Methane-derived carbonate conduits from the late Aptian of Salinac (Marne Bleues, Vocontian Basin, France): Petrology and biosignatures

Marine and Petroleum Geology xxx (2015) 1e12

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Methane-derived carbonate conduits from the late Aptian of Salinac (Marne Bleues, Vocontian Basin, France): Petrology and biosignatures €fer a, Joachim Reitner a, *, Martin Blumenberg a, 1, Eric-Otto Walliser b, Nadine Scha Jan-Peter Duda a €ttingen, Goldschmidtstr. 3, 37077 Go €ttingen, Germany Geobiology Group, Geoscience Centre, Georg-August-University Go Department of Applied and Analytical Paleontology, Earth System Science Research Center, Institute of Geosciences, University of Mainz, Johann-Joachim-Becher-Weg 21, 55128 Mainz, Germany

a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 March 2015 Received in revised form 25 May 2015 Accepted 28 May 2015 Available online xxx

Peculiar carbonate bodies occur in distinct marl layers of the Marnes Bleues Formation (AptianeAlbian, Vocontian Basin, Southern France). The carbonate conduits exhibit pipe- or sausage-like forms and a central channel. Their sizes range between 30 and 60 cm in length and 5e10 cm in diameter. The conduit carbonates consist of automicrite authigenically formed within the sediment. Millimeter-sized aggregates of framboidal pyrite are abundant within the conduit automicrites, probably representing former colonies of sulfate reducing bacteria. The central channel reflects former pathways of reduced fluids in the carbonate conduit. Ni-enrichments at the margins of the central cavity are may be due to the activity of methane-related metabolism as Ni is an important bio-element for respective microbes. Light stable carbon isotope ratios of the conduit automicrites (
25.86‰ to
23.10‰ VPDB) point to carbonate precipitation linked to anaerobic oxidation of methane (AOM), while less depleted stable carbon isotope ratios of microspar in marginal zones of the central opening (
8.96‰ VPDB) are in line with microbial sulfate reduction. A methane-related origin of the conduit carbonates is confirmed by the presence of authigenic lipid biomarkers tentatively sourced by archaea most of which are characterized by strong 13C depletions (d13C values down to
104‰). The presence of organically bound sulfur is well in line with microbial sulfate reduction. Isorenieratane potentially point to the presence of brown pigmented green sulfur bacteria. The methane was probably sourced by older OAE black shales which are known to contain isotopically (d13C) heavy biomarkers of archaea as reported elsewhere. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Vocontian Basin Conduits Anaerobic oxidation of methane (AOM) Biomarker Methane Carbonates

1. Introduction 1.1. Study aims The anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SR) is a key process for the generation of microbial carbonates at recent and ancient hydrocarbon seep sites (e.g. Kulm and Suess, 1990; Hinrichs et al., 1999; Peckmann et al., 1999, 2001; Michaelis et al., 2002; Reitner et al., 2005a,b; Knittel and Boetius, 2009). Methane seepage-related carbonates can be grouped into two main types. The first type is formed at the sedimentewater interface and thus more or less in the water column. Carbonates of this * Corresponding author. E-mail address: [email protected] (J. Reitner). 1 Present address: Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, 30655 Hannover, Germany.

type can even form reef-like build-ups, therefore commonly referred to as “chemoherms”. Good examples are tower-like carbonate buildups in the euxinic Black Sea (Michaelis et al., 2002; Reitner et al., 2005b) and chemoherms at the Hydrate Ridge (Teichert et al., 2005; Leefmann et al., 2008). The second type grows within the sediment and is probably related to micro-seepage of CH4-, CO2-, and H2S-rich fluids (cf. Reitner et al., 2005a). Carbonate bodies of this type typically exhibit pipe- or chimney-like shapes and are often several meters in size. Interestingly, peculiar carbonate bodies with such shapes occur in a distinct marl layer of the Lower Cretaceous Marnes Bleues Formation (Vocontian Basin, southern France) (Capozzi et al. 2013). Although the intrasedimentary formation of conduit-like methane-derived authigenic carbonates (MDAC) is a widespread phenomenon and observed through time, the formation of these carbonate bodies is not yet fully understood (Capozzi et al., 2012). Sub-recent and modern MDAC are e.g. common at the Gulf of Cadiz n and have been studied in detail (e.g. Díaz-del-Río et al., 2003; Leo

