Bacterially mediated formation of diagenetic aragonite and native sulfur in Zechstein carbonates (Upper Permian, Central Germany)

ELSEVIER

Sedimentary Geology 126 (1999) 205–222

Bacterially mediated formation of diagenetic aragonite and native sulfur in Zechstein carbonates (Upper Permian, Central Germany) J. Peckmann a,Ł , J. Paul a , V. Thiel b a

Institut und Museum fu¨r Geologie und Pala¨ontologie, Georg-August-Universita¨t, Goldschmidtstrasse 3, D-37077 Go¨ttingen, Germany b Institut fu ¨ r Biogeochemie und Meereschemie, Universita¨t Hamburg, Bundesstrasse 55, D-20146 Hamburg, Germany Received 9 March 1998; accepted 3 March 1999

Abstract Neoformed diagenetic aragonite associated with organic-rich aggregations and native sulfur occur within cavities of the Ca2 and Ca3 Zechstein carbonates in Central Germany. The cavities were formerly filled with gypsum. Bacterial sulfate reduction favored the precipitation of the carbonate phase. This can be attributed to an increase in alkalinity accompanied with sulfate reduction. The high Sr concentrations of the neoformed aragonites compared to the low concentrations in the carbonate host rocks point to sulfate dissolution as the cation supplying process for the precipitation of aragonite. Low δ13 C values ( 10‰ PDB) of the aragonite indicate that some of its carbon is derived from organic matter that has been oxidized by bacterial sulfate reduction. Aragonite inclusions bear rhomb-shaped crystals of calcite, replacing former dolomite. Elevated Mg=Ca ratios due to this dedolomitization may have promoted the precipitation of aragonite instead of calcite. The aragonite precipitated in the near-surface meteoric–vadose zone in recent times. Aragonite crystals display a platy habit. SEM analyses show that two types of micro-rods are associated with these plates. The mineralized micro-rods are interpreted to be fossilized bacteria. Aragonite inclusions, most of which contain organic-rich aggregations, yield a distinctive biomarker pattern. High concentrations of specific unsaturated fatty acids are clearly indicative of newly produced organic matter and reflect the presence of a discrete microbial community being associated with the formation of the aragonite. At one of the studied localities the aragonite is accompanied by native sulfur. The formation of sulfur was mediated by H2 S-oxidizing bacteria. This is corroborated by the presence of densely packed curved rods representing permineralized bacterial cells on and within the sulfur.  1999 Elsevier Science B.V. All rights reserved. Keywords: aragonite; sulfur; bacteria; gypsum replacement; Zechstein; Germany

1. Introduction A strong link between gypsum deposits and neoformed carbonate minerals is well established in the literature. Diagenetic carbonates have been reported from sulfate-dominated host rocks and soils. A well Ł Corresponding

author. Fax: C49 551 397918; E-mail: [email protected]

known example are the carbonates recognized in the Permian evaporites of the Castile and Salado Formations from the Delaware Basin. These carbonates have been interpreted as replacing gypsum and anhydrite (Davis and Kirkland, 1979; Kirkland and Evans, 1976) and are associated with biogenic sulfur (Hentz and Henry, 1989). Similar associations of diagenetic carbonates (calcite, aragonite, dolomite, and magnesite) and sulfur deposits like these Amer-

0037-0738/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 9 9 ) 0 0 0 4 1 - X

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ican examples occur on the Gemsa Peninsula on the west coast of the Gulf of Suez in a cap rock of a diapir (Pierre and Rouchy, 1988; Youssef, 1989). The δ18 O values of the carbonates indicate that the replacement of sulfate minerals by carbonates took place at both an early and late diagenetic stage (Pierre and Rouchy, 1988; Youssef, 1989). In the case of the Tripoli Formation in Sicily, replacement occurred very soon after deposition (McKenzie et al., 1979/1980; McKenzie, 1985). Sulfur within lagoonal evaporite–carbonate sequences in Sicily has been interpreted to be of biogenic origin (Dessau et al., 1962). The process of transforming sulfate minerals in the presence of petroleum to carbonate minerals and native sulfur is attributed to sulfate-reducing bacteria (Thode et al., 1954). Carbonate replacement of gypsum is not restricted to marine sediments, but occurs likewise in lacustrine environments. Calcite and dolomite pseudomorphs after aragonite crystals occur embedded in paleo– lacustrine sediments of the rift basins on the east coast of North America (Triassic–Jurassic; Riccioni et al., 1996). Much of the original crystal form is still preserved. Formation of aragonite is attributed to the release of CO2 by bacterial decomposition of organic matter. Elevated Mg=Ca ratios were found to be crucial for the preferential formation of aragonite over calcite (Riccioni et al., 1996). In the lacustrine deposits of the Miocene Teruel and Cabriel Basins, gypsum is replaced by aragonite and calcite (Anado´n et al., 1992). Replacement occurred soon after deposition in the Libros gypsum (Teruel Graben), respectively at shallow burial depths in contact with meteoric waters (Cabriel Basin). Sulfate reduction is believed to be the driving mechanism for carbonate precipitation in the Libros gypsum, which is supported by the high content of organic matter and the associated native sulfur (Anado´n et al., 1992). Sulfate-rich soils are another reactor for the neoformation of carbonates. In soils derived from gypsum-bearing Keuper strata in east-central Spain, fibrous radial aragonite nodules are common (Laya et al., 1992). The quality of preservation of the nodules decreases downward, indicating that aragonite formation is a very recent near-surface process. Within gypsum-bearing horizons in soil profiles in the Tamoa Valley on the western coast of New Caledonia, calcite (in situ pseudomorphs after lenticular

gypsum) and aragonite are forming (Podwojewski, 1995). However, Laya et al. (1992) and Podwojewski (1995) do not presume that carbonate precipitation might be mediated by soil bacteria. Both soil bacteria and soil fungi were found to precipitate calcite when cultured on a Ca-rich medium (Boquet et al., 1973; Monger et al., 1991). Applying the ‘biomarker-concept’, Newman et al. (1997) demonstrated that calcites formed in fractures in the vadose zone were derived from microbial activity. The authors suppose that the precipitation of other types of pedogenic calcites is also controlled biologically. Aragonite aggregations within Zechstein carbonates are known as ‘Schaumspat’ (Ko¨hler, 1930). It is described as a leafy and loose pseudomorphism of aragonite after gypsum. Ko¨hler suggested that the aragonite formed during burial diagenesis with enhanced pressure and temperature. In this paper we report on neoformed aragonite and native sulfur in a vadose–meteoric environment within Zechstein carbonates. Petrographic and stable isotope analyses indicate that diagenesis was mediated by biological processes involving sulfate-reducing and sulfur bacteria.

