, H and N) and carbonates (C and O) of drill cores from the Kupferschiefer of the Lower Rhine Basin with the results of previous organic geochemical and isotopic investigations. These data are used to reconstruct the palaeoceanographic record of the sedimentary basin during Kupferschiefer deposition. Variations in facies and in temperature histories are shown to influence carbon cycling and, therefore, the isotopic composition of organic matter and carbonates.

2. Geological setting and sample description Comprehensive descriptions of the overall geological setting of central Europe during the Permian are given by Wedepohl (1964), Oszczepalski and Rydzewski ( 1987L and Vaughan et al. (1989). The pre-Permian basement was folded during the Variscan orogeny in the Late Carboniferous. At the end of this orogenic phase (Westphalian C), large depressions were filled with Lower Permian pyroclastic sediments, lava flows and clastic sediments eroded from the Variscan mountains. These are typical Permian red beds, forming a lithostratigraphic unit of high rank, the Rotliegendes. In most parts of the basin fluviatile or eolian sediments, forming the Weissliegendes strata, another lithostratigraphic unit of central Europe, were deposited during the Early Late Permian. The intracontinental Rotliegendes depressions were rapidly flooded by sea water from the north and northwest (Fig. 1) during the Late Permian trans-

A. Bechtel, W Piittmann / Palaeogeography, Palaeoclimatology, Palaeoecology 136 (1997) 331 358

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A Fig. 1. Extension and morphology of the Zechstein Sea basin of North Central Europe during the Kupferschiefer deposition (from Schwark. 1992).

gression (Glennie, 1986). Also, areas which formed palaeohighs during the Early Permian were partly flooded by the invading Zechstein Sea. The marine sediments deposited after transgression built up the lithostratigraphic unit of the Zechstein (Late Permian).

The Zechstein Sea transgressed the folded Carboniferous rocks within the study area of the Lower Rhine Basin. Therefore, Lower Permian sediments are only deposited in very few areas as thin layers and there is a general lack of volcanosedimentary rocks (Diedel and Piittmann, 1988).

334

A. Bechtel, W. Pfittmann, Palaeogeography, Palaeoclimalology, Palaeoecology 136 (1997)331 358

The geological structure of the Lower Rhine Basin is characterized by a folded and faulted Carboniferous basement and a gently northwarddipping Permian, Mesozoic and Cenozoic sedimentary cover (Vaughan et al., 1989). The Kupferschiefer, a bituminous calcareous or dolomitic shale, represents the first fully marine sediment after a long period of arid to semi-arid conditions. In the area investigated it is separated only by a thin bed of fluviatile sediments from the Carboniferous basement. Post-Variscan movements along NW--SE striking faults resulted in the development of a horst and graben morphology (Fig. 2). The thin Zechstein conglomerate under-

LEGEND:

Faults ~

Areas without Kupferschiefer

~

•

lying the Kupferschiefer is coeval with the Weissliegendes sandstone of the central part of the Zechstein basin. The main components of the conglomerate are very poorly rounded and sorted fragments of Carboniferous limestones, sandstones and schists. The lower part of the Kupferschiefer shows lamination due to alternating calcareousdolomitic and argillaceous-bituminous laminae, whereas the top is characterized by increasing bioturbation. The Kupferschiefer grades upward into overlying dolomitic limestones, the Zechstein first-cycle carbonate (Zechstein limestone). The upper parts of the Zechstein succession (Z1 Z4 cyclotherms) show facies thickness development

Iso-maturation lines with relative amount of meso-pristane Drilling locations

•

Boreholessampled in this study

Fig. 2. Map showing locations of the drill sites and the main tectonic structures in the Lower Rhine Basin (from Bechtel and Pi)ttmann, 1992). Analyses presented in this study were carried out on samples from wells marked with triangles. Iso-maturation lines were obtained from measurement of the proportions of meso-pristane (Piittmann and Eckardt. 1989).

A. Bechtel, W. Piittmann / Palaeogeography, Palaeoclimatology, Palaeoecology 136 (1997) 331 358

and distribution typical of morphologically structured, shallow water environments (Wolburg, 1957). The study area is situated at the southwest border of the past Zechstein Sea (Fig. l). In this part of the basin the Kupferschiefer was deposited during the Permian under anoxic conditions. Previous investigations using petrological and organic geochemical methods have revealed that the proposed basic intrusion near Krefeld (northwest Germany), the so-called Krefeld High (Fig. 2), has been responsible for modifying the maturity of organic matter present in the Kupferschiefer (Piittmann and Eckardt, 1989; Ptittmann et al., 1989). Proportions of the mesopristane within the extractable organic matter indicated a general depth-related increase in rank of the organic matter towards the north (Fig. 2). In addition, in the western part of the study area, a zone of maturation higher than expected according to its present depth is detected (Fig. 2). This enhanced maturation was obviously caused by the higher heat flow associated with the intrusive body of the Krefeld High. Fox the present investigation, core samples within closely sampled profiles from six drilling locations (Fig. 2) in the area of the Lower Rhine Basin were selected for isotopic and organic geochemical studies. Five of the selected drill sites are located in an area where maturation of the organic matter was influenced solely by burial. The profiles were selected at different distances from the former Zechstein Sea shoreline (Fig. 2). At drill site No. 26 the Kupferschiefer was deposited on top of a palaeohigh inside the Zechstein basin (swelltype Kupferschiefer). Samples of Kupferschiefer from drill site No. 132 are included to study the possible thermal influence of the Krefeld High.

3. Analytical methods

3.1. Carbon and hydrogen isotope investigations of organic matter Bitumen was extracted over 24 h from aliquots of the pulverized and homogenized samples using

335

dichloromethane in a Soxhlet apparatus. Parts of the extracts were used for C and H isotope investigations. Carbon isotope analyses were also performed on kerogen isolates, prepared by use of standard techniques (Durand and Nicaise, 1980). The extracted samples were treated with 6 N HCI and subsequently with 30% HF to remove carbonate and silicates from the samples. After ultrasonic treatment with dichloromethane-methanol mixture (1:1), kerogen isolates were washed with distilled water, dried and homogenized. Samples of kerogen and organic extracts were prepared for mass spectrometric (MS) analyses for C and H isotopic composition by sealing a mixture of sample and CuO in evacuated quartz glass tubes and heating to 1050°C to produce CO2 and H20. The H20 was trapped together with CO2 at liquid nitrogen temperature. Carbon dioxide was subsequently separated from H20 at dry ice-methanol temperatures and was frozen into glass tubes for mass spectrometric analyses. The remaining H20 was reduced to H2 by passing it over hot uranium (Bigeleisen et al., 1952). The hydrogen gas was trapped in a sample container with activated charcoal at liquid nitrogen temperature. C-isotope measurements were carried out on VG SIRA-9 triple-collector mass spectrometer. MS analyses of the hydrogen gas were performed using a VG PRISM, 60 ° instrument. The results are reported relative to the PDB standard for 613C, and relative to SMOW for 6D. The overall reproducibility of the whole analytical procedure was in the range +0.1-0.2%0 for 613C and generally between _ 1 and 2%0 for 6D.

3.2. Nitrogen isotope investigations of organic extracts Aliquots of the total extractable organic matter from the Kupferschiefer samples were analysed for N isotopic composition using an Europa Scientific TRACERMASS mass spectrometer online to a ROBOPREP TCD. The samples were oxidized in a combustion chamber by oxygen gas, resulting in formation of C02, NOx and H20. Water was removed from the other gases by a MgC10 4 trap. Separation of nitrogen-containing gas was accom-

336

,4. Bechtel, 14~ Pfittmann

Polaeogeograp/ty, Pakwoclm~atoh~g),. Palaeoecolo,~:v 136 (IC)97) 331 358

plished by gas chromatography. N isotopic composition was determined by the mass spectrometer coupled to the combustion and gas chromatographic device. The results are reported relative to the N isotope composition of atmospheric nitrogen (air). The reproducibility of the results (6~5N) was in most cases better than 0.3%,,.