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et al., 2007; Merinero et al., 2008; Magalhaes et al., 2012). MDAC pipes in this setting are linked to large mud volcanoes and were mainly formed deeper within the sediment. The MDAC pipes mainly consist of dolomite and high-Mg calcite and exhibit light d13C values down to
50‰ VPDB, clearly demonstrating a methane-related origin. Another important example are dolomitic conduits within Miocene sedimentary rocks of New Zealand (Nyman et al., 2010). These carbonates were formed in the subsurface via AOM/SR and/or methanogenesis as evidenced by d13C values ranging from
20‰ to þ5‰ VPDB. Temperature reconstructions based on stable oxygen isotope signatures of the carbonates (d18O of ca. 5‰) indicate that the methane originated from the dissociation of gas hydrates. Plioto Pleistocene conduits from the northern Apennines (Enza River) exhibit comparable growth forms and d13C patterns as the ones investigated herein (cf. Blumenberg et al., in press, Oppo et al., in press and Viola et al., in press). Although these conduits were also formed in greater sediment depths, their formation was not solely restricted to AOM but also due to the anaerobic oxidation of higher hydrocarbons (Blumenberg et al., in press). This study is focused on the formation of peculiar carbonate bodies that occur within a distinct marl layer of the Lower Cretaceous (Late Aptian) Marnes Bleues Formation in the area of Sisteron-Salinac. Based on petrographic and geochemical evidences we show that these hydrocarbon conduits were formed due to microbially induced AOM, thus belonging to the second type of methane-related carbonates. Based on petrographical and (bio-) geochemical data we suggest a linkage between the deposition of organic rich sediments during earlier OAEs, seepage of methane microbially formed in these shales, and subsequent AOM, causing the formation of the conduit carbonates. 1.2. Geological frame The Vocontian Basin is located between the Rhone River and the Vercors-, Ventoux- and southern subalpine mountain ranges (Fig. 1). Geologically it was probably a special strike-slip fault basin situated on the north-western shelf of the European plate as part of the western Alpine Ocean, positioned at a paleolatitude of 25e30 N during the lower Cretaceous (e.g. Savostin et al., 1986; Arnaud and Lemoine, 1993; Herrle et al., 2003). Due to subsidence, the water depth increased during the AptianeAlbian and ranged between several hundred meters down to 2000 m (Cotillon and Rio, 1984; Wilpshaar, 1995). Consequently, cyclic hemipelagic to pelagic marly sediments were deposited during this time, represented by the Marnes Bleues Formation. Fresh unweathered marlstones of this formation are generally blueegrey in color, while more weathered ones are rather pale. However, some layers, including ones that represent Cretaceous oceanic anoxic events (OAEs), are much darker in color and therefore easily recognizable in the field he ret, 1988, 1997; Urbat, 2009). In the area of Sisteron(e.g. Bre Salinac, however, the late Aptian OAE 1b horizons “Niveau Jacob” and “Niveau Kilian” are not well preserved (Fig. 2). This might be due to a hiatus as evidenced by an erosional surface at the base of he ret, 1997). the Lower Albian OAE level (“Niveau Paquier”) (Bre Another explanation, however, could be that the respective horizons are not distinctly developed. In this case, the layer with the peculiar carbonate bodies could represent the “Niveau Kilian” and/ or “Niveau Jacob” of the OAE1b.

