Palaeoenvironment of the Eocene Eckfeld Maar lake (Germany): implications from geochemical analysis of the oil shale sequence

Organic Geochemistry Organic Geochemistry 36 (2005) 873–891 www.elsevier.com/locate/orggeochem

Palaeoenvironment of the Eocene Eckfeld Maar lake (Germany): implications from geochemical analysis of the oil shale sequence M. Sabel a, A. Bechtel b

a,*

, W. Pu¨ttmann b, S. Hoernes

a

a Mineralogisch-Petrologisches Institut, Universita¨t Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany Institut fu¨r Mineralogie – Umweltanalytik, J.W. Goethe-Universita¨t, Georg-Voigt-Str. 14, 60054 Frankfurt a.M., Germany

Received 2 August 2004; accepted 5 January 2005 (returned to author for revision 13 October 2004) Available online 31 March 2005

Abstract Drill core samples from the depth interval between 19.4 and 32.0 m of the laminated central lake facies of the Eocene Eckfeld Maar were investigated for biomarker and stable isotope composition. Bulk organic geochemical parameters (C/N, HI) and the molecular composition of the soluble organic matter indicate a dominance of particulate organic matter from land plants and microbially derived lipids in the lower part of the sedimentary succession. An angiosperm-dominated vegetation is indicated from the terpenoid biomarker composition. Abundant 4-methylsteroids in the 25.6–30.8 m section of the oil shale sequence reflect a contribution of algal-derived biomass. Samples with high concentrations of methylsteroids are characterized by low amounts of triterpenoids related to the arborane skeleton, and vice versa. This pattern is interpreted as reflecting differences in autochthonous organic matter production vs. microbial activity. In the lowermost section (32.0–30.6 m), a trend towards heavier d18O and d13C values indicates the evolution of permanently meromictic conditions in the lake and an increase in methanogenesis. High d13C values of siderites (>10&) throughout most of the sequence are consistent with permanently anoxic conditions at the sediment–water interface. The lighter d18O values of siderites from turbidites, relative to siderites from biogenic laminites, are postulated to have been caused by temporary phases of increased precipitation, followed by landslides and improved circulation within the lake. Depletion of the organic matter in 13C (average d13C =
29.4&), in comparison with the fossil wood (ranging from
23.1& to
26.6&), is explained by the dominance of waxy, lipid-rich land plant material (e.g., leaf waxes, resins, bark) over wood supplied to the lake. Carbon cycling during anoxic decomposition of organic matter is assumed to further affect the d13C values through the activity of anaerobic (e.g., methanogenic) bacteria, resulting in a depletion of the biomass in 13C. The overall trend in the isotopic composition of organic carbon towards heavier values in the depth interval between 26.0 and 3.2 m is accompanied by decreasing C/N ratios, indicating an increase in aquatic organic matter production.

*

Corresponding author. Present address: Angewandte Geowissenschaften und Geophysik, Prospektion und Angewandte Sedimentologie, Montanuniversita¨t Leoben, Peter-Tunner-Str. 5, A-8700 Leoben, Austria. Tel.: +43 3842 402 6307; fax: +43 3842 402 6302. E-mail address: [email protected] (A. Bechtel). 0146-6380/$ - see front matter
2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2005.01.001

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M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891

The geochemistry of the oil shale succession points towards a highly productive, eutrophic ecosystem with intense microbial activity under meromictic conditions in the Maar lake. Beside the high input of land plants to the biomass, major contributions from bacteria and phytoplankton are indicated by the biomarker composition.
2005 Elsevier Ltd. All rights reserved.

1. Introduction Stable isotopic compositions of organic matter and carbonates are widely used in palaeoenvironmental studies of ancient ecosystems and for the assessment of palaeoclimate (Anderson and Arthur, 1983; Arens et al., 2000; Degens, 1970; Hoefs, 1987; Schleser, 1995; Veizer et al., 1999). However, interpretation of the stable isotope records is often complicated by the effect of facies-dependent variations in the sedimentary environment (Eh, pH, salinity, temperature, etc.) and post-depositional events during and after early diagenesis, which can substantially influence the isotopic composition of organic matter and carbonates (Degens, 1970; Veizer, 1983; Benner et al., 1987). The results of previous studies of organic matter-rich sedimentary successions indicate that combined isotopic and organic geochemical analysis can be used to overcome these difficulties (Hayes et al., 1987; Westerhausen et al., 1993; Bechtel and Pu¨ttman, 1997; Ro¨hl et al., 2001). The molecular composition of organic matter can be used to study the thermal history of sedimentary basins, the type and origin of organic matter, and the sedimentary facies (Hunt, 1979; Radke, 1987; Tissot and Welte, 1984). Maar lakes often provide excellent conditions for the preservation of palaeoenvironmental information in their sediments, in particular because of their great water depth combined with a relatively small surface area. Anoxic conditions easily develop in the bottom water and facilitate the extraordinarily good preservation of fossil remains and of high resolution palaeoenvironmental information. Inside the Eckfeld crater, a typical Maar lake with an estimated water depth of 160–210 m and a diameter of 800–1050 m formed during the middle Eocene (Lutz et al., 2000). Eutrophic conditions and permanent meromixis became established in this deep lake (Wilde et al., 1993). Restricted circulation caused anoxic conditions in the monimolimnion and prevented bioturbation. Excavation of fossil remains from the sedimentary succession has been carried out by the Museum of Natural History (Mainz, Germany) since 1987. A great number of very well preserved floral and faunal fossil remains have been collected since, documenting a highly diverse terrestrial flora and fauna (Franzen, 1994; Lutz et al., 1998). In contrast, the aquatic biocenosis was rather poor in species (Lutz et al., 2000). Fossil remains of palms and crocodiles

indicate a humid paratropical climate. The palaeogeographical position of the Eckfeld Maar during Middle Eocene was around 42–44 N (Mingram, 1998), about 10–15 farther south than present. It is the aim of this paper to present information about the palaeoenvironmental evolution of the lake from geochemical investigation of siderites, bulk organic matter and wood particles. Siderites were analyzed with respect to their carbon and oxygen isotopic composition. Bulk organic parameters (C/N wt% ratios, hydrogen index (HI) from Rock–Eval pyrolysis) and d13C values are presented together with the molecular composition of aliphatic and aromatic hydrocarbon fractions as well as of heterocompounds in selected samples. The results were expected to extent our knowledge about the evolution of the continental environment in central Germany following the Tertiary climatic optimum, which is presently primarily based on data from other oil shale sequences (e.g., Messel) and coal seams (e.g., Geiseltal area).