2. Geological setting and stratigraphy Sediments of the Upper Permian Zechstein Group crop out in a small belt south of the Variscan Harz Mountains in Central Germany (Fig. 1). The Zechstein Basin is a giant evaporite–carbonate basin in which huge masses of rocksalt and sulfate (anhydrite and gypsum), and smaller amounts of carbonate were deposited. Evaporitic cycles, which document the changing conditions within the basin, were used to establish a stratigraphic division of the Zechstein (Richter-Bernburg, 1955). Each cycle starts with either a clay (T) or carbonate (Ca) horizon, both deposited under normal marine conditions. These horizons are overlain by sulfate (A) and rocksalt (Na) deposits, which precipitated from hypersaline waters more or less isolated from the open sea. During the Mesozoic, an overburden of several kilometers of clastic and carbonate rocks accumulated in this area. This resulted in the conversion of the gypsum deposits to anhydrite. Beginning in the Late Cretaceous, the evaporites were uplifted and exposed

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Fig. 1. Geological sketch map of the Harz Mountains in Central Germany with the localities of aragonite occurrences.

to surface conditions. The humid climate of Central Europe favors the dissolution of the rocksalt to a depth of several hundred meters and the near-surface reconversion of anhydrite to gypsum. During the deposition of evaporites, the central part of the Zechstein belt south of the Harz Mountains was influenced by a palaeo-high, the Eichsfeld High (Paul, 1993). The evaporite accumulation from the first cycle (A1) is strongly influenced by this high. Here, a thick succession of shallow water gypsum was deposited, generating a sulfate platform (Fig. 2). The carbonates of the second cycle (Ca2) draped this platform. The Ca2 oolitic grain- and packstones on the platform, which reach more than 50 m in thickness, pass basinward into thin-bedded mud- and wackestones. The edge of the platform is characterized by an intense brecciation of the carbonate, due to early slumping and sliding processes. The pre-existing differences in the platform and basin facies were intensified by variable diagenesis and resulted in two different lithologies. The Ca2 unit of the platform is preferentially dolomitic, whereas the basinal facies was converted to dedolomite (Clark, 1980; Huttel and Mausfeld, 1991). The sulfate of the second cycle (A2) is up to 50 m thick in the basin and wedges out on top of the

platform. The rocksalt of this cycle (Na2), which once filled up the basin, has been dissolved in the study area. The third evaporitic cycle starts with claystones and thin carbonate beds (Ca3), which have a uniform thickness, like the overlaying sulfate (A3) which terminates the evaporite sequence south of the Harz Mountains. 2.1. Walkenried Aragonitic inclusions within the second cycle carbonate (Ca2) were collected 3 km southwest of the village of Walkenried. Unfortunately, these strata are not well exposed; therefore the exact position of the inclusions in the carbonate sequence is not clear. Palaeogeographically, the rocks were deposited on top of the Eichsfeld High (Fig. 2). They consist of about 50 m of oolitic and oncolitic grain- and packstones (dolomites and dedolomites). 2.2. Mu¨hlberg Aragonite occurrences of the Mu¨hlberg locality are found near the top of an abandoned quarry at the northwestern margin of the village of Niedersachswerfen, north of Nordhausen. They are also

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Walkenried

A

Mühlberg

B

0

Alter Stolberg 200

400 m

5 km

Fig. 2. Stratigraphic and palaeogeographic position of the host rocks containing neoformed aragonite on the eastern slope of the Eichsfeld Palaeo-High. Ca1 D first cycle carbonate; A1 D first cycle sulfate; Ca2 D second cycle carbonate; A2 D second cycle sulfate; T3 D third cycle clay; Ca3 D third cycle carbonate; A3 D third cycle sulfate.

embedded in second cycle carbonates (Ca2), which were deposited at the steep eastern slope of the Eichsfeld High. As a consequence of this, the carbonate sequence consists of sedimentary breccias in the lower part, followed by fossiliferous wackestones, which are overlain by oncolitic and peloidal dolomite bearing the aragonite inclusions. Today the Ca2 crops out in a steep slope of 50 m in height with the Bere Creek at its base, indicating that the recent water table is almost 50 m below the occurrences of neoformed aragonite. 2.3. Alter Stolberg The aragonite inclusions of the Alter Stolberg area were found between the villages of Steigerthal and Stempeda, northeast of Nordhausen. In contrast to the other occurrences, aragonite is hosted in carbonates of the third evaporite cycle (Ca3). The Ca3 has a thickness of a few meters and consists of alternating well-bedded grayish, platy calcitic mudstones, massive limestones and grayish marly claystones. The limestones are porous due to the dissolution of gypsum nodules. The aragonite occurs in these dissolution cavities.