sampling site: 44 100 44.5400 N; 005 59016.9300 E). The Marnes Bleues Formation is well exposed in this area and exhibits a thickness of ca. 700e800 m (Flandrin, 1963). The carbonate conduits analyzed in this study were collected ca. 4 m below the Leymeriella tardefurcata event horizon which marks more or less the AptianeAlbian boundary (Reboulet et al., 2011) (Figs. 1 and 2). The carbonate conduits are concentrated in a ca. 1 m thick layer (Fig. 2). In addition to the conduits typical carbonate concretions with septaria-like shrinkage-structures are common. However, the conduits do not exhibit any of these features and are therefore easily distinguishable from the carbonate concretions. Barite concretions, as observed in other parts of the upper Aptian Marnes Bleues Formation, are lacking. The carbonate conduits exhibit pipeor sausage-like shapes. On average, their sizes range between 30 and 60 cm in length and 5e10 cm in diameter. 2.2. Petrography Petrographic analyzes were conducted on several thin sections (10  5 cm and 5  5 cm) from the carbonate conduits using a Zeiss SteREO Discovery.V8 stereomicroscope (transmitted- and reflected light) linked to an AxioCam MRc 5-megapixel camera. Some thin sections were stained with Alizarin Red S (pink to red calcite stain) and potassium ferrocyanid (II) (blue ferroan calcite stain) (Reid, 1969). Cathodoluminescence microscopy (CL) was carried out with a Citl CCL 8200 Mk3A cold-cathode system (operating voltage of ca.15 kV; electric current of ca. 250e300 mA) linked to a Zeiss Axiolab microscope. Images were created with a cooled SPOT-CCD camera. Raman spectra were recorded using a Horiba Jobin Yvon LabRam-HR 800 UV spectrometer (focal length of 800 mm) attached to an Olympus BX41 microscope. For excitation in the visible range (488 nm) an Argon ion laser (Melles Griot IMA 106020B0S) with a laser power of 20 mW was used. The laser beam was focused with an Olympus MPlane 100 objective with a numerical aperture of 0.9 onto the sample. Because of severe fluorescence artifacts in the visible range, the 244 nm line of a frequency doubled ion laser (Coherent Innova 90C FreD) was used afterwards (laser power of 30 mW). Here, the laser beam was focused with an OFR LMV-40-UVB objective with numerical aperture of 0.5. The laser beam was dispersed by a 600 l/mm grating on a CCD detector with 1024  256 pixels, yielding a spectral resolution of <2 cm
1 per pixel. Data were acquired over 60 s for a spectral range of 100e2000 cm
1. The spectrometer was calibrated by using a diamond standard with a major peak at 1332.0 cm
1. All spectra were recorded and processed using the LabSpecTM database (version 5.19.17; Jobin-Yvon, Villeneuve d'Ascq, France). 2.3. Element-geochemistry Main- and trace element concentrations were analyzed with laser ablation inductively coupled plasma mass spectrometry (LAICP-MS) using a PerkineElmer SCIEX ELAN DR II linked to a COMPex 110 ArF excimer laser from Lambda Physik and an optical bench Geolas from Mikrolas (laser operating conditions: 27 kV, 10 Hz; pit ø 120 mm). NBS 610 was used as internal reference standard (Jochum et al., 2005).

2. Material and methods

2.4. Stable isotope-geochemistry (d13C, d18O)

2.1. Study area and sample material

For carbon and oxygen stable isotope measurements, individual carbonate samples of 50e80 mg were drilled from the conduit slabs using a high precision drill (ø 0.8 mm). The isotope measurements were performed at 70  C using a Thermo Scientific Kiel IV carbonate

The study area is located near the small village Salinac (Les Coignets) close to Sisteron in southern France (exact position of the

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Fig. 1. Location of the Salinac-section within the Vocontian Basin in southern France (detailed map from Google Earth).

device coupled to a Finnigan DeltaPlus gas isotope mass spectrometer. Both d13C and d18O values were normalized to the international standard NBS19. Reproducibility, based on replicate analysis of standard NBS19, was better than 0.1‰ for d13C and d18O. d13C was measured relative to Vienna Pee Dee Belemnite (VPDB) while d18O was measured relative to standard mean ocean water (SMOW). However, d18O values were subsequently converted to VPDB using the equation by Coplen et al. (1983) (d18OVPDB ¼ 0.97002  d18OSMOW
29.98). 2.5. Organic geochemistry Carbonate rock samples were carefully sawn to remove the diagenetically altered and potentially contaminated surfaces. In

case of the conduit carbonates external and internal parts were individually analyzed. The remaining interior portions were crushed into small pieces and ultrasonicated in acetone before they were pulverized using a swing mill. The samples were subsequently decalcified in order to release the organic matter from the microcrystalline rock matrix. In order to allow for a gentle reaction, dichloromethane (DCM)-extracted water was added to approximately 100 g of each sample, before concentrated hydrochloric acid (HCl) was slowly poured on the samples. After 24 h, the residue was separated from the solution by centrifugation and washed two times with DCM pre-extracted water to approach a neutral pH. The residues were subsequently extracted by ultrasonication in DCM/ methanol (1:3, v:v), DCM and n-hexane. The combined extracts were separated by column chromatography (2 g silica gel 60;

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Fig. 2. The Salinac-section. The exposed marls and marly limestones of the Marnes Bleues Formation span the Aptian/Albian boundary. The conduit layer (marked by asterisk) could be equivalent to the “Niveau Kilian” and/or “Niveau Jacob” which are both not developed at this site.

