A methane-derived carbonate build-up at a cold seep on the Crimean slope, north-western Black Sea

    A Methane-Derived Carbonate Build-Up at a Cold Seep on the Crimean Slope, North-Western Black Sea Sofya A. Novikova, Yevgeny F. Shnyukov, Ella V. Sokol, Olga A. Kozmenko, Dina V. Semenova, Vladimir A. Kutny PII: DOI: Reference:

S0025-3227(15)00044-4 doi: 10.1016/j.margeo.2015.02.008 MARGO 5267

To appear in:

Marine Geology

Received date: Revised date: Accepted date:

23 September 2014 16 December 2014 18 February 2015

Please cite this article as: Novikova, Sofya A., Shnyukov, Yevgeny F., Sokol, Ella V., Kozmenko, Olga A., Semenova, Dina V., Kutny, Vladimir A., A Methane-Derived Carbonate Build-Up at a Cold Seep on the Crimean Slope, North-Western Black Sea, Marine Geology (2015), doi: 10.1016/j.margeo.2015.02.008

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ACCEPTED MANUSCRIPT A Methane-Derived Carbonate Build-Up at a Cold Seep on the Crimean Slope, North-Western Black Sea

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Sofya A. Novikovaa,*, Yevgeny F. Shnyukovb, Ella V. Sokola, Olga A. Kozmenkoa, Dina V. Semenovaa, Vladimir A. Kutnyb a

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V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3, Akademik Koptyug ave., Novosibirsk, 630090, Russia; b Government Scientific Institution Department of Marine Geology and Sedimentary Ore Formation, National Academy of Sciences of Ukraine, 55B, O.Gonchar street, Kiev, 01601, Ukraine

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*Corresponding author. E-mail address: [email protected] (S. Novikova)

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Abstract

A unique chimney-shaped carbonate build-up was produced by microbially mediated

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anaerobic oxidation of methane at a deep-sea cold seep. The build-up was sampled from

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1600 m water depth in the area of the Lomonosov Rise (NW Black Sea, Crimean shelf slope).

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The carbonate chimney grew free into the anoxic water column, with its base attached to a steep slope composed of plagiogranite and void of sediments. The perfectly preserved 1.5 m high chimney stores reliable records of the diversity of mineralogy, geochemistry, and stable

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isotope composition of a deep-sea methane-related carbonates never studied before. The build-up consists of Mg-calcite (MgCO3 = 9-13 mol.%) with minor aragonite. The carbonate matrix encloses organic matter, and Emiliania huxleyi coccoliths, as well as minor framboidal pyrite, gypsum, barite, and diatomite fragments. The contribution of detrital silicate material is negligible. Micritic Mg-calcite in the inner zone of the chimney forms obtuse rhombohedrons clustered in hemispherical aggregates (clots) and hosts isolated prismatic aragonite (<< 1 vol.%). In the outer zone, Mg-calcite exists as foliated crystals and spherulites, while aragonite spherulites are restricted to the top surface. The carbon isotope compositions of carbonates (δ13С = -46.5 to -33.0 ‰ VPDB) and remnant bacterial mats (δ13С = -76.9 to -81.6‰ VPDB) provide evidence of a biogenic methane source for the build-

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ACCEPTED MANUSCRIPT up formation. Oxygen came mainly from sea water, judging by a narrow range of δ 18O values in carbonates (+0.2 to 1.3‰ VPDB). The prevalence of Mg-calcite throughout the build-up

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indicates an ANME-2/SBR consortium being the main component of the bacterial mats living

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at the seep. Aragonite grew mostly late in the seep history, which hints to environmental

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changes and the ensuing predominance of the ANME-1/SBR consortium. The observed relationship between the prokaryotic communities and the main carbonate phases is common to the microbial mats of the Black Sea, but is actually opposed to the pattern of AOM-

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community and AOM-related carbonates found in sediments. Different morphologies of

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carbonates in the outer and inner zones of the build-up point to variations in the methane flux gradient, which controlled the biomass density of the AOM consortia. The REE+Y patterns of Mg-calcite from the inner zone are typical of carbonates crystallised in a freshened sea-water

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environment, while both Mg-calcite and aragonite from the outer zone show distinct seawater signatures. Inasmuch as the time span of build-up growth (radiocarbon ages from

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9000±180 to 7500±180 yr BP) overlaps the period when the Black Sea ‗Lake‘ reconnected to the Mediterranean Sea, the progressive arrival of sulphate-rich waters can be inferred to be

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another possible control of REE+Y patterns in seep carbonates besides the methane flux gradient and related activity of AOM consortia.

Key words: Cold seep, methane-derived carbonates, Back Sea, rare earth elements

1. Introduction Hydrocarbon seeps are ubiquitous features of many active and passive continental margins in both the present-day marine environment and throughout the geological record (e.g. Hovland et al., 1987; Matsumoto, 1990; Roberts and Aharon, 1994; Campbell et al., 2002; Mazzini et al., 2004; Moore et al., 2004; Peckmann and Thiel, 2004; Reitner et al.,

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ACCEPTED MANUSCRIPT 2005; Judd and Hovland, 2007; Neahr et al., 2007; Bahr et al., 2009; Campbell et al., 2010; Magalhães et al., 2012; Pierre et al., 2012). They represent discrete sites where methane-rich

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fluids in chemical disequilibrium with sea water migrate within the bottom sediments, reach

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the seafloor and emanate at the sediment-water interface (Naehr et al., 2007). Near the sea

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floor and within the sediment column in anoxic and sulphate-rich environments, anaerobic oxidation by a consortium of methanotrophic archaea and sulphate reducing proteobacteria (SRB) consumes the methane contained in ascending fluids, according to the reaction

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(Boetius et al., 2000; Elvert et al., 2000; Valentine and Reeburgh, 2000; Orphan et al., 2001; Bahr et al., 2009):

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CH4 + SO42- → HCO3- + HS- + H2O (1).

Anaerobic oxidation of methane (AOM) produces bicarbonate, which leads to

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authigenic carbonate precipitation:

Ca2+ + Mg2+ + 2HCO3- → (Ca,Mg)CO3 (↓) + CO2 + H2O (2).

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Hydrocarbon seepage is accompanied by carbonate precipitation very often. A variety of methane-derived carbonates have been observed at numerous cold seeps, with shapes of

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isolated plates (slabs), irregular crusts, thinly lithified pavements, vertical pillars (chimneys or towers), flat pancake-shaped and mushroom-shaped deposits, dispersed crystal aggregates, tabular constructions and concretions (e.g. Hovland et al., 1987; Neahr et al., 2007; Pierre and Fouquet, 2007; Bahr et al., 2009; Kocherla, 2012; Magalhães et al., 2012; Pierre et al., 2012). Depending on temperature and geochemical conditions (sulphate, phosphate, and organic matter concentrations, magnesium-to-calcium ratio, and oxygenation degree), authigenic lowand high-magnesian calcite, aragonite, proto-dolomite or dolomite can precipitate in different geochemical zones of bottom water column, on the sediment surface, or within water- and gas-saturated bottom sediments (Burton, 1993; Greinert et al., 2001; Magalhães et al., 2012). The carbon and oxygen isotope compositions of authigenic carbonates have bearing on the

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ACCEPTED MANUSCRIPT source of these elements and origin and temperature of fluids from which carbonates were precipitated (Reeburgh, 1980; Anderson and Arthur, 1983; Ritger et al., 1987; Naehr et al.,

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2000; Pierre, Fouquet, 2007).

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Trace and rare earth element (REE) concentrations recorded in seep-related carbonates

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can provide more information on marine palaeoenvironments and palaeofluids (Feng et al., 2009a,b; Ge et al., 2010; Himmler et al., 2010; Rongemaille et al., 2011). However, we have found few studies of trace-element and REE compositions of seep-related carbonates (Ge et

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al., 2010; Himmler et al., 2010) from different age zones of a build-up growing within gas-

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saturated sediments, but the analysed fragments were several cm in size (Ge et al., 2010) and the ages were only relative (younger/older). Other publications reported analyses of small isolated nodules or randomly sampled carbonates difficult to place into a particular part of a

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build-up (Chen et al., 2005; Feng and Chen, 2008; Feng et al., 2009a,b, 2010; Rongemaille et al., 2011).