2. Geological setting and samples The Eckfeld Maar is situated at the southwestern margin of the Tertiary Eifel volcanic field (Meyer et al., 1994). The lake is surrounded by folded lower Devonian sand- and siltstones (Fig. 1). Mammal stratigraphy (Franzen, 1993, 1994) and palynological methods (Nickel, 1994) revealed a middle Eocene age for the Eckfeld sediments (MP13 of European mammal stratigraphy). This age was confirmed using 40 Ar/39Ar dating of a weakly weathered alkali basalt from the drill core E1/96, which resulted in an age of 44.3 ± 0.4 Ma (Mertz et al., 2000). The uncertain origin of the Eckfeld structure was the reason for a scientific drilling programme in 1980 that revealed the existence of mostly reworked pyroclastics below the sedimentary series (Negendank et al., 1982; Pirrung, 1998). Detailed geological and geophysical investigations by Pirrung (1992, 1993, 1998) provided additional evidence for the volcanic origin of the Eckfeld structure. Today, the Maar hypothesis is generally accepted. In 1996 six additional drill cores (E1/96–E6/96) penetrated the sediments of the central lake and the marginal lake, respectively. A maximum well depth of 123 m was reached in the central area of the Maar (E1/96; Fig. 1). The sedimentary sequence starts with

M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891

875

Fig. 1. Geological sketch map of the Eckfeld maar (modified after Pirrung, 1998) with position of boreholes (E1/96, E2/96).

volcanic breccias consisting of lower Devonian clasts, mafic lapilly and fragments of the Mesozoic–Tertiary weathering horizon (Pirrung, 1998; Fischer, 1999). At a depth of about 80 m, the initial phase of the lake is indicated by the appearance of a laminated clay layer (Pirrung, 1998; Fischer, 1999). The sediments up to a depth of about 41 m are dominated by turbiditic layers consisting of volcanic and Devonian clasts. From 41 to 38 m the sequence consists of closely alternating layers of mud and fine sand-sized quartz grains. The stabilization of the tephra rim by vegetation is documented by the increasing content of land plant detritus in the sediments from 38 to 33 m (Bullwinkel and Riegel, 2001). Dark organic matter-rich laminites, occasionally interrupted by graded layers, are characteristic of the final ‘‘biogenic laminites’’ (Bullwinkel and Riegel, 2001) from 33 m to the top. These ‘‘biogenic laminites’’ are the subject of this study. The samples derive from two adjacent drill cores (E1/ 96 and E2/96; Fig. 1). The core from borehole E1/96 was continuously sampled from 20.4 to 32.0 m. Drill core sections of 1–5 cm were homogenized to form one sample. Because of core loss from borehole E1/96, samples

from different lithotypes within the depth interval from 3 to 20 m were taken from borehole E2/96: laminated and graded layers were sampled separately. The total oil shale sequence sampled records a period of 60,000–80,000 years, based on an overall sedimentation rate of 0.4 mm/year (Mingram, 1998). Additional samples were taken from the excavation site, in particular for collection of wood fragments and siderite concretions.

3. Methods Portions of borehole samples were ground and homogenized prior to further treatment. The total carbon and total nitrogen (N) contents were determined with a Roboprep elemental analyser. The organic carbon content (Corg) was measured with the same instrument on samples pretreated with concentrated hydrochloric acid. Pyrolysis was carried out on several of the collected samples using a Delsi Rock–Eval instrument Version RE II. Using this method, the amount of hydrocarbons

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M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891

(mg HC/g rock) released from kerogen during gradual heating in a helium stream is normalized to Corg to give the HI. In a similar way, the CO2 released upon pyrolysis is to Corg (mg CO2/g Corg) normalized to give the Oxygen Index (OI). These indices are related to the H/ C and O/C atomic ratios of kerogen, respectively (Espitalie´ et al., 1977). Due to the possible interference of carbonate minerals (especially siderite) on OI values, these parameters are not presented. As a pyrolysis maturation indicator, the temperature of maximum hydrocarbon generation (Tmax) was measured. Selected samples were sonicated two times for 15 min with dichloromethane/methanol (5:1; v/v). The soluble organic matter (SOM) was filtered and concentrated using a rotary evaporator and then under gas flow using dry nitrogen gas. Aliquots of the total extracts (SOM) from several of the extracted samples were converted to methyl derivatives by reaction with trimethylsulfoniumhydroxide (TMSH) and pyridine for 1 h at 60 C. The total extracts were also separated into NSO compounds, saturated hydrocarbons and aromatic hydrocarbons using medium pressure liquid chromatography (MPLC) with a Ko¨hnen–Willsch instrument (Radke et al., 1980), after precipitation of asphaltenes from a n-hexane/dichloromethane (80:1; v/v) solution. Gas chromatography–mass spectrometry (GC–MS) of the derivatized total extracts was performed using a ThermoQuest model 8000 GC coupled to a ThermoQuest model 800 quadrupole MD. Separation was achieved with a fused silica capillary column coated with SGE-BPX-5 (30 m · 0.25 mm i.d., 0.25 lm film thickness). The GC operating conditions were temperature hold at 80 C for 2 min, increase from 80 to 300 C at a rate of 4 C min
1 with final isothermal hold at 300 C for 20 min. Helium was used as carrier gas. The sample was injected splitless with the injector temperature at 300 C. The spectrometer was operated in the electron ionization (EI) mode at 70 eV ionization energy and scanned from m/z 50 to m/z 600 (0.5 s total scan time). Data were acquired and processed with the Chemstation software. Individual compounds were identified by comparison of mass spectra with literature and library data, comparison with authentic standards and interpretation of MS fragmentation patterns. Absolute concentrations of selected NSO compounds were calculated using peak areas from the gas chromatograms in relation to those of squalane, added prior to analysis as internal standard. The saturated and aromatic hydrocarbon fractions were analyzed on a GC equipped with a 30 m DB-1 fused silica capillary column (i.d. 0.25 mm; 0.25 lm film thickness) and coupled to a Finnigan MAT GCQ ion trap mass spectrometer. The oven temperature was programmed from 70 to 300 C at a rate of 4 C min
1 followed by an isothermal period of 15 min. Helium was used as carrier gas. The sample was injected splitless

with the injector temperature at 275 C. The spectrometer was operated in the EI (electron ionization) mode over a scan range from m/z 50 to m/z 650 (0.7 s total scan time). Data were processed with a Finnigan data system. Identification of individual compounds was accomplished on the basis of retention time in the total ion current (TIC) chromatogram and comparison of the mass spectra with published data (LaFlamme and Hites, 1979; Philp, 1985; Wolff et al., 1989). Relative percentages and absolute concentrations of different compound groups in the saturated and aromatic hydrocarbon fractions were calculated using peak areas from the gas chromatograms in relation to those of internal standards (deuteriated n-tetracosane and 1,1 0 -binaphthyl, respectively). The concentrations were normalized to Corg. For carbon and oxygen isotope analysis of siderite, about 100 mg of fine grained powder were dissolved in 100% H3PO4 according to McCrea (1950). Complete conversion of carbonate to CO2 was obtained after 12 h at a temperature of 60 C (Rosenbaum and Sheppard, 1986; Bahrig, 1989). Determination of C and O isotopic ratios of the released CO2 was carried out on a VG Isogas (SIRA-9) gas mass spectrometer. The analytical error is less than 0.2&. The data were corrected to the PDB standard using the O isotope fractionation factor involved in the dissolution of siderite in H3PO4 at 60 C (Carothers et al., 1988). Carbon isotope measurements of organic matter were made on homogenized samples after removal of siderite by treatment with concentrated hydrochloric acid. Portions of each sample (between 1 and 4 mg) were packed into tin capsules and combusted in excess oxygen at 1000 C using a Roboprep elemental analyser. Residual oxygen was removed by reaction with reduced copper at 600 C. After passing through a H2O trap (MgClO4), the resulting CO2 was analyzed on line with an Europa Scientific isotope ratio mass spectrometer. The 13C/12C isotope ratio of the CO2 was compared with the corresponding ratio for a reference, calibrated against the PDB standard. The reproducibility of the total analytical procedure is in the range of 0.1–0.2&.