i.d.1.2 cm), using n-hexane (saturated hydrocarbons), n-hexane/ DCM (1:1, v:v) (aromatic hydrocarbons) and DCM/methanol (1:1, v/ v) (polar residue). Gas chromatography-mass spectrometric (GCeMS) analyses were carried out using a Varian CP-3800 gas chromatograph coupled to a Varian 1200L mass spectrometer. The system was equipped with a silica column (Phenomenex Zebron ZB-5MS, 30 m, 0.25 mm film thickness, inner diameter 0.32 mm). Samples were injected on column. The carrier gas was helium. The temperature program was 80  C (3 min) to 310  C at 4  C min
1 (held 25 min). Mass spectra were produced using electron energy of 70 eV, a mass range of m/z 50e650 and a scan time of 0.25 s in full scan mode. Compounds were identified by comparison of the mass spectra and GC retention times with published data (e.g. a Toarcian black shale € ster et al., 1995). from Austria which contains isorenieratane; Ko The d13C signatures of hydrocarbons were measured via gas chromatography-combustion isotope ratio mass spectrometry (GCC-IRMS) using a Trace GC gas chromatograph coupled to a Delta Plus isotope ratio mass spectrometer (both Thermo Scientific). The combustion reactor contained CuO, Ni and Pt and was operated at 940  C. The chromatograph was equipped with the same column and operated under the same conditions as for conventional GCeMS analyses. The d13C values are measured relative to VPDB. The precision of the replicate measurements was usually better than ± 0.5‰. 3. Results and discussion 3.1. Petrographic evidences for an AOM induced formation of the conduits The Marnes Bleues Formation in the working area is generally characterized by organic-rich marlstones. Carbonate contents of the marlstones, however, vary significantly and so concretions and carbonate banks are locally abundant. The described carbonate

conduits consist of mudstones and wackestones with few planktonic foraminifera, calcispheres and shell debris (Fig. 3). No benthic organisms were observed. The micrite is Mg-poor as evidenced by a slight reddish Alizarine staining, Raman spectroscopy, and X-ray diffraction (XRD). The micritic matrix is weakly luminescent while allochems as e.g. foraminifer tests are commonly even nonluminescent (Fig. 4), both indicating relatively low amounts of Mn2þ which usually acts as a luminescence-activator (Richter and Zinkernagel, 1981; Haberman et al., 2000). In contrast, late blocky cements exhibit a strong red luminescence due to higher amounts of Mn2þ (Fig. 4). Given the concretion-like appearance within marly sediments, the micrite is interpreted as automicrite that was authigenically formed within the sediment. All analyzed carbonate conduits exhibit a central opening or channel that is cemented with a blocky calcite sparite (Figs. 5 and 6). The inter-crystal space is filled with yellow-to brownishcolored organic matter, often enriched in millimeter-sized spots (Figs. 5e7). Raman spectroscopic analysis of this organic matter with excitation in the visible range (488 nm) exhibits a very strong fluorescence which is may be related to high amounts of aromatic components, being in good accordance with the observed biomarkers (see 3.2.3) and published data (Okano et al., 2008). Therefore, excitation in the UV range (244 nm) was applied to analyze the organic matter and mineral components in more detail (Fig. 6). In addition to a well-developed G-band peak, the organic matter indeed exhibits a broad D-band consisting of numerous single peaks, additionally pointing to the presence of abundant diverse aromatic compounds. Calcite crystals, in contrast, are characterized by clear Raman spectra with only little organic matter, underlining the distinct spatial differentiation of organic matter and mineral phases. The sparite is generally a ferroan calcite as indicated by an intense blue potassium ferrocyanid (II) staining. The inner margin, however, is characterized by higher amounts of Fe2þ reflected by a stronger deep blue potassium ferrocyanid (II) staining, while the

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Fig. 3. Microfacies of the conduit automicrite (transmitted light; blue potassium ferrocyanid (II) stain indicates ferroan calcite). (a) Large aggregates of framboidal pyrite (p) evidence microbial sulfate reduction. (b) Remaining pore spaces of calcispheres and foraminifers are cemented with ferroan calcite (blue stain) and/or pyrite (p). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Cathodoluminescence behavior of the conduit carbonates (scale in (a) applies to both images). (a) The central channel of the conduit (1) exhibits only a weak to nonluminescent behavior. The marginal zone of the central channel (2) shows a strong luminescence, indicating high contents of Mn2þ. The conduit automicrite (3) shows only a very weak luminescence. Low-Mg calcite tests of foraminifers (arrow) are non-luminescent. (b) Detailed view of the conduit automicrite. While the automicrite is generally characterized by a weak red luminescence, some small carbonate crystals exhibit a strong red luminescence. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