3.3. Carbon and oxygen isotope investigations oJ carbonates The decomposition of carbonate minerals to CO2 for MS analyses was done by reaction of 100% H3PO4 with the untreated, pulverized samples in evacuated glass tubes (McCrea, 1950) at 2 5 C . Carbon and oxygen isotope compositions of coexisting calcite and dolomite were obtained from one sample by MS analyses of the resulting CO2 after different reaction times, following the procedure described by Degens and Epstein (1964). The results are reported relative to the PDB standard for 613C as well as for ~5180. The reproducibility of the analytical procedure was in most cases better than +0.1%,,.

coupled to a Finnigan MAT 8200 mass spectrometer using the same column type and oven temperature program as described for GC analysis. The mass spectrometer was operated in the EI (electron impact) mode at 70eV, an emission current of I mA, and a scan range from 50 to 700 daltons ( l.l s total scan time). Data were processed with a Finnigan Incos data system. The total carbon content was determined on a Leco CR 12. The organic carbon content (Corg) was measured with the same instrument on samples that were treated with concentrated hydrochloric acid. The loss of HCl-soluble organic carbon during this treatment of the Kupferschiefer samples is negligible in relation to the total organic carbon content. The amount of carbonates (Ccarb) was calculated from the difference between total and organic carbon contents. The nitrogen and carbon contents of the kerogen isolates were determined with a Carlo Erba NA 1500 multielement analyzer. From these data the C/N ratios of the organic matter of the samples were calculated.

3.4. Organic geochemical mvestigatums

4. Results and discussion

Finely ground samples ( < 0 . 2 r a m ) were extracted in a Soxhlet-apparatus for 24 h using dichloromethane. The total extracts were separated into three fractions by low-pressure column chromatography over silica gel. Saturated hydrocarbons were eluted with hexane, aromatic hydrocarbons were eluted with dichloromethane and heterocomponents (NSO) with methanol (30 ml of each solvent was used). The saturated and aromatic hydrocarbon fractions were analyzed by gas chromatography (GC) on a Carlo Erba 5160 Mega Series gas chromatograph equipped with a 25 m SE 54 fused silica capillary column (i.d. 0.25 mm). The oven temperature program was operated from 8 0 to 300'C at a rate of 4'~C min 1 followed by an isothermal period of 20 min. Hydrogen was used as carrier gas. The compounds were quantified by adding internal standards prior to GC analysis. GC MS analyses for peak identification were performed with a Varian 3700 gas chromatograph

The 613C values of the kerogen concentrates range from - 2 9 . 2 to -24.4%,, (Table 1). These values are consistent with the C isotope compositions of ancient marine organic matter composed mainly of phytoplankton (algae) and photosynthetic bacteria with minor contribution of higher land plants (Galimov et al., 1975; Deines, 1980). The majority of the data obtained from the organic matter of the Kupferschiefer samples (Table l) compares well with the values generally reported for Paleozoic type II kerogen (Galimov, 1980). The 6~3C values determined for the extracted bitumens yield, as expected, lower values in the range of - 3 0 . 2 to -25.0%o (Table 1 ). Plots of the 613C values of kerogen versus depth indicate that the Kupferschiefer profiles can be subdivided in two groups (Fig. 3). Low 613C values of kerogen ( - 2 9 . 2 to -27.6%0) are obtained on samples from drill sites 26, 49, 127 and 134 (Fig. 3). From the bottom to the top of the Kupferschiefer, a slight enrichment in ~3C is

A. Bechtel, lie Piittmann / Palaeogeography, Palaeoclimatology, Palaeoecology 136 (1997) 331 358

337

Table 1 Characteristic geochemical data and stable isotope composition of organic compounds Sample

Depth

Co,g

Cca~b

No.

(m)

(wt%)

(wt%)

26/6 (Ca0 26/7 (Ca0 26/8 (Ca1) 26/9 (TO 26/10 (T1) 26/11 (TO 26/12 (TO 26/13 (TO 26/14 (TO 26/15 (CO 41/7 (Ca0 41/13 (T 0 41/16 (T 0 41/27 (TO 41/32 (T 0 41/39 (Ct) 49/13 (T 0 49/21 (TO 49/34 (T 0 49/51 (T 0 49/66 (T~) 127/18 (T0 127/28 (TO 127/32 (T 0 132/3 lTt) 132/13 (TO 132/19 (T0 132/21 (T 0 134/10 (TO 134/14 (TO 134/19 (TO 134/29 (T~) 134/33 (CO

559.5 560.2 560.8 561.2 561.4 561.7 561.9 562.2 562.4 563.0 738.0 738.3 738.7 739.0 739.2 739.7 748.8 749.1 749.6 750.3 750.8 769.2 769.7 770.0 383.1 383.8 384.1 384.3 595.2 595.6 595.8 596.6 596.9

0.4 0.5 0.7 0.9 1.7 2.7 3.4 4.0 2.2 0.1 n.a. 1.1 3.7 6.5 7.7 n.a. 0.5 1.5 1.0 2.8 5.5 1.7 6.0 3.7 0.8 1.3 n.a. 4.4 0.9 4.6 5.2 4.9 n.a.

7.0 7.7 8.5 8.4 5.8 6.0 7.2 5.7 6.1 1.0 n.a. 8.3 6.5 3.3 4.3 n.a. 6.8 1.9 8.8 8.7 6.2 6.3 5.6 5.8 5.9 5.3 n.a. 3.8 8.8 5.1 7.0 5.9 n.a.

mg Ext./ g Co~g

C/N

CaO/MgO a

(org)

n.a. n.a. n.a. 36.8 42.2 41.7 34.5 33.6 45.4 n.a. n.a. 34.8 48.1 42.2 46.4 n.a. 26.6 30.7 55.6 57.5 52.2 56.4 57.3 47.7 32.9 49.5 n.a. 38.4 57.2 40.8 39.8 52.2 n.a.

n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n,a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 23.2 30.8 27.6 24.1 21.4 n.a. n.a. n.a. 11.6 15.0 n.a. 8.7 26.3 31.8 25.7 16.9 n.a.

n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 1.76 1.61 1.70 1.51 n.a. 1.56 1.53 1.77 1.62 1.81 3.70 15.25 14.52 1.83 1.92 4.33 4.43 1.40 1.49 1.54 1.45 n.a.

Kerogen

Bitumen

613C (%,, vs. PDB)

()13C (%0 vs. PDB)

6D (%,,vs. SMOW)

615N (%, vs. air)

-27.3 -27.4 -27.7 -27.8 -27.7 -27.6 -27.7 -27.9 -28.2 -27.6 -28.0 -27.5 -27.0 -26.2 -25.9 -25.2 -27.6 -27.9 -28.3 -28.5 -28.5 -28.4 -28.5 -28.7 -26.5 -26.1 -25.3 -24.4 -27.9 -28.3 -28.8 -29.2 -28.6

n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. --28.5 -27.7 -26.7 -26.1 n.a. n.a. n.a. -29.8 -29.5 -28.9 -30.2 -29.8 -29.4 -27.5 -27.4 n.a. -25.0 -29.8 -29.5 -29.1 -29.6 n.a.

n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. -125 -118 n.a. - 109 n.a. n.a. n.a. -132 - 130 - 128 -137 - 133 -129 - 115 -110 n,a. -92 -135 - 132 -130 -125 n.a.

n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. +4.4 +2.6 +0.5 -0.2 + 1.4 n.a. n.a. n.a. +5.7 +4.5 n.a. +3.9 +3.3 +1.3 +0.5 +2.4 n.a.

