Methane-derived carbonates and authigenic pyrite from the northwestern Black Sea

Marine Geology 177 (2001) 129±150

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Methane-derived carbonates and authigenic pyrite from the northwestern Black Sea J. Peckmann a,*, A. Reimer a, U. Luth b, C. Luth b, B.T. Hansen c, C. Heinicke c, J. Hoefs d, J. Reitner a a

Institut und Museum fuÈr Geologie und PalaÈontologie, Georg-August-UniversitaÈt, Goldschmidtstrasse 3, D-37077 GoÈttingen, Germany b Institut fuÈr Hydrobiologie und Fischereiwissenschaft, UniversitaÈt Hamburg, Zeiseweg 9, D-22765 Hamburg, Germany c Institut fuÈr Geologie und Dynamik der LithossphaÈre, Georg-August-UniversitaÈt, Goldschmidtstrasse 3, D-37077 GoÈttingen, Germany d Geochemisches Institut, Georg-August-UniversitaÈt, Goldschmidtstrasse 1, D-37077 GoÈttingen, Germany Received 27 August 1999; accepted 29 December 2000

Abstract Methane seeps in the northwestern Black Sea are accompanied by carbonate and pyrite precipitates. Sediments were sampled at locations on the Romanian (120 m depth) and Ukrainian (180±200 m depth) shelf and slope. Layered carbonate crusts are formed of (i) carbonate-cemented siliciclastic sediment containing dreissenoid bivalves, (ii) microcrystalline high-Mg-calcite or aragonite, and (iii) aragonitic cement. The Dreissena sediment is subrecent and was deposited during the freshwater-phase of the Black Sea. It has been affected by seepage of methane-rich ¯uids, which induced intergranular precipitation of authigenic carbonates. The microcrystalline carbonates exhibit an intense auto¯uorescence. High-Mg-calcite contains 11±14 mol% MgCO3. The aragonitic cement (8300±9500 ppm Sr) forms either isopachous layers or botryoids. Microbial ®laments about 10±20 mm in diameter and up to 900 mm in length are preserved within and on carbonate crusts. The carbonates are depleted in 13 C. Microcrystalline carbonate ranges from 227 to 241½ PDB, and botryoidal aragonite ranges from 226 to 238½ PDB. The 13C depletion indicates that the carbonates predominantly derive from the microbial oxidation of methane. Carbonate deposits do not project up into the oxic water column. They are restricted to the anoxic water column and to anoxic sediments revealing the crucial role of anaerobic methane oxidation for carbonate precipitation. 14C contents give apparent radiocarbon ages of 20,640 ^ 180 a BP for a sample of botryoidal aragonite and 19,110 ^ 180 a BP for a sample of microcrystalline carbonate, compatible with minimum ages of carbon derived from a fossil hydrocarbon source. The d 18O values of methanederived carbonates show a narrow range from 11.2 to 10.2½ PDB. 87Sr/ 86Sr ratios of microcrystalline carbonate (mean 0.70927) and aragonitic cement (mean 0.70918) are indistinguishable from ambient seawater (mean 0.70917) and thus indicate a shallow Sr source. Higher 87Sr/ 86Sr ratios of the Dreissena sediment (mean 0.71005) are probably caused by Sr derived from detrital mica. Carbonates are accompanied by blocks and crusts composed of pyrite. The framboidal sulphide exhibits a palisade-like fabric with framboids arranged to parallel pillars. Sulphur isotopic ratios (d 34S) ranging from 116.8 to 119.7½ CDT indicate that the sulphur derives not from the 34S-depleted H2S of the water column or the uppermost sediment layers. Most likely, pyrite formed in the lacustrine sediments after the ®rst incursion of Mediterranean seawater. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbonates; Pyrite; Cold seeps; Methane; Anaerobic methane oxidation; Black Sea; Romania; Ukraine

* Corresponding author. Fax: 149-551-397918. E-mail address: [email protected] (J. Peckmann). 0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(01)00128-1

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1. Introduction Hydrothermal `hot vents' from oceanic spreading centres and `cold seeps' from a variety of settings may have been some of the most intriguing discoveries in earth sciences forthcoming to the new millennium. A dominant component of many cold seeps is dissolved or free methane, but heavier hydrocarbons may also predominate (Davis and Spies, 1980; Paull et al., 1985; Roberts and Aharon, 1994). Cold seeps fuel chemosynthesis-based benthic communities that consist of invertebrates containing bacterial symbionts, which depend on H2S or methane. A review of the faunal communities of cold seeps and their geographic distribution is given by Sibuet and Olu (1998). Carbonate precipitation is a striking phenomenon that occurs at hydrocarbon seeps. It has been assigned to coupled bacterial sulphate reduction and methane oxidation (Ritger et al., 1987; Paull et al., 1992; von Rad et al., 1996). This reaction is associated with an increase in alkalinity which favours carbonate precipitation. Hydrocarbon-derived seep carbonates are known from the Middle Devonian to Recent (Peckmann et al., 1999a). The typical carbonate species at cold seeps are Mg-calcite, aragonite, and dolomite (Hovland et al., 1987; Roberts et al., 1993). The chemophysical factors controlling the mineralogy of seep carbonates are still insuf®ciently determined, but should generally be the same as in other environments (cf. Kitano et al., 1962; Berner, 1975; FernaÂndez-DiÂaz et al., 1996). Carbonate precipitation at cold seeps is a consequence of microbial activity (Ritger et al., 1987; Peckmann et al., 1999b). The general concept of chemosynthesis is based on a chemical oxidation that provides the energy for biosynthesis (Jannasch, 1984). Chemolithotrophic bacteria use H2S or other inorganic compounds as a source of energy, whereas chemoorganotrophs depend on organic substrates. Microbial mats near hydrocarbon seeps were found to be formed of Beggiatoa and other H2S-oxidising bacteria (Sassen et al., 1993; Larkin et al., 1994). However, the metabolism of these microbes does not induce carbonate precipitation as it is proposed for methanotrophs. Analyses by LaRock et al. (1994) indicate that methane serves as an energy source for microbial populations at cold seeps.

Signatures of aerobic methanotrophs have been found in a Tertiary cold seep carbonate (Peckmann et al., 1999b). Anaerobic oxidation of methane at an ancient cold seep was assigned to a microbial consortium including archaea (Thiel et al., 1999). The ability of archaea to use methane as a source of energy is also substantiated by examples from Recent sediments (Hoehler et al., 1994; Harder, 1997) and by biogeochemical evidence (Elvert et al., 1999; Hinrichs et al., 1999). Here we report on the occurrence of methanederived carbonates from the northwestern Black Sea. In contrast to most other seep sites the Black Sea seeps are partly situated in the anoxic zone which signi®cantly in¯uences carbonate precipitation. The microbial carbonates are accompanied by pyrite precipitates. Carbonates and pyrite will be described in terms of petrography, mineralogy, elemental composition, and isotope geochemistry.