inner central portion exhibit a brighter blue stain indicating varying amounts of iron (Fig. 5). The high amounts of intracrystalline Fe2þ in cements of the central channel opening are also reflected in a low luminescence, as Fe2þ is known to quench the Mn2þ induced luminescence (Richter and Zinkernagel, 1981; Haberman et al., 2000). The most intensive luminescence is observed in the marginal zone between the central opening and the micritic matrix characterized by microspar. However, the outer zones of these microsparitic crystals exhibit a very strong luminescence whereas their cores are non-luminescent, reflecting differences in the chemical composition of the fluids (Fig. 7). An important characteristic of the carbonate conduits are millimeter-sized aggregates of framboidal pyrite, which is generally common in various parts of the samples. For instance, tests of foraminifers and calcispheres are frequently filled with framboidal pyrite. These pyrite framboids are interpreted as end products of bacterial SR (Berner, 1984; Merinero et al., 2008) and probably represent former colonies of sulfate reducing bacteria (Fig. 3). A further important observation are abundant rows of framboidal pyrite with lengths of 5e6 mm, which are generally radially orientated and linked to the central opening in the central part of the carbonate conduit. The formation of these pyrite rows therefore rather represents former pathways of reduced fluids moving through the central channel (Figs. 3 and 5).

3.2. Geochemical evidences for an AOM origin of the conduits 3.2.1. Elemental evidences for fluid composition The conduit automicrite does not show any significant excursions of fluid critical elements (e.g. Ni, Ba, P, Sr, S, Mn) (Fig. 8). In contrast, one side of the central channel and the marginal zones are characterized by remarkably higher concentrations of all of these elements. The enrichment of Ba, P, Sr and S in the final stages of the conduit formation is well in line with the widespread occurrence of barite- (BaSO4), celestite- (SrSO4), strontianite- (SrCO3) and francolite- (((Ca, Mg, Sr, Na)10(PO4, SO4, CO3)6F2e3); Benmore et al., 1983) cements and concretions in the near surroundings he ret, 1988, 1991; Bre he ret and Brumsack, 2000). The Ni(Bre excursion (Fig. 9) is particularly interesting in this regard, as Ni is an important bio-element for methane-related metabolisms. The coenzyme M reductase with the cofactor F430 is the key-enzyme for metabolisms performing methanogenesis and AOM (e.g. Krüger et al., 2003; Heller et al., 2008). The cofactor F430 has a porphinoid-ring with a Ni-ion in its center (i.e. Ni-tetrapyrrole; Hausinger et al., 1984; Friedmann et al., 1990). Methanogenesis and anaerobic oxidation of methane is one of the most prominent metal-rich enzymatic processes and the use of Ni is therefore one of the most critical elements for methanogenesis and AOM (Glass and Orphan, 2012; Glass et al., 2013). As the Ni-enrichments go along

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outer margin (
18.12‰). The surrounding background sediment generally exhibits d13C mean values of þ3‰ (Herrle et al., 2003; he ret, 1991). Therefore, the less negative values Weissert and Bre in the external parts of the conduits are probably due to a mixture of AOM-related automicrite and allomicrite from the surrounding sediments, supporting the formation of the conduit structures within the sediment fairly comparable to a concretion (cf. Raiswell, 1987). The microspar of the marginal zone between the central opening and the micritic matrix is characterized by depleted d13C values of about
8.96‰, being well in line with microbial SR (Raiswell and Fisher, 2000; Hoefs, 2009). This is also in good accordance with the observed framboidal pyrite (see above). In contrast, the ferroan calcite cements in the central channel exhibit slightly positive d13C values (0.70‰e1.25‰). These d13C signatures evidently reflect the relative enrichment of 13C in fluids due to the progressive withdrawal of 12C through AOM and SR (Fig. 11). The stable oxygen isotope values (d18O) of the marly and micritic background sediment are slightly negative around
4.5‰ he ret, 1991). While ferroan calcite cements of (Weissert and Bre the central channel are characterized by fairly comparable values (
4.86‰), the methane-related automicrite of the conduit exhibits heavier values which range from
0.37‰ to
1.44‰. Under the assumption that the carbonates were not substantially altered by later diagenetic processes, fluid temperatures were calculated from these data using the equation provided by Anderson and Arthur (1983). Interestingly, the d18O signatures evidence precipitation under slightly warmer temperatures ranging between 13  C and 33  C, pointing against dissociation of hydrates as methane source.