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Thus seep-related authigenic carbonates represent one of few permanent records of the ephemeral phenomenon such as submarine (deep-water) methane flow. This valuable archive

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of mineralogy, geochemistry, and isotopy stores a wealth of evidence about the composition and origin of the source fluids and the changes in seep activity (Naehr et al., 2007; Pierre and Fouquet, 2007; Bayon et al., 2009; Bahr et al., 2009, 2010; Liebetrau et al., 2010). Authigenic carbonates from marine seeps studied worldwide in modern and past sedimentary successions differ in extremely low δ13C values (down to -75‰ VPDB), which rely on the methane source of carbon (e.g. Greinert and Derkachev, 2004; Mazzini et al., 2004; Campbell; 2006; Pierre and Fouquet, 2007; Naehr et al., 2000; 2007; Bahr et al. 2009; Magalhães et al., 2012; Pierre et al., 2012). Gas saturation of sediments, hydrocarbon seepage, and authigenic carbonate precipitation are common processes in the north-western (e.g. Shnukov et al., 1995; Michaelis

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ACCEPTED MANUSCRIPT et al., 2002; Shnyukov and Kutniy, 2003), central, north-eastern (e.g. Ivanov et al., 1998; Bohrmann et al., 2003), and south-eastern (e.g., Kruglyakova et al., 1993; Ergun et al., 2002)

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parts of the Black Sea. The related well-shaped carbonate build-ups are widespread in deep-

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water environments. They are commonly connected with mud diapirs and volcanoes, and fault

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network systems. From the data now available, it is evident that the chemical environment inducing seep-related carbonate precipitation in the Black Sea results from microbial metabolism (Reitner et al., 2005 a,b).

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For the NW Black Sea area the best documented are the mineralogy as well as C and O

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isotope compositions of AOM-related seep carbonates. The studied build-ups grew at the depths from 180 to -2000 m, both within anoxic bottom sediments (slabs, crusts) and in the column of anoxic water (chimneys) (Peckmann et al., 2001; Michaelis et al., 2002; Mazzini et

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al., 2004; Pape at al., 2005; Gulin et al., 2005; Reitner et al., 2005b; Bahr et al., 2009, 2010). Lipid biomarkers from authigenic carbonates were also analyzed for identification of archaea

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and SRB in microbial consortia (Pape at al., 2005; Reitner et al., 2005a; Bahr et al., 2009). Chemical analyses for individual grains of seep-related carbonates were reported in a single

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publication (Peckmann et al., 2001). There exist carbonate formation models for different settings of (i) a deep-sea chimney grown free into anoxic water column (Reitner et al., 2005b) and (ii) slabs and crusts originated within bottom sediments (Mazzini et al., 2004). However, as far as we know, comprehensive analysis of mineralogy, morphology, carbon and oxygen isotope compositions, and geochemical patterns of seep-related carbonates from the NW Black Sea has never been performed. This paper is a synthesis of anatomy, mineralogy, isotope (δ13C and δ18O), and chemical (major and trace elements, including REE+Y) features of a unique 1.5 m high carbonate chimney. The build-up was located at ~1600 m water depth on a steep slope of the submarine Lomonosov Rise (NW Back Sea), almost void of sediments (Shnyukov and Kutniy, 2003;

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ACCEPTED MANUSCRIPT Shnyukov et al., 2013). The carbonates produced by a methane seep build a chimney grown free into the anoxic water column below the chemocline. Therefore, the build-up has always

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remained in a marine environment being never involved in any significant interaction with

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bottom sediments and never contaminated with clastic deposits. The perfect preservation of

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the chimney, with all its growth zones, allows tracing variations of morphological, mineralogical, isotope, and geochemical features over the build-up and the effects of

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microbial metabolism on the carbonates.

2. Geological setting and sampling area

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The Black Sea originated during Cretaceous times is the world‘s largest anoxic marine basin, which became anoxic as a result of recent events. The postglacial inflow of high

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salinity waters from the Mediterranean Sea invaded the Black Sea via the Bosphorus around 9000-9400 yr BP in response to the Holocene global sea level rise (Major et al., 2006; Soulet

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et al., 2011). Permanent stratification was established between the original fresh-water body and the underlying marine waters, allowing anoxia to develop in the bottom layers

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accompanied by a rise of the halocline (Ross and Degens, 1974). At present, the oxic–anoxic interface in the Black Sea varies in depth between 130 and 180 m, and 80% of the water column is anoxic (Ross et al., 1978, Mazzini et al., 2004; Bahr et al., 2009). Samples of AOM-carbonates were taken in the northwestern part of the Black Sea basin on the continental slope of the Crimean peninsula (Fig. 1). Here the seepage-related carbonate precipitates have different shapes depending on medium oxygenation. They are small and pancake-shaped in the oxic zone above 60 m water depth, where anoxic conditions are only present within the sediment column, and plates or small chimneys in the suboxic zone between 110 and 190 m. In the anoxic zone below 230 m, where the carbonates may grow free into the water column, the precipitates build tall chimneys or towers up to several meters

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ACCEPTED MANUSCRIPT in height (Fig. 2) (Luth et al., 1999; Peckmann et al., 2001; Lein et al., 2002; Michaelis et al., 2002; Reitner et al., 2005a,b).

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The studied authigenic carbonate build-up was sampled in 2001 at 1600 m of water

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depth on the slope of the submarine Lomonosov Rise during dredging in the 56th trip of R/V

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Professor Vodyanitskiy (station 5390; 44°27.4‘ N; 32°48.6‘ E) (Shnyukov and Kutniy, 2003; Shnyukov et al., 2013) (Fig. 1). The offshore Lomonosov Rise lies in the north-western part of the Black Sea basin on the extension of onshore orogenic structures of the Crimean

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Peninsula. The site occurs at the intersection of the West Crimean and Lomonosov faults

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which separate the continental slope and the Black Sea basin. Other faults on the slope are pre-Cenozoic N—S and recent W—E ones. The slope has a convex terraced transversal profile with alternated steep (dipping from 20-45° to 80°) and almost flat surfaces (Shnyukov

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et al., 1997; Shnyukov and Kutniy, 2003; Shnyukov and Pasynkov, 2003; Lukin, 2007; Shnyukov et al., 2013). The Lomonosov Rise is composed mainly of island arc igneous rocks:

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gabbro, diorite, quartz diorite, tonalite and plagiogranite at the depths 1400-1750 m and boninite-, calc-alkaline and shoshonite-series volcanics at depths 700-800 m. Sedimentary

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rocks are of limited occurrence and are mainly Carboniferous mudrocks, Taurian flysch, Late Cretaceous carbonate clay and marl, and Early Miocene limestone (Shnyukova and Pasynkov, 2003).

The Lomonosov Rise lies in a zone of fractured sea floor, with numerous sites of active methane emission. Gas seepages are presumably responsible for an acoustic anomaly at the slope foot. Gas emission through the sea floor between 9000 and 3000 yr BP is also indicated by 14C ages of AOM carbonates found earlier in the area (Shnyukov et al., 1997; Shnyukov and Kutniy, 2003; Lukin, 2007; Shnyukov et al., 2013). The studied fragment of carbonate build-up is a 1 meter high chimney, with its top and root 40 cm and 15 cm in diameter, respectively, and a distinct central channel (Fig. 3a).

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ACCEPTED MANUSCRIPT Dredging failed to tap its basement attached immediately to plagiogranite (Fig. 2), but the total height of the build-up was inferred to reach 1.5 m (Shnyukov and Kutniy, 2003;

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Shnyukov et al., 2013). The chimney has an irregular surface covered with pink and orange

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microbial mats (Figs. 3b and 3e). The walls are 1-4 cm thick, layered, porous and cavernous,

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brittle and with grey colour (Figs. 3b and 3d). The external surface and the cavities are encrusted with yellowish spherulites (Figs. 3b and 3c). 14C contents give apparent radiocarbon ages of 9000±180 and 7500±180 yr BP in the lower and upper parts of the build-up

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(Shnyukov and Kutniy, 2003). Mineralogical, petrological, geochemical and isotopic analyses

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were applied to 3-10 cm3 fragments from the top and the root (Fig. 3a).