4. Results and discussion 4.1. Bulk organic geochemistry The Corg contents of the samples from the oil shale sequence are generally high (between 3.4 and 35.8 wt%; Table 1). The positive correlation (r2 = 0.96) with total nitrogen content suggests that nitrogen is predominantly fixed in the organic matter. The C/N (wt/wt) ratios of the organic matter vary between 20 and 45 (Fig. 2), equivalent to atom/atom ratios between 23 and 52. C/N atomic ratios less than about 8 have typically been found in marine sediments with organic matter

M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891

877

Table 1 Depth, Corg, HI and Tmax from Rock–Eval analysis, and SOM yields and d13C of total organic carbon of samples from borehole E1/96 Sample

Depth (m)

Corga (wt%)

HIb (mg HC/g Corg)

Tmaxc (C)

SOM yield

30 33 34 43 44 46 49 51 57 D 06 D 07 D 08 D 14 D 23 D 24 D 25 D 26 D 27 D 28 D 34 D 41 D 42 D 43 D 44 D 45 D 46

19.43 20.73 21.43 24.63 24.88 25.63 26.68 27.10 29.93 30.80 30.83 30.85 30.99 31.22 31.25 31.28 31.30 31.32 31.34 31.58 31.78 31.81 31.84 31.86 31.88 31.91

24.7 26.8 24.5 25.7 8.2 22.6 29.9 19.4 15.8 29.1 35.8 13.2 3.4 18.2 20.8 11.5 32.5 14.4 7.8 17.0 26.8 14.0 19.9 8.3 20.3 12.9

239 110 317 282 163 344 350 389 343 129 251 684 315 263 224 537 207 324 203 286 263 319 291 334 276 243

432 431 437 434 435 437 434 434 437 435 431 433 423 434 431 433 434 434 430 435 431 432 433 431 433 432

29.7 21.4 29.0 71.6 14.2 60.8 51.4 52.3 31.4 46.7 36.7 76.7 24.9 24.0 22.5 73.2 25.1 25.5 15.9 20.8 32.9 26.8 36.4 47.2 30.9 19.8

a b c d e

d

(mg/g Corg)

d13CTOCe (& vs. PDB)
29.9
30.4
29.3
31.4
29.3
28.6
27.6
28.0
30.1
28.7
29.2
29.5
28.0
28.7
28.8
29.2
29.4
28.8
27.7
29.9
28.8
28.6
28.6
28.2
28.9
28.0

Organic carbon content. HI, hydrogen index. Tmax, temperature of maximum pyrolysis yield. SOM, soluble organic matter. TOC, total organic carbon.

predominantly from algae or microorganisms, whereas ratios greater than about 20 are commonly interpreted as being related to detrital organic matter from land plants (Premuzic et al., 1982; Meyers and Benson, 1988). Based on this, the C/N ratios of the sediments from the Eckfeld Maar indicate a high contribution of terrigenous organic matter. However, biogeochemical reworking by bacteria is known to result in enhanced C/N ratios (Meyers and Ishiwatari, 1993). In drill core E2/96, decreasing C/N ratios within the sequence towards the top of the profile were observed (Fig. 2). This trend is interpreted as indicating an increasing contribution of algal organic matter. Organic petrographical results showing increasing alginite content with decreasing depth within the oil shales are in agreement with this interpretation (Bullwinkel and Riegel, 2000). The results of Rock–Eval pyrolysis are presented in the HI vs. Tmax diagram of Espitalie´ et al. (1984; Fig. 3), which shows that the samples range from type II to type III organic matter. The low Tmax values, around 430 C, indicate the low maturity of the organic matter. Based on organic petrography, organic matter of the

Eckfeld Maar sediments is a mixture of type I (Botryococcus braunii has been identified) and type III (macerals of the huminite group) kerogen (Bullwinkel and Riegel, 2000), consistent with previous findings for the Messel oil shale sequence (Rullko¨tter et al., 1988). Considering these results, the low HI values of most samples, between 200 and 350 mg HC/g Corg (Table 1), indicate a high contribution of humic material from land plants (type III kerogen) to the sedimentary organic matter. These results are in agreement with the high C/N ratios of organic matter, especially for the sediments from drill core E1/96. The amounts of SOM vary between 14.2 and 76.7 mg/g Corg (Table 1). High normalized SOM yields, as measured via high concentrations of free lipids, were obtained from samples with high HI values, indicating high H/C ratios for the kerogen. The relationship may be explained by the high concentrations of extractable polar compounds of high molecular weight within the organic matter (see Section 4.2), which are often released as an early component of the S2 peak during Rock–Eval pyrolysis. The relative proportions of hydrocarbons are

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M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891 2

2

C/N - Total Organic Matter

4

6

6 8

8

Eckfeld E1 Eckfeld E2 Eckfeld E1 - Detail

10

Eckfeld E1 Eckfeld E2 Eckfeld E1 - Detail

10

12

12

14

14

16

30.6

18

30.8

Depth

Depth

δ13Corg [‰]- Total Organic Matter

4

16

30.6

18

30.8

20

31.0

20

31.0

22

31.2

22

31.2

24

31.4

24

31.4

26

31.6

26

31.6

28

31.8

28

32.0

30 0

10

203

04

05

20

30

40

32.0

30 -32

0

C/N

32

31.8

-30

-29

-28

-27

13

δ Corg[‰]

32

50

-31

-32

-31

-30

-29

-28

-27

13

δ Corg[‰]

C/N

Fig. 2. Variation in C/N ratio and d13C values of the total organic matter with depth (composite profile of drill cores E1/96 and E2/96).

generally very low (<5% of the SOM), consistent with the low maturity. The SOM is mainly composed of NSO compounds (in most samples >30%) and asphaltenes (between 33% and 70%).

1100

1000

type I

4.2. Molecular composition of lipids

Hydrogen Index [mg HC/g TOC]

900

800

700 type II 600

500

400

300

200

100

0 390

type III

410

430

450

470

490

Tmax [°C]

Fig. 3. Relationship between HI and Tmax from Rock–Eval analysis. Fields in diagram are labelled according to predominance of respective kerogen type (after Espitalie´ et al., 1984).