2. Sampling, in situ observations, and description of the study area 2.1. Sampling sites and sampling procedure Samples were taken at locations known for gas bearing sediments (Polikarpov et al., 1989, 1992) on the shelf and slope of the northwestern Black Sea within the scope of the multinational MEGASEEBS project (Methane Gas Seep Explorations in the Black Sea). In October 1993 and June 1994 the R/V Professor Vodyanitskiy (Institute of Biology of the Southern Seas, Sevastopol, Ukraine) operated in the Dnieper Canyon region southwest of the Crimean peninsula. The Romanian shelf near the Vityaz Canyon was investigated in April 1994 during a cruise of the R/V Poseidon (Institut fuÈr Meereskunde, Kiel, Germany). The samples were obtained from seep sites at 120 m water depth close to the shelf edge off Romania and from the Ukrainian slope at 190 m water depth (Fig. 1). Bottom topography and gas seeps were mapped by echosounding. Visual underwater surveys were carried out using a remote operative vehicle (ROV) equipped with a TV-camera. The seep sites were sampled with a 2.5 m beam trawl (mesh size

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occurs at varying depths between 130 and 180 m. Therefore, it has major in¯uences on the biogeochemical conditions and the distribution of the benthic fauna of the lower shelf and upper slope environments. Likewise, hydrophysical and hydrochemical measurements in the oxic±anoxic interface zone can only describe a temporary situation. The two sampled seep sites are both strongly affected by the Black Sea rim current. Hydrophysical and hydrochemical parameters are given in Table 1. The Romanian site is situated in the suboxic zone with oxygen values from 50 to 5 mM l 21. Within the anoxic zone at the Ukrainian site no free oxygen was detectable.

Fig. 1. Sampling sites in the northwestern Black Sea (Ro: Romanian shelf; Ukr: Ukrainian slope).

0.5 cm), as lithi®cation at the gas seeps was very massive. 2.2. Oceanography and sedimentation 2.2.1. Oceanography In response to global sea level rise in the Holocene, high salinity waters from the Mediterranean Sea invaded the Black Sea via the Bosphorus. This postglacial in¯ow is believed to have occurred 9000 yr BP (Ross et al., 1970) or about 7000 yr BP (Ryan et al., 1997), respectively. Permanent strati®cation was established between the original freshwater body and the underlying marine waters. Anoxia developed in the bottom layer accompanied by a slow rise of the halocline (Ross and Degens, 1974). Today, surface water has a mean salinity of 18 PSU, and oxygen is almost totally absent below 150 m water depth. More than 80% of the water column (max. depth 2100 m) is anoxic. Salinity in the anoxic deep layer is about 22 PSU (Sorokin, 1982) and H2S concentration amounts up to 380 mmol L 21. Throughout the Black Sea, the depth of the oxic± anoxic interface is largely determined by the meandering cyclonic rim current (Skopintsev, 1975; Murray, 1991). This highly dynamic interface zone

2.2.2. Sedimentation Sediments in the northwestern Black Sea mainly derive from the rivers Danube, Dnestr, and Dnieper. Due to eutrophication caused by high nutrient load of the contributing rivers sedimentation rates are generally high (Zaitzev, 1993). However, at the shelf edge and in the vicinity of submarine canyons, strong variations in sedimentation rates have been described (Shopov et al., 1986). The meandering cyclonic rim current induces a system of anticyclonic eddies and sediments may either not accumulate or may be eroded (Oguz et al., 1992, 1994; Sur et al., 1994; Latun, 1990). The Dnieper Canyon region is characterised by frequent lateral transport and resedimentation along the shelf and represents a source area of turbidity currents (Domanov et al., 1996). A similar situation is observed in the Romanian Vityaz Canyon region (Panin, 1996; Luth, unpubl. data). In undisturbed strata, shell debris of dreissenoid bivalves, derived from the lacustrine period of the Table 1 Hydrophysical and hydrochemical data of the bottom water at the two seep sites (Ro: Romanian shelf; Ukr: Ukrainian slope; n.d.: not detectable; temperature and salinity values obtained by CTD, oxygen and hydrogen sulphide values determined from the overlying water in multiple corer tubes) Parameter

Temp. [8C] sal. [PSU] O2 [mmol l 21] H2S [mmol l 21]

Seep site Ro

Ukr

8.3 21.3 11 n.d.

8.7 21.4 n.d. 140

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Black Sea, appear below layers containing modern species (Luth and Luth, 1998). At the Romanian shelf station (120 m), Dreissena valves occur mixed with modern bivalves (e.g. Modiolus phaseolinus, Mytilus galloprovinciales, Abra nitida milachewichi, Plagiocardium simile) in the upper sediment layers. Dreissena layers even form the sediment surface at the Ukrainian slope station (190 m). Currents predominantly account for erosion and reworking of sediment, but gas seepage may cause additional disturbance (cf. Dando and Hovland, 1992). The stratigraphy of the deep-water sediments in the Black Sea has been described by Ross and Degens (1974). Carbonate-rich microlaminated sediments mainly consisting of the coccolithophore Emiliana huxleyi form the so-called Unit-1, which is underlain by a marine sapropel (Unit-2) and freshwater sediments (Unit-3). 2.3. Description of seeps and gas-related features Although a considerable number of gas seeps have been reported from the Romanian shelf and slope (Polikarpov et al., 1992), very little is known about their morphological, biological, and lithological features. Seeps occur along the entire oxic±anoxic gradient (Egorov et al., 1998). The gas seeps can easily be traced by the distinct signal of echosounder recordings caused by rising gas bubbles. In calm seas, bubble streams derived from strong gas emissions are

even visible at the sea surface. According to previous investigations the gas consists of 80% methane (Ivanov et al., 1991). Measurements or estimates of the amounts of released gases or ¯ow rates are not available. Video observations and trawl data show the presence of various types of carbonate deposits and metal sulphides around the gas emission sites on the sea ¯oor (Fig. 2). At 60 m water depth, in the oxic zone, the carbonates are ¯at, pancake-shaped deposits, 20±30 cm in diameter. These precipitates are con®ned to the anoxic sediment and form no positive sea¯oor relief. Usually, the crusts exhibit a conspicuous, central, 1±2 cm wide channel. Gas seepage through these central holes has been recognised during ROV observations (Luth et al., 1999). At water depths of 110±130 m, carbonate deposits show an increased thickness of up to 10 cm. Most of these samples are porous and cavity walls are abundantly lined by microbial ®lms. In the anoxic zone at about 190 m water depth the carbonates form irregular, small chimneys (Fig. 3). Some of them arise from ¯at carbonate plates which may be up to 20 cm thick and about 1 m in diameter. At greater depths in the anoxic zone (230 m) the precipitates are tall chimneys up to 1 m high that are penetrated by active seepage (Ivanov et al., 1991). The chimneys protrude from thick platforms. Carbonate deposits are commonly associated with authigenic metal sulphide which have been described by Lein et al. (1995).