3. Methods Thin sections (15 ð 10 cm) and polished thin sections (48 ð 28 mm) were studied petrographically.

Fluorescence microscopy was carried out on a Zeiss Axiolab (lamp: HBO 50; filter: BP 450-490 FT 510 LP 520). Thin sections and polished rocks were stained with combined potassium ferricyanide and alizarin red, dissolved in 0.1% HCl solution (Fu¨chtbauer, 1988, pp. 240–241), and with Feigl’s solution to stain aragonite (Feigl, 1958). SEM analyses were performed with gold-coated samples using a Hitachi S-2300. A LEO 1530 Gemini was used for fieldemission (FE)–SEM analyses on uncoated samples. Mineralogy was determined by X-ray diffraction on unoriented slurries using a diffractometer with CuKa radiation (Philips PW 1800). Chemical composition was measured by atomic absorption spectrometry (AAS) on a Philips PU 9200X. Samples for carbon and oxygen stable isotope ratios were taken from polished slabs using a hand-held microdrill. CO2 was liberated by the standard phosphoric acid technique (McCrea, 1950) at 75ºC. Isotope measurements were made with a Finnigan Mat 252 mass spectrometer, using a Carbo-Kiel carbonate preparation technique, at the University of Erlangen. The δ13 C and δ18 O results are reported relative to the PDB standard and appropriate correction factors were applied (Craig, 1957). The standard deviation ranges for carbon and oxygen from 0.01 to 0.04‰. A correction factor for δ18 O values of 0.7‰ (Grossman and Ku, 1986) was added to the aragonite samples and a correction factor of 0.8‰ (Sharma and Clayton, 1965) was added to the δ18 O ratios of dolomitic samples. 3.7 g of aragonite were

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mechanically prepared and subjected to biomarker analysis. After saponification of the ground sample in 6% KOH in methanol (60ºC, 2 h), the supernatant was decanted and the sample was ultrasonically extracted with dichloromethane=methanol (3 : 1; v=v). The combined supernatants were acidified to pH2 and extracted with dichloromethane. The resulting organic phase was fractionated by silica gel column chromatography into hydrocarbons, alcohols (not discussed here) and an acidic fraction (elution with dichloromethane=methanol; 3 : 1, v=v). Methyl esters of the organic acids were prepared by treatment with 1 ml methanol=trimethylchlorosilane (8 : 1, v=v; 60ºC, 2 h). After filtration over silica gel, the hydrocarbon and methyl ester fractions were subjected to gas chromatography (GC) and combined gas chromatography–mass spectrometry (GC– MS). GC analyses were run on a Carlo Erba Fractovap 4160 gas chromatograph, using ‘on-column’ injection and a flame ionization detector. The GC was equipped with a 30-m DB5 column (0.3 mm i.d., 0.25 µm film thickness; carrier gas H2 ). The GC–MS system was a Finnigan Mat CH7A mass spectrometer interfaced to a Carlo Erba 4160 GC (60 m XLB, 0.25 mm i.d., 0.32 µm film thickness, carrier gas He, ‘on-column’ injection). The identification of the compounds was based on a comparison of their mass spectra and of GC retention times with those of published data or with reference compounds. Quantification was carried out by GCpeak area integration (Bruker Chromstar software) and comparison with an internal standard of known concentration.

4. Carbonate petrography 4.1. The host rocks (Ca2 and Ca3) The host rocks consist of a crystal mosaic of carbonate, gypsum and anhydrite. The carbonate is calcite at the localities Walkenried and Alter Stolberg and dolomite at the Mu¨hlberg locality. Neomorphic fabrics prevail. Carbonate crystals mostly exhibit irregular crystal boundaries typical for a neomorphic spar. Their crystal size distribution is patchy with a mean of about 20 µm. At Mu¨hlberg rhombshaped dolomite crystals, which are about 150 µm

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in diameter, are common. Occasionally remains of a dark-gray, homogenous micrite are preserved within the neomorphic fabric (Alter Stolberg) or the primary fabric of a former oolitic limestone is still obvious (Mu¨hlberg). Acicular gypsum lines the ooids. Locally granular gypsum may be volumetrically even more important than carbonate. Its crystal sizes range from 10 to 150 µm. Less common than gypsum is anhydrite, the crystal size of which may reach 1.2 mm. Gypsification of some of the anhydrite crystals is obvious with transformation directed from the crystal surface to the center. Veins crosscutting the rock consist dominantly of anhydrite and to a lesser extent of gypsum. Framboidal pyrite is locally abundant. The only fossils found in the thin sections are foraminifera of the genus Nodosaria (Mu¨hlberg). 4.2. Aragonite and native sulfur Aragonite inclusions occur in voids of the host rocks, which were formerly filled with gypsum. These inclusions of white aragonite may reach 3 cm in diameter (Fig. 4a). Joint surfaces may also be covered by aragonite over areas of about 10 cm2 . The aragonite is not indurated and is always closely associated with gypsum. Internally the aragonite inclusions are formed of platy crystals (Fig. 4b). Plates may exhibit the same general orientation in one inclusion (Fig. 4c) or may be arranged in bundles with the orientation of the plates changing from one bundle to the other (Fig. 4d). Perfectly preserved aragonite crystals also occur on heavily weathered joint surfaces (Fig. 4e). Only one example has been found of fibrous aragonite crystals (Fig. 4f). Some aragonite inclusions enclose rhomb-shaped crystals of calcite replacing former dolomite (Fig. 4g). Within some veins there is a sequence from acicular gypsum to granular gypsum to aragonite towards the center of the vein. Aragonite crystals are oriented orthogonally or at an obtuse angle to solution fronts on the gypsum crystals (Fig. 4h). Replacement of gypsum by aragonite is obvious from these solution fronts along gypsum crystals (Fig. 5a,b). Acicular gypsum appears to be less affected by replacement than the granular gypsum. Two types of micro-rods are associated with the aragonite plates (Fig. 5c). The first type of micro-rod is about 0.1 µm in diameter and 1 µm in length. Rods

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Fig. 3. Gas chromatograms of the total hydrocarbons (a) and the fatty acids (b) extracted from neoformed aragonite (Alter Stolberg).

of this type are frequent. Often these rods were found to be arranged in nest-like structures (Fig. 5d). They are cylindrical in cross section, have rounded terminations, and may be straight or curved along their length. The second type of micro-rod is less common. Rods with larger dimensions of about 0.8 µm in diameter and 4 µm in length are attached to the crystal

surfaces or even appear to be slightly embedded in depressions on the crystal surfaces (Fig. 5e). Both at the margins of the aragonite inclusions and between the bundles of aragonite, dark-brown to black organic-rich aggregations occur. These very characteristic residues are restricted to the aragonite. Within the host rocks nothing similar was