4.2.1. Fatty acids, n-alkanes, isoprenoids Partial gas chromatograms of the derivatized total extracts of samples 46 and D42 are shown in Fig. 4. (see Table 2) High concentrations of long chain (>nC26) saturated fatty acids (Table 3) with a marked even over odd carbon number predominance are present and probably derive from leaf epicuticular waxes of land plants (Eglinton and Hamilton, 1967). Microalgae are also suspected to be a source of long chain saturated fatty acids having an even carbon number predominance, but would be characterized by long chain n-alkanes with no odd over even carbon number predominance (Volkman et al., 1998). The gas chromatogram of the non-aromatic hydrocarbons of sample 30 (Fig. 5a) shows that the n-alkane distributions are dominated by long chain n-alkanes with a marked odd over even predominance (CPI = 3.2–5.0; carbon preference index in the n-C23 to n-C34 range according to Bray and Evans, 1961) and maximum abundance at n-C31. High relative proportions of long chain C27–C31 n-alkanes comprising about 60% of C15–C33 n-alkanes (Table 4) are typical for higher terrestrial plants, where they

M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891

879

n-C28-FA

(a) 100

Sample: 46

HO

O

O

S-6 H

HO

HO

S-1

m

27

S-2

n-C27-FA

n-C22-FA

m OH

n-C25-FA

n-C24-FA

S-3 Std.

O

%

S-5

S-4

31

O

A

n-C32-FA

H1

n-C30-FA

Relative Intensity

n-C26-FA

O

29

0 H-1

Sample: D42

n-C28-FA

Std.

A-5

A-1

27

OH

O

n-C29-FA

31

n-C27-FA

n-C26-FA n-C25-FA

n-C24-FA

%

n-C22-FA

Relative Intensity

HO

O-3

A-4

n-C32-FA

100

n-C30-FA

(b)

H

A-3

29

0 46.00

48.00

50.00

52.00

54.00

56.00

58.00

60.00

62.00

64.00

66.00

68.00

Fig. 4. Partial gas chromatograms (TICs) of derivatized total extracts of (a) sample 46 and of (b) sample D42 from drill core E1/96. Peak assignments refer to Table 2. n-alkanes are labelled according to carbon number. FA, fatty acid; Std, standard (squalane).

occur as the main components of waxes (Eglinton and Hamilton, 1967). The dominance of n-C31 has been suggested as being characteristic of either grasses or warmclimate plant waxes (Cranwell, 1973; Schwark et al., 2002). A predominance of grassland in the vegetation can be excluded because of the high content of macerals of the huminite group (Bullwinkel and Riegel, 2000) and of the results of palaeobotanical investigations. Within the investigated section of the oil shale sequence, no stratigraphic change in the chain length distribution of n-alkanes was noticed. The n-alkanes of low to medium molecular weight (
calibrated for mature source rocks (Didyk et al., 1978). However, the interpretation of pristane/phytane ratios as reflecting anoxic conditions within the sedimentary environment is consistent with the proposed anoxic bottom water in the meromictic Eckfeld Maar (Wilde et al., 1993). High d13C values of siderites (d13C > 10&; Bahrig, 1989, and this study) further argue for high microbial activity of methanogenic archaea. 4.2.2. Steroids, hopanoids In the derivatized total extracts, 4-methylsterols and 4-methylsterones were identified (Fig. 4) in highly variable concentration (Table 3). The concentrations of the C30 methylsteroids (4-methyl-24-ethyl-cholestan-3bol, 4-methyl-24-ethyl-cholestan-3b-one) and of the C28 methylsteroids (4-methylcholestan-3b-ol, 4-methylcholestan-3b-one) are nearly equal, while lower concentrations of the C29 pseudohomologues were measured. Interestingly, desmethylsterols and desmethylsterones

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M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891

Table 2 Biomarkers in SOM from sediments of the oil shale sequence of the Eckfeld Maar (drill core E1/96) Peak

Compound

Figs.

A A-1 A-2 A-3 A-4 A-5 A-6 D H H-1 H-2 H-3 H-4 L O O-1 O-2 O-3 O-4 O-5 O-6 O-7 O-8 O-9 U-1 U-2 U-3 U-4 U-5 U-6 S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 S-9 S-10 S-11 S-12 S-13 S-14 S-15 S-16 S-17 S-18

Arborinone Des-A-arbora-9(11)-ene Des-A-arbora-5,7,9-triene 24,25-Dinorarbora-1,3,5,7,9,11-hexaene 24,25-Dinorarbora-3,5,7,9-tertraene 24,25-Dinorarbora-1,3,5,7,9-pentaene 24,25-Dinorarbora-5,7,9-triene Simonellite Diplopterol Hop-17(21)-ene 22,29,30-Trisnorhopane 17b(H), 21b(H)-Hopane 17b(H), 21b(H)-Homohopane Des-A-lupane b-Amyrin Des-A-olean-12-ene Des-A-olean-13(18)-ene Olean-12-ene Olean-13(18)-ene 2,2,4a,9-Tetramethyl-octahydrochrysene 3,3,7-Trimethyl-tetrahydrochrysene 24,25-Dinoroleana-1,3,5(10),12-tetraene 2,2,4a,9-Tetramethyl-octahydropicene 2,2,9-Trimethyl-tetrahydropicene Urs-12-ene 1,2,4a,9-Tetramethyl-octahydrochrysene 3,4,7-Trimethyl-tetrahydrochrysene 24,25-Dinorursa-1,3,5(10),12-tetraene 1,2,4a,9-Tetramethyl-octahydropicene 1,2,9-Trimethyl-tetrahydropicene 4-Methylcholestan-3b-ol 4-Methylcholestan-3b-one 4,24-Dimethylcholestan-3b-ol 4,24-Dimethylcholestan-3b-one 4-Methyl-24-ethylcholestan-3b-ol 4-Methyl-24-ethylcholestan-3b-one (D4,D5)-Cholestene 24-Methyl-(D4,D5)-cholestene 24-Ethyl-(D4,D5)-cholestene 4-Methyl-(D4,D5)-cholestene 4,24-Dimethyl-(D4,D5)-cholestene 4-Methyl-24-ethyl-(D4,D5)-cholestene Diacholest-13(17)-ene 24-Methyldiacholest-13(17)-ene 24-Ethyldiacholest-13(17)-ene 4-Methyldiacholest-13(17)-ene 4,24-Dimethyldiacholest-13(17)-ene 4-Methyl-24-ethyldiacholest-13(17)-ene