3. Methods

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The texture and morphology of carbonate precipitates were studied in hand specimens and polished thin sections using optical petrography and scanning electron microscopy (SEM)

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(Figs. 3-6). The SEM analyses of Au-coated freshly broken samples and C-coated polished thin sections were made on a JEOL JSM 6380LA microscope coupled with an energy

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dispersive X-ray spectrometer.

Bulk mineralogy and relative abundances of minerals in each sample were determined by powder X-ray diffraction (XRD) combined with SEM observations. For mineral identification by XRD, all sample materials were rinsed with distilled water to remove any salts and then crushed and pulverised under iso-propanol in an agate mortar. XRD was performed on a DRON-3 diffractometer with CuKα radiation (1.5405 Å wavelength). Scans were recorded from 7° to 60° 2θ at 0.017°/s, using 30 kV accelerating voltage and 25-30 mA beam current. Mineral phases were identified with Crystallographica Search-Match software. In-situ chemical compositions of individual mineral grains were assessed by electronmicroprobe analysis (EMPA) of C-coated polished thin sections, on a CAMECA Camebax-

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ACCEPTED MANUSCRIPT Micro probe equipped with a single EDS spectrometer and five WDS spectrometers with LiF, PET, and TAP crystals. The EPMA compositions were obtained for individual ≥ 10 μm

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grains, at the operation conditions of 20 kV, 15 nA, 10 s count time at each analytical line,

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and 4-10 μm incident beam diameter. Peak overlaps for CaKβ-РKα and SiKα-SrLα were

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automatically compensated by the instrument software, and a matrix correction using ZAF algorithm was applied to raw data prior to recalculation into major oxides. The analytical accuracy was within 2 %-relative for >5 wt% elements, and about 5 %-relative for < 2 wt%

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elements, especially Na (Table 1).

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Rare earth and trace element analyses of authigenic carbonates were performed on a Finnigan MAT ELEMENT high resolution ICP-MS (Table 2, Fig. 7). For analytical details, see (Nikolaeva et al., 2008). The acid digested BHVO-2 standard material was used for

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external calibration. The detection limits for Rb, Sr, Y, Zr, Ba, REE, Th, U were in the range 0.01–0.2 μg/l. The analytical precision was about 10% for all elements. Possible spectral

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overlaps in ICP-MS measurements (including the overlaps of BaO signals (135Ba16O+ and Ba16O+) on the signals of 151Eu+ and 153Eu+) were taken into account during data analysis.

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Selective dissolution of calcite and aragonite followed a modified procedure of Feng et al. (2009a). Namely, powder samples (0.2 g) were rinsed in 50 ml water (Milli-Q) in an ultrasonic bath. The obtained solutions were centrifuged and the supernatants were carefully decanted to remove any water soluble salts. Then the insoluble residues in the centrifuge tube were dissolved in 5% HNO3 for 2–3 h to isolate the carbonate phase. The solutions were centrifuged and the supernatants were stored for analysis. All mentioned analytical work was done at the Institute of Geology and Mineralogy (IGM), Siberian Branch of the Russian Academy of Sciences, in Novosibirsk. For the analyses of carbon and oxygen stable isotopes, the powdered calcite samples were dissolved in 100% phosphoric acid and analyzed on a Thermo Finnigan Gas Bench II

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ACCEPTED MANUSCRIPT coupled with a Thermo Finnigan MAT 253 stable isotope mass spectrometer at the IGM Analytical Center. The C and O isotope compositions of organic carbon samples were

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analyzed on a Thermo Finnigan Flash 2000 HT Elemental Analyzer interfaced through a

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ConFlo IV open-split interface with a continuous flow Thermo Finnigan Delta V Advantage

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isotope ratio mass spectrometer (the analyses were made at the Centre of Cenozoic Geochronology, Institute of Archaeology & Ethnography, in Novosibirsk). All isotope values are quoted per mill (‰) using δ-notation relative to the Vienna-PeeDee Belemnite standard

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(VPDB), with a standard deviation of >0.1‰ for δ13C and 0.2‰ for δ 18O in carbonate and

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less than 0.3‰ for δ13C of organic matter (Fig. 8, Table 3). The trace-element contents (V, Cr, Ni, Zn, Mo) of the bulk samples (authigenic carbonates ± associated minor phases) were determined at the Siberian Synchrotron and

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Terahertz Radiation Centre (SSTRC) based on the laboratories of Budker Institute of Nuclear Physics, Novosibirsk, using precise synchrotron radiation X-ray fluorescence analyses (SR

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XFA) with energy-dispersion spectroscopy (EDS), at 23 kV and 42 kV excitation energies. The element abundances were measured in 30 mg fine powder (<50 µm) samples rinsed with

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distilled water to remove salts and compressed into 6 mm pellets at 120 - 150 kg/cm2. Samples of dolomitic limestone (SI-1, SI-2 provided by the Vinogradov Institute of Geochemistry, Irkutsk, Russia) were used as external standards (Barishev et al. 1986; Phedorin et al. 2000). The analytical precision was about 10-15%.

4. Results 4.1. Petrography and mineralogy. The mineralogy of the studied chimney fragments consists mainly of high-magnesian calcite (Table 1) and minor aragonite. Mg-calcite builds chimney walls at its root and top (Fig. 3). Within the inner zone of the grey layered walls (Fig. 3d), it is commonly micritic Mg-calcite existing as 50-200 µm-sized hemispherical

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ACCEPTED MANUSCRIPT aggregates (irregular clots), showing brown or deep-brown coloration under polarised light (Figs. 4a and 4c). The clotted Mg-calcites are 3-10 µm obtuse rhombohedrons with curved

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faces (Fig. 4d). The space between loosely packed clots is often filled with later generations

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of sparitic (10-25 µm), spherulitic or foliated (50-200) Mg-calcite (Fig. 4a). Mg-calcites in the

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outer zone of the chimney walls are 5-25 µm foliated acute rhombohedrons (Figs. 3b, 3c, 4e and 4f). Aragonite (<< 1 vol.%) forms isolated prismatic crystals (<5 µm) set in the micritic Mg-calcite matrix and closely associated with organic clots and films (Fig. 5a).

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The outer surface and cavities of the chimney walls at the root are encrusted with 100-

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500 µm yellowish spherulites and foliated crystals of Mg-calcite (Fig. 3b, 3c and 4b). On the top, the micritic Mg-calcite walls are encrusted with later 1.5 mm grey and white aragonite spherulites.

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The carbonate matrix is often impregnated by 5-10 µm pyrite framboids (<< 1 vol.%) and contains Emiliania huxleyi coccoliths (< 5 vol.%), and films and clots of organic matter

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(locally reaching 5 vol.%), which are most probably remains of bacterial mats (Figs. 5a and 5b). The negligible percentages of coccoliths excluded distortions to bulk isotopic and

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element compositions in authigenic carbonates. Barite exists as up to 5 µm sporadic elongate crystals in the carbonate matrix or as spherulites buried in organic films on the chimney surface (Fig. 5c). It regularly contains about 2.5 wt% of SrO. Gypsum and diatomite fragments are extremely rare. Gypsum occurs as isolated 20-30 µm elongate, flattened and curved crystals in the calcite matrix (Fig. 5d). Minor norsetite (BaMg(CO3)2) was identified in the sample by XRD (Shnyukov and Kutniy, 2003). The contribution of detrital silicate material (quartz and rarer feldspar) is negligible. 4.2. Major oxides were determined by EMPA in calcite, which is the only mineral in the chimney to form large crystals of ≥10 µm (Table 1). The analyses show MgCO3 contents from 9 to 13 mol.%; FeO reaching 0.11 wt% in early micritic calcite but below detection limit

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ACCEPTED MANUSCRIPT (<0.02 wt%) in the later sparitic, spherulitic and foliated varieties; SrO, Na2O and MnO ranges of 0.11-0.28 wt%, 0.13-0.24 wt%, and 0.03-0.09 wt%, respectively, similar in all

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calcite generations and morphological varieties; BaO 0.07-0.20 wt% and ≤ 0.05 wt% in early

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and late calcites, respectively.