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found. The organic-rich aggregations have an irregular shape and may reach 1 mm in diameter. Half of the volume of some aragonite inclusion may be composed of these organic residues. Their fabric is peloidal, and at their margins it becomes obvious that they are formed of accumulations of globules (Fig. 5f). The diameters of globules show a continuous spectrum from 3 µm to smaller sizes. Epifluorescence-microscopy applied on these thin sections is a useful tool to prove the organic composition of these aggregations within the aragonite. Irradiation with ultraviolet light causes an intense fluorescence of the organic residues (Fig. 5g,h and Fig. 6a,b). Another feature of the Ca3 from the locality Alter Stolberg is the local occurrence of native sulfur within micro-cavities. The latter is found associated with aragonite, but more commonly it occurs in cavities without any aragonite phases (Fig. 6c). Where aragonite and sulfur are associated, the formation of sulfur postdates the former. Globular aggregates consisting of microcrystalline sulfur reach 800 µm in diameter. The surfaces of sulfur are densely covered with mineralized curved rods (Fig. 6d). These rods are about 7 µm in length and 3 µm in width. Their size and shape are extremely constant. Many of them exhibit small depressions on their outer surface (Fig. 6d). With ongoing precipitation of sulfur, the rods become enveloped (Fig. 6e). Associated with them are coccoid-shaped bodies. The latter have a diameter of about 0.4 µm (Fig. 6d). At the locality Walkenried some aragonite inclusions exhibit a small central cavity caused by the solution of aragonite. These cavities are lined with microcrystalline gypsum growing on the aragonite and partially replacing the carbonate.

5. Geochemistry 5.1. Mineralogy and chemical composition Chemical analyses of the host rocks and the aragonite inclusions are shown in Table 1. Measured Sr concentrations in the samples of the host rock are smaller than 100 ppm, whereas the aragonite inclusions exhibit elevated Sr concentrations ranging from 820 to 2800 ppm. The high concentrations of Fe and Mn in the sample Sto-A are related to

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Table 1 Mineralogy and chemical composition of host rocks (Mu¨h-B, Sto-C) and aragonite inclusions Sample

Mineralogy

Ca2C (%)

Mg2C (%)

Sr2C (ppm)

Fe2C (ppm)

Mn2C (ppm)

Mu¨h-A Mu¨h-B Sto-A Sto-B Sto-C

aragonite dolomite aragonite aragonite calcite

38.7 22.5 37.6 36.7 34.9

0.374 13.3 0.190 0.051 0.093

2800 90 820 1620 80

20 310 2820 140 980

20 250 730 85 920

the occurrence of Fe–Mn-oxy-hydroxides within the aragonite inclusion analyzed. XRD studies indicate that the black phases associated with aragonite are X-ray amorphous. The Fe–Mn minerals were found in association with only one aragonite inclusion, whereas the overwhelming majority of the aragonite inclusions contained no additional mineral phase, except for rhomb-shaped calcites. The high Mg content (0.051–0.374 wt.%) of the aragonite samples is probably related to these dedolomite rhombs. 5.2. Stable isotopes Carbon and oxygen isotopes of Zechstein carbonates and the aragonite inclusions are given in Table 2. The δ13 C differ significantly from host rock to aragonite at the localities Mu¨hlberg and Alter Stolberg. The host rock is slightly enriched in 13 C at the Mu¨hlberg (C0.96 to C0.97‰) and depleted at Alter Stolberg ( 5.28 to 5.63‰). At both localities δ13 C values of aragonite ( 9.67 to 10.20‰) are lower than those of the surrounding rock. The aragonite from Walkenried yielded similar δ13 C values, ranging from 9.24 to 9.64‰. In contrast to the other localities, the host rock here does not differ significantly from the aragonite values ( 9.45 to 9.99‰). Oxygen isotope measurements from aragonite inclusions yield rather uniform values, ranging from 6.17 to 7.21‰. The host rocks exhibit a broader range, with values for Mu¨hlberg of 3.66 to 4.84‰, for Walkenried of 6.64 to 6.66‰, and for Alter Stolberg of 6.60 to 6.63‰. 5.3. Organic geochemistry A gas chromatogram of the total hydrocarbon fraction is shown in Fig. 3a. The aragonite with or-

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J. Peckmann et al. / Sedimentary Geology 126 (1999) 205–222 Table 2 Stable carbon (δ13 C) and corrected oxygen (δ18 O) isotope analyses of Zechstein carbonates and aragonite inclusions Sample Wal-1 Wal-2 Wal-3 Wal-4 Mu¨h-1 Mu¨h-2 Mu¨h-3 Mu¨h-4 Mu¨h-5 Mu¨h-6 Sto-1 Sto-2 Sto-3 Sto-4 Sto-5

Carbonate phase calcitic matrix calcitic matrix aragonite inclusion aragonite inclusion dolomitic matrix dolomitic matrix dolomitic matrix aragonite inclusion aragonite inclusion aragonite inclusion calcitic matrix calcitic matrix aragonite inclusion aragonite inclusion aragonite inclusion

δ13 C (‰ PDB)

δ18 O (‰ PDB)

9.45 9.99 9.24 9.64 C0.96 C0.97 C0.97 9.67 9.97 9.98 5.28 5.63 10.08 10.16 10.20

6.64 6.66 7.21 7.02 3.66 3.68 4.84 6.69 6.17 6.54 6.63 6.60 6.56 6.75 6.26

ganic-rich aggregations shows a simple compound pattern comprising a modal n-alkane distribution with a first concentration maximum at n-heneicosane (n-C21 , 92 ng=g dry sample). Such pattern may derive from a petroleum source and=or microbial reworking of organic matter rather than from a specific input. A conspicuous feature of the hydrocarbon fraction is the presence of a second, more distinctive concentration maximum at n-octacosane (n-C28 , 70 ng=g) and the occurrence of an unusual suite of terminally (2- and 3-methyl- D iso- and anteiso-) branched alkanes in the range of C21 to C25 . Although no information on biomarker significance, age and=or the biological function of these compounds is available, they are likely to reflect a primary contribution of lipids from discrete microbial sources.