Samples are labelled according to the drill hole number in combination with the core number (increasing with depth). Cal=Zechstein limestone; Tl=Kupferschiefer; C~ =Zechstein conglomerate; Ext.=extract; n.a.=not analysed.aData from Diedel (1986).

d e t e c t e d in t h e s e p r o f i l e s ( F i g . 3). S a m p l e s f r o m drill sites 41 a n d 132 y i e l d h i g h e r •13C v a l u e s o f

4.1. Facies variations versus post-sedimentary alteration

kerogen between -27.5 Kupferschiefer samples these two profiles show 13C ( F i g . 3) in c o n t r a s t

T h e d i f f e r e n c e s in t h e trends of organic carbon

organic

carbon

boreholes.

within

and -24.4%o (Table from the bottom part significant enrichment t o t h e t r e n d in 6~3C the

majority

of

1). of in of the

depth-related isotopic within a stratigraphic

u n i t , as o u t l i n e d a b o v e , a r e v e r y u n c o m m o n a n d a r e u s u a l l y t h o u g h t t o r e s u l t f r o m d i f f e r e n c e s in composition and origin of the biomass deposited

338

A. Bechtel. 14"~Pfiltmann Palaeogeography Pahwoc/imatology, PalaeoecoloKv 136 (1997) 331 358

Kerogen I

I

I

I

I

Drill site

26

t

Ca1

49

X

127

~,

134

\

132

Ii

T1

C1 -30

I

I

I

I

I

-29

-28

-27

-26

-25

-24

513C (PDB) Fig. 3. Isotopic composition of the insoluble organic carbon kerogen) within the Kupferschieter drill hole profiles.

in the sedimentary basin (Peters et al., 1978; Jasper and Gagosian, 1990). Another possible explanation is the occurrence of post-depositional events, such as higher heat flow or contact with hot and/or oxidizing basinal fluids (Schoell et al., 1983: Clayton and Bostick, 1986; Ohmoto, 1986; Bechtel and Ptittmann, 1991 ). The results of previous organic geochemical and stable isotope analyses suggest post-sedimentary alteration of organic matter within specific sites of the study area, which at least at drilling location 132 was associated with a thermal maturation of the organic matter exceeding the heat caused by burial (Ptittmann et al., 1989: Bechtel and Ptittmann, 1992), The organic geochemical data

indicate that, in the area affected by the intrusive body of the Krefeld High (Fig. 2), maturation of the extractable organic matter has been increased significantly. This is clearly pointed out by a plot of the relative amounts of meso-pristane within the saturated hydrocarbon fractions from the basal Kupferschiefer samples (a useful maturation parameter; Ptittmann and Eckardt, 1989) against depth (Fig. 4). In the sample from drill site 132, a maturation of organic matter much higher than expected for its burial depth is detected (Fig. 4). An increase in thermal maturation is reported to result in increasing 613C values of organic carbon (Table 1), due to the preferential release of isotopically light hydrocarbons through progressive

A. Bechtel, W. Piittmann / Palaeogeography, Palaeoclimatology, Palaeoecology 136 (1997) 331-358 300

=

,

,

I

I

=

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I 132 •

400

500 26 A

E

600

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700

22

800

900

1000 0.2

I 0.3

I 0.4

I 0.5

Proportion

I 0.6

I 0.7

I 0.8

I 0.9

1.0

of meso-pristane

Fig. 4. Proportionsof meso-pristaneversusburial depth of the Kupferschiefer.The data points of the samplesfromdrill sites,located in an area where maturation of the organic matter was influencedsolelyby burial, fall within a field delimitedby the two curves (modifiedafter Diedel, 1986).

degradation of organic matter (Schoell et al., 1983; Clayton and Bostick, 1986). Furthermore, evidence has been provided that the present isotopic trend might be the result of organic matter maturation by localized heating in combination with alteration of organic matter by ascending Carboniferous formation waters (Diedel and Pt~ttmann, 1988; Bechtel and Pt~ttmann, 1992). The influence of ascending brines is supported by higher Pb, Zn and Ba contents at the bottom of the Kupferschiefer affected by the intrusive body of the Krefeld High (Diedel and PtRtmann, 1988). The biomarker compositions suggest that the divergence of the 613C values at drill site 41 from the tendency obtained in Kupferschiefer profiles distant from the intrusive body (Fig. 2) cannot be explained by temperature effects (Bechtel and Pt~ttmann, 1992). The data from organic geochemical investigations indicate that the maturation of

organic matter in this profile was influenced solely by burial (Fig. 4; POttmann et al., 1989). The isotope data of organic carbon obtained from the Kupferschiefer at drill site 41 has been tentatively interpreted as the result of the oxidative potential of ascending formation waters from the Rotliegendes unit, underlying the Kupferschiefer exclusively at this drilling location (Bechtel and Piittmann, 1992). The post-depositional effects on isotopic composition of organic matter from Kupferschiefer at drill sites 41 and 132 (altered samples) are further indicated by the results of stable isotope (C, H) analyses of extracted bitumens (Table 1). The extractable organic matter from drill sites 49, 127 and 134 yield low 61~C and 6D values. Hydrogen isotope composition ranges from - 137 to - 125%0 (Fig. 5). Highest enrichment in deuterium in the bottom part of the Kupferschiefer is shown by the

340

,4. Bechtel, 14: Piittmann Palaeogeogral~tlv. Palaeoc/imalologT, PakwoecoloKv 136 f 1997) 331 358

-90

i

I

i

Drill site

O -100

O =E

41

•

49

X

127

•

132

z~

134

=/ •

-110

o~ Q

/

/

/

¢

/

/

-120 /

/

//

A

-130

-140

-32

I

I

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-30

-28

-26

.

-24

13 C ( P D B ) Fig. 5. Cross-plot of the J*'~C vs, bD ~alues from extractable organic matter from Kupferschiefer samples (modified alter Bechtel and P0ttmann. 1992).

H isotope data of the total extracts from drill sites 41 and 132. A good positive correlation ( R = 0.981) between 613C and 6D exists (Fig. 5). The results are in good agreement with D/H and 13C/L'C fractionation trend lines of light hydrocarbons obtained during laboratory simulation experiments (Sackett, 1978). Therefore, the data are interpreted to reflect the isotopic sensitivity of extractable organic matter to thermocatalytic production. The effects of post-sedimentary alteration on molecular and stable isotopic composition of organic matter have been discussed in detail by Bechtel and POttmann (1992). In the present study, the potential of biomarkers and stable isotope signatures for reconstruction of the sedimentary environment during Kupferschiefer deposition is

evaluated at drill sites 26, 49, 127 and 134 (unaltered samples). 4.2. Source ojorganic matter

In most of the profiles the 613C values of the organic matter (kerogen) show a tendency towards an isotopically heavier composition from the bottom to the top of the Kupferschiefer (Fig. 3). This isotopic trend is most pronounced at drill site 134. whereas in the profile from drill site 26 only a slight depletion in 13C is detected in the basal Kupferschiefer unit (Fig. 3). The 6J3C trend-lines of both other profiles fall in between. In the upper part of the Kupferschiefer sequence, organic matter from all investigated drilling locations yield nearly identical 613C values (Fig. 3). The organic