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4.3. REE+Y and trace elements were analysed in six samples from different chimney parts: two from the top and four from the root. The former were monomineral fractions of spherulitic aragonite (7/4-7-2, surface of the build-up) and micritic Mg-calcite of the matrix

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(sample 7/4-7-3) (Figs. 3a and 6). Out of the four root samples (Fig. 3a, 3b and 6), only one

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(sample KM-4) was monomineral spherulitic and foliated Mg-calcite that grew on the outer surface (Fig. 3b, 3c and 6) while three others (samples KM-1, KM-2, KM-3) represented an aggregate of micritic Mg-calcite (Fig. 3c, 3d and 6) with few sporadic aragonite micro-

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crystals detected by SEM (Fig. 5a). Powder XRD failed to identify aragonite in any of these samples, which means that its amount must be below 3 rel.%. Thus, the presence of aragonite

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should affect neither trace-element nor isotopic geochemistry of Mg-calcite. The REE, Th, Zr, Rb, Sr, Ba, U and Y contents determined by ICP-MS are listed in

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Table 2a, and shale-normalised REE+Y patterns of calcite and aragonite are shown in Fig. 7. The results of SR XRF determination of Ni, Zn, V, Cr and Mo in the bulk carbonate samples (authigenic carbonate ± minor phases) are listed in Table 2b. Compared to typical marine carbonates (Turekian and Wedepohl, 1961), the samples we analysed have lower Rb, Th and Zr (0.03-2.39, <0.24 and 0.14-2.30 ppm, respectively) and much higher Ba (444-1300 ppm), while Sr (772-1387 ppm) is within the marine carbonate range. Most of redox-sensitive elements (Ni, V, Mo) vary only slightly and are higher than the average marine carbonate values: 12-18 ppm Ni, 47-63 ppm V, and 4.0-11.8 ppm Mo. Uranium is in the range 1.88-3.34 ppm in most of carbonates but reaches 34.3 ppm in sample 7/4-7-3, which far exceeds that in marine carbonates. The contents of Zn (4-9 ppm) are lower than in marine carbonates; Cr is

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ACCEPTED MANUSCRIPT usually below the detection limit (< 0.1 ppm), except sample KM-1 where it reaches 3.6 ppm (Table 2).

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The carbonates fall into two distinct groups according to their REE+Y patterns

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normalised to shale (PAAS): Mg-calcite from the inner zone and Mg-calcite and aragonite

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from the outer zone. The Mg-calcite samples (KM-2, KM-3, 7/4-7-3) of the former group show similar REE+Y signatures (Figs. 6 and 7), with total REE between 2.3 and 20.8 ppm. Their flat or nearly flat REE+Y patterns (PrSN/YbSN = 0.50-0.84) are characterised by (i)

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positive La anomaly (La/La* = 1.18-2.05) in all samples; (ii) Ce anomaly positive in sample

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7/4-7-3 (Ce/Ce* = 1.33) and lacking from samples KM-2 and KM-3; (iii) Eu anomaly positive in samples KM-2 and KM-3 (Eu/Eu* = 1.78-5.09) and slightly negative in sample 7/4-7-3 (Eu/Eu* = 0.77); (iv) positive Gd anomalies, same in all samples (Gd/Gd* ~ 1.2). The

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Y/Ho ratios are from 24 to 39 (Table 2, Fig. 7). Samples of Mg-calcite (KM-1, KM-4) and aragonite (7/4-7-2) from the outer zone (Fig.

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6) likewise demonstrate similar REE+Y signatures. Their total REE concentrations are in a range of 2.1 to 7.2 ppm. Their shale-normalised REE+Y patterns show the following

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characteristics: (i) distinct HREE enrichment over LREE (PrSN/YbSN = 0.03-0.16); (ii) positive La anomaly (La/La* = 1.15-7.07); (iii) Ce anomaly positive (Ce/Ce* = 1.13-1.26) in samples KM-4 and 7/4-7-2 and lacking from sample KM-1 (Ce/Ce* = 0.95); (iv) positive Eu anomaly (Eu/Eu* = 1.60-3.28); (v) Gd anomaly from 0.9 to 1.8. The Y/Ho ratios vary from 30 to 36 (Table 2, Fig. 7). 4.4. Oxygen and carbon stable isotopes were determined in authigenic carbonates and carbon isotopic composition was studied in the associate organic matter of remnant bacterial mats (Table 3). The range of 18O values is very narrow (0.2 to 1.3‰ VPDB) in all analyzed carbonates. Both calcite and aragonite are strongly depleted in

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C in comparison with any

other marine carbonates. The lowest 13C values of -46.5‰ VPDB were observed in late

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ACCEPTED MANUSCRIPT spherulitic aragonite (sample 7/4-7.2) encrusting the chimney top (Figs. 3a and 6). The coexisting Mg-calcites (sample 7/4-7.3), which act as seeds for aragonite, have a higher δ13C

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of -40.0 ‰ VPDB (Table 3, Fig. 8). Carbon isotopic composition of calcites from the build-

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up roots vary significantly. The 13C value in late shperulitic Mg-calcite (sample KM-4) from

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the build-up surface is as low as -41.5 ‰ VPDB. Micritic foliated Mg-calcites (samples KM1, 6-8) making the base of shperulitic aggregates have 13C between -40.0 and -37.8 ‰ VPDB. Micritic Mg-calcites from the inner zone of the chimney roots have 13C values from

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-38.2 to -33.0‰ VPDB (samples KM-2-3, 5, 9-11) (Figs. 3 and 6; Table 3). Carbon in organic

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matter, which is most likely a dry residue of bacterial mats, has extremely low δ13С from

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-76.9 to -81.6‰ VPDB.

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5. Discussion

The chimney shape of the submarine carbonate build-up in the Lomonosov Rise

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(Crimean slope, NW Black Sea), its mineralogy with coexisting abundant Mg-calcite and minor sporadic aragonite and pyrite, the predominant morphology of authigenic carbonates

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with hemispherical crystal aggregates (clots) and their carbon isotope composition (13C = -33 to -46.5 ‰ VPDB) provide solid evidence for its formation by anaerobic oxidation of methane at an ancient cold seep. Methane-derived carbonates are widespread in the Black Sea (Peckmann et al., 2001; Egorov et al., 2003; Mazzini et al., 2004; Reitner et al., 2005a,b; Bahr et al., 2009; 2010), but this build-up is exceptional by growing free into anoxic water column 1600 m below the present sea level upon a steep slope almost devoid of sediments. These conditions prevented the authigenic carbonates from interaction with the gas-saturated bottom sediments and from contamination with clastic and biogenic carbonate materials. Therefore, the isotope and trace-element data we report may be considered as features of abyssal methane-derived carbonates typical of the anoxic environment of the Black Sea. The big size

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ACCEPTED MANUSCRIPT and good preservation of the chimney allowed us to determine for the first time the variety of mineral, trace-elements, and stable isotope compositions of the carbonates, which has never

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been done before.

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5.1. Microbially induced carbonate precipitation. The mineralogy and morphology of carbonates and associated phases change from the interior to the outer surface of the build-up (Figs. 3-6). The inner zones of layered grey walls consist mostly of micritic Mg-calcite. It

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occurs as well-shaped rhombohedra clustered into hemispherical aggregates known as clots

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(Figs. 4a and 4c). Such carbonate clots were reported from many deep-sea seep-related buildups elsewhere (Peckmann et al., 2001; Campbell et al., 2002; Reitner et al., 2005b; Magalhães et al., 2012).

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The model of Reitner et al. (2005b) predicts precipitation of these carbonate clots at the sites where chemical environment inducing carbonate precipitation is created by metabolism

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of specific microbial mats. In the anoxic zone, carbonate build-ups (towers) develop above bottom sediments where gaseous methane discharges into the water column. At the vent,

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consortium of methanotrophic archaea and sulphate-reducing bacteria form microbial mats, which are most often of spherical shapes, hollow and pervaded by gas. The initial growth of the carbonate towers starts within these spherical mats. Their metabolism induces the precipitation of calcium carbonate being stimulated by an increase in carbonate alkalinity upon sulphate-dependent AOM, according to reactions 1 and 2 (see above). The early calcification process results in the formation of delicate walls composed of CaCO3 around the microbial spheres of 100-200 µm in size. Further gradual inward lithification leads to the formation of hemispherical and spherical carbonate aggregates (clots). Mature spheres generate buds from which new juvenile spheres emerge. Continuous budding and ongoing calcification eventually create tall tower-like build-ups (Reitner et al., 2005b). Within

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ACCEPTED MANUSCRIPT spherical microbial mats, the precipitation of Mg-calcite begins with accumulations of iron sulphides, which then become enclosed in the Mg-calcite as fine framboidal pyrite aggregates

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(Reitner et al., 2005b), like those we observed in inner zones of studied chimney (Fig. 5b).