4(a) 4(b), 5(a) 5(b) 4(b) 4(b), 5(b) 4(b), 5(b) 5(b) 5(b) 4(a) 4(a), 5(a) 5(a) 5(a) 5(a) 5(a) 4(b) 5(a) 5(a) 4(b), 5(a) 5(a) 5(b) 5(b) 5(b) 5(b) 5(b) 5(a) 5(b) 5(b) 5(b) 5(b) 5(b) 4(a) 4(a) 4(a) 4(a) 4(a) 4(a) 5(a) 5(a) 5(a) 5(a) 5(a) 5(a) 5(a) 5(a) 5(a) 5(a) 5(a) 5(a)

were not found in quantities sufficient for peak integration. High concentrations of methylsterols and methylsterones were detected in samples from the depth interval 25.6–30.8 m (Table 3). The lowermost (30.9–31.9 m) and uppermost (19.4–24.9 m) sections of the oil shales from borehole E1/96 contain methylsteroids in low

abundance; 4-methyl-(D4,D5)-sterenes and 4-methyldiaster-13(17)-enes were identified in the non-aromatic hydrocarbon fractions (Fig. 5a) in slightly higher concentrations than the corresponding desmethylsterenes and desmethyldiasterenes, respectively (Table 4). The C30 methyl-(D4,D5)-sterenes predominate over the C28 pseudohomologues. The methyldiasterenes (C28, C30) were found in low quantities, with higher abundances of the C30 methyldiasterenes (Table 4). The (D4,D5)-sterenes and the diaster-13(17)-enes show similar carbon number distributions (C29 > C27  C28). Algae have been discussed as the predominant primary producers of C27 sterols, while C29 sterols are more typically associated with land plants (Volkman, 1986). However, numerous recent results add to the growing list of microalgae that contain high proportions of 24-ethylcholesterol (Volkman et al., 1999). Methylsteroids with a C30 dinosterol structure were considered as biomarkers of dinoflagellates (Robinson et al., 1984), while other C30 4-methylsteroids appear to be related to marine and lacustrine precursors (Mackenzie et al., 1982; Volkman et al., 1990; Peters and Moldowan, 1993). C28 4-methylsterols, found in the derivatized total extracts, have been associated with methane oxidisers (Bouvier et al., 1976; Coolen et al., 2004). Furthermore, Auras and Pu¨ttman (2004) suggest that methylsteroids may have been formed in the chemocline of meromictic lakes. Hopanoids are important constituents of the nonaromatic cyclic triterpenoids (Fig. 5a). The samples show comparable hopanoid patterns characterized by the occurrence of 17b,21b(H)- and 17a,21b(H)-type hopanes from C27 to C31 with the C28 hopanes being absent. The predominant hopanoids are the 17b,21b(H)-C29 hopane, the 17b,21b(H)-C30 hopane and hop-17(21)ene (Table 4). Other constituents are the C27 and C29 neohop-13(18)-enes. The ratio of 17b,21b(H)-hopanes to 17b,21b(H) + 17a,21b(H)-hopanes is in the range generally measured for lignites (0.5–0.7; Mackenzie et al., 1981). The most probable biological precursors of the hopane derivatives are bacteriohopanepolyols (Ourrison et al., 1979; Rohmer et al., 1992). These compounds have been identified in bacteria and fungi, as well as in cryptogams. The biological source of hop-17(21)-ene has not been clarified, although the compound is known to occur in many immature sediments. A direct input to the sediment by bacteria or in some cases by ferns and moss (Bottari et al., 1972; Volkman et al., 1986; Wakeham, 1990), as well as a diagenetic origin from hop22(29)-ene, have been proposed (Brassell et al., 1980). It might originate from diplopterol found in several eukaryotic phyla (e.g., ferns, mosses, lichens, fungi) as well as in hopanoid-producing bacteria (Bottari et al., 1972; Ourrison et al., 1979; Rohmer and Bisseret, 1994). Diplopterol occurs in the derivatized total extracts

M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891

881

Table 3 Concentrations of n-C16–24 and n-C26–32 fatty acids, 4-methylsterols, 4-methyl-sterones, diplopterol, b-amyrin and arborinone in total extracts of selected samples from borehole E1/96 Sample

Depth (m)

n-C16
24 Fatty acids (lg/g Corg)a

n-C28
32 Fatty acids (lg/g Corg)

4-Methyl-sterols (lg/g Corg)

4-Methyl-sterones (lg/g Corg)

Diplopterol (lg/g Corg)

b-Amyrin (lg/g Corg)

Arborinone (lg/g Corg)

34 46 49 51 57 D 06 D 08 D 26 D 28 D2 D4

21.43 25.63 26.68 27.10 29.93 30.80 30.85 31.30 31.34 31.81 31.86

200 145 147 162 93 109 72 84 89 53 63

777 447 463 413 362 367 336 389 367 280 296

24 267 253 393 22 125 n.d. n.d. n.d. n.d. n.d.

44 235 287 328 33 110 n.d. n.d. n.d. n.d. n.d.

89 56 7 41 68 56 82 102 71 68 63

n.d. n.d. n.d. n.d. n.d. n.d. 22 14 5 22 14

14 81 59 72 3 34 36 44 n.d. n.d. n.d.

n.d., Not detectable. a Organic carbon content.

Fig. 5. Gas chromatograms (TIC) of (a) saturated hydrocarbon fraction and (b) the aromatic hydrocarbon fraction of sample 30 from drill core E1/96. Peak assignments refer to Table 2. n-alkanes labelled according to carbon number. Std, standards (n-tetracosane for saturated hydrocarbons, 1,1 0 -binaphthyl for aromatic hydrocarbons).

882 Table 4 Concentration of n-alkanes, proportions of n-C15–24 and n-C27–31 relative to the sum of n-alkanes, carbon preference index (Bray and Evans, 1961), concentrations of steroid and hopanoid hydrocarbons, angiosperm derived terpenoid biomarkers and arborane derivatives in the hydrocarbon fractions of samples from borehole E1/96 n-Alkanes (lg/g Corg)

n-C15–24/ n-C15–33

n-C27–31/ n-C15–33

CPIa

Sterenes (lg/g Corg)

Diasterenes (lg/g Corg)

Methyl-sterenes (lg/g Corg)

Methyl-diasterenes (lg/g Corg)

Hopanes (lg/g Corg)

Hop-17(21)-ene (lg/g Corg)

Oleanenes + Ursenes (lg/g Corg)

Arborenes (lg/g Corg)

30 33 34 43 44 46 49 51 57 D 06 D 07 D 08 D 14 D 23 D 24 D 25 D 26 D 27 D 28 D 34 D 41 D 42 D 43 D 44 D 45 D 46

152 93 116 95 108 203 135 185 174 101 83 173 144 74 123 162 76 173 101 73 79 113 96 172 84 126

0.12 0.07 0.08 0.12 0.08 0.10 0.10 0.08 0.14 0.10 0.05 0.15 0.08 0.11 0.10 0.10 0.08 0.08 0.10 0.04 0.14 0.12 0.13 0.09 0.13 0.09

0.67 0.61 0.67 0.66 0.64 0.64 0.64 0.64 0.61 0.61 0.62 0.49 0.58 0.56 0.60 0.55 0.60 0.59 0.56 0.55 0.54 0.55 0.51 0.56 0.54 0.60