Fig. 2. Schematic pro®le of shelf/slope transition of the northwestern Black Sea. The shape and the dimensions of carbonate precipitates vary in dependence of the bottom water chemistry. The precipitates in the permanent oxic zone are con®ned to the anoxic sediment (after Luth et al., 1999).

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Fig. 3. Drawing of a carbonate precipitate from the Ukrainian slope (188 m depth, upper anoxic zone).

At all depths, microbial mats of pink-brown colour are associated with the carbonate precipitates. With increasing water depth mat thickness also increases reaching several centimetres in the anoxic zone. Here, the mats cover outer surfaces of precipitates as well as channel and cavity walls. Pimenov et al. (1997) have studied the structure and ecology of the microbial mats sampled at the 190 m slope station (study site). Dominating microorganisms of the mats are very similar in ultrastructure to methanogenic archaea of the genus Methanosaeta (formerly referred to as Methanothrix). However, in experiments on the sampled microbial mats under strictly anaerobic conditions signi®cant rates of 14 CH4 oxidation were found. Besides, sulphate reduction and methanogenesis were proved using 14C carbonate and 14C acetate as substrates. Apart from the microbial mats no seep biota, a predominant factor at many seep sites (Paull et al., 1984; Kennicutt et al., 1985; Suess et al., 1985; Jensen et al., 1992; Dando and Hovland, 1992; Sibuet and Olu, 1998), were found associated with the carbonate precipitates at any depth. 3. Methods Thin sections were studied using conventional and

¯uorescence techniques on a Zeiss Axiolab. Some thin sections and polished rocks were stained with combined potassium ferricyanide and alizarin red, dissolved in 0.1% HCl solution (FuÈchtbauer, 1988, pp. 240±241), with Feigl's solution (Feigl, 1958), and with Titan-Yellow (Choquette and Trusell, 1978). A LEO 1530 Gemini was used for ®eldemission (FE)-SEM analyses on uncoated samples. After microscopical examination about 1000 mg of material was drilled out of the corresponding rock sample. Mineralogy was determined by X-ray diffraction on unoriented slurries using a diffractometer with CuKa radiation (Philips PW 1800). The chemical composition was analysed by atomic absorption spectrometry (AAS) on a Philips PU 9200X after digestion of powdered samples in hydrochloric acid (15%) at 608C. The standard deviation for replicate analyses is ^2%. Electron microprobe analyses were performed on polished thin sections using a JEOL JXA 8900. The 320 individual measurements had a resolution of 5±10 mm. 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 758C. Measurements were made with a Finnigan Mat 252 mass spectrometer using a Carbo-Kiel carbonate preparation technique at the University of Erlangen. The d 13C and d 18O results are reported relative to the

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PDB standard and appropriate internal correction factors were applied (Craig, 1957). The standard deviation is 0.02½ for carbon and oxygen for replicate analyses of the NBS 19 standard. 14C analyses were performed at the Erlangen AMS facility (for details see: Kretschmer et al., 1997). Results are given as uncalibrated 14C radiocarbon years calculated on the base of t1/2 14C ˆ 5730 a. Sr isotopic analyses were carried out at the Institut fuÈr Geologie und Dynamik der LithosphaÈre in GoÈttingen on a Finnigan 262 RPQ 1 mass spectrometer. Rb/Sr determinations were made on powdered rock samples (2.5±50 mg) using conventional column chemistry. All samples analysed were spiked with a 84Sr± 87Rb mixed spike prior to dissolution. A detailed description of sample preparation and analytical procedures is given in Zeck and Hansen (1988). Measured ratios were corrected to the obtained value of the NBS SRM 987 (0.710261, n ˆ 7) standard. For S isotope analyses pyrite was reacted with V2O5 at 10008C. The resulting SO2 was measured with a Finnigan Mat 251. d 34S values are expressed in the conventional d-notation as deviation in per mil from the CD troilite. The standard deviation for replicate analyses is ^0.1½.

4. Carbonate crusts and pyrite 4.1. Petrography The carbonate crusts consist of three different phases, which are (i) carbonate-cemented siliciclastic sediment containing dreissenoid bivalves, (ii) microcrystalline carbonate, and (iii) aragonitic cement (Fig. 4a). Crusts often exhibit a regular layering with the microcrystalline carbonate being enclosed between the Dreissena sediment and the aragonitic cement. A more complex fabric due to the intermesh of the three phases is observed in precipitates that exhibit a more pronounced vertical aggregation (Fig. 5). Domed structures or even chimneys are preferentially formed of aragonitic cement. Domes and chimneys extend from the Dreissena sediment and thus probably formed a positive sea¯oor relief. By using a submersible, chimneys of about 1 m in height that arise from carbonate crusts have been documented (Ivanov et al., 1991). Samples obtained with the

beam trawl are up to 1.5 m in diameter and 0.3 m in height. Pyrite precipitates were dredged from the same surface sediments which harbour the carbonate crusts. In samples that contain both carbonate and pyrite the carbonate precipitates surround a sulphidic nucleus. 4.1.1. Carbonate cemented Dreissena sediment The dreissenoid bivalves are subrecent and date from the freshwater phase of the Black Sea. The primary shell structure and mineralogy is preserved. The dominant mineral of the sediment that contains Dreissena shells is detrital quartz which is accompanied by pyrite, glauconite, chlorite, mica, and feldspars. An internal layering of the sediment is caused by a change in composition from shell-dominated to quartz-dominated sublayers. The sediment is cemented by microcrystalline aragonite and calcite and to a minor extent by small botryoids of brownish-yellow or clear aragonite up to 300 mm in diameter. Larger portions of microcrystalline carbonate within the matrix of the Dreissena sediment occasionally exhibit a peloidal fabric. Individual peloids have a diameter of about 200 mm. 4.1.2. Microcrystalline carbonate Layers and zones of microcrystalline carbonate are either pure or contain detrital quartz and minor amounts of feldspars (Fig. 4b). Pure microcrystalline carbonate is formed of highly irregular clots strongly varying in size (Fig. 4c). The space between loosely packed clots is predominantly open. Some pores may be partly ®lled by a tiny rim cement originating from the clots. The crystals of the rim cement are stubby and smaller than 30 mm. Microcrystalline carbonates enclose microbial ®laments that vary in diameter from 10 to 20 mm and are up to 900 mm in length. These ®laments also occur attached to the surface of crusts and within aggregates of aragonitic cement. They exhibit a strong auto¯uorescence (Fig. 4d). The pure microcrystalline carbonate may also exhibit an intense ¯uorescence when excited with ultra-violet light (Fig. 4e and f). 4.1.3. Aragonitic cement In thick crusts, domed structures, and chimneys, aragonitic cement is the dominant phase. XRD and