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According to Reitner et al. (2005b), ongoing calcification of spherical microbial mats leads to

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precipitation of carbonates as fibrous aggregates. Thus formed build-ups have chaotic textures and lack lamination. Mg-calcite in the inner zone exists as faceted rhombohedral crystals of larger and uniform sizes (Figs. 4c and 4d), which allows us to assume its re-crystallization

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from precursor fibrous aggregates, though rhombohedral habit may be primary as well. The

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lamination observed in the inner zone (Figs. 3b and 3d) is likewise secondary, resulting from re-crystallization of the primary fibrous Mg-calcite. The surfaces of calcite clots are sometimes covered with organic films that enclose

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aragonite microcrystals (Fig. 5a). The films must be remains of microbial mats formed after calcification of the precursor spherical mats. Metabolism of the later bacterial community

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apparently created an environment for the growth of aragonite inside the build-up. The phases and crystal morphologies in the outer zone of the chimney are very diverse.

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In the build-up root Mg-calcite exist as foliated crystals and spherulites (Figs. 3b, 3c, 4b, 4e and 4f). These growth forms and aggregates indicate highly supersaturation settings (Chernov, 1984). The top surface is encrusted with aragonite spherulites growing upon the Mg-calcite matrix. Late carbonates precipitated on the walls surface in the conditions of interaction with sea water. In the same way as in the inner zone, AOM reactions in the outer zone were controlled by consortia of methanotrophic archaea and sulphate-reducing bacteria. The shapes of these consortia at the mat-water interface are unknown, but they hardly were spherical microbial colonies judging by the lack of specific carbonate clots in the outer zone. More likely they were small local associations (Boetius et al., 2000; Blumenberg et al., 2004; Nauhaus et al., 2005; Treude et al., 2007), as the biomass density is much lower on the mat-

16

ACCEPTED MANUSCRIPT chimney surface than inside it. Nevertheless, the microbial activity was high enough to produce local supersaturation with respect to HCO3- sufficient for crystallization of

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spherulites and foliated crystals of CaCO3.

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5.2. Environmental control on carbonate mineralogy. The mineralogy of the AOMrelated carbonates is influenced by the precipitation medium chemistry. Specifically, dissolved sulphate was shown experimentally to inhibit calcite precipitation and to give rise to

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aragonite-rich carbonate deposits (Burton, 1993; Savard et al., 1996). Note that the chimney

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rooted in the Lomonosov rise consists mostly of Mg-calcite and only has its top encrusted with aragonite. Two models can be invoked to explain the chemical evolution of the methanederived carbonate precipitation environment during the seep activity.

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According to one model, this environment evolved mainly under the chemical control of the AOM consortia that lived at the methane seep. The microorganisms mediating anaerobic

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oxidation of the seep methane are of several groups. Two main groups of methane-oxidising archaea have been termed ANME-1 and ANME-2. The sulphate-reducing bacterial (SRB)

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partners associated with ANME belong to the Desulfosarcina-Desulfococcus and Desulfobulbus groups (Nauhaus et al., 2005; Niemann et al., 2006; Knittel and Boetius, 2009; Rossel et al., 2011). The microbial mats discovered earlier at the methane seeps in the NW Black Sea are mainly composed of ANME-1 and ANME-2 in association with SRB. The third group (ANME-3) has never been found in the Black Sea so far (Knittel et al., 2005; Bahr et al., 2009; Rossel et al., 2011). Studies of authigenic carbonates from the anoxic deep-water environment of the NW Black Sea led Reitner et al. (2005b) to suggest that the compositions of AOM consortia affect the carbonate mineralogy indirectly in the way that different groups of archaea have different rates of methane turnover, with methane consumption slower in ANME-1 and faster in

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ACCEPTED MANUSCRIPT ANME-2 (Nauhaus et al., 2005; Reitner et al., 2005b). Accordingly, a high methane turnover rate reduces the sulphate concentration in the carbonate precipitation medium faster than a

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low AOM rate. Thus aragonite precipitation in the Black Sea anoxic water column is

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indicative of environments with lower rates of sulphate-dependent AOM mediated by an

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ANME-1/SBR dominated community, whereas the predominance of calcite records the environments where ANME-2/SBR dominated communities rapidly converted sulphate into sulphide upon AOM (Reitner et al., 2005a,b; Roberts et al., 2008). It should be stressed that

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this kind of relationship between dominant carbonate phases and prokaryotic communities is

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restricted to the deep anoxic water column of the Black Sea, but it differs from the AOM pattern in the sediments. Within the anoxic bottom sediments overlain by an oxic water column, AOM communities dominated by ANME-2 favour the precipitation of aragonite

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cement, whereas ANME-1 are typically associated with calcite (Peckmann et al., 2009, Haas et al., 2010, Guan et al., 2013).

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The absolute predominance of Mg-calcite in the studied build-up points to ANME2/SBR consortium being the main component of the bacterial mats during the whole activity

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period of the ancient seep. The precipitation of minor spherulitic aragonite was restricted to the late history of the seep, when the environmental conditions apparently changed to become favourable for ANME-1 proliferation. At the time being, it remains unclear how the activity of prokaryotic communities depended on the methane flux in the anoxic deep-sea environment. However the morphological diversity of carbonates indicates a notable methane gradient across the channel confined by the walls of the growing chimney. That very gradient controlled the difference in the biomass density of AOM consortia and determined the evolution of spherical mats restricted to the chimney inner zone. The concentrations of sulphate were higher in the outer zone, at the chimney-water interface, and lower inside it, where microbial sulphate consumption was the most intense.

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ACCEPTED MANUSCRIPT This inference is supported implicitly by the location of barite precipitated at the sulphatemethane transition zone (Pierre et al., 2012). The transition zone corresponds to a strong

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redox front which may move up- or downwards depending on the relative upward flux of

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methane and the corresponding penetration depth of sulphate (Riedinger et al., 2006; Kasten

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et al., 2012; Pierre et al., 2012). The location of barite on the chimney surface only indicates that the redox front did not penetrate inward the chimney/mat due to an upward flux of methane and intense sulphate consumption by the ANME-2/SBR consortium.

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The anaerobic oxidation of methane by ANME/SBR consortia and related carbonate

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precipitation produced fresh water by reactions (1) and (2). This water must have diluted sea water, especially inside the bacterial mat, where the AOM consortia had a maximum biomass density. Correspondingly, the dilution of sea water with AOM-related fresh water was very

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weak in the outer zone of the chimney/mat with a poor biomass of the AOM consortia. This idea agrees well with the REE+Y patterns of carbonates (Fig. 7). The sea-water geochemical

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signatures appear in aragonite and calcite from both younger and older (top and root) parts of the outer zone, while calcites of different ages in the inner zone show REE+Y spectra typical

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of markedly freshened sea water. According to the other model, the precipitation medium of AOM-related carbonate changed its chemistry in response to global environmental effects in the Holocene rather than to the local biochemical variations. In principle, this hypothesis is consistent with

14

C dating

of the chimney from the Lomonosov Rise (9000-7500 yr BP) as it possible formed in the time when the Black Sea reconnected to the global ocean (Soulet et al., 2011). This very model was earlier used in reconstructions for the evolution of AOM-related carbonate build-ups in Marmara Lake and in the eastern Mediterranean (Bayon et al., 2013, Cremiere et al., 2013). Namely, Cremiere et al. (2013) write that fluid emission dynamics and hydrocarbon oxidation at cold methane seeps can be directly related to changing

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ACCEPTED MANUSCRIPT environmental conditions through time. Dissolved sulphate concentrations in the Marmara ―Lake‖ pore waters during glacial time were too low to promote significant anaerobic

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methane oxidation, thereby preventing sedimentary carbonate authigenesis. Sea-water inflow

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from Mediterranean to Marmara ―Lake‖ provided a source of dissolved sulphate that allowed

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anaerobic oxidation of methane, which was a key factor to enhance microbial activity and associated carbonate precipitation at that time (Cremiere et al., 2013). During the last glacial period, and the associated oceanic low-stand, the Black Sea