4.9 4.5 4.5 3.6 4.0 4.5 4.5 3.6 4.5 5.3 4.5 4.1 3.9 3.2 4.5 3.5 3.6 5.0 4.8 4.5 4.4 3.6 5.5 5.3 4.2 3.8

8 19 9 24 15 14 15 98 29 32 9 13 12 9 37 21 18 22 6 3 10 16 9 14 18 9

n.d. 2 3 4 3 11 13 10 4 2 4 3 4 4 3 7 4 3 2 n.d. 2 3 4 2 2 5

15 27 15 30 22 32 43 72 59 63 22 25 19 20 49 34 22 29 13 8 19 21 15 22 20 14

2 3 7 6 4 13 11 26 8 5 6 5 3 6 6 10 4 7 2 n.d. 3 5 7 5 8 3

63 31 34 42 36 68 66 99 60 36 32 80 36 38 60 86 37 62 23 16 27 46 36 44 25 33

57 82 51 81 56 152 89 169 181 96 51 109 118 103 157 158 53 97 31 26 49 82 86 55 88 40

108 46 61 84 47 131 52 125 108 80 66 106 93 73 79 142 69 106 72 39 65 116 91 126 80 71

16 26 69 45 19 9 6 12 5 27 44 185 168 81 64 173 116 133 173 101 161 149 203 157 184 165

n.d., Not detectable. a Carbon preference index.

M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891

Sample

M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891

4.2.4. Non-hopanoid triterpenoids The following tetra- and pentacyclic triterpenoids of the oleanane and ursane types (Fig. 5(a)) were found in the non-aromatic hydrocarbon fractions: des-A-oleanenes, des-A-urs-12-ene, olean-12-ene, olean-13(18)ene, and urs-12-ene (ten Haven et al., 1992; Logan and Eglinton, 1994; Philp, 1985; Rullko¨tter et al., 1994). The following aromatic tetra- and pentacyclic triterpenoids occur in the aromatic hydrocarbon fractions (Fig. 5b): tetramethyl-octahydro chrysenes, trimethyltetrahydro chrysenes (Spyckerelle et al., 1977; Wakeham et al., 1980), monoaromatic pentacyclic triterpenoids assigned as 24,25-dinor-oleana-1,3,5(10), 12-tetraene, 24,25-dinor-ursa-1,3,5(10), 12-tetraene, and 24,25-dinor-lupa-1,3,5(10)-triene (Wolff et al., 1989), as well as tri- and tetraaromatic pentacyclic triterpenoids of the oleanane and ursane types (i.e., tetramethyl-octahydro picenes, trimethyl-tetrahydropicenes; LaFlamme and Hites, 1979; Wakeham et al., 1980). Non-hopanoid triterpenoids containing structures typical of the oleanane skeleton, the ursane skeleton, or the lupane skeleton are known as biomarkers for angiosperms (Karrer et al., 1977; Sukh Dev, 1989). These compounds are significant constituents of wood, roots and bark (Karrer et al., 1977); b-amyrin, one of the possible biological precursors, was found in several samples (Fig. 4). Efforts to use the triterpenoid composition for chemotaxonomy of angiosperms remain unsuccessful because most angiosperms contain b- as well as a-amyrin in considerable quantities (Karrer et al., 1977; Gu¨lz et al., 1992). A tendency towards higher concentrations of angiosperm-derived triterpenoids (Table 4) with increasing Corg-normalized concentrations of hopanoids exists (Fig. 6). This relationship provides evidence for a supply of microbial biomass (predominantly from aerobic bac-

260 240

Hopanoids (µg/g Corg)

4.2.3. Diterpenoids Diterpenoids of the abietane-type (i.e., simonellite, retene; Philp, 1985) were found in very low concentration, between 0.6 and 3.1 lg/g Corg (Fig. 5b). In plants of extant coniferales families, abietane-type diterpenoids are widespread in species of Pinaceae, Podocarpaceae, Taxodiaceae and Cupressaceae, but they are of little use for the taxonomic differentiation (Otto et al., 1997 and references therein). The very low abundance of diterpenoids in the hydrocarbon fractions argues for the scarcity of gymnosperms in the vegetation around the Eckfeld Maar.

280

220 200 180 160 140 120 100 80 60 40 20 0 0

20

40

60

80

100

Triterpenoids (µg/g Corg) Fig. 6. Cross-correlation of concentration of angiospermderived triterpenoid biomarkers vs. concentration of hopanoids within the Eckfeld oil shale sequence from borehole E1/96.

teria) together with detrital organic matter of land plant origin. The interpretation is in agreement with previous results indicating higher amounts of hopanoids within turbidites of the oil shale sequence (Zink and Pu¨ttmann, 1994). Interestingly, a general tendency towards higher HI values with increasing concentrations of angiosperm-derived triterpenoid hydrocarbons is observed (Fig. 7). The relationship argues that enhanced HI values are at least partly caused by the supply of waxy, lipid-rich organic matter from land plants (e.g., leaf 700

600

500

HI

(Fig. 4; Table 3) and is suggested as the most probable biological precursor of hop-17(21)-ene. Recently, diplopterol and bacteriohopanetetrol were found as membrane rigidifiers in strictly anaerobic bacteria capable of ammonium oxidation (Sinninghe Damste´ et al., 2004).

883

400

300

200

100 0

20

40

60

80

100

Triterpenoids (µg/g Corg)

Fig. 7. Relationship between concentration of angiospermderived triterpenoids and HI from Rock–Eval pyrolysis.

884

M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891

waxes, resins, bark) to the lake. Consistent with these results, a positive relationship exists (r2 = 0.75) between the % of cutinite plus sporinite plus resinite macerals, measured in the oil shale sequence drill cores (Bullwinkel and Riegel, 2000), and the concentrations of land plant biomarkers. The occurrence of arborinone (Fig. 4) corroborates the identification of the remaining triterpenoid hydrocarbons, found in the aromatic hydrocarbon fractions, as being related to the arborane skeleton. Specific compounds are (Fig. 5b): des-A-arbora-5,7,9-triene; 24,25dinor-arbora-5,7,9-triene; 24,25-dinor-arbora-3,5,7,9tetraene; and 24,25-dinor-arbora-1,3,5,7,9-pentaene (Hauke et al., 1992a,b). The Corg-normalized concentrations of the arborane derivatives vary within the profile (Table 4). Interestingly, high concentrations of arborane derivatives are exclusively found in sections of the sedimentary succession (between 19.4 and 24.9 m; as well as between 30.9 and 31.9 m) characterized by low abundances of methylsteroids (Fig. 4; Table 4). The concentrations of arborane triterpenoids further correlate with the occurrence of diatom frustules, which are abundant in the lowermost part of the oil shale sequence. The origin of triterpenoids related to the arborane skeleton remains controversial with respect to their possible biological precursors (Hauke et al., 1992b, 1995; Vliex et al., 1994). Arborane derivatives are derived from isoarborinol or arborinone during early diagenesis (Jaffe´ and Hausmann, 1994). Isoarborinol, although present in various families of higher plants (Ohmoto et al., 1970; Hemmers et al., 1989), is a rather unique compound. Geochemical and biosynthetic features have led to the proposal that fossilized isoarborinol (and other sedimentary arborane derivatives) might originate from as yet unknown aerobic bacteria (Hauke et al., 1992b; Jaffe´ and Hausmann, 1994). Similar low d13C values of monoaromatic arborenes and benzohopanes obtained using GC–isotope ratio MS have been used to argue for a bacterial origin of the arborane derivatives (Hauke et al., 1992b). The relationship between arborane triterpenoids concentration and the abundance of diatom frustules, recognized in this study, may imply a genetic linkage between the biological precursors of arborane derivatives (e.g., arborinone, isoarborinol) and ecosystems favourable for diatom communities. 4.3. C isotopic composition of organic matter The d13C values of the bulk organic matter range from
27.5& to
31.4&, yielding an average of
29.4& (Fig. 2). Separated wood particles from the excavation site have average d13C values of
24.9& (ranging from
23.1& to
26.6&). However, a significant contribution of particulate organic matter of land