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Table 3 Concentrations of selected molecular markers present in the hydrocarbon and fatty acid fractions of the neoformed aragonite (Alter Stolberg) cements Compound Hydrocarbons n-C21 n-C28 Fatty acids 16 : 1ω7c 16 : 1ω7t 16 : 1ω5c 16 18 : 2ω9 18 : 1ω9c 18 : 1ω7c 18

(ng=g cement) 92 70 750 430 16780 7880 1240 3340 1840 2250

Fatty acids: c, t refer to cis- and trans-configuration of the (terminal) double bond, respectively.

The fatty acid pattern of the aragonite is shown in Fig. 3b. Fatty acids act as building blocks of the cell membranes in most living organisms. In the sample studied, the compounds are present in remarkably high concentrations which exceed those of individual hydrocarbons by more than two orders of magnitude. The fraction contains mainly even-chain saturated and monoenoic n-acids with chain lengths from 14 to 18 carbon atoms (C16 predominant, see Table 3). The pattern found yields some valuable implications on the nature and the source of the organic inclusions present in the neoformed aragonite: (1) The prominent concentrations and the high relative amounts of comparably unstable, unsaturated fatty acids are clearly indicative of fresh, newly produced organic matter. (2) The simple fatty acid pattern points to a single organism, or a simple consortium

Fig. 4. (a) Polished slab of the Ca3 from the locality Alter Stolberg with several bright inclusions of aragonite embedded in the dark matrix of the host rock. Scale bar D 2 cm. (b) SEM image of aragonite plates. The arrow points to a nest-like aggregation of M micro-rods. Ca3, Alter Stolberg. Scale bar D 20 µm. (c) Sharp transition between the granular, calcitic host rock and an aragonite inclusion (stained with Feigl’s solution). Ca3, Alter Stolberg. Scale bar D 200 µm. (d) Arrangement of bundles of plates in groups with the orientation of the plates changing from one bundle to the other. Ca2, Walkenried. Scale bar D 200 µm. (e) Aragonite on a weathered joint surface. Ca3, Alter Stolberg. Scale bar D 1 cm. (f) Aragonite inclusion made of fibrous crystals. Ca3, Alter Stolberg. Scale bar D 1 mm. (g) Rhomb-shaped crystals of calcite replacing former dolomite within an aragonite inclusion. The bright object on the left is an organic inclusion. Thin section excited with ultraviolet light. Ca3, Alter Stolberg. Scale bar D 200 µm. (h) Gypsum crystals (left) and platy aragonite. Center: aragonite crystals are oriented at an obtuse angle to the solution front along the gypsum crystal below the center. Ca2, Mu¨hlberg. Scale bar D 100 µm.

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of organisms as the key biota associated with the formation of the aragonite cements. (3) The molecular characteristics of the acids found strongly suggest a bacterial source input, since all of them have been observed as bacterial lipid constituents, or from recent sediments showing a pronounced microbial activity (e.g., Volkman et al., 1980; Nichols et al., 1986 and refs. cited therein). Notably, the 18 : 1ω7 fatty acid was previously reported by Russell et al. (1997) from diagenetic carbonates replacing gypsum and was interpreted as a marker of bacterial activity.

6. Discussion 6.1. Diagenesis of the host rocks A general scheme of successive diagenetic processes of Zechstein carbonates and anhydrites was proposed by Clark (1980) and Huttel and Mausfeld (1991). Their model is also valid for the carbonates of the Eichsfeld High. They assume an early pervasive dolomitization of carbonates in response to the reflux of brines from sabkhas and playas. Due to this process, microcrystalline dolomites were formed and later recrystallized. During burial diagenesis anhydritization of gypsum formations, pressure solution, leaching and calcitization of dolomite due to the generation of CO2 from microbial oxidation of organic matter occurred. The latter process took place preferentially at basinal sites, where the available amount of organic matter was higher than on top of the platforms, resulting in present-day calcitic lithologies in the basinal settings and in dolomitic lithologies on the platforms. In addition, uplifting, erosion, and access of water enriched in Ca-ions due to the leaching of anhydrite led to calcification of former dolomites under near-surface, meteoric conditions.

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6.2. Diagenetic aragonite The delicate aragonite crystals are not affected by weathering. This is surprising, as they are directly exposed to the changing conditions at the surface including episodic soaking by rainfall. Even aragonite crystals originating on weathered joint surfaces were found. This indicates that the aragonite formation postdates at least part of the weathering. All inclusions are not indurated and the crystal aggregates readily disintegrate. Taking the metastability of aragonite under surface conditions into account, the Recent formation of this mineral phase is likely. Aragonite deposits, that were formed in pre-Holocene times and still preserve their primary mineralogy, are not uncommon. However, to the best of our knowledge, all of these aragonite deposits were protected from alterations by the surrounding rock or mineral phases. Comparing the development at the three localities, the process of aragonite formation is least advanced in the Ca2 from the Mu¨hlberg locality. Here the aragonite is growing in cavities and veins which are still partly filled with gypsum. Aragonite crystals are found to protrude into both granular and fibrous gypsum crystals, replacing the sulfate minerals. The granular gypsum crystals are significantly more affected by this process. The interface between the gypsum crystals and aragonite is very irregular, indicating the dissolution of gypsum. The plates of aragonite are probably directly related to the replacement of the gypsum. This is indicated by an almost constant angle of the aragonite plates to the dissolution fronts along the gypsum crystals and the circumstance that aragonite is always in direct contact with these fronts. If the aragonite formation did not occur immediately after dissolution of gypsum, a more indiscriminate infill of the cavities should be expected.