Fig. 5. Microfacies of the central tube (reflected light). The central channel initially was a pathway for reduced fluids (CH4, HS
) and is now cemented by later sparitic ferroancalcite (indicated by blue colors due to potassium ferrocyanid (II) staining). Note that the intensity of the staining varies due to varying amounts of Fe2þ in the calcite crystal lattices. Within remaining pore spaces yellow-to brownish-colored organic matter is concentrated. While the bright blueish ferroan-calcite shows positive d13C values (1.1‰), dark blueish stained ferroan calcites of the pyrite-rich channel margin are distinctly lower (
9‰). Note also the rows of framboidal pyrite (yellow arrows) which radiate from the central channel, probably representing fluid pathways associated with the main conduit body. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

with increased P concentrations (Figs. 8 and 9), the signature is may be due to the activity of methane-oxidizing or -producing archaea. The higher concentrations of Mn in the marginal zone of the central channel are in good accordance with the observed strong cathodoluminescence behavior of the microsparite (Fig. 10). The enrichments can best be explained by Mn2þ-rich fluids which seeped through the channel and affected the adjacent automicrites during early diagenesis. This scenario would be well in line with the observed ferroan-calcite cements that fill the central channel, thus representing a second fluid phase. Both fluids, however, must have been reducing as Mn and Fe are only soluble in their reduced oxidation states (i.e. Mn2þ and Fe2þ). 3.2.2. Stable isotopic (d13C, d18O) evidences for AOM and fluid temperatures The stable carbon isotope ratios (d13C) of the conduit micrites are generally light and range between
25.86‰ and
23.10‰ (Fig. 11). This is in good accordance with the suggested microbially mediated formation of the micrites (i.e. “automicrites”, see above) due to the anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SR) (cf. Peckmann and Thiel, 2004) and strongly supported by the lipid biomarker data (see 3.2.3). The conduit carbonate is characterized by slightly higher d13C values at the

3.2.3. Organic biomarker evidences for geomicrobiological processes In case of the carbonate conduits, lipid biomarkers were only found in the internal parts so that external contamination can be excluded. The authigenic lipid biomarkers are generally dominated by structures tentatively sourced by archaea (e.g. Thiel et al., 1999; Elvert et al., 2000) (Fig. 12). The respective compounds commonly exhibit strong 13C depletions (d13C down to
104‰), supporting AOM metabolism as source (Fig. 12). The observed irregular C25 isoprenoid 2,6,10,15,19-pentamethylicosane (PMI) (d13C ¼
62‰) is e.g. often present at sites with high methanogenic and methanotrophic activity (Tornabene et al., 1979; Elvert et al., 1999), although it is not clear whether this is due to the originally high abundances of archaea or to the relatively high resistance against diagenetic attack (Peters et al., 2004). Furthermore, a cyclized phytane derivative highly depleted in 13C (d13C ¼
104‰) was observed, which is either a degradation product of cyclized C40-biphytanes (Blumenberg et al., 2004) or a phytane derivative. The former presence of archaea is further indicated by C40:0 biphytane. In this case the d13C values are relatively high (d13C ¼
57‰) and thus not distinct for AOM, but this is probably due to the co-elution with nC35 or C40:0 from other archaeal sources. AOM is further supported by hydrocarbons that are as yet not known from living organisms but appear to be typical for AOM environments. Squalene thiophene (a sulfurized squalene derivative) was e.g. reported in fossil stromatolites that were influenced by AOM (Arp et al., 2008). Additional tentative sulfur-containing squalene derivatives were also found and were highly negative in their d13C values. The incorporation of reduced sulfur species into these archaeal isoprenoids is probably an early diagenetic process (Brassell et al., 1986; Kohnen et al., 1992). As organically bound sulfur is an indicator for excess sulfide during  and De Leeuw, 1990; Duda early diagenesis (e.g. Sinninghe Damste et al., 2014), microbial SR has probably been an important process in the setting.

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Fig. 6. Raman spectra (UV-laser, 244 nm) of materials in the central channel. The left spectrum shows a strong G-band indicative for carbonaceous material. The broad D-band exhibits many additional peaks which might be due to aromatic compounds. The right spectrum shows a clear calcite peak and only little carbonaceous material (c ¼ c).

Fig. 7. Detailed view from Fig. 5. (a) Transmitted light photograph showing organic matter (brownish color) locally enriched within remaining spaces between sparitic ferroancalcite crystals. (b) Cathodoluminescence photograph showing that the ferroan calcite exhibits a stronger luminescence at the crystal boundaries, which is probably due to lesser Fe2þ contents in these zones. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The setting at Salinac was obviously sulfidic which is confirmed by high amounts of framboidal pyrite and the organically bound sulfur. The observed isorenieratane could point to the presence of brown pigmented green sulfur bacteria, thus possibly indicating photic zone euxinia in the water column (Koopmans et al., 1996a;  and Ko €ster, 1998). Isorenieratane, however, Sinninghe Damste cannot only derive from the b-isorenieratene biosynthesized by Chlorobiaceae, but also from the aromatization of b-carotene (Koopmans et al., 1996b). An unequivocal interpretation of the compounds without their specific d13C signatures is therefore not possible (Koopmans et al., 1996b). A potential source rock for migrating biogenic methane during the formation of the conduit carbonates is the underlying late Aptian “Niveau Fallot”. However, additional biomarker analyses on material from this horizon in the frame of this study did not reveal any lipid biomarkers of methanogenic archaea (e.g. PMI or squalane; e.g. Thiel et al., 1999; Elvert et al., 2000). Likewise, lipid