plant origin to the biomass of most samples from the oil shale sequence is suggested by the results from Rock–Eval pyrolysis and biomarker analyses, as well as being supported by published d13C and C/N values of major sources of plant organic matter to lake sediments (Meyers, 1994). The observed depletion in 13C of the total sedimentary organic matter, in comparison with the fossil wood, can be explained by a high proportion of lipid-rich land plant material (leaves, bark, resins) and low contribution of wood to the Maar lake. This conclusion is in accordance with high concentrations of long chain fatty acids and n-alkanes, derived from leaf epicuticular waxes (Fig. 8). In comparison to the data for the wood particles, lower d13C values around
27.5& were reported for leaf material from Eocene land plants (Hayes et al., 1987). Aeolian transport is assumed to be responsible for most of the terrestrial organic input, although the Maar lake was probably at least temporarily connected to a stream (Goth et al., 1988). Leaves are easily transported by wind, in contrast to wood. This explains the selective input of leaves and the low d13C values of 
29& contrasting with the carbon isotopic composition of the fossil wood fragments. The isotopic composition of the wood fragments is in agreement with d 13C values of
25.0& reported by Spiker and Hatcher (1987) for Eocene wood. The average carbon isotopic composition of the wood particles from Eckfeld Maar (d13C =
24.9&) is higher than the mean d13C data for angiosperms of
26.0& from the Middle Miocene Garzweiler Seam of the Lower Rhine Embayment (Lu¨cke et al., 1999). The biomarker composition of the fossil wood remains allows them to be taxonomically classified as angiosperms. Biomass from algae and bacteria also contributes to the organic matter, as indicated by biomarker analysis and the microscopic investigations by Bullwinkel and Riegel (2001). The contribution of autochthonous organic matter to the biomass is reflected in high concentrations of methyl steroids in core samples from borehole E1/96 from depth interval 25.6–30.8 m (Fig. 8). However, samples characterized by different biomarker composition, suggesting different ratios of terrestrial, microbial, and algal organic matter (Fig. 8), show no systematic differences in d13C of Corg (Table 1). The overall trend in the organic carbon isotopic composition towards slightly heavier values from depths around 26 m to the top of the sequence is consistent with generally decreasing C/N ratios in this depth range (Fig. 2) and with the observed continuous increase in abundance of remains of green algae in the sedimentary sequence (Bullwinkel and Riegel, 2001). Quantification of the proportions of allochthonous and autochthonous organic material using d13C values for the bulk organic matter is complicated by carbon recycling processes within the lake. Methane that is

M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891

885

Fig. 8. Relative proportion of specific compound groups normalized to sum of quantified compounds within SOM of samples selected for biomarker analyses on total extracts. n-FA, n-fatty acids.

produced by methanogenic archaea can be extremely depleted in13C (Whiticar, 1999). The methane rises from the sediment and is assimilated by methanotrophic bacteria living at the anaerobic/aerobic boundary of the lake. The methanotroph biomass is characterized by extremely negative d13C values due to its derivation from methane. Carbon isotopic analysis of individual compounds from Eocene Messel shale resulted in a d13C value of
65.3& for a C29 hopane. The strong depletion in 13C was explained by the origin of this compound from methanotrophs (Freeman et al., 1990). Recent results for the Toarcian Posidonia Shale of SW Germany indicate the co-occurrence of maximum oxygen depletion of the depositional environment and negative shifts in d13Corg values (Ro¨hl et al., 2001). This finding was explained by the recycling of 12C-enriched carbon derived from remineralization of organic matter during a highly elevated redox boundary within the water column. The activity of methanogenic archaea within the oil shale sequence of the Eckfeld Maar is confirmed by high d13C values of siderites (see Section 4.4). Abundant C28 4-methylsteroids in the SOM of the sediments are most probably associated with methane oxidisers.

4.4. Stable isotopic composition of siderites The formation of siderite is restricted to reducing environments (Eh <100 mV), low sulfide concentrations (<10
7 mol/l HS
) and high bicarbonate concentrations (CO2 partial pressure > 10
6 atm; Curtis and Spears, 1968; Stumm and Morgan, 1981). Variations in the oxygen isotopic composition of siderite can be caused by changes in the d18O values of the bottom water of the lake or by temperature variation at the time of siderite formation. Increased evaporation can cause significant enrichment of 18O at the sediment–water interface (Mozley and Wersin, 1992; Bahrig, 1989). During the microbial degradation of organic matter, CO2 reduction to CH4 by methanogenic archaea is associated with a kinetic isotope effect for carbon, leading to a significant 13C enrichment in the residual Co2 (Whiticar, 1999). The activity of methanogens is, therefore, often reflected in positive d13C values of siderite (Bahrig, 1989; Mozley and Wersin, 1992). The d18O values of siderite from drill cores E1/96 and E2/96 vary between
11.4& and +4.6& vs. PDB. The d13C values range from
3.3& to

886

M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891 2

2 13

Siderite - δ C

4 6

6

8

8

Eckfeld E1 Eckfeld E2 Eckfeld E1 - Detail

10

10

12

12

14

14

16

30.6

Depth

Depth

Siderit - δ18 O

4

Eckfeld E1 Eckfeld E2 Eckfeld E1 - Detail

16

30.6

18

30.8

18

30.8

20

31.0

20

31.0

22

31.2

22

31.2

24

31.4

24

31.4

26

31.6

26

31.6

31.8

28

31.8

28

32.0

30 -4

0

4

8

12

16

20

δ13C [‰]

32 -4

0

4

8

12

16

32.0

30 -12

-12

13

δ C[‰]

-4

0

4

8

δ18O [‰]

32

20

-8

-8

-4

0

4

8

δ18O [‰]

Fig. 9. Variation in d13C and d18O values of siderites with depth within composite profile of drill cores E1/96 and E2/96.