Fig. 5. (a) Aragonite (dark) growing in a micro-cavity surrounded by acicular gypsum. Ca2, Mu¨hlberg. Scale bar D 250 µm. (b) Detail from Fig. 4a. Solution front on a gypsum crystal with aragonite crystals orientated orthogonally to this surface. (c) SEM image of two types of micro-rods attached to aragonite plates. Ca3, Alter Stolberg. Scale bar D 6 µm. (d) Detail from Fig. 4c. M micro-rods arranged in a nest-like cluster. Scale bar D 5 µm. (e) FE–SEM image of a micro-rod. Ca3, Alter Stolberg. Scale bar D 4 µm. (f) Organic-rich aggregations (bright) within an aragonite inclusion. Note the flaky fabric of the organic aggregate in the center. Excited with ultraviolet light. Ca3, Alter Stolberg. Scale bar D 100 µm. (g) Aragonite inclusion containing several organic-rich aggregations (dark). The long axes of organic-rich aggregations are parallel to the orientation of aragonite plates. Ca3, Alter Stolberg. Scale bar D 200 µm. (h) Same view as Fig. 4c now excited with ultraviolet light, causing the organic-rich inclusions to emit an intense fluorescence.

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Fig. 6. (a) Organic-rich aragonite replacing gypsum crystals (right). Ca3, Alter Stolberg. Scale bar D 100 µm. (b) Same view as Fig. 5a now excited with ultraviolet light. Organic compounds cause an intense fluorescence. (c) Globular aggregation of native sulfur within a micro-cavity. Ca3, Alter Stolberg. Scale bar D 500 µm. (d) SEM image of native sulfur with curved rods (arrows) and coccoid-shaped bodies. Ca3, Alter Stolberg. Scale bar D 10 µm. (e) SEM image of native sulfur. Curved rods (arrow) become enveloped by extracellular precipitation of sulfur. Ca3, Alter Stolberg. Scale bar D 10 µm.

These processes are most advanced in Walkenried, where the return to oxidizing conditions caused some aragonite to be replaced by secondary gypsum. At the contact surface between aragonite and gypsum, which is very irregular, microcrystalline gypsum protrudes into the laminated aragonite fabric. The process of secondary replacement of aragonite by diagenetic

gypsum under aerobic conditions was already described by Anado´n et al. (1992) from a similar setting. 6.3. Aragonite vs. calcite precipitation Several factors such as the presence of bacteria and organic matter, the magnesium concentration of

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the solution, high temperature and the degree of carbonate supersaturation have been suggested to favor aragonite precipitation instead of calcite. The relationship between microbial activity and the formation of aragonite or calcite still remains unclear. Buczynski and Chafetz (1991) found that media with the same bacterial composition yield aragonite or calcite precipitates dependent on the viscosity of the media. Oppenheimer (1961) noted the formation of aragonite in the presence of bacteria, whereas several others report on microbially-derived calcites (Boquet et al., 1973; Monger et al., 1991; Newman et al., 1997). Bacteria may not be able to influence the mineralogy of the precipitate, but calcifying organic substances derived from the decay of bacteria might trigger matrix-mediated precipitation as described for microbialites from Lizard Island (Reitner, 1993). Rising Mg=Ca ratios have been demonstrated to promote the precipitation of aragonite instead of calcite (Riccioni et al., 1996; Laya et al., 1992). Kitano et al. (1962) found that the presence of MgCl2 in Ca(HCO3 )2 solutions favors the formation of aragonite. This is supported by Berner (1975), who studied the crystal growth of aragonite and calcite in sea water, magnesium-depleted sea water, and magnesium-free sea water. He reported that dissolved magnesium at sea water levels reduces the crystal growth of calcite. The process which supplies excess magnesium in the Zechstein carbonates is suggested to be dolomite replacement. This is corroborated by the presence of calcitized dolomite crystals within the aragonite inclusions. High temperature has been demonstrated to promote precipitation of aragonite relative to calcite (Kitano et al., 1962), but is very unlikely to be responsible for the aragonite formation in the nearsurface setting as discussed here. The degree of carbonate supersaturation predetermines the mineralogy of the precipitates, with very high supersaturation levels promoting aragonite formation (Chafetz et al., 1991). In the case of our example high degrees of supersaturation in Ca2C (due to the solution of gypsum), and an elevated PCO2 (due to microbial activity), cannot be proven, but may have occurred.

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6.4. Implications on the elemental composition The contents of Sr in subsurface Zechstein carbonates vary from less than 100 ppm to some hundred ppm, with extremely rare values above 1000 ppm (Clark, 1980; Huttel and Mausfeld, 1991). Carbonates from outcrops exhibit concentrations up to 110 ppm. Limestones have higher Sr contents than dolomites, whereas Fe and Mn are enriched in microcrystalline dolomites indicating reducing conditions and saline solutions favoring the transport of Fe and Mn as metal complexes. The values in subsurface samples are between 200 and 1000 ppm for Mn and 300–2000 ppm for Fe, for both dolomite and calcite. Ranging from 820 to 2800 ppm, the neoformed aragonites exhibit lower Sr concentrations than marine aragonites, which exhibit values of about 10,000 ppm (Veizer, 1983). The host rocks analyzed in this study yield much lower Sr concentrations, ranging from 80 to 90 ppm. This indicates that the neoformed aragonites have sources of cations other than the calcitic or dolomitic matrices. Botz and Mu¨ller (1981) report Sr concentrations in the range of 1300–1700 ppm of the gypsum-rich transition horizons below and above the Zechstein carbonates. Local enrichment of Sr is obvious from celestite crystals which occur within gypsificated anhydrite deposits (Langbein, 1968). Jung and Knitzschke (1961) report that the A2 contains up to 5300 ppm Sr. The distribution of Sr in carbonates and sulfates elucidates the crucial part of sulfate dissolution as the process supplying cations for the precipitation of aragonite. 6.5. Implications on the stable isotope compositions The aragonite that replaces gypsum is characterized by distinctly lower δ13 C values than the host rocks, except for the Ca2 from Walkenried. The narrow range of the δ13 C values of aragonite suggests that precipitation at the different locations was triggered by the same process. The values reported here of about 10‰ indicate two different carbon sources. δ13 C values of about 25‰ are typically associated with bacterial sulfate reduction and depend on the composition of the organic matter that becomes oxidized (Irwin et al., 1977). Accordingly, the isotopic composition of the aragonites studied implies a mixing with a less depleted carbon source, probably the