biomarkers which potentially indicate water column stratification (e.g. gammacerane for ciliates living at or below a chemocline; e.g.  et al., 1995) and/or water Schoell et al., 1994; Sinninghe Damste column anoxia (e.g. okenane, chlorobactane or isorenieratane for phototrophic sulfur bacteria; e.g. Summons and Powell, 1986; Grice et al., 1996; Koopmans et al., 1996a; Brocks and Schaeffer, 2008) have not been found. However, since various maturity indices as for instance the carbon preference index (CPI ¼ 1.50; calculated after Bray and Evans, 1961), the methylphenathrene index (MPI
1 ¼ 0.36; calculated after Radtke and Welte, 1983) as well as the calculated vitrinite reflectance equivalent (Rc ¼ 0.47; calculated from the MPI-1 after Boreham et al., 1988) all point to a low thermal maturity, a preservational bias due to thermal maturity can be excluded. On the other hand, PMI and 2,6,15,19tetramethylicosane (TMI) were reported from the “Niveau Goguel” (OAE1a), the “Niveau Kilian” and the “Niveau Paquier” (OAE1b, respectively) close to Castellane (Vink et al., 1998; Okano

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Fig. 8. LA-ICP-MS analysis of fluid-critical elements (S, Sr, P, Ba, Ni; scale: ppm log10; dotted line: laser path). All shown elements exhibit an excursion at the margins of the channel, and some as e.g. S and Ba also in the center. These enrichments relative to the conduit automicrite are interpreted as signs for enhanced microbial metabolism at the channel margin.

Fig. 9. Clear Ni-excursion at the margin of the central channel. The anomaly is related to organic matter enrichments in remaining pore spaces (see Figs. 5 and 6). Ni is an important micro nutrient for methanogenic and methanotrophic archaea and the enrichment therefore interpreted as a proxy for AOM processes.

he ret, et al., 2008), where all of the OAEs are well developed (Bre 1994, 1997). Interestingly, the d13C signatures of the PMI shown by Vink et al. (1998) are heavy (ca.
20‰), suggesting methanogenic archaea as source organisms. The stable isotope signature is in strong contrast to the PMI in the conduit carbonates investigated herein, which is characterized by a distinctly low d13C signature (
62‰) and thus rather derived from methanotrophic archaea. These OAE layers therefore appear as plausible source horizons for the methane required for the formation of the carbonate conduits. 3.3. Formation model for the carbonate conduits (Fig. 13) Petrographic evidences, trace elemental data, stable carbon isotope signatures, as well as lipid biomarkers and compound specific d13C values are all in line with an authigenic formation of

the peculiar carbonate bodies within the Marnes Bleues Formation due to AOM. Apparently the investigated carbonate conduits (Fig. 2) were formed in the sulfate-methane-transition zone within the sediment. Sulfate plausibly derived from the marine water body, while the required methane was probably sourced from organicrich sediments deposited during earlier OAEs. Dissociation of hydrates in the sediment appears implausible as methane source given that the conduits were formed in slightly warm water as evidenced by the d18O-based temperatures. However, AOM related biofilms utilized the methane and might locally formed small conduits as observed in comparable modern conduits within Black Sea sediments (Reitner et al., 2005a; their Fig. 2 AeD). These initial conduits then acted as AOM reactor space as the fluid was successively concentrated, causing increasing rates of AOM. This model is well in line with the abundant occurrence of aggregated framboidal pyrite within the AOM automicrite and in radially orientated

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Fig. 10. Distinct Mn-excursions at the margins of the central channel. The higher Mn-concentrations are well in line with the cathodoluminescence behavior of the respective parts (see insert photo).

rows linked to the central channel (Figs. 3 and 5). The high AOM activity is also supported by the observed Ni and P enrichments at the margin of the central channel (Figs. 8 and 9). Later fluids within the conduit where enriched in critical elements like S, Sr, Mn, and Ba as reflected in geochemical composition of the ferroan calcites (Fig. 8). These calcite phases were not or only partially precipitated due to AOM as indicated by their slightly positive d13C values (0.70‰e1.25‰; Fig. 11). The formation pathway of the carbonate

conduits investigated herein is thus generally comparable with other sedimentary settings as for instance the Black Sea (Reitner et al., 2005a), the Gulf of Cadiz (e.g. Díaz-del-Río et al., 2003; n et al., 2007; Merinero et al., 2008; Magalhaes et al., 2012), Leo the Gulf of Mexico (Bohrmann and Schenck, 2003), Miocene strata in New Zealand (Nyman et al., 2010), and the northern Apennines (Blumenberg et al., in press; Oppo et al., in press; Viola et al., in press).