A. Bechtel, W. Piittmann / Palaeogeography, Palaeoclimatology, Palaeoecology 136 (1997) 331 358

carbon from the Zechstein conglomerate (C 1) generally show higher ~13C values in comparison with the lowermost Kupferschiefer sample from the same drilling location (Fig. 3). For the interpretation of these stable isotope signatures with respect to changes in sedimentary environment, the effects of possible variations in the type of organic matter input on the 6~3C values must be evaluated (Degens, 1970). The observed isotopic trend in 613C within the profiles could be interpreted as the result of maximum input of terrigenous organic matter, primarily produced by photosynthesis of C3 plants, at the beginning of Kupferschiefer deposition (Jasper and Gagosian, 1990). A possible clarification of the validity of this explanation can be drawn from previous microscopical investigations of 20 Kupferschiefer samples which have shown that bisaccate conifer pollen are the dominant morphologically preserved organic matter in the near-shore Kupferschiefer (Fig. 6; Wolf et al., 1989). In contrast to the possible implication from isotopic tendencies, outlined above, the observed pollen concentrations are highest in the upper part of the Kupferschiefer. This is in agreement with the results already reported by Grebe (1957), and reflects the enhanced density of vegetation near the Zechstein Sea shorelines due to the change from a flora typical of arid to semi-arid climatic conditions to a flora of near-shore areas with relatively high humidity (Grebe, 1957). Horsetails and conifers predominate, and an increasing amount of this pollen was transported during Kupferschiefer deposition from the surrounding shorelines into the basin of the Zechstein Sea, presumably both by wind and by rivers (Grebe, 1957). The depth-related trends of the C/N ratios of the organic matter further support the results of the microscopic studies (Fig. 7). The C/N ratios of the Kupferschiefer samples vary between 8.7 and 30.8 (Table 1). They are consistent with the proposed dominance of the organic matter of the Kupferschiefer by marine phytoplankton (algae) and photosynthetic bacteria (Ptittmann et al., 1989; Wolf et al., 1989). The C/N ratios of three borehole profiles yield a maximum in the uppermost middle part of the Kupferschiefer, indicating

341

an enhanced input of terrigenous organic matter (Sweeney et al., 1980). The samples from drill site 132 show lower C/N ratios (between 8.7 and 15.0; Table 1) as compared with the other samples (Fig. 7). Most probably, this is the result of the already discussed enhanced maturity of the organic matter, which is known to be associated with the enrichment of nitrogen in relation to organic carbon (Patience et al., 1990). Under consideration of these findings, the variation in terrigenous organic matter input is reflected by the C/N ratios, in contrast to the observed 13C depletion of organic carbon at the base of the Kupferschiefer. Therefore, the ~13C trends in the Kupferschiefer profiles are not governed by the amount of terrigenous organic matter.

4.3. COz recycling Correlation plots involving •13C of carbonate (~13Ccarb) and organic matter (613Cker) as well as the contents of the carbon species are widely used in the literature for identification of the processes responsible for isotopic trends in organic matter (Kelts and HsiL 1978; MacKenzie, 1985; Imbus et al., 1992). In order to use such diagrams in the present study, the 613C values of carbonate from the Kupferschiefer profiles of the Lower Rhine Basin are required (Table 2). Carbon isotope data of kerogen reveal that, apart from the type of organic matter input, carbon isotopic variations are dependent on the primary productivity in the water column and the availability and species of dissolved inorganic carbon (DIC) used by algae for photosynthesis (Galimov, 1980; Schoell, 1984; Fogel and Cifuentes, 1993). Variations in the availability of DIC are interpreted by the 'planktonic bloom' model (Fogel et al., 1988; Elmore et al., 1989), or have been reported to be the result of global changes in the atmospheric CO2 content (Poppet al., 1989; Jasper and Hayes, 1990). Possibly influential parameters (e.g. productivity and speciation of DIC) must be ruled out in order to reconstruct changes in the CO2 content of the Earth's atmosphere from 613C variations in organic carbon. Such information cannot be assessed using the available data from the Kupferschiefer in the study area. However, cross-

342

A. Be~4tteL W. Piittmann / Palaeogeography, Palaeoclimatology, Palaeoecology 136 [ 1997) 331 358

0

50

I pm 100

Fig. 6. Photomicrograph of bisaccate conifer pollen, Kupferschiefer. UV-beam reflected light.

correlation of the parameters Corg, Ccarb, 613Cker and •13Ccarb may enable a better understanding of the ancient Kupferschiefer carbon cycle. Positive correlations between the carbonate content (% Ccarb) and the ~5~3Cc,rb values are interpreted as an indicator of calcite precipitation in response to late spring-early summer mass planktonic blooms (Kelts and Hs/L 1978; MacKenzie, 1985). An apparent positive correlation between organic carbon content (% Corg) and carbonate content has been reported as further support to a link between planktonic blooms and carbonate precipitation (Imbus et al., 1992). The correlation diagrams indicate that these parameters are almost uncorrelated in the Kupferschiefer samples investigated (Figs. 8a,b). Therefore, the planktonic bloom model may be rejected as an explanation of the observed stable isotope records in the Kupferschiefer. The rather good inverse correlation between c~3C of organic matter and the organic carbon

content suggests accumulation of dissolved C O 2 in the water column, originating from degradation of organic matter by bacterial oxidation or sulphate reduction (Fig. 8c). This process is known as 'CO 2 recycling' of degradation products. Subsequent incorporation of this CO2 into the biomass by photosynthetic organisms results in isotopically light 6~3C values of the organic matter preserved in the sediments (Eewan, 1986; Fogel et al., 1988). In Paleozoic and younger sediments a light carbon isotopic composition (< - 24%o) of kerogen is often observed in fully marine sediments, especially in black shales deposited under euxinic conditions. Lewan (1986) discussed various possible explanations and favours the concept of CO2 recycling in the water column. According to this concept, amorphous kerogen with 6~3C values ranging from -20%o to -24%0 are expected to occur in basins with well circulated water columns, where atmospheric-derived CO2 is predominantly

A. Bechtel, W. Piittmann / Palaeogeography, Palaeoclimatology, Palaeoecology 136 (1997) 331-358

100 ~ T

~)

t

T

I'

80

q) C

._q ,.1: ,-

60

._~ r

~

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0

~

D r i l l site

20

49

--0--

5

10

15

20

25

30

~32

35

C/N (or(j) Fig. 7. Variation in C/N elemental ratios of organic matter from Kupferschiefer samples within selected borehole profiles.

used for photosynthesis. On the other hand, kerogen with 6~3C values from -26%0 to -35%o is explained as having originated from restricted basins with a stratified water column, where recycled, organic-derived CO/is the dominating source of carbon in the photic zone. This relationship is attributed to additional isotopic fractionation during microbial degradation of organic matter in strictly anaerobic environments (Lewan, 1986; Imbus et al., 1992). According to this concept, the carbon isotopic data would indicate sedimentation under euxinic conditions in a restricted basin at the beginning of Kupferschiefer deposition. During progressive transgression of the Zechstein Sea, sedimentation took place under normal marine conditions (Paul, 1982). In agreement with this interpretation, samples with high amounts of organic carbon in general show lower 6~3C values of carbonate (Fig. 8d). The negative correlation provides evidence for the