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carbonate host rocks. The sulfate sources for the bacterial sulfate reduction are the minerals gypsum and anhydrite. The organic matter that became oxidized was probably derived from the Ca2 and Ca3 and not from deeper strata. Indeed, the basinal facies of the Ca2 contains up to 1.75% organic carbon (Botz and Mu¨ller, 1981) and has thus been discussed for its oil and gas potential (Botz et al., 1981). The negative δ18 O values of aragonite of about 6 to 7‰ indicate that precipitation occurred under the influence of meteoric waters. Today, all outcrops are above the ground water table, implying that aragonite formation occurs in the vadose–meteoric zone. Data from Botz and Mu¨ller (1981) regarding the δ13 C values of the Zechstein carbonates show a range from C7 to C4‰. Only those samples which contain considerable amounts of sulfate yielded values that range from C2 to 0‰. These positive δ13 C values of Zechstein carbonates are interpreted by Huttel and Mausfeld (1991) to be derived from the relative enrichment of 13 C in the seawater. This is due to the preferential fixation of 12 C in the anoxic sediments as organic carbon (Magaritz et al., 1983). In comparison with the data from Botz and Mu¨ller (1981) and Huttel and Mausfeld (1991), the depletion of 13 C in the host rocks of this study indicates a higher carbon content that was derived by the oxidation of organic matter. Only the dolomitic Ca2 of the Mu¨hlberg is slightly enriched in 13 C (¾C1‰), whereas the matrices at the other localities are depleted in this isotope. Studies on the isotopic composition of co-existing dolomites and calcites have revealed that dolomites are slightly enriched in 13 C with respect to associated calcites (Fritz and Smith, 1970; Tan and Hudson, 1971). However, this enrichment is not sufficient to explain the offset in the δ13 C values between the Mu¨hlberg-Ca2 and the other calcitic host rocks. The 13 C isotopic composition of the Mu¨hlberg-Ca2 indicates precipitation from marine sources. At Walkenried the calcitic host rock exhibits a similar depletion in 13 C of about 10‰ like the aragonite inclusions. This low value indicates that some carbonate carbon was derived from the oxidation of organic matter. Probably the host rock at Walkenried has also been affected by bacterial sulfate reduction. This implies that similar processes account for formation of aragonite as well

as the alteration of the host rock in earlier times. To a lesser extent, this also holds true for the locality Alter Stolberg (δ13 C 5 to 6‰). For subsurface samples Huttel and Mausfeld (1991) report δ18 O values in the range of C4 to 4‰ in Zechstein carbonates. The δ18 O values of the dolomite host rock from Mu¨hlberg have to be lowered by 3–7‰, because of the differential oxygen isotope fractionation during crystallization of calcite with respect to dolomite (O’Neil and Epstein, 1966; Fritz and Smith, 1970; Tan and Hudson, 1971; Land, 1980). The low temperature precipitation of protodolomite yields an enrichment in 18 O by 3– 4‰ relative to coexisting calcite (Fritz and Smith, 1970), which corresponds well with the data of Irwin et al. (1977). Taking this into account, all the host rocks described here exhibit δ18 O values of about 6 to 7‰, indicating an exchange with isotopically light meteoric waters during diagenesis (cf. Tan and Hudson, 1971). 6.6. Bacterial mediation The bacterial mediation in the diagenetic processes taking place within Zechstein carbonates is evidenced by the co-occurrence of characteristic phenomena. These are carbonate replacement of gypsum, low δ13 C values of the replacing carbonates, and the presence of native sulfur. The crucial role of bacteria is further corroborated by bacterial body- and chemo-fossils associated with the diagenetic phases. The smaller micro-rods attached to the aragonite plates are organized in nest-like accumulations, lack any crystallographic form or indication of cleavage, and their form exhibits similarities to bacteria. This indicates a biotic origin. They correspond to the M micro-rods described by Verrecchia and Verrecchia (1994). These M micro-rods are thought to be calcified bacteria or physicochemical nuclei. Phillips and Self (1987) interpret them as calcified rod-shaped bacteria and notice their frequent association with organic matter. They also have been attributed to bacteria by their form and their types of organization (Chafetz et al., 1998). Micro-rods of the second type (about 4 µm in length) found on the aragonite plates exhibit the typical size and form of rod-shaped bacteria and are probably also calcified bacteria.

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Biomarker studies on aragonite including organicrich aggregations yielded high concentrations of specific unsaturated fatty acids, which are clearly indicative of newly produced organic matter and reflect the presence of a discrete microbial community being associated with aragonite formation. Bacterial sulfate reduction leads to the production of H2 S, which might be converted to native sulfur under aerobic conditions or by the reaction with Fe–Mn–oxy-hydroxides even under anaerobic conditions (Jørgensen, 1987). However, as iron and manganese primarily occur in reduced state in Zechstein carbonates, an exposure to oxygen is required anyway. A shift from anaerobic conditions during aragonite formation to an aerobic micro-environment during sulfur formation within some cavities is most likely related to an exposure to oxygen due to progressive weathering of the sulfate-rich carbonate rocks. As sulfate-reducing bacteria are obligate anaerobes, the oxidation of H2 S probably did not occur at the same time as the sulfate reduction and aragonite precipitation at the locality Alter Stolberg. Formation of native sulfur might have taken place at a later stage, when H2 S, trapped in cavities, came into contact with oxidizing waters. A contemporaneous formation of aragonite and sulfur due to the upward seepage of H2 S into an aerobic environment cannot be excluded, but is considered to be unlikely as cavities represent rather isolated traps. The process of sulfur formation is also bacterially mediated. Several groups of bacteria are able to use H2 S as an electron donor. We interpret the curved rods which are associated with the native sulfur as fossilized bacterial bodies. The small depressions on their outer surface indicate that shrinkage of the bacterial bodies occurred at a low extent before they were completely mineralized. The permineralization of bacterial bodies is followed by an extracellular mode of precipitation enveloping the rods. The coccoid-shaped bodies in the sulfur deposits may also represent fossilized bacteria. With a diameter of 0.4 µm, these bodies exceed the size of nannobacteria described by Folk (1993). Apart from chemolithotrophic sulfur-oxidizing bacteria, purple sulfur bacteria are also able to oxidize H2 S to elemental sulfur. However, as the anoxigenic purple bacteria are phototrophic, it is highly unlikely that they formed the sulfur deposits