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evolved as a giant fresh-water lake (e.g. Stoffers et al., 1978, Schrader, 1979, Soulet et al.,

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2011). It experienced voluminous inflow of marine waters from the Mediterranean in the Holocene, about 9000-9400 yr BP (Major et al., 2006; Soulet et al., 2011) and gradually became more saline after it had reconnected to the ocean (Soulet et al., 2011). According to C ages of the carbonates from the chimney root (about 9000 yr BP), the build-up

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possibly began its growth in freshened water with low concentrations of dissolved sulphate,

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where the AOM processes were restricted to high-Mg calcite precipitation. Later, about 7500 yr BP, sulphate became rather high in bottom water thus providing favourable conditions for

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precipitation of AOM-related aragonite. This reasonable and elegant hypothesis looks very tempting, but it contradicts the observed REE+Y features in the earlier and later precipitated carbonates of the chimney. The contradiction is discussed in more detail below in Section 5.4. 5.3. Carbon and oxygen isotopic compositions of methane-derived carbonates and organic matter. The carbon isotopic composition of the analysed authigenic calcite and aragonite, with the 13C range from -46.5 to -33.0 ‰ VPDB (-38.5 ‰ on average), is typical of carbonates formed at methane seeps (e.g. Peckmann et al., 2001; Lein et al., 2002, Egorov et al., 2003, Reitner et al., 2005b, Bahr et al., 2010, Magalhaes et al., 2012), indicating their origin by oxidation of strongly 13C-depleted methane.

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ACCEPTED MANUSCRIPT The 13C values of methane-derived authigenic carbonates have several controls. Number one is the biogenic or thermogenic origin of methane in ascending fluids: δ13C is

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extremely negative in the former and higher in the latter (-60 ‰ or less against -30 to -50‰

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C-depleted carbon in bacterial mats and

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microbially-mediated AOM process, with more

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VPDB, respectively). Another factor is the degree of carbon isotope fractionation during the

heavier carbon in carbonates. This distribution of carbon isotopes, with

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С being higher in

organic matter and lower in carbonates relative to the primary methane, was proven for active

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seeps of the Black Sea (Lein and Ivanov, 2005). Similar carbon isotope fractionation is

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likewise common to many other microbial processes, e.g., photosynthetic reactions (Pimenov et al., 2008; Hoefs, 2009). Another factor, that of mixing with carbon from other sources,

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such as normal marine or biogenic (skeletal material) carbonates or sediment organic matter,

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may play some role (Whiticar, 1999), but it was negligible in our case. The δ13C values of source methane must lie between the averages for the analysed

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carbonates (δ13Cav. ≈ -38.5‰ VPDB) and the related bacterial mats (δ13Cav. ≈ -79‰ VPDB). The very low δ13C values in the organic matter of the bacterial mats indicate mostly biological

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origin of the seep methane. Biogenic methane with δ13C = -60 to -70‰ VPDB was reported for cold seeps in the neighbour areas of NW Black Sea as well (Lein et al., 2002; Egorov et al., 2003).

The narrow oxygen isotope range in the carbonates (δ18O = +0.2 to 1.3‰ VPDB) indicates their precipitation affected by a single oxygen source (Table 3, Fig. 8) and

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contents are generally compatible with precipitation from sea water. Some of the carbonates have δ18O above that in the present sea water: about + 1‰ VPDB against ≈0.0 ‰ ± 0.2‰ SMOW (Anderson and Arthur, 1983). However, significant addition of

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O enrichment is very small and rules out

O from water released on gas hydrate decomposition or clay

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ACCEPTED MANUSCRIPT dehydration (Bohrmann et al., 1998; Aloisi et al., 2000; Han et al., 2004; Orphan et al., 2004; Teichert et al., 2005).

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Oxygen isotopic data from authigenic carbonates are widely used to reconstruct

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precipitation temperatures. We estimated the palaeotemperatures of Mg-calcite formation

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using an equation by Epstein et al. (1953) modified by Anderson and Arthur (1983): T(ºC) = 16.0 − 4.14(δ18Ocalcite - δ18Owater) + 0.13(δ18Ocalcite - δ18Owater )2

(3)

The temperatures of aragonite formation were calculated with the equation of Grossman

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and Ku (1986)

(4).

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T(°C) = 20.6 - 4.34 (δ18Oaragonite - δ18Owater),

The obtained estimates (Table 3) are quite realistic, being between 4.9 and 8.3°C (7.2°C on average), which is slightly below the present Black Sea bottom-water temperature of about

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9°C (Pape et al., 2010; Korshenko, 2013).

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5.4. Trace-element compositions of methane-derived carbonates. Calcites and aragonites store geochemical records informative for genetic reconstructions, as the

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abundances and distributions of REE+Y apparently remained relatively stable and uniform (Banner et al., 1988; Nothdurft et al., 2004; Zhao et al., 2009; Webb et al., 2009; Azmy, et al., 2011 and references therein). The geochemical characteristics of carbonates from the chimney's inner and outer zones are quite diverse (Fig. 7, Table 2). In the group of inner-zone Mg-calcite, only the REE+Y patterns of the latest precipitates from the build-up top (sample 7/4-7.3) show distinct (though smooth) sea-water signatures (Fig. 6) (Zhang 1996; Van Kranendorn et al., 2003; Johannesson et al., 2006). The REE+Y-spectra of the earliest Mg-calcites from the root (samples KM-2 and KM-3) lack any sea-water signatures but record strong dilution with water from a source undifferentiated with respect to (REE+Y)SN. Generally, the (REE+Y)SN

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ACCEPTED MANUSCRIPT patterns of older and younger Mg-calcites from the inner zone bear evidence of growth in a considerably diluted sea water environment. Unlike the samples from the inner zone, both

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early and late Mg-calcite and aragonite from the outer zone have uniform REE+Y patterns

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(Fig. 7) typical of sea water (Webb and Kamber, 2000; Bolhar et al., 2004; Shields and Webb,

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2004).

This geochemical diversity of the carbonates may be due to the diluting fresh-water input from four hypothetical sources: (1) clay dehydration water coming with methane

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through the seep; (2) water released by gas hydrate dissociation; (3) fresh water existing in the

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―lake‖ of Black Sea before the Mediterranean marine water inflow; and (4) fresh water produced by biochemical reactions of AOM and carbonate precipitation (equations 1 and 2). The two former sources likely caused a minor effect judging by δ18O values between 0.2 and

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1.3‰ VPDB (Table 3, Fig. 8) inconsistent with 18O-rich water input (Bohrmann et al., 1998; Aloisi et al., 2000; Han et al., 2004; Orphan et al., 2004; Teichert et al., 2005; Mansour,

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2009).

At first glance the Holocene Black Sea salinityzation appears to be a reasonable

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explanation for the geochemical variations in carbonates (Soulet et al., 2011). If this hypothesis were valid, the REE+Y spectra of carbonates would exhibit gradual evolution trends along the chimney height, from fresh-water signatures in the oldest precipitates at the root (all samples KM) to obvious sea-water signatures in all young carbonates at the top (samples 7/4-7.3 and 7/4-7.2 from the inner and outer zones, respectively). However, this is not the real distribution of fresh-water and sea-water REE+Y signatures in the carbonates, which does not depend on their location either in the chimney top or root (and hence on their age). The samples rather have REE+Y spectra typical of fresh water in the inner zones that confine the methane flux channel, while more or less prominent sea-water signatures appear in the samples from outer zones, both at the top and in the bottom (Figs. 6 and 7). Therefore,

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ACCEPTED MANUSCRIPT the sulphate gradient existed all over the seep history, but remained lateral across the buildup, with (SO4)2- concentration increasing abruptly on transition from inside the methane channel

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to the outside sea-water environment. Therefore, the biochemical control was principal for the

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mineralogy of carbonate precipitates, and was also critical for their geochemistry.

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At the same time, La/La*, Pr/Yb, and Y/Ho variations in the carbonates we analysed (Table 2) may reflect joint effects from both AOM-related fresh water and general changes in the bottom-water composition in Holocene. The sea-water signatures of higher Y/Ho and

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La/La* but lower Pr/Yb become progressively more prominent from early calcites at the base

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of the chimney to later calcites at its top (Fig. 6), and this trend may be due to gradually increasing bottom-water salinity.