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Fig. 4. Modern cold seep carbonates from the Black Sea. (a) Lower side of a carbonate crust formed of aragonite. The scale bar corresponds to 2 cm. (b) Microcrystalline carbonate and detrital quartz (arrows), FE-SEM image. The scale bar corresponds to 100 mm. (c) Clotted micrite in status nascendi, plane-polarised light. The scale bar corresponds to 200 mm. (d) Filaments attached to the surface of a carbonate crust. The ®laments show a bright ¯uorescence, epi¯uorescence image. The scale bar corresponds to 100 mm. (e) Microcrystalline carbonate (dark) and botryoidal aragonite (clear), plane-polarised light. The scale bar corresponds to 200 mm. (f) Same section as (e). The microcrystalline carbonate exhibits an intense ¯uorescence, epi¯uorescence image.

microprobe analyses reveal that this phase is monomineralic. Aragonite occurs in several varieties with botryoids being much more common than isopachous layers. A brownish-yellow aragonite exhibits extremely thin ®brous crystals (Fig. 6a). They form small botryoids

up to 100 mm in diameter which are arranged to densely packed aggregates. The irregular outline of these small botryoids is due to a hindered growth. Larger botryoids about 300±600 mm in diameter exhibit a regular spherical outline. Botryoids and isopachous layers of

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Fig. 5. Sketch of a vertical section through a carbonate precipitate.

clear aragonite originate on aggregate surfaces of yellow aragonite (Fig. 6a). Clear aragonite often forms a radial ®brous layer around botryoids made of yellow aragonite. Its crystals are signi®cantly larger than those of yellow aragonite (Fig. 6b and c) and exhibit concentric growth zones (Fig. 6d). Many of the botryoids formed of yellow aragonite exhibit an intriguing deformation pattern. Cracks ®lled by microcrystalline aragonite are directed from the centre to the periphery of botryoids (Fig. 6e). Cracks are widest in centres and narrow towards the outer edge of the botryoid. Occasionally some later formed cracks crosscut botryoids and subradial crack®llings. The microcrystalline aragonite that ®lls up the cracks shows an intense ¯uorescence when excited with ultra-violet light (Fig. 6f). It contains abundant dark inclusions of a few mm in size. The walls of thin cracks represent a ®tted fabric. Larger, triangular zones of microcrystalline aragonite within the botryoids point toward the periphery with their acute angle. Here, microcrystalline aragonite appears to replace the yellow aragonite. At an extreme there is only a rim of yellow aragonite left at the periphery of some of the affected botryoids which are completely ®lled by microcrystalline aragonite. 4.1.4. Pyrite Most of the studied pyrite crusts are pure. Some crusts contain detrital quartz that is cemented by the sulphide. In cross-sections some pyrite crusts exhibit a palisade-like fabric (Fig. 7a) and others are massive.

The pillars of the former type are orientated perpendicularly to the long axis of the crusts and thus, most probably, were originally vertical structures. All aggregates consist of framboidal pyrite (Fig. 7b). Individual framboids vary in diameter from 20 to 30 mm. They are formed of smaller globules about 3±4 mm in diameter or of hypidiomorphic cubes (Fig. 7c). Some surfaces of pyrite aggregates are affected by weathering indicated by the brownish colour of Fe-oxy-hydroxides. XRD analyses of the sulphide precipitates without weathered portions or carbonate coatings yielded pyrite and quartz. Both microcrystalline carbonate and botryoidal aragonite may cement the peripheral portions of the sulphides and form enveloping crusts (Fig. 7d).

5. Geochemistry 5.1. Elemental composition AAS analyses are given as carbonates (Ca, Sr, Mg) or oxides (Mn, Fe) in Table 2. Aragonitic cements represent the purest carbonate phase. They exhibit high Sr (up to 8500 ppm) and low Mg, Mn, and Fe contents. Chemical compositions of the microcrystalline carbonates and, in particular, the Dreissena sediment re¯ect varying mineralogies. High MgCO3 contents and the XRD patterns reveal that microcrystalline carbonates mainly consist of

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Fig. 6. Varieties of aragonite in carbonate crusts from the Black Sea. (a) Two varieties of aragonite. Yellow aragonite (dark) and clear aragonite originating on the yellow aragonite, plane-polarised light. The scale bar corresponds to 200 mm. (b) and (c) Clear aragonite, FE-SEM images. The scale bars correspond to 100 mm. (d) Crystal of clear aragonite broken perpendicular to the c-axis, FE-SEM image. The scale-bar corresponds to 10 mm. (e) Fractured botryoid of yellow aragonite. The fractures are ®lled with microcrystalline aragonite, plane-polarised light. The scale-bar corresponds to 200 mm. (f) Same section as (e). The microcrystalline aragonite emits an intense ¯uorescence, epi¯uorescence image.

high-Mg-calcite. Nevertheless, aragonite is found as well as trace amounts of quartz and feldspar. In the Dreissena sediment low CaCO3 contents display the predominance of detrital silicates. MgCO3 contents are low and vary according to the amount of either

Mg-calcite or aragonite. FeO is clearly enriched in phases bearing detrital silicate while higher MnO values are obviously associated with Mg-calcite. Chemical compositions of carbonate minerals were determined in detail by microprobe analyses on

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Fig. 7. Authigenic pyrite precipitates around Black Sea seeps. (a) Authigenic pyrite with a palisade-like fabric. The scale bar corresponds to 1 cm. (b) The pyrite exhibits a framboidal microstructure, FE-SEM image. The scale bar corresponds to 30 mm. (c) Individual framboids may be formed of hypidiomorphic crystals, but mm-sized globules (arrow) are also common, FE-SEM image. The scale-bar corresponds to 10 mm. (d) Pyrite (black) surrounded by authigenic carbonate (bright), crossed-polarised light. The dark areas on the lower left and right sides correspond to uncovered glass. The scale-bar corresponds to 200 mm. Table 2 Elemental (AAS) and mineral (XRD) composition of carbonate phases. Samples: A1±A4: aragonitic cements, M1±M4: microcrystalline carbonate, S1±S4: cemented siliciclastic sediment. Minerals: Ara ˆ aragonite, Mg-Cc ˆ high-Mg-calcite; Cc ˆ calcite, Qz ˆ quartz, Fsp ˆ feldspar, Mi ˆ mica, Chl ˆ chlorite Sample