343

generation of carbonates from isotopically light CO 2 through degradation of organic matter (Irwin et al., 1977; Pierre, 1989). Carbonate minerals in sediment-hosted sulphide deposits are usually characterized by a large variation in 613C with a predominance of negative values, indicating the well known processes involving the incorporation of organic degradation by-products into early carbonate cements (Ohmoto, 1986; Imbus et al., 1992). In Fig. 9, the 613C values of carbonate are plotted versus depth. A tendency towards lower 6~3C values with increasing depth is observed, suggesting maximal incorporation of light carbon in carbonates of the basal Kupferschiefer (Fig. 9). This is further supported by the inverse correlation of ~13C values of carbonates with V/Cr ratios in the Kupferschiefer from the Lower Rhine Basin (Fig. 10; Schwark, 1992). V/Cr ratios are well established in the literature as a facies indicator in organic-rich mudstones (Stribrny and Puchelt, 1991). Sediments deposited under strictly anoxic conditions generally show ratios higher than 2, whereas V/Cr ratios <2 indicate sedimentation under anoxic to weakly oxidizing conditions. The inverse correlation of the t~13C values with this redox parameter suggests that the C isotope composition of carbonate was governed by the redox conditions in the sediment (Fig. 10). The 613C~arb depleted values in the lower part of the Kupferschiefer indicate increasing remineralization of organic-derived CO2 together with an increase of the reduction potential (Figs. 9 and 10). The results from cross-correlations, in summary, point towards the influence of variations in the sedimentary environment on C isotope composition of both organic matter and carbonates (Figs. 8 and 10). Therefore, the isotopic trends in the unaltered Kupferschiefer profiles (see Section 4.1 ) are suggested to be mainly controlled by changes in redox conditions in the sediment and the water column (Figs. 3 and 9). The C isotope data are consistent with the presence of a stratified water column at the beginning of Kupferschiefer deposition. Sedimentation took place in a shallow water basin with restricted water exchange, in which euxinic conditions were presumably established first in depressions inside the morphologically

344

A. Bechtel, W Piittmann Palaeogeography, Palaeoclimatoh)gy, Palaeoecology 136 (19971 331 358

Table 2

Carbon and oxygen isotope composition of carbonates Sample

Depth

Calcite

No.

(m)

(~3(C,}, vs. PDB)

dLsO (%, vs. PDB)

~3 C (%, vs. PDB)

~5~sO (%o vs. PDB)

26/6 (Ca1) 26/7 (Ca0 26/8 (Ca 0 26/9 (TO 26/10 (TL) 26/11 (Y~) 26/12 (T 0 26/13 (T~) 26/14 ( T~ ) 26/15 (C~) 41,7 (Ca~) 41/13 (T~) 41/'16 (T~) 41/27 (T 0 4132 (T~) 41/39 (C~) 49/13 (T,) 4 9 2 l (T~) 49/34 {T~) 49/51 ( T~ 1 49/66 (TO 127/18 (T l) 127/28 ( T t ) 127/32 (T 0 132/3 (TO 132,'13 (T~) 132/19 (Tl)

559.5 560.2 560.8 561.2 561.4 561.7 561.9 562.2 562.4 563.0 738.0 738.3 738.7 739.0 739.2 739.7 748.8 749.1 749.6 750.3 750.8 769.2 769.7 770.0 383.1 383.8 384. I 384.3 595.2 595.6 595.8 596.6 596.9

+ 2.8 + 2.6 + 3.2 + 3.3 +2.7 +3.3 +3.2 + 1.7 + t).9 4.2 + 2.8 +3.0 +3.0 +2,5 +3.0 +1/.4 +3.7 +3.3 + 3.5 + 3.0 +2.9 +2.6 + I. 1 + 1.5 0.3 1.1 - 2.2 -2.2 ~-3.8 ÷ 3.2 +2.8 + 2.1 n.a.

0.2 +0.6 - 1.5 -2.0 + 1.6 +3.1 ~3.0 +0.5 ~-11.5 5.8 1.6 1.5 1.6 2.4 1.7 2.7 1.5 1.7 - 1.8 2.2 1.3 3.4 5.5 5.3 2~0 2.7 4.8 5.1 r0.7 ~ 0.6 +0.5 + 0.2 n.a.

+4.5 +4.3 +4.3 +4.8 +4.1 +4.0 +3.9 +3.6 + 2.4 +0.7 + 3.8 +3.9 +4.2 +3.5 +3.7 + 1.4 +4.6 +4.3 +4.0 + 4.0 +3.5 +4.9 + 2.4 +2.3 +2.6 +2.2 + 1.9 + I.I +4.7 + 4.2 +3.6 + 2.8 + 2. I

+ 1.4 + 1.5 + 1.9 +2.8 +2.7 +3.5 +3.5 +3.1 + 2.5 + 1.2 0.6 0.6 -0.9 -2.4 - 1.8 - 1.7 - 1.1 -0.1 - 1.6 - 1.6 - 1.1 - 1.4 - 4.4 -5.6 -2.1 -2.2 --4. I -4.6 + 1.3 + 1.2 + 1.2 + I. 1 -0.8

132/21 (TI) 134/10 134/14 134/19 134/29 134/33

(T~) (Tt) (T 0 (T~) (C 1)

Dolomite

For explanation of the abbrevations and the numbering system of the samples see Table I.

structured basin. In deeper parts of the water column an extensive zone with free H2S was evolved. Fig. 11 shows the palaeogeographic reconstruction of the study area with the morphology of the Lower Rhine Basin during Kupferschiefer sedimentation (Schwark, 1992). Water exchange with the Zechstein Sea in the north was restricted by palaeohighs, such as the Ratum-Winterswijk Swell and the Bentheim Swell. Progressive transgression of the Zechstein Sea resulted in the evolution of

marine conditions within the sedimentary basin (Paul, 19821. The geochemical (Corg, Ccarb, V/Cr) and isotopic data in the upper part of the Kupferschiefer (Figs. 3 and 9) indicate deposition under normal marine conditions, with the chemocline ( O 2 / H 2 S ) below the sediment-water interlace. Enhanced carbonate precipitation, due to invading sea water and progressive evaporation, indicate the beginning of the first evaporation cycle of Zechstein. The outlined evolution in sedimentary environment gave rise to differences in

-29.5

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4.

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i

@

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4. O •

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Fig. 8. Kupferschiefer samples. Cross-plots of: (A) 613C of carbonate vs. carbonate carbon content; (B) carbonate carbon content vs. organic carbon content; (C) 613C of kerogen vs. organic carbon content; (D) 613C of carbonate vs. organic carbon content.

C.

#

r

A.

,m

3

4

t~

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346

A. Bechtel, W. Pt~ttmatm

Palaeo~eograpttr, Palaeoclimahdogy, Palaeoecology 136 ( 1997~ 331 358 Carbonate I

I

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i 2

i 3

Drill site

26

Ca1

41

V

49 m

- X -

L~

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134

T1

C1 0

i 1

6 13C (PDB) Fig. 9. Carbon isotope composition of tile carbonates within the Kupferschiefer profiles.

the ecosystems in the water column and consequently in the local carbon cycle.

4.4. Palaeoecology The interpretation of the isotopic data with respect to changes in palaeoenvironmental conditions is supported by earlier organic geochemical investigations of the biomarker composition of saturated and aromatic hydrocarbon fractions in Kupferschiefer from the Lower Rhine Basin (Schwark and Ptittmann, 1990: Bechtel and Ptittmann, 1992). Here, only the main results concerning the facies variations are summarized. Analyses of the saturated hydrocarbon fractions

of four samples from the 1.9 m thick succession in drill hole 134 indicate variations in the polycyclic hydrocarbon composition (Bechtel and PiJttmann, 1992). Three major sets of compounds are identiffed using GC-MS analyses, The results of quantification of individual compounds are graphically presented in Fig. 12. Based on the mass spectral data, the dominating compound among a set of tetracyclic hydrocarbons was tentatively identified as des-A-arborene (Bechtel and Pfittmann, 1992). The amount of this tetracyclic alkene decreases significantly from the bottom to the top of the Kupferschiefer profile ( Fig. 12). The compound probably originates from the ring-A degradation of pentacyclic hydro-

A. Bechtel, W. Piittmann / Palaeogeography, Palaeoclimatology, Palaeoecology 136 (1997) 331-358 \

LEGEND

IX

\\

•

347

G~ x

~qO

~

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\

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A m £3 n

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,,....=b x

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~

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. .'..- ,:,.- :..,,:.'.