The marine redox environment in which the methane seep carbonates were growing was

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reconstructed from their Се/Се* anomaly calculated using the equation of Bulhar (2004) (Table 2), without the concentration of La known to be above the PAAS values in minerals

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crystallised from water. All carbonate samples we analysed show a Се/Се* anomaly to be either positive or absent. According to Webb and Kamber (2000), Frimmel (2009), and Ge et

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al. (2010), no fractionation of Ce occurs under reducing conditions, resulting in Ce concentrations from normal to enriched in anoxic environments. The narrow range of Се/Се* anomalies indicates similar redox conditions of both precipitation of late carbonates in the outer zone and re-crystallization of inner-zone early minerals. The trace-element patterns of five (out of six) carbonate samples contain a positive Eu anomaly, the highest (Eu/Eu* = 5.09-1.78) in Mg-calcites from the chimney root and the lowest for both Mg-calcite and aragonite from the top (Eu/Eu* of 0.77 and 1.60, respectively). Eu enrichment (positive Eu anomaly) in the chimney root may result from leaching of plagiogranites of the Lomonosov rise by bottom water. Sea water and organic acids (Huang and Kiang, 1972) produced by metabolism of biota living around the seep dissolved

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ACCEPTED MANUSCRIPT plagiogranite minerals, whereby Eu was easily leached from plagioclase, the main Eu hostphase (Shiata et al., 2006; Feng et al., 2009a). Therefore, bottom water could rapidly uptake

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Eu (Shiata et al., 2006), which appears to be the main cause of the prominent Eu positive

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anomaly restricted to the carbonates of the build-up root.

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6. Conclusion

We have investigated a unique chimney-shaped carbonate build-up produced by microbially mediated anaerobic oxidation of methane at a deep-sea cold seep. The samples

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were retrieved from 1600 m water depth in the area of the Lomonosov Rise (NW Black Sea,

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Crimean continental slope). The carbonate chimney grew free into the anoxic water column upon a steep sediment-free slope composed of plagiogranite. The perfectly preserved 1.5 m high chimney stores reliable records of variations in mineralogy, geochemistry, and isotope

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compositions in the deep-sea methane-derived carbonates never studied before. The presence of large faults in the area maintains active and long-lasting gas venting through the sea bottom

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and the existence of the methane seep. The carbon isotope compositions of carbonates (δ13С = -46.5 to -33.0 ‰ VPDB) and remnant bacterial mats (δ13С = -76.9 to -81.6‰ VPDB) show

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signatures of origin mostly from biogenic methane. The mineralogy of the carbonate chimney grown at the seep was apparently controlled biochemically by AOM microbial consortia. The environmental conditions have been favourable for development and high productivity of an ANME-2/SBR consortium in bacterial mats for the whole history of the seep. Rapid sulphate-to-sulphide conversion upon AOM within the bacterial mats created a chemical medium favouring Mg-calcite crystallization, while aragonite grew late in the seep history when an ANME-1/SBR consortium began to predominate in the mats as the environmental conditions changed. Although Mg-calcite is the main phase throughout the build-up, carbonates have distinctly different morphologies and geochemistry in the outer and inner zones of the

25

ACCEPTED MANUSCRIPT chimney. Most likely the difference is due to variations in the methane flux gradient. Persistently high methane concentrations inside the channel provided the maximum biomass

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density of the AOM consortium and gave rise to spherical mats. Active microbial

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consumption of sulphate ions led to their low concentrations inside the mats and eventually

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resulted in calcium fixation in the form of Mg-calcite clots in the inner zone. The high microbial activity inside the seep channel had another important consequence: high production of fresh water by AOM reactions 1 and 2. The fresh water input was large enough

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for the inner-zone Mg-calcite to acquire the signatures of SN-distribution of REE+Y typical

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of carbonates crystallised in a freshened sea water environment. The lack of other fresh water inputs from gas hydrate decomposition or clay dehydration is indicated by the stable oxygen isotope composition of calcites with δ18O = 0.2 to 1.2‰ VPDB. The methane concentrations

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and the AOM-consortia biomass density were much lower outside the channel, where the AOM consortia apparently existed as isolated small associations and interacted immediately

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with the sea water. The concentrations of (SO4)2- were higher outside the mat/chimney than inside it and barite precipitated at the sulphate-methane transition. The sea water was the least

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diluted with fresh AOM-related water in outer zone, as confirmed by REE+Y SNdistributions in calcite and aragonite with distinct sea-water signatures. Thus, the biochemical control determined the mineralogy of carbonate precipitates and largely affected their geochemistry, while increasing bottom-water salinity in the Holocene, when the Black Sea previously evolving as a fresh water lake reconnected to the Mediterranean, may have strengthened the REE+Y sea-water signatures in late carbonates.

Acknowledgements This study has become possible due to joint efforts by many people, besides the authors, and we appreciate their aid greatly. Yu. Kolmogorov, M. Khlestov, E. Nigmatulina

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ACCEPTED MANUSCRIPT and E. Simonova (IGM SB RAS, Novosibirsk) performed the analytical work. The authors are grateful to Dr. Jörn Peckmann and the anonymous reviewer for critically reviewing and

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correcting the manuscript and significant intellectual contribution to the present work. We

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wish to acknowledge Editor-in-chief Gert J. De Lange. We further wish to thank

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T. Perepelova (IGM, Novosibirsk) for cooperation and helpful advice. The study was supported by grant No. 12-05-90403_ukr from the Russian Foundation for Basic Research

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and was carried out as part of Integration Project # 1-2013 SB RAS – NAS of Ukraine.

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Fig. 1. Location of the sampling site in the northwestern Black Sea. The carbonate build-up was retrieved by dredging during the 56th trip of R/V Professor Vodyanitskiy (14.07.2001 – 15.07.2001) from the slope of the submarine Lomonosov rise (station 5390; 44° 27.4‘ N; 32° 48.6‘ E). Red circles are sites of gas bubble streams; yellow circle is sampling site. The map and the gas bubble stream locations are after Egorov et al. (2003).

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Fig. 3. The carbonate chimney from the Lomonosov rise (see Fig. 1). a: general view of the chimney with a distinct central channel; white boxes frame the locations of materials sampled for mineralogical, petrological, geochemical and isotopic analyses; b-e: carbonate morphology and samples from the chimney root: c: Mg-calcite foliated crystals and spherulites (sph/fol Cal; samples, KM-1, KM-4, KM-6, KM-7, KM-8) predominant in the outer zone; the surface and the cavities of calcite aggregate are covered by organic films remaining from bacterial mats (OM); d: in the inner zone gray, porous and cavernous brittle chimney walls of a layered structure mainly composed of micritic Mg-calcite (micr. Cal); samples KM-2, KM-3, KM-5, KM-9-11; e: organic films (OM; samples KM-12, KM-13) remaining from bacterial mats and barite spherulites (Brt) on the wall surface.

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Fig. 4. Morphology of authigenic Mg-calcites from the chimney root. a is a thin section photomicrograph in polarized transmitted light; b-f are SEM images. a: clotted micritic Mg-calcite (micr. Cal) from the inner zone; voids between clots are filled with sparitic (spar. Cal) and spherulitic or/and foliated (sph/fol Cal) Mg-calcite; the dash-line oval outlines one of the clots; b: foliated Mg-calcites (fol. Cal) encrusting the chimney surface; c, d: clots of micritic Mg-calcite (micr. Cal) from the inner zone; Mg-calcites are obtuse rhombohedrons (c) with curved faces coexisting with organic films and clots (OM), Emiliania huxleyi coccoliths (Coc), and aragonite (Arg) microcrystals; e, f: foliated acute rhombohedrons of Mg-calcites (Cal) from the outer zone.

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Fig. 5. Diversity of minor phases from the chimney root (SEM images).

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a: aragonite (Arg) prismatic microcrystals and Emiliania huxleyi coccoliths (Coc) in the organic clots and films (OM) upon Mg-calcites; b: a framboid of pyrite (Py) and Emiliania huxleyi coccoliths (Coc) in a matrix of micritic Mg-calcite (Cal); c: Sr-barite (Brt) spherulites buried in organic films (OM) on the chimney walls; d: elongate, flattened and curved gypsum (Gp) crystals in a calcite matrix (Cal).