CaCO3 (wt%)

SrCO3 (wt%)

MgCO3 (wt%)

MnO (ppm)

FeO (ppm)

XRD mineral composition Main

A1 A2 A3 A4 M1 M2 M3 M4 S1 S2 S3 S4

97.9 94.8 96.4 96.8 51.1 53.5 68.3 88.6 26.9 24.2 62.0 15.3

1.41 1.39 1.41 1.42 0.21 0.11 0.79 1.27 0.06 0.05 0.86 0.09

0.30 0.27 0.35 0.28 6.99 7.37 3.11 1.49 2.94 2.96 0.91 1.81

8 19 16 4 465 334 103 52 404 124 58 57

269 1050 333 333 19500 3440 6570 1490 4150 3410 1050 5610

Ara Ara Ara Ara Mg-Cc Mg-Cc Mg-Cc Ara Mg-Cc Mg-Cc Ara Qz, Fsp

Minor

Trace

Qz Qz Ara Mg-Cc Qz, Fsp Qz Mg-Cc, Qz Mg-Cc, Mi

Qz, Mg-Cc Qz, Mg-Cc Qz, Mg-Cc Qz Fsp, Mi Fsp, Mi Qz Qz, Cc Mi, Chl Fsp, Mi Fsp, Cc Cc, Chl

Fig. 8. Elemental composition of carbonate phases determined by electron microprobe analyses; statistical distribution of 240 measurements of botryoidal aragonite and 80 measurements of microcrystalline high-Mg calcite. Vertical lines mark mean (dashed) and median (straight), box boundaries indicate 25th and 75th percentiles, whiskers indicate 10th and 90th percentiles, and horizontal lines denote range of outliers.

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Fig. 9. Plot of the carbon and oxygen isotope values.

polished thin sections (Fig. 8). Several pro®les covered yellow aragonite, clear aragonite, and microcrystalline high-Mg-calcite. Yellow aragonite and clear aragonite, which are identi®ed under the microscope, are not signi®cantly differentiated by microprobe analysis. Aragonite contains 96±98 wt% CaCO3 and MgCO3 does not exceed 0.3 wt%. Mean SrCO3 content is 1.5 wt% or 8900 ppm Sr (range: 8300±9500 ppm). These values are similar to the results of AAS analyses. Likewise, low MnO and FeO contents are found for aragonite. Mean P2O5

content is below 0.1 wt% and, with a few exceptions, SiO2 and Al2O3 are low due to the lack of detrital silicates. CaCO3 and MgCO3 contents of the microcrystalline carbonates exhibit a higher variation, which can be attributed to a signi®cant contribution of detrital silicate minerals. MgCO3 contents average around 10.5 wt% which corresponds to 12.2 mol% Mg in the calcite lattice. Calculation on a 100 mol% scale results in the formula Ca0.89±0.86Mg0.11±0.14CO3 for 80 individual analyses. High-Mg-calcites exhibit low

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Fig. 10. Plot of the 87Sr/ 86Sr ratios of seawater and carbonate phases.

SrCO3 contents averaging at 0.2 wt% (equivalent to 1200 ppm Sr), but contain considerably more SiO2, Al2O3, FeO, MnO, and P2O5 than the aragonite (Fig. 8). 5.2. Isotope geochemistry The carbonate phases of the Black Sea seep deposits are strongly depleted in 13C (Fig. 9). The d 13C values of aragonite range from 228.9 to 237.3½ PDB at the Romanian site and from 225.9 to 237.6½ at the Ukrainian site. The microcrystalline carbonates exhibit a similar depletion. Samples from the Romanian shelf range from 230.1 to 231.5½, those from the Ukrainian slope range from 227.1 to 241.0½. The d 13C values of Dreissena sediment depend on the degree of cementation by methanederived carbonates. Two Ukrainian samples analysed yielded values of 214.8 and 233.8½. Dreissena shells show values of 12.4 and 21.2½. A mixture of a Dreissena shell and botryoidal aragonite yielded a d 13C value of 224.5½. d 18O values of microcrystalline carbonates and botryoidal aragonite exhibit a narrow range. Aragonite

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from the Romanian site ranges from 10.5 to 10.2½ PDB and from 10.7 to 10.5½ at the Ukrainian site. Microcrystalline carbonates vary from 10.5 to 10.3½ (Romania) and from 11.2 to 10.5½ (Ukraine). The two Ukrainian samples of the Dreissena sediment yielded values of 10.9 and 10.5½. The Dreissena shells exhibit d 18O values of 23.1 and 23.5½, the mixture of a Dreissena shell and botryoidal aragonite exhibits a value of 10.2½. One sample of botryoidal aragonite yielded a radiocarbon age of 20,640 ^ 180 a BP and a sample of microcrystalline carbonate yielded an age of 19,110 ^ 180 a BP. The 87Sr/ 86Sr compositions of aragonite, microcrystalline carbonates, Dreissena sediment, and seawater samples from the Ukrainian site show a bimodal distribution (Fig. 10). Analysed seawater samples range from 0.70915 to 0.70918 (mean 0.70917, n ˆ 4). Aragonite exhibits 87Sr/ 86Sr ratios in the range from 0.70915 to 0.70921 (mean 0.70918, n ˆ 4) and microcrystalline carbonates range from 0.70915 to 0.70938 (mean 0.70927, n ˆ 5). The 87Sr/ 86Sr compositions of the Dreissena sediment vary strongly depending on the degree of cementation by methane-derived carbonates, which correlates positively with the Sr content of the analysed samples. Indurated samples with a high Sr content (3317±5576 ppm) exhibit a less radiogenic composition (mean 0.70933, n ˆ 3) than samples with a low carbonate and Sr content (393±501 ppm) which show a more radiogenic composition (mean 0.71059, n ˆ 4). Three pyrite samples from the Ukrainian slope have been analysed for sulphur isotopic ratios. The crusts yielded d 34S values of (i) 119.1 and 116.8½, (ii) 119.7 and 118.7½, and (iii) 118.7 and 118.0½ CDT.