Fig. 11. Carbon and oxygen stable isotope signatures of the conduit carbonates and surrounding sediments. Stable isotope signatures of the conduit automicrite (i.e. sampling spots 2, 3, 4, 9) are well in line with AOM and SR. In contrast, the outer margin of the conduit (i.e. sampling spot 1) is stronger influenced by allomicrite of the background sediment, thus representing a mixing signal. The margin of the central channel (i.e. sampling spot 5) points to sulfate reduction, which is in good accordance with the increased pyrite-content in this zone. The late sparitic ferroan-calcite in the central channel (i.e. sampling spots 6, 7, 8) does neither point to AOM nor to SR. However, stable oxygen signatures of the respective cements point to an increased fluid temperature compared to the ambient allomicrite from the Marnes Bleues, which derived from the upper water column.

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Fig. 12. Total ion chromatogram (TICs) of the organic inventory of the conduit automicrite. The presence of authigenic lipid biomarkers tentatively sourced by archaea (PMI, C40:0 biphytane), most of which are characterized by strong 13C depletions (d13C down to
104‰), is well in line with a methane-related origin of the conduit carbonates. The presence of organically bound sulfur is in good accordance with microbial sulfate reduction. Isorenieratane potentially point to the presence of brown pigmented green sulfur bacteria, although compound specific stable carbon isotope signatures are required for an unequivocal interpretation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 13. Comprehensive formation model for the carbonate conduits. CH4 is formed by methanogenic archaea within OAE black shales and subsequently migrates upwards into a 2
zone characterized by higher SO2
4 -concentrations. In this CH4 e SO4 - transition zone AOM led first to the precipitation of a weakly indurated automicrite before the conduit carbonate became further indurated during early diagenesis. The remaining fluids (CH4, CO2, HS
) were concentrated within the central channel of the conduit, were further metabolized and released into the surrounding sediment and partly into the overlying oxygen-depleted water column. It is assumed that the water column was stratified and possibly enriched in reduced sulfur compounds.

4. Conclusions Conduit carbonates in the Marnes Bleues Formation consist of automicrite that was authigenically formed via AOM in the sulfateemethane transition zone within the sediment. The presence of methane-based metabolism is evidenced by Ni-enrichments, light stable carbon isotope ratios of the conduit automicrites (
25.86‰ to
23.10‰ VPDB), as well as by lipid biomarkers tentatively sourced by archaea (PMI, C40:0 biphytane) that are commonly characterized by strong 13C depletions (d13C down to
104‰). Sulfate reduction is evidenced by abundant framboidal pyrite, less depleted stable carbon isotope ratios of microspar in marginal zones of the central opening (
8.96‰ VPDB) and organically bound sulfur. Isorenieratane potentially point to the presence of brown pigmented green sulfur bacteria in the water column, although compound specific stable carbon isotope signatures are required for an unequivocal interpretation. Potential source horizons for the methane-enriched fluids could be black shales of older oceanic

anoxic events, and indeed biomarkers of archaea characterized by heavy d13C signatures were reported from these layers elsewhere. Acknowledgements €ttingen) is C. Hundertmark (Geobiology Group, University of Go thanked for help in figure preparation. J. Dyckmans (Centre for €ttingen) is Stable Isotope Research and Analysis, University of Go acknowledged for GC-IRMS-measurements. We are greatly indebted to L. Bulot (CNRS, University of Marseille) for assistance in the field and help with stratigraphic problems of the investigated sections and P. Imbert (Total, Pau, France) for important hints of possible conduits in the Sisteron area. M. Taviani is greatly acknowledged for field assistance, fruitful discussions and logistic help. This project was financially supported by the Deutsche Forschungsgemeinschaft (grant BL971/3-1), the Courant Research €ttingen, the ISMAR-CNR Bologna and the Centre Geobiology Go Italian PRIN 2009 Project (MIUR research grants to R. Capozzi).

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Please cite this article in press as: Reitner, J., et al., Methane-derived carbonate conduits from the late Aptian of Salinac (Marne Bleues, Vocontian Basin, France): Petrology and biosignatures, Marine and Petroleum Geology (2015), http://dx.doi.org/10.1016/j.marpetgeo.2015.05.029