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Fig. 6. A sketch cross section of the methane-derived carbonate build-up from the Lomonosov rise, with details of inner and outer structure of its walls and points of sampling for mineralogy, geochemistry, and isotopes (on the right). Spherulites, mat organic films, calcite clots and layers are not to scale. Dash lines show inferred extension of the build-up (the parts not tapped by dredging).

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Fig. 7. Shale (PAAS)-normalized REE+Y patterns of authigenic carbonates from different zones of the chimney. a: REE+Y patterns of calcite and aragonite samples from the outer zone: KM-1 is micritic Mg-calcite forming foliated crystals at the contact with yellowish Mg-calcite spherulites (root, Figs. 3b, 3c and 6); KM-4 is spherulitic and foliated Mg-calcite (root, Figs. 3b, 3c and 6); 7/4-7-2 is spherulitic aragonite (top, Figs. 3a and 6); b: REE+Y patterns of calcite samples from the inner zone: KM-2, KM-3 (root, Figs. 3a and 6) and 7/4-7-3 (top, Figs. 3b, 3d and 6) are micritic Mgcalcite forming hemispherical crystal aggregates (clots).

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Fig. 8. Carbon and oxygen stable isotope compositions of authigenic Mg-calcite (bold symbols) and aragonite (open symbols) from the Lomnosov rise chimney and carbonate build-ups elsewhere in the NW Black Sea (δ13C and δ18O values quoted relative to VPDB). Data from (Peckmann et al., 2001; Bahr et al., 2010; Lein et al., 2002) are given for comparison.

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Table 1. EMPA analyses (wt.%) of authigenic calcites from the chimney root

0.06 0.05 0.09 0.09 0.06 0.05 0.06 0.02

S 37 47 51 52 53 58 X

S

SrO

Total

Late sparitic, spherulitic and foliated calcite, n = 20 4.71 <0.05 50.83 0.22 0.13 55.98 3.76 <0.05 49.99 0.13 0.18 54.22 4.96 <0.05 49.58 0.21 0.20 55.03 4.59 <0.05 50.11 0.23 0.14 55.19 4.81 <0.05 50.65 0.21 0.17 55.96 5.45 <0.05 49.88 0.19 0.17 55.76 4.76 <0.05 50.25 0.21 0.13 55.50 4.74 <0.05 50.53 0.16 0.12 55.67 4.35 <0.05 51.08 0.24 0.12 55.87 4.66 <0.05 50.32 0.20 0.14 55.45 0.42 0.82 0.03 0.04 0.94 Early micritic calcite forming clots, n = 8 4.04 <0.05 51.08 0.16 0.28 55.63 4.82 0.07 49.81 0.19 0.19 55.12 4.20 0.19 50.81 0.19 0.18 55.78 4.42 0.20 50.07 0.24 0.17 55.30 4.53 0.09 51.34 0.20 0.11 56.34 4.49 0.11 51.85 0.21 0.12 56.88 4.54 0.10 50.17 0.21 0.17 55.35 0.37 0.07 1.38 0.03 0.05 1.13

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<0.02 <0.02 0.11 0.11 <0.02 0.04 0.06 0.05

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<0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 0.04 <0.02 <0.02 -

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13 14 17 20 21 23 24 41 44

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MgCO3, CaCO3, mol.% mol.% 11 9 12 11 12 13 12 12 11 11 0.01

89 91 88 89 88 87 88 88 89 89 0.01

10 12 10 11 11 11 11 0.01

90 88 90 89 89 89 89 0.01

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7.2 30 0.95 1.15 0.91 1.84 0.16

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ΣREE Y/Ho Ce/Ce* La/La* Gd/Gd* Eu/Eu* (Pr/Yb)N

0.65 0.59 0.10 0.51 0.17 0.09 0.32 0.06 0.58 6.15 0.17 0.67 0.12 0.89 0.16

2.1 32 1.26 2.30 1.22 3.28 0.08

5.1 36 1.13 7.07 1.76 1.60 0.03

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0.30 0.49 0.06 0.27 0.09 0.07 0.10 0.02 0.18 1.46 0.05 0.17 0.03 0.22 0.03

carbonates from inner zone KM 2 KM 3 7/4-7-3 Cal Cal Cal 1.92 1.34 ≤0.02 772 857 1387 0.81 0.56 1.53 900 1300 444 0.17 0.11 0.05 1.88 2.02 34.3 0.70 1.43 0.18 0.78 0.18 0.07 0.19 0.03 0.16 0.63 0.03 0.07 0.01 0.07 0.01

0.41 0.82 0.11 0.47 0.09 0.10 0.10 0.01 0.08 0.35 0.01 0.04 0.01 0.04 0.01

4.72 7.73 0.75 3.31 0.65 0.14 1.05 0.15 0.95 7.88 0.20 0.55 0.06 0.48 0.06

3.9 24 0.96 1.20 1.26 1.78 0.82

2.3 25 0.96 1.18 1.25 5.09 0.84

20.8 39 1.33 2.05 1.22 0.77 0.50

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1.10 2.13 0.27 1.16 0.28 0.12 0.34 0.07 0.47 3.69 0.12 0.43 0.08 0.55 0.09

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carbonates from outer zone KM-1 KM 4 7/4-7-2 Cal Cal Arg 2.39 0.30 0.03 911 988 1021 1.30 0.14 2.30 1200 850 511 0.24 ≤0.02 ≤0.02 3.34 2.94 2.44

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Note. KM-1 is micritic Mg-calcite forming foliated crystals at the contact with yellowish Mg-calcite spherulites; KM-4 is spherulitic and foliated Mg-calcite from the chimney surface; 7/4-7-2 is spherulitic aragonite from the chimney surface; KM-2, KM-3 and 7/4-7-3 are micritic Mg-calcites forming hemispherical crystal aggregates (clots) (Fig. 6); Anomalies calculated using equations from (Bulhar, 2004): Ce/Ce*=CeN/(2PrN-1NdN); La/La* = LaN/(3PrN2NdN); Gd/Gd* = GdN/(2YbN-1DyN); Eu/Eu* = EuN/(0.5SmN+0.5GdN); N refers to normalization against PAAS (Taylor and McLennan, 1985);

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ACCEPTED MANUSCRIPT Table 2b. Concentrations of the redox-sensitive trace elements (in ppm) in the bulk probe of carbonate samples (SR XRF-analysis)

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carbonates from outer zone carbonates from inner zone КМ-1 КМ-4 КМ-2 КМ-3 Cal Cal Cal Cal 50 63 60 47 3.6 <0.1 <0.1 <0.1 13 18 12 16 9 4 7 7 6.3 11.8 4.3 4.0

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δ13С(VPDB), ‰

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Carbonates sampled from the chimney top (Figs. 3a and 6) late aragonite spherulites -46.5 1.3 7.4 7/4-7.2 encrusting chimney surface micritic Mg-calcite forming clots -40.0 1.2 4.2 7/4-7.3 in inner zone Carbonates sampled from chimney root (Fig. 3a, 3b and 6) late yellowish spherulitic and foliated Mg-calcite encrusting -41.5 0.3 8.0 KM-4 chimney surface (Fig. 3b, 3c and 6) late foliated micritic Mg-calcite -38.2 0.6 6.9 KM-1 making seeds for yellowish Mg-37.8 1.0 5.7 KM-6 calcite spherulites; chimney outer -40.0 0.8 6.2 KM-7 zone (Figs. 3b, 3c and 6) -39.6 0.3 8.0 KM-8 clotted early micritic Mg-calcite of -36.6 0.5 7.3 KM-2 gray layered walls, chimney inner -38.0 0.2 8.3 KM-3 zone (Figs. 3b, 3d and 6) -33.0 – – KM-5 -35.6 0.6 6.9 KM-9 -35.3 0.6 6.9 KM-10 -38.2 0.7 6.6 KM-11 Brown films of organic matter (remains of bacterial mats) upon chimney surface (Figs. 3b and 3e) -76.9 – KM-12 -81.6 – KM-13

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Graphical abstract

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1. A carbonate buildup grown in a deep-water environment in the NW Black See was studied. 2. Authigenic carbonates precipitated at a site of ancient cold methane seepage. 3. Carbonate precipitation results from microbially-mediated anaerobic methane oxidation. 4. Carbonates morphology and geochemistry were controlled by the methane flux gradient. 5. The carbonates were derived from biogenic methane.

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