6. Discussion 6.1. Methane-derived carbonates 6.1.1. C and O isotopes and origin of methane The negative d 13C values of microcrystalline carbonates and botryoidal aragonite as low as 241½ reveal that the precipitates predominantly derive from the microbial oxidation of methane. The d 13C values of samples of the Dreissena sediment

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depend on the degree of cementation by methanederived carbonates, as indicated by the value of about 234½ obtained from a strongly cemented sediment. Methane from deep-sea sediments of the Black Sea shows an average carbon isotopic composition of 268½ and has been interpreted to be microbially produced (Hunt, 1974). Free methane sampled from the water column in the area of gas emissions is less depleted in 13C yielding a d value of 258.2½ (Ivanov et al., 1991). Although, considering the geotectonic and sedimentary setting, a biogenic origin of this methane is likely, this value is not negative enough to preclude a contribution of thermogenic methane (cf. Suess and Whiticar, 1989; Aharon et al., 1997). The source of the water column methane is the sediment between the oxic±anoxic interface and the continental slope±abyssal plain transition (Reeburgh et al., 1991). The authors suppose that high carbon deposition sediments of the northwestern shelf and Danube fan are important sources of water column methane. The 14C ages of botryoidal aragonite and microcrystalline carbonate reported here do not represent the time of carbonate formation. This is caused by the fossil carbon source. Due to the unknown degree of mixing between methane-derived carbon and marine bicarbonate the given ages of about 20,600 and 19,100 a BP represent minimum ages for the fossil methane source. It is well known that sulphate reduction recedes in freshwater environments and that methanogenic fermentation is the dominant process of mineralisation of buried organic matter. Intriguingly, the freshwater phase of the Black Sea developed 25,000 years BP (Ross and Degens, 1974). Hence, we propose that the methane of the Black Sea seeps is of a shallow source and mainly derives from lacustrine organic-rich sediments. However, the seeps occur close to the mid-Cretaceous graben bounding fault zone mapped by Robinson and Kerusov (1997). Towards the north, the east±westtrending extensional high (Kalamit Ridge) includes the Lebada oil ®eld. The source rock is believed to be Tertiary (Upper Eocene?) in age (Robinson and Kerusov, 1997). The authors also report on accumulations of dry gas in Oligocene strata. Considering the geotectonic setting and the potential Tertiary source rocks, a contribution of gas from a deeper source is possible.

The d 18O values of methane-derived carbonate phases range from 11.2 to 10.2½ PDB and are compatible with precipitation from marine waters. Variations do not appear to be related to the mineralogy of the samples. We have to stress that neither the aragonitic samples nor the high-Mg-calcites were corrected relative to calcite. We refused this correction due to the mineralogical heterogeneity of microcrystalline carbonates with changes on a microscale in the Mg-calcite to aragonite ratios and in the Mg content of calcites. The two Dreissena shells analysed are depleted in 18 O relative to the methane-derived carbonates and are thus compatible with precipitation in a freshwater environment. However, the 18O values were expected to be even more negative considering the contributing rivers at the given latitude. The values may thus indicate an isotope exchange with marine water. 6.1.2. Strontium isotope ratios The 87Sr/ 86Sr ratios of seep carbonates have been used to identify the strontium source. This is possible, because the strontium isotopic composition of carbonate minerals re¯ects the composition of the water in which they formed (Hess et al., 1986). 87Sr/ 86Sr compositions of modern seep carbonates have been found to be similar to modern non-seep marine carbonates and the ambient sea water (Aharon et al., 1997; Naehr et al., 2000), thus indicating a shallow Sr source. 87Sr/ 86Sr ratios have also been used to prove the hydrothermal origin of some Palaeozoic vent carbonates (Mounji et al., 1998). The strontium isotopic composition of methanederived carbonates analysed here re¯ects a shallow, e.g. marine Sr source. 87Sr/ 86Sr ratios of microcrystalline carbonate (mean 0.70927) and aragonitic cement (mean 0.70918) indicate that Sr is derived from the ambient seawater (mean 0.70917) or shallow pore waters. The very radiogenic 87Sr/ 86Sr composition of the Dreissena sediment (mean 0.71005) is probably caused by radiogenic 87Sr from detrital mica. Less radiogenic values of Unit-3 carbonates reported by Cox and Faure (1974) were interpreted to be derived from redeposited late Cretaceous to early Tertiary coccoliths. The lack of these coccoliths and the dominance of mica in the Dreissena sediment may explain the strong variance in 87Sr/ 86Sr ratios between the Dreissena sediment and the corresponding deepwater sediments of Unit-3.

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6.1.3. Carbonate mineralogy and chimneys Controls on the mineralogy of the Black Sea seep carbonates are probably manifold. However, a preliminary evaluation of some factors is possible on the basis of the data obtained in this study. A factor that may in¯uence the mineralogy of the seep precipitates is the viscosity of the growth medium. Liquid media have been found to favour aragonite precipitation, whereas gelatinous media favour calcite precipitation (Buczynski and Chafetz, 1991). As the viscosity of a sediment is mainly a function of its water content, aragonite should preferentially form closer to the sea¯oor than calcite. The degree of carbonate supersaturation has also been discussed to in¯uence the mineralogy, with high degrees promoting aragonite precipitation (Chafetz et al., 1991). At cold seeps high degrees of supersaturation with respect to carbonate minerals are probably related to high rates in sulphate reduction, giving additional indication that aragonite is favoured in the zone of sulphate reduction. Walter (1986) found that Mg-calcite precipitation is favoured in high-phosphate, low-sulphate solutions which are characteristic of more restricted sedimentary environments. Burton (1993) pointed out that this dependence is also in¯uenced by the pH of the porewaters. The high content in P2O5 of microcrystalline high-Mg calcites in relation to the lower content in the aragonite cement indicates a higher phosphate concentration of the pore waters that precipitated calcite. Around methane seeps, sulphate is believed to be locally exhausted by bacterial reduction within the sediment (Ritger et al., 1987; Stakes et al., 1999). It has been suggested that ¯uctuations in ion activity (a carbonate/a sulphate) control the mineralogy of early precipitates at methane seeps (Savard et al., 1996). Thus two important factors favouring the local precipitation of calcite at the Black Sea seeps may be high-phosphate and low-sulphate concentrations of pore waters. Aragonite forms above this zone in porewaters or even in seawater, where sulphate concentrations are higher and phosphate concentrations are lower. This is in good accordance with the observation that the large chimneys that form above the sea¯oor are predominantly made of aragonite. Mineral chimneys are a frequent feature of hot vents and some cold seeps. Roberts and Aharon (1994) reported chimney-like carbonate deposits at