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\

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ols

i

125

2

zs

";'.q

f....'.;:. ~~.~.

a

V/Cr

;.'~':"

'=..~ ~i, t , ~ ' ~

.~:::i!]Dortmund

Fig. 10. Cross-plot of the (~13C values of carbonate vs. the V/Cr ratios of the Kupferschiefer of the Lower Rhine Basin (modified after Schwark, 1992).

"""'":"~ii iiiiiiiiiii!iii;iiiSXOLigseldorf ~'/<~/)(~O~, carbons, which is thought to be mediated either by bacteria or by photochemical activity (Corbet et al., 1980). Recent experiments have shown that ring-A degradation of pentacyclic triterpenoids is mediated by the microbial activity of sulphatereducing or methanogenic bacteria rather than by photochemical activity (Wolff et al., 1989). Therefore, the observed high quantity of des-Aarborene in the bottom part of the Kupferschiefer most likely is caused by the activity of anaerobic bacteria (Fig. 12). These bacteria were able to degrade the related pentacyclic precursor primarily under the anoxic conditions at the beginning of Kupferschiefer sedimentation. The activity of anaerobic bacteria decreased significantly, together with a shift to more oxic conditions with ongoing sedimentation as reflected by decreasing amounts of des-A-arborene (Fig. 12). Four diasterenes were detected in extracts of Kupferschiefer samples from drill site 134; the compounds elute with the so-called saturated hydrocarbon fractions. The variation of their Corg-normalized amounts within the profile is shown in Fig. 13. The concentrations of the two C27-diasterenes remain largely constant, whereas

Fig. 11. Palaeogeographic reconstruction of the study area during the Late Permian (from Grice et al., 1996). • = Location of boreholes sampled in this study.

both C29-diasterenes show parallel variations with the highest concentration in the uppermost middle part of the Kupferschiefer and decreasing amounts towards the bottom and the top (Fig. 12). This pattern suggests that the organisms which produce predominantly C29-sterols had a maximum of their productivity during deposition of the middle part of the shale. The predominant primary producers of C29-sterols are phytoplankton and photosynthetic bacteria, living in the photic zone of the water column (Volkman, 1986). In contrast, the decreasing Corg-normalized concentrations of the hopanes towards the top of the shale suggest a decreasing activity of bacteria having hopanoids as membrane constituents (Fig. 12). Hopanoids have been found in many procaryotes including some growing anaerobically

348

A. Bechtel. H[ PtTillmam1 Palac,A~c,,v*rupln. Palaeoclimamlo~y, PedaeoecoloAu" 136 ( 1997~ 331 35~

594.5

I

I

I

--~-~

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I I C27-diasterene (20S)

~-

C27-diasterene (20R)

-~]~

C29-diasterene (20S)

I

tetracyclicalkene

595.0

i

i

]

i

17 (H), 21 (H)-homohopane(22R)! _~t

17 (H), 21 (H)-homohopane(22R) i

C29-diasterene (20R)

T \

"--"

E

\\,

595.5 /

"v CI

\

596.0

~ f

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/

\ \

1 \

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597.0 0

I 5

I 10

I 15

I 20

25 0

I 10

I 20

I 30

I 40

t

50 5

10

15

20

25

Fig, 12. Concentrations(microgram gram C,,~g)of individual biomarkcr compoundsdetermined within the extracts of Kupferschiefcr samples from drill site 134 (from Bechteland P/:lttnaann, 19~)2t. (Ourisson et al., 1979). The sterene/hopanc ratio increased significantly during Kupferschiefer sedimentation, indicating an increasing marine influence in the basra during the initial stage of Zechstein transgression (Bechtel and P0ttmann, 1992). This corresponds very well with the already discussed tendencies in the ~93C values of the organic matter and carbonates from the Kupferschiefer profiles. In a series of monomethylated to trimethylated 2-methyl-2-trimethyl-tridecylchromans ( M T T C ) , a trimethylated C>-chroman (tri-MTTC) occurs in the Kupferschiefer samples in significant amounts and has been identified by comparison of the mass spectrum of the aromatic hydrocarbon fractions with published data (Sinninghe Damste et al., 1987: Schwark and Piittmann, 1990). This compound dominates by far over its monomethylated and dimethylated counterparts, indicating euhaline to mesohaline (30 40%.) conditions ill the early Zechstein Sea (Schwark and Piittmann, 1990). The results from quantitation of trimethylMTTC in the Kupferschiefer profiles 26 and 212 are shown in Fig. 13. The absolute amounts of the

compound increase by a factor of about 2.5 from the bottom to the top. The uniform increase in the trimethyl-MTTC has been interpreted as reflecting an increase in palaeosalinity in the sea water during Kupferschiefer deposition (Schwark and Ptittmann, 1990). The aromatic hydrocarbon composition of nearshore Kupferschiefer profiles is further characterized by the occurrence of trimethyl-substituted aryl isoprenoids in the range from C~1 to Ce~ (Schwark and Ptittmann, 1990), These compounds have been suggested to be degradation products of isorenieratane, which was tentatively identified in low concentrations (Schwark and Piittmann, 1990). In order to investigate the influence of facies changes on the aryl isoprenoid distribution, quantitation of the C~3- and C14-aryl isoprenoids has been carried out for six samples from well 212 ( Fig. 2; Schwark and Ptittmann, 1990). The results show that the absolute amounts of these compounds significantly decrease towards the top of the Kupferschiefer to approximately 30% of the concentration at the base (Fig. 14). Aryl isoprenoids are thought to be derived from

A. Bechtel, W. Piittmann / Palaeogeography, Palaeoclimatology, Palaeoeeology 136 (1997)331 358 100

r-mu)

I

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Jk

Drill site 26

100

Drill site 212 ~ i

I

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A

J

C13"arylis°pren°id - ' ,d ' [

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g I g Corg tri-MTTC Fig. 13. Absolute amounts of the trimethylated C:9-chroman (tri-MTTC) in wells 26 and 212 (modified after Schwark and Pfittmann, 1990).

carotenoids specific for the photosynthetic green sulphur bacteria Chlorobiaceae and purple sulphur bacteria Chromatiaceae (Summons and Powell, 1987). These organisms are phototrophic anaerobes and thus require both light and H2S for growth. In modern environments they appear in sulphate-containing water bodies which are sufficiently quiescent and organic-rich to enable sulphide production close to the photic zone (Summons, 1993). Thermal and salinity stratification is usually involved, such as may be found in lagoonal environments. Euxinic conditions in the deep-water zone are required and the intensity of the green spectral component of the light used for photosynthesis should be reduced by particulate organic matter or vegetation in the water column (Pfennig, 1977). Therefore, purple and green sulphur bacteria are preferentially found in nearshore environments. The proposed appropriate conditions for the development of purple/green sulphur bacteria are

0

0

I 10

I 20

I 30

I 40

50

g I g Corg Fig. 14. Absolute amounts of the C13- and C~4-aryl isoprenoids within the extracts of the Kupferschiefer from drill site 212 (data from Schwark and Pt~ttmann, 1990).

illustrated in Fig. 15 (Schlegel, 1985). In more basinward areas other photosynthetic organisms, which do not require HzS, predominate (Fig. 15). Therefore, at drill sites 49 and 127, representing the conditions in the deeper parts of the basin, only lower amounts of aryl isoprenoids have been detected in the Kupferschiefer. The low 613C values of the organic matter obtained in the basal Kupferschiefer, most pronounced at well 134 (Fig. 3), cannot be the result of the higher concentrations of aryl isoprenoids originating from photosynthetic green/purple sulphur bacteria, because these are reported to produce a biomass enriched in 13C (Hartgers et al., 1994). Recent investigations of sedimentary organic matter have shown that, in sediments with a high abundance of biomarkers from green/purple sulphur bacteria, the bulk organic matter is not significantly enriched in ~3C (Hartgers et al., 1994). Instead, the isotopic shift in 613C in bottom samples from well 134 might rather reflect a contri-

350

.4. Bechtel, 14~ Piittmam7 ' Palewogeography+ P¢th~eoclimatology, Palaeoecology 136 I097) 331 358

~

c--- Particulate organic matter : ....