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in the completely dark cavities within Zechstein carbonates. The following processes of gypsum solution (Eq. 1), reduction of sulfate ions (Eq. 2), aragonite precipitation (Eq. 3), and sulfur formation (Eq. 4) are envisaged: CaSO4 ð 2H2 O ! Ca2C C SO24 C 2H2 O

(1)

The gypsum involved here comes from the nodules embedded in the carbonate host rocks. The widespread dissolution of the sulfate deposits of the A1, A2 and A3 may generate additional sources of Ca2C and SO4 ions. SO24 C 2CH2 O ! 2HCO3 C H2 S

(2)

Due to the utilization of organic material by sulfate-reducing bacteria, alkalinity increases (see Eq. 2). Together with the availability of Ca2C ions (see Eq. 1), precipitation of aragonite will be induced (Eq. 3): Ca2C C 2HCO3 ! CaCO3.ara/ C H2 O C CO2

(3)

If the product of sulfate reduction, H2 S, gets trapped in the aerobic zone for a sufficient length of time, it can be oxidized to sulfur and sulfur-oxides (Eq. 4): 3H2 S C 4O2 ! S0 C 3H2 O C SO2 C SO3

(4)

The latter process can accumulate sulfur to deposits of commercial value, such as in the Permian anhydrite formations of West Texas (cf. Davis and Kirkland, 1979), as well as in the Tertiary of the Mediterranean area (Dessau et al., 1962), and in the Middle Miocene of the Gulf of Suez (Pierre and Rouchy, 1988).

7. Conclusions Bacterial mediation is a widespread phenomenon in the formation of diagenetic minerals in different sedimentary environments. Neoformed aragonite and native sulfur in the Zechstein carbonates from Central Germany described here, highlight the influence of microbial activity on diagenesis. The precipitation of diagenetic aragonite and sulfur within cavities of the carbonates was mediated by bacteria. All occurrences of the neoformed, metastable aragonite and of

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sulfur are located far above the watertable, indicating that aragonite formation occurred in the vadose– meteoric zone. The processes which led to the formation of aragonite started with the dissolution of sulfate nodules embedded in the carbonate host rocks. Solution of gypsum provided both Ca ions for the precipitation of aragonite and sulfate ions for the metabolic processes of sulfate-reducing bacteria. The substrate for these bacteria was organic matter, probably derived from the carbonate host rocks. The oxidation of organic matter was coupled with sulfate reduction in an anaerobic micro-environment. Aragonite forms as a consequence of the increase in alkalinity accompanied with bacterial sulfate reduction. The importance of this process is corroborated by the low δ13 C values of the aragonite, which indicate that some of its carbon is derived from organic matter. The bacterial oxidation of H2 S (product of sulfate reduction) resulted in the formation of native sulfur, which occurs as globular aggregates at one of the study localities. Aragonite crystals exhibit a platy habit. SEM analyses show that two types of micro-rods are associated with the plates. The smaller type (1 µm long and 0.1 µm in diameter) corresponds to the M microrods described by Verrecchia and Verrecchia (1994). This type and a larger type of mineralized rods (4 µm long and 0.8 µm in diameter) are interpreted to be bacterial fossils. The volumetrically important aggregations of organic-rich material, which are limited to the aragonite inclusions, are derived from bacterial sources. This is indicated by biomarker analyses which evidenced molecular characteristics of a microbial community. SEM analyses show that bacterial bodies are densely packed on and within the sulfur. With ongoing sulfur formation, the permineralized curved rods (7 µm long and 3 µm in diameter) become enveloped by an extracellular mode of precipitation. High Sr concentrations in sulfate deposits and aragonite precipitates indicate that sulfate dissolution, and not the dissolution of the carbonate host rocks, was the cation supplying process. On a microscale, solution fronts on the gypsum crystals appear to be the interfaces where these processes take place. The platy aragonite crystals, which replace the gypsum, are oriented orthogonally or at an obtuse angle to the solution fronts. This and the ubiqui-

tous loose and platy appearance of the aragonite inclusions are believed to be caused by the replacement of gypsum by aragonite. The crucial factor for the precipitation of aragonite instead of calcite was probably an elevated Mg=Ca ratio.

Acknowledgements We are indebted to M. Plache, A. Quast and K. Tra¨ger, who discovered the rocks which are the subject of this study. We thank H. Becker and K.-H. Faber for the preparation of thin sections. M. Joachimski is acknowledged for stable isotope analyses and H. Huckriede and C. Gross for their comments. Special thanks to I. Harding for a careful review of an earlier version of the manuscript and to H. Lehnert and K. Techmer for their electron microscope support. We acknowledge the logistical support provided by the IMGP, Go¨ttingen (AG Reitner) and the IfBM, Hamburg (AG Michaelis). Comments by J.M. Rouchy and an anonymous reviewer greatly improved the manuscript. This paper is a contribution to the SFB 468 ‘Wechselwirkungen an geologischen Grenzfla¨chen’ (publ. no. 8) at the University of Go¨ttingen.

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