17

15

13

13 δ C (o/oo)

11

9

7

5

3

1

Drill core E2/96 Drill core E1/96

-1

-3 -5 -12

-10

-8

-6

-4

-2

0

2

4

δ O ( /oo) 18

o

Fig. 10. Cross plot of d13C vs d18O values of siderites from composite profile of drill cores E1/96 and E2/96.

+16.5& vs. PDB (Fig. 9). Isotopically lightest siderites occur exclusively in the lowermost section (between 32.0 and 30.6 m depth). In this part of the profile, a trend towards heavier d18O and d13C values with

decreasing depth can be observed (Fig. 9). The d18O–d13C cross plot illustrates this tendency in the isotopic composition of siderite within the lowermost part of the profile (Fig. 10). A considerable variability in d18O but fairly uniform d13C values were obtained for siderite of the remaining sections from the oil shale sequence (Fig. 10). The trend towards more positive d13C values reflects the increasing extent of methanogenesis due to the evolution of constantly meromictic conditions in the water column. Permanently anoxic conditions at the sediment water interface are the result of restricted circulation and continuous input of organic matter from land plants (Bullwinkel and Riegel, 2001). Apart from the lowest part of the sequence, consistently heavy d13C values of siderite prevail throughout the sedimentary sequence and show only minor variation. Thus, once established, the methanogenic conditions within the sediment continued throughout the sequence. In contrast, the d18O values show significant variations (Fig. 10). Samples from borehole E2/96 were taken with special regard to the sedimentary facies. As shown in Fig. 11, siderites from laminated sediments without exception show heavier d18O values compared to siderites from graded or turbiditic sedimentary layers of corresponding depth. The d13C values of siderites from drill

M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891

887

ing d13C values and the sedimentary facies indicates that the activity of methanogens at the sediment–water interface and within the sediment was not affected by these events.

2

4

6

5. Conclusions 8

Depth

10

12

14

16

laminated layer graded layer

18

20

22 -8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

δ18O[ ‰ ]

Fig. 11. Depth trends in d18O values of siderites from laminated and turbiditic sequences, respectively, within E2/96 drill core section.

core E2/96 do not show a correlation with the sedimentary facies. The turbiditic events were probably caused by periods of increased precipitation, leading to a destabilization of the crater wall. This process caused landslides and increased input of clastic material from the crater wall into the lake, resulting in the observed graded, turbiditic sedimentary layers. Landslides could have caused a partial turnover of the stratified water column. This turnover would result in a partial exchange between the bottom water and the surface water, influenced by the light d18O values of precipitation. These temporary phases of increased precipitation, followed by landslides and improved circulation within the lake, may explain the lighter d18O values of siderites from graded layers. However, the isotopic differences in d18O of siderites with regard to sedimentary facies may also represent a secondary signal, due to differences in sediment–water interaction. In connection with diagenetic alteration of basalts and volcanic ash to smectite, a decrease in the oxygen isotopic composition of pore waters of up to
8& has been observed for modern marine sediments (Lawrence and Gieskes, 1981; Mozley and Carothers, 1992). The lack of correlation between the correspond-

Bulk organic geochemical parameters (C/N, HI) indicate a predominant origin of organic matter from land plants in the lower part of the organic-rich, laminated sedimentary succession of the central lake facies of the Eocene Eckfeld Maar. Enhanced HI values are at least partly caused by the supply of waxy, terrigenous organic matter (e.g., leaf waxes, resins, bark) to the lake, as indicated by a positive relationship between HI and the sum of oleanane, ursane and lupane type triterpenoid concentrations. Slightly decreasing C/N ratios towards the top of the profile are interpreted as being the result of an increasing contribution of biomass from algae and microorganisms. Abundant terrigenous organic matter (e.g., cuticular leaf waxes, wood of angiosperms) in the sediments is further confirmed by the molecular composition of lipids (n-fatty acids, n-alkanes, non-hopanoid triterpenoids) of selected samples. Steroid composition points to the activity of photosynthetic organisms, including dinoflagellates, within the lake. Abundant hopanoids indicate the high microbial activity within the depositional environment. Triterpenoids related to the arborane skeleton are found in high proportions, especially in the lowermost part of the oil shale sequence that is characterized by the abundance of frustules of diatoms. The molecular compositions of the derivatized total extracts reveal that samples with high concentrations of 4-methylsteroids contain low amounts of arborinone plus arborane derivatives. The results may imply a genetic linkage between arborane triterpenoids and ecosystems favourable for diatom communities. The observed depletion in13C of the total organic matter of the sediments (d13C from
27.5& to
31.4&), in comparison with the fossil wood (average d13C values of
24.9&), is explained by the dominance of waxy, lipid-rich land plant material (e.g., leaf waxes, resins, bark) over wood in the terrigenous organic matter supplied to the lake. No systematic difference in d13C corresponding to different ratios of terrestrial, microbial, and algal organic matter, estimated for samples within the depth interval 21.4–31.9 m on the basis of biomarker analysis of the total extracts, was obtained. The overall trend in the isotopic composition of organic carbon towards heavier values from depths around 26 m to the top of the sequence is consistent with generally decreasing C/N ratios in this depth range, indicating increasing autochthonous organic matter production. Carbon cycling during anoxic decomposition of organic matter is

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M. Sabel et al. / Organic Geochemistry 36 (2005) 873–891

assumed to further affect the d13C values of the sediments through the activity of anaerobic (e.g., methanogenic) bacteria, resulting in a 13C depletion in the biomass. The isotopically heavy d13C values of siderite argue for the activity of methanogenic bacteria. The trend towards heavier d18O and d13C values of siderites in the lowest part of the profile indicates an increasing extent of methanogenesis due to the evolution of meromictic conditions in the water column. The lighter d18O values of siderites from turbiditic layers are explained by temporary phases of increased precipitation, followed by landslides and an improved water circulation within the lake. A high productivity ecosystem with intense microbial activity under meromictic conditions in the Maar is indicated by the geochemistry of the oil shale succession. The results are consistent with palaeontological data from the Eckfeld Maar, the Messel oil shale and coal seams from the Geiseltal area, documenting a highly diverse terrestrial flora and fauna indicating a warm and humid climate during middle Eocene in central Germany.

Acknowledgements The study was funded by the German Research Foundation (Grant No. Ho 868/18). The scientific drilling programme in 1996 was made possible by financial support of the Geological Survey of Rheinland-Pfalz (Mainz) and the Geoscience Research Center (Potsdam), which also provided technical support and a wealth of geological information during sampling. Scientific discussions with all co-workers within the framework of the interdisciplinary ÔEckfeldÕ-project helped to improve the paper. The authors thank especially H. Lutz (Museum of Natural History, Mainz). Furthermore, the paper benefitted greatly from critical remarks by Phil Meyers and Richard Pancost. Associate Editor—M. Fowler

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