7--- an-dl°v-eget r -ati°n -~ ~!~-A~_

-a,." Purple sulfur bactena : 5ae-~ ~,~

-Hz,S

Purple sulfur bacteria Algae, Cyanobacteria. ! ~ "

Sulfate reduction

Sapropel

Fig. 15. Illustration of the proposed appropriate conditions for the development of purple sulphur bacteria (adopted from Schtegel. 19851.

bution of methane oxidizers to the CO2 recycling than the activity of green/purple sulphur bacteria. The activity of methane-oxidizing bacteria in the water column requires the formation of methane in the sediments below. The presence of methanogenic bacteria in the sediment has been indicated by the presence of C40-isoprenoids in the Kupferschiefer of the Lower Rhine Basin (unpublished results). An alternative explanation for the progressing depletion of kerogen in 13C with approach to the Zechstein Sea shoreline is the higher contribution of terrestrial organic matter and of aerobic bacteria to the sediment. This is supported by higher amounts of hopanes within the extracts from near-shore Kupferschiefer ( Bechtel and PiJttmann, 1992). The isotopically heavier C isotope data of drill site 26 can be explained taking into account that

this borehole is situated over a palaeohigh. The location of the palaeohighs are marked in the reconstruction of the physiography during the initial stage of Zechstein transgression (Fig. 11). Kupferschiefer in the area of drill site 26 was deposited on top of a swell, presumably under more oxic conditions (Fig. 111. The Kupferschiefer succession at this drilling location shows a higher thickness, and is characterized by lower organic carbon content (Table 1) and high carbonate content. Atmospheric-derived CO2 was the predominating source of organic carbon fixed by photosynthesis. 4.5+ Nitrogen ~Tcle

Analysis of the total organic extracts from 12 Kupferschiefer samples for organically bound

A. Bechtel, W. Piittmann / Palaeogeography, Palaeoclimatology, Palaeoeeology 136 (1997)331-358

nitrogen elemental abundances (% N, C/N) and stable isotope composition (615N) was conducted in an attempt to infer ancient nitrogen flux and biochemical incorporation mechanisms (Table 1). The samples selected for analysis represent thermally altered (drill site 132) and unaltered Kupferschiefer successions and varying ~13Cker composition (Fig. 3). The major nitrogen-containing compounds in the extractable organic matter are Ni and V---O porphyrins (Schwark and Ptittmann, 1990; Ptittmann et al., 1991). The organic material of the Kupferschiefer deposited in the Lower Rhine Basin is enriched in porphyrins with contents up to 3700 ~tg/g Corg (Eckardt, 1989). The content of N i + V = O porphyrins reaches about 5.5% of the total extract in the bottom sample of well 212 (Fig. 2). A decrease in the content of Ni porphyrins is accompanied by an increase in V = O porphyrins towards the top of the Kupferschiefer (Fig. 16). The total amounts of porphyrins remain largely constant and show only a slight minimum in the middle part of the Kupferschiefer (Fig. 16). The porphyrins identified in Kupferschiefer by direct probe MS analysis largely originate from

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chlorophylls of photosynthetic bacteria (Eckardt, 1989). As already discussed, the lower C/N ratios of the samples from drill site 132 have been attributed to the enhanced thermal maturity of organic matter (Fig. 7). The organic extracts of these samples yield the highest 615N values of all investigated bitumens, which is proposed to be the result of maturation of organic matter due to the preferential loss of lSN-depleted heterocompounds during thermal degradation (Fig. 17). In contrast, both other profiles (49 and 134) show nearly identical variation with the lowest 615N values of organic extracts in the lowermost middle part of the Kupferschiefer and ~SN enrichment towards the top and downward. Isotopically enriched 6aSN values are associated with low C/N ratios (Table l; Figs. 7 and 17) and in the basal Kupferschiefer, also with depleted 613Ckervalues (Fig. 3). The total 6aSN range observed for the Kupferschiefer of wells 49 and 134 (-0.2 to +4.4%0) is largely coincident with the range observed for marine sediments (Schidlowski et al., 1983). Isotopically depleted 6~5N values are reported to be suggestive of the incorporation of

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352

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Kupferschiefer are best explained as reflecting the increasing contribution of isotopically enriched marine biomass (Fig. 17). The 6tSN values approaching 0%o in the middle part of the shale would suggest primary contribution of nitrogen fixers (e.g. cyanobacteria; Fig. 17). In the basal Kupferschiefer, the biomarker distribution suggests a maximum of the productivity of anaerobic bacteria probably under euxinic conditions. Conceptually, therefore, tSN_enriched organic extracts from the bottom of the profiles 49 and 134 most probably indicate additional isotopic fractionation during bacterial degradation. In the lower part of the Kupferschiefer depleted 6~3Ck~r values, which are suggested to originate via recycling of CO+ from degrading organic matter, are associated with enriched 61SN values (Figs. 3 and 17). That implies the occurrence of greater microbial degradation and subsequent re-incorporation of the products into the biomass of primary producers. A comparable nitrogen cycling scenario has recently been outlined from the results of stable isotope studies on the Precambrian Nonesuch Shale (Wisconsin, Michigan, U.S.A.) by lmbus et al. (1992).

4.6. Water stratification antt salinity molecular nitrogen (6~SN,~=0<'/,,,,) via the activity of symbiotic nitrogen-fixing bacteria associated with higher plants and/or the ability of some freeliving prokaryotes (e.g. cyanobacteria) to fix atmospheric nitrogen (Fogel and Cifuentes. 1993). In contrast, isotopically enriched values approaching filSN= +10%,+ suggest planktonic growth in marine regimes where nitrogen metabolism is effectively limited to the assimilation of nitrate. However, the possibility that these values might be influenced by biogenic N-recycling must be taken into account, in order to apply nitrogen isotope composition of organic matter to reconstruct palaeoecology (lmbus et al.. 1992). As indicated by the biomarker distribution pattern (see Section 4.4), only algae and microbial organisms lived in the early Zechstein Sea. Increasing marine conditions in the study area are proposed to have been established during the Zechstein transgression. Therefore, the increasing (5~SN values from the middle part to the top of the

The results of stable isotope analyses on carbonate minerals from the Kupferschiefer might be useful in detecting facies variations and overprinting by diagenetic processes. A plot of the bL+C. 6~sO data of the carbonates shows that most of the samples yield 6t3C values of calcite in the range of +0.9 to +3.8%. (Table 2: Fig, 18). The 6~80 values of the calcites vary from -5.5 to +3.1%,,, related to the PDB standard (Table2), The carbonates fall into the field of marine carbonate in the classification by Keith and Weber (1964). In general, the calcites and dolomites show almost parallel variations in the Kupferschiefer profiles with t3C and 180 enrichment of dolomite ( Fig. 18). Dolomite is the dominating carbonate in most of the samples. The amount of calcite falls generally in the range of 15- 30% of the total carbonate content, except in the samples from the lower part of the Kupferschiefer of the profiles 127 and 132, where the calcite content reaches up to ~60% of