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cold seeps on the upper continental slope in the Gulf of Mexico and conclude that the seep deposits were exposed by erosion. Calcite-cemented sandstones recovered from methane seeps off Japan partly represent pipes and chimneys (Sakai et al., 1992). Dolomite-cemented sandstone pillars around methane seeps of the Kattegat are believed to be exposed on the sea¯oor by erosion (Jùrgensen, 1989). Dolomite chimneys that occur on the slope off New Zealand formed below the seabed and were subsequently exhumed by erosion (Orpin, 1997). Carbonate chimneys of Monterey Bay formed within unconsolidated sediments and were exposed later (Stakes et al., 1999). The Black Sea carbonate chimneys appear to be peculiar in terms of size and their genesis. Only the chimneys that have been reported from the Oregon accretionary prism (Kulm and Suess, 1990) are similar. After careful evaluation the authors conclude that these chimneys mainly formed above the sea¯oor. It cannot be excluded that the Black Sea chimneys were exposed by erosion. However, due to their conical shape and the active seepage penetrating these carbonate deposits, current carbonate precipitation is much more likely. 6.1.4. Microbially induced carbonate precipitation It is widely accepted that methane-derived carbonates preferentially form within the sediment (Paull et al., 1992; Gaillard et al., 1992). Methane oxidation has been inferred to be coupled to sulphate reduction. This reaction is associated with an increase in alkalinity which is held responsible for carbonate formation (Ritger et al., 1987; Thiel et al., 1999). Field and laboratory studies indicate that methane is oxidised by a consortium of archaea and sulphate reducers (Hoehler et al., 1994). Considering that marine bottom waters are usually oxic, carbonate formation induced by anaerobic oxidation of methane will be con®ned to anoxic sediments. Thus, seep carbonates forming a positive sea¯oor relief should be an exception. Due to its peculiar oceanography the Black Sea offers an unique opportunity to study the in¯uence of chemical environments on the carbonate precipitation at methane seeps. Depending on the bottom-water chemistry in the transect from the permanent oxic to the anoxic zone carbonate deposits exhibit different

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shapes and sizes (see Fig. 2). The distribution of carbonate deposits reveals that their formation is restricted to oxygen-free, sulphate-containing environments. The d 13C values of the seep carbonates as low as 241½ PDB demonstrate that the precipitates are predominantly methane-derived. Moreover, the restriction of carbonate deposits to the anoxic zone (carbonates con®ned to anoxic sediment beneath an oxic water column in contrast to carbonate chimneys projecting up into an anoxic water column) reveals a pronounced anaerobic oxidation of methane. The anaerobic oxidation at seeps has been assigned to archaea (Elvert et al., 1999; Hinrichs et al., 1999; Peckmann et al., 1999b; Thiel et al., 1999). Recent biomarker analyses of the carbonate deposits studied here prove the presence of archaea and sulphate reducing bacteria. Archaeal biomarkers found are the isoprenoids crocetane and PMI, ether-bound biphytane, and cyclic C40-isoprenoids (Thiel et al., 2001). In addition, the d 13C signatures of the archaeal marker molecules as low as 2112½ PDB reveal the use of carbon derived from methane (Thiel et al., 2001). The distribution of carbonate deposits, the archaeal biomass enclosed in the carbonates, and the presence of `methanotrophic' archaea in microbial mats associated with carbonates (Pimenov et al., 1997) elucidate that archaea are an important factor for the cycling of carbon at the Black Sea seeps. The ®laments described here are mostly preserved in the carbonate matrix and are signi®cantly larger than the microbial ®laments of the jelly-like mats which are attached to the carbonates from the same location (cf.Pimenov et al., 1997). A striking difference is the lack of any carbonate cementation in the jelly-like mat. It is not yet clear whether the mat-forming microbial consortium is contemporaneously active with the archaea that induce carbonate precipitation. It should also be stressed that it is not proven, whether the archaeal markers extracted from the carbonate really derive from the enclosed ®laments. On the one hand, their large size is rather unusual for archaea. On the other hand, it is hard to ignore the coincidence between the large amounts of enclosed ®laments and the enormous content of archaeal chemofossils. Organic residues appear to be preferentially preserved in microcrystalline carbonates. This is

indicated by the intense auto¯uorescence of microcrystalline carbonates in relation to the non¯uorescent cements. Intriguingly, the degree of ¯uorescence is positively correlated to the phosphorus content. Aragonite cements show an average P2O5 content of 500 ppm, whereas the fractured, partly microcrystalline botryoids contain about 1000 ppm P2O5. The P2O5 content of pure microcrystalline carbonates mainly ranges from 2200 to 2500 ppm with maxima up to 3500 ppm. Phosphate is believed to largely originate from the breakdown of organic matter (Jarvis et al., 1994) and microbial communities are, in part, held responsible for elevated phosphate concentrations in pore waters (Krajewski et al., 1994). 6.1.5. Fractured botryoids Fractured botyroids are ubiquitous in the Black Sea carbonates. Due to the uniform pattern it is likely that deformation was caused by a single process. This process is believed to be in situ brecciation, but desiccation as an alternative mechanism can not be ruled out. The latter process is indicated mainly by the remarkable similarity to the deformation pattern of septarian concretions. Cracks in septarian concretions result from tensional failure as a consequence of localised excess pore pressure within the concretion body (Hounslow, 1997). Excess pore pressure, associated with the ¯uid phase, is achieved by the reduction of permeability initiated by cementation. The pore pressure gradient is driving dewatering. However, applying this model to botryoids raises some problems. It is unlikely that an excess pore pressure was generated in a botryoid, because the cement grows from a centre to the periphery and besides forms close to the sea¯oor lacking any burial history. Likewise, the cement does probably not contain suf®cient amounts of water or other inclusions that could be affected by an excess pore pressure. To the best of our knowledge, aragonite is neither known to exist in a hydrated variety nor have pseudomorphs been reported after hydrated calcium carbonate. Thus, reduction of volume due to desiccation is improbable. Consequently, in situ brecciation of botryoidal aragonite is held responsible for the crack formation. This is substantiated by the ®tted fabric of the crack walls. Dewatering, on the other hand, would lead to a loss in volume and the resulting crack walls would ®t