A. Bechtel, W. Piittmann / Palaeogeography, Palaeoclimatology, Palaeoecology 136 (1997)331-358 I

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the total carbonate. The depletion in dolomite in these samples is indicated by an increase in the CaO/MgO ratio (Table 1; Diedel, 1986). These variations in calcite/dolomite ratios are strongly reflected by the measured 61sO values (Table 2). In drill sites 127 and 132 the depletion in dolomite is accompanied by a depletion in 1sO. This correlation is clearly outlined by a plot of the 6~80 values of carbonate versus the CaO/MgO ratios of the Kupferschiefer samples (Fig. 19). Therefore, the mineralogical composition of the carbonates obviously governs the variation in the &180 values (Fig. 19). Similar observations have been reported from isotopic investigations of the Marl Slate in England, which is the lateral equivalent of the Kupferschiefer of North Central Europe (Sweeney et al., 1987). Indications for a primary formation of dolomite

in the Kupferschiefer and the Marl Slate have been found recently (Sweeney et al., 1987; Schwark, 1992). Therefore, the observed correlation of the 6~80 values with the mineralogy of carbonates can be explained by contrasting amounts of calcite versus dolomite precipitation in the basin. Dolomite is assumed to be formed primarily in the reducing, deeper zone of the water column (Sweeney et al., 1987). In contrast, calcite is thought to precipitate in near-surface, oxygen-rich water. The variation in lSO-enrichment in carbonate with dolomite content would, therefore, reflect 6180 differences in the water column, probably associated with temperature and salinity stratification (Broecker, 1974; Faure, 1977). The array of the data points of the Kupferschiefer samples in the 613C-6180 diagram indicates almost no correlation between 5180 and

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513C for the carbonates from drill sites which show only minor variation in calcite dolomite ratios (wells 26, 41. 49 and 134: Fig. 18). The carbon isotopic trends seem to be uncorrelated with oxygen isotope variations, most probably due to presumed incorporation of organic-derived carbon into early carbonate cements of the basal Kupferschiefer. The carbonate minerals from drill sites 26 and 134 yield ~180 values, which are higher (on average 3.5%,, and 2%,,, respectively) compared to the samples from the other Kupferschiefer profiles (Fig. 18). These differences in 6180 can be related either to carbonate formation from 10 15'C colder sea water in these parts of the Zechstein basin (Craig, 1965), or to recrystallization during early diagenesis under the influence of isotopically heavy, hypersaline formation waters (Anderson and Arthur, 1983). Temperature variations of the above mentioned magnitude seem unrealistic within this regionally restricted sedimentary basin

Fig. 2). Broecker (1974) and Faure (1977) have presented relationships between the 5180 values and the salinity of the water in the North Atlantic and the Red Sea, respectively, Compared with the data from the Red Sea, an increase of 2 3.5%,, in ~i~sO of the sea water correlates with a salinity increase of approximately 5 10%,,,due to preferential loss of H~60 during evaporation ( Faure, 1977 ), According to these observations, the enriched 6'SOc,rb values might be the result of precipitation from 1SO-enriched ocean water due to enhanced salinity. The later explanation seems very reasonable because the carbonates with high 6180 values are measured in wells 26 and 134, which represent swells and near-shore areas inside the basin, respectively. In these zones of shallow-water marine basins a higher salinity of the water is often observed, caused by evaporation (Broecker, 1974). This is further supported by the high dolomite content (low CaO/MgO ratios: Table 1) of the Kupferschiefer closest to the Zechstein Sea shore

A. Bechtel, 14~ Piittmann / Palaeogeography, Palaeoclimatology, Palaeoecology 136 (1997) 331-358

line (well 134), indicating hypersaline conditions (Sweeney et al., 1987). The C isotope compositions of calcite from drill site 132, located near the Krefeld High (Fig. 2), differ significantly from those of the carbonate samples from the other boreholes (Fig. 18). The samples from profile 132 are characterized by isotopically heavy organic carbon and by an enhanced thermal maturity of organic material. The low 613C values of calcite from these samples provide evidence that isotopically light COz, possibly generated through thermal degradation of organic matter, may have been incorporated into diagenetic calcites (Bechtel and Piattmann, 1992).

5. Conclusions

The analysis of the stable isotope (C, O, H, N) composition of organic matter and carbonates in combination with molecular investigations on saturated and aromatic hydrocarbon fractions in the Kupferschiefer of the Lower Rhine Basin provides information about the sedimentary environment and biochemical carbon and nitrogen cycling during the initial stage of the Late Permian Zechstein transgression. The study area during the beginning of Kupferschiefer deposition is best characterized as a shallow-water basin, in which water exchange with the Zechstein Sea was restricted by palaeohighs. The morphology, composed of swells and sub-basins, combined with salinity stratification of the water column, caused regional differences in the ecosystems. Apart from swell localities, the activity of sulphate-reducing bacteria prevailed in the sediment and resulted in euxinic conditions with free H2S in the bottom water. In deeper sections of the sediment methanogenic bacteria produced CH4, which was subsequently oxidized to CO2 by methane oxidizers living in the water column. Recycling of CO2 was further accomplished by organic matter oxidation and sulphate reduction. Evidence for the occurrence of microbial degradation and subsequent re-incorporation of the products into the biomass of primary producers is provided for the basal Kupferschiefer. Low 6t3C values of kerogen are associated with

355

enriched 615N values of the organic extr:~,cts and with depleted ~13Ccarb values. The progressive depletion of kerogen in 13C in the direction of the Zechstein Sea shoreline could be due to increasing activity of methane-oxidizing bacteria incorporating preferentially light organic carbon into their biomass. An alternative explanation is the higher contribution of terrestrial organic matter to the sediment. Both processes are in agreement with the geochemical data. During progressive Zechstein transgression an increasing marine influence in the study area is indicated by enhanced activity of organisms living in the photic, oxygen-rich zone (e.g. marine phytoplankton) of the water column, as well as by the higher 613C values of organic matter and carbonate. Finally, normal marine conditions were established, characterized by the location of the O2/H2S chemocline below the sediment-water interface. In the prevailing marine environment, the regional variations in ecosystems were reduced due to minimized importance of physiographic differences and water stratification on the organisms living in the water column. This is indicated by the nearly identical 613C values of organic matter at the top of the Kupferschiefer of all investigated profiles. Invasion of sea water and progressive evaporation resulted in increased salinity in the early Zechstein Sea, initiating the first of the Zechstein evaporation cycles. Slightly increasing salinity with progressive Kupferschiefer deposition is indicated by increasing amounts of trimethylated 2-methyl-2-trimethyl-tridecylchromanes (tri-MTTC) in the extracts. High fi180 values of carbonates from near-shore and swelltype Kupferschiefer are suggestive of hypersaline conditions.

Acknowledgements

Support of this study by S. Hoernes (University of Bonn) is gratefully acknowledged. This article profited from the critical remarks of S.W. Imbus (Houston, Texas, USA), F. Surlyk (Copenhagen, Denmark) and J. Trichet (Orlrans, France). We thank G. Friedrich, R. Diedel and P. Redecke (RWTH Aachen) for providing samples and for

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,t. Becht,.'/. B'. Ptitlmann

Palaeok,eo,tg"aphv, t'alaeoclimalolo:~.l, P,:tlaeoeeolo
information about the geology, petrology and inorganic geochemistry. Thanks also go to T. Gedding ( University of Bonn) for performing nitrogen isotope analysis on organic extracts. Financial support by the Deutsche Forschunjzsgemeinschqlt (grant nos. Ho 868:6, Be 1485:1 and Pu 73:2) is gratefully acknowledged.

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