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only approximately. A possible factor triggering brecciation is the gas seepage itself. It is possible that excess pressures can be created due to a plugging effect of carbonate precipitates. Hence, deformation in botryoids would be caused by an external force and not by excess pressure in the botryoids itself. This model is based on the same mechanism as the genesis of pockmarks proposed by Hovland et al. (1987). 6.2. Inferred origin of authigenic pyrite Several contributions add to the knowledge of how pyrite forms (Berner, 1970; Rickard, 1975; Leventhal, 1983; Raiswell, 1982; Berner, 1984; Raiswell and Berner, 1985; Calvert and Karlin, 1991; Wilkin et al., 1996). Sedimentary pyrite formation results from the reaction of H2S, derived from bacterial sulphate reduction, with reactive detrital iron minerals (Berner, 1984). The initial products of this reaction are metastable iron monosulphides, mackinawite and greigite, which readily transform to pyrite under most conditions (Berner, 1974). The dominant control on pyrite formation under euxinic conditions is the availability of reactive iron minerals (Berner, 1984). Applying the degree of pyritisation (DOP) as proposed by Raiswell and Berner (1985) to modern sediments, Calvert and Karlin (1991) have corroborated that pyrite formation in the Black Sea is Fe-limited. This was con®rmed by Lyons and Berner (1992) who analysed pyrite of Unit-1. In the Black Sea pyrite forms within the anoxic water column as well as at the sediment±water interface due to the presence of H2S and reactive iron minerals (Leventhal, 1983). Muramoto et al. (1991) have reported the occurrence of framboidal pyrite within the anoxic water column. Based on isotope evidence they concluded that both water columnderived and authigenic pyrite are present in Black Sea sediments. DOP pro®les of the Unit-1 indicate that the majority of the pyrite forms in the sulphidic water column (Lyons and Berner, 1992). Measurements of the stable isotope ratio of sulphur in hydrogen sulphide from water samples yielded d 34S values in the range from 238.7 to 24.9½ (Sweeney and Kaplan, 1980). Fry et al. (1991) have reported rather constant d 34S values for H2S near 241.5½. Pyrite enclosed in the marine sediments of Unit-1 and

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Unit-2 ranges from 225.1 to 237.4½ (Calvert et al., 1996). The isotopic ratios of the framboidal pyrite described here range from 116.8 to 119.7½. This elucidates that the sulphur does not derive from the 34 S-depleted H2S of the water column or the upper sediment layers. Isotopes are only fractionated to a very small extent when, reduced sulphur species react to form iron phases (Price and Shieh, 1979), so that the isotopic composition of pyrite is close to that of the dissolved HS 2 from which it formed. Intriguingly, similar highly positive values like those found in this study have been reported for pyrite in the upper part of the freshwater deposits (Unit-3) from the central Black Sea (Calvert et al., 1996). These sediments formed more or less contemporaneously to the Dreissena beds on the Romanian and Ukrainian shelf and upper slope. Vinogradov et al. (1962) already reported similar, positive values for porewater sulphide from layers immediately below the Unit-2. Like Calvert et al., Vinogradov and co-authors analysed deep-water cores. Calvert et al. (1996) have concluded, that the isotopic composition of the lake deposits pyrite re¯ects formation under closed system conditions in the iron-rich freshwater sediments triggered by downward diffusing sulphate or sulphide from the overlying organic-rich sapropel. However, although this model is a reasonable explanation for the central Black Sea, it fails to explain the shelf and slope occurrence of 34S-enriched pyrite reported here. This is due to the observation that neither the sapropel (Unit-2) nor the Unit-1 deposited on the continental shelf and upper slope of the northwestern Black Sea (Panin et al., 1983; Shopov et al., 1986; A. Reimer, unpubl.). We propose that the isotopic signature of pyrite re¯ects an early, limited incursion of seawater followed by the almost complete consumption of seawater sulphate by bacterial sulphate reduction in the lacustrine sediments. The generated H2S reacted with the excess reactive iron minerals of the freshwater sediments, which led to the formation of 34S-enriched pyrite. This scenario represents closed system conditions causing the pyrite to exhibit a 34S signature similar to that of marine sulphate. Additional evidence for the origin of the framboidal pyrite results from its size distribution. Wilkin et al. (1996) have shown that pyrite framboids in sediments

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of modern euxinic basins are on average smaller and less variable in size than those of sediments underlying oxic water columns. A study on Black Sea pyrite revealed that the framboids of the Units-1 and -2 are remarkably uniform having a mean diameter of 5 mm (Wilkin et al., 1997). The framboids of the lacustrine Unit-3, which corresponds to the Dreissena sediment, on the other hand, were found to be much larger in size. They range from 7 to 25 mm in diameter, but framboids up to 50 mm are also present. This corresponds well with the size of the framboids from the northwestern shelf and slope. The association of pyrite with the Dreissena sediment, highly positive d 34S values, and the large size of framboids account for a formation in lacustrine sediments after the ®rst incursion of seawater into the basin following the reconnection with the Mediterranean Sea.

which are characteristic of the more restricted environments. Aragonite forms where sulphate concentrations are higher and phosphate concentrations are lower. Other factors that may in¯uence mineralogy of Black Sea carbonates are the viscosity of the growth medium, with more liquid media favouring aragonite precipitation, and the degree of carbonate supersaturation, with higher degrees favouring aragonite. 6. The authigenic pyrite that is frequently associated with the carbonate deposits formed in lacustrine sediments after the ®rst incursion of seawater following the reconnection with the Mediterranean Sea and is not related to the present-day seepage.

7. Conclusions

We are grateful to H. Becker and S. Klautzsch for the preparation of thin sections. C. Kaubisch prepared the drawings for Figs. 2 and 3 and M. Hundertmark the photos for Figs. 4a and 7a. We acknowledge the support provided by M. Pache (AAS analysis), Dr A. Kronz (microprobe analysis), and G. Schmidt. Dr M. Joachimski (Erlangen) provided C/O stable isotope measurements and Dipl. Phys. H. Kerscher (Erlangen) 14C measurements. Drs M.L. Coleman (Reading) and T. H. Naehr (Monterey Bay) provided constructive reviews which are gratefully acknowledged. Special thanks to Drs C. Hensen and S. Kasten (both Bremen) for editorial work. U. Luth and C. Luth were funded by the German Volkswagen Foundation (grant I/68 809). This paper is a contribution to SFB 468 `Wechselwirkungen an geologischen Grenz¯aÈchen' (publication no. 21) at the University of GoÈttingen.

1. Methane seeps in the northwestern Black Sea are accompanied by authigenic carbonate deposits. The carbonate formation is induced by the microbial oxidation of methane as indicated by d 13C as low as 241½. 2. The shape and size of carbonate deposits depend on the bottom water chemistry. In the oxic zone precipitates are ¯at crusts and form only within the anoxic sediment. In the oxic±anoxic interface zone carbonate deposits show an increased thickness. In the anoxic zone the carbonates form tall chimneys that grow into the water column. Hence, carbonate formation is restricted to anoxic environments and is induced by the anaerobic oxidation of methane. Filamentous microbes preserved within the carbonate are signi®cantly larger than mat-forming ®laments attached to carbonate deposits and are believed to represent `anaerobic methanotrophs'. 3. The methane is of a shallow source and mainly derives from the lacustrine organic-rich sediments of the freshwater phase of the Black Sea. 4. The isotopic composition of Sr in the methanederived carbonates indicates a shallow Sr source. 5. Several factors may in¯uence the mineralogy of calcium carbonates. Mg-calcite precipitation is favoured in high-phosphate, low-sulphate solutions

Acknowledgements

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