Methane-Derived Authigenic Carbonates (MDAC) in northern-central Adriatic Sea: Relationships between reservoir and methane seepages

Marine Geology 332–334 (2012) 174–188

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Methane-Derived Authigenic Carbonates (MDAC) in northern-central Adriatic Sea: Relationships between reservoir and methane seepages Rossella Capozzi a,⁎, Francesco L. Guido a, Davide Oppo a, Giovanni Gabbianelli a, b a b

Dipartimento di Scienze della Terra e Geologico-Ambientali, University of Bologna, Italy I.G.R.G. Integrated Geoscience Research Group, University of Bologna, Ravenna, Italy

a r t i c l e

i n f o

Article history: Received 19 September 2011 Received in revised form 23 May 2012 Accepted 14 June 2012 Available online 23 June 2012 Keywords: Adriatic Sea methane seepage Methane-Derived Authigenic Carbonates reservoir

a b s t r a c t In the Adriatic basin, gas venting characterized by pockmarks, gas bubbling and other fluid escape related features, is diffused and has been linked or to shallow gases trapped in the Late Pleistocene and below the Holocene maximum flooding surface or to leakage from more deeply buried Pliocene to Pleistocene foredeep succession. Some seepages are associated with carbonate crusts, but a possible correlation between methane leakage and carbonate precipitation in this basin never reached a clear definition. Carbonate concretions recovered by different areas of the Adriatic Sea (northern, central and southern) at different depths (from ca 30 to ca 300 m) have been analysed. Different fabrics, petrography and mineralogy can be evidenced for the different sets of samples; all of them show a carbonate content mainly represented by high-Mg calcite: up to 66% in weight of total carbonates for the samples from central Adriatic (close to Bonaccia gas field, offshore Ancona), and 79 to 96% of the total carbonate weight for the other samples that are mainly biogenic. These differences are also reflected by the δ13C values, ranging between −19‰ and −29‰ VPDB, for the samples collected in the Bonaccia area and moderately positive values for all the other samples. Bonaccia samples contain carbonate cements that record the contribution of methane-derived carbon and have been interpreted as formed below the sea floor, in correspondence of the Sulphate-Methane Transition Zone. Carbonates recovered from the southern and northern Adriatic basin, do not show clear evidence of methane-derived carbon and are mainly made up by organisms typically recovered in the Adriatic coralligenous assemblage. Nonetheless micro-morphological evidences suggest influence of bacterial sulphate reduction and methanogenesis in precipitation of some of these carbonates. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Carbonate slabs and crusts, in many cases associated with conduits and chimneys, have been reported where methane leakage occurs at the seafloor in various tectonic settings (e.g. Campbell, 2006). They can also develop in passive margins at shallow depth, as in shelf environments where their evolution can be related to sea level oscillations (Orpin, 1997). The methane source is often diagnostically fingerprinted by a substantial 13 C-depletion when carbonate precipitation is generated by anaerobic methane oxidation (AOM) and sulphate reduction (SR). The anaerobic oxidation of methane produces HCO3¯ ion favouring the precipitation of carbonates. This increase in alkalinity is thought to result in the precipitation of 13 C-depleted authigenic carbonates at seep sites depending on the isotopic signature of methane (Baker and Burns, 1985; Stakes et al., 1999; Greinert et al., 2002; Lein, 2004; Gieskes et al., 2005; Naehr et al., 2007; Kinnaman et al., 2010). Resulting carbonates show a variety ⁎ Corresponding author at: Dipartimento di Scienze della Terra e Geologico-Ambientali, University of Bologna,40127, Bologna Italy. Tel.: +39051209451. E-mail address: [email protected] (R. Capozzi). 0025-3227/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2012.06.006

of lithofacies and structures which range from pervasive lithification of sediment to crusts, chemoherms etc., depending on the processes of fluid expulsion and the type of microbial consortia involved and the geologic substrate. The bacterial activity that promotes the anaerobic methane oxidation, together with the sulphate reduction process, has been studied on carbonate crusts, mounds and thrombolites (e.g. Reitner et al., 2005, with references therein). However, there are evidences that support the hypothesis of carbonate formation in association with methanogenesis instead of AOM-SR processes (Burns and Baker, 1987; Morse and Mackenzie, 1990). In the Adriatic sea, fluid vents have been observed and widely studied for years (e.g. Conti et al., 2002 and references therein). They are characterized by pockmarks, gas bubbling and other features related to fluid escape (Hovland and Curzi, 1989; Conti et al., 2002) and are frequently associated with carbonate crusts (Casellato and Stefanon, 2008). These carbonate patches are known since about two centuries and, during the last two decades, many samples have been recovered and studied. Notwithstanding some descriptions of the biogenic calcareous components, the possible correlation between methane leakage and carbonate precipitation in the Adriatic Sea never reached a clear definition. The diffused gas venting in some cases has been linked

R. Capozzi et al. / Marine Geology 332–334 (2012) 174–188

175

Apennines and Dinarides thrusts

46°

Holocene wedge Bonaccia field samples 25

Others analysed samples Close up in Fig. 1b

GIU

CROP M17 Others seismic profiles analysed

MED_01

Wells

50

44° 75

BONACCIA 200 175

100

100

200 175

100 125

200

MED_03

1000 100

1200

42°

MED_00

a) 12°

16°

14°

18°

CROP M17-C

Alessandra 44 °00 ’ Corinna

Carlo1 Carlo2

BP4-83-05 Bon_3

Bon_5 Bon_1

BP4-83-01

Bon_2 43°30’

b) 13°30’

14°00’

14°30’

15°00’

Fig. 1. Index map of the study area, generated with GeoMapApp 3.1.2 (http:/www.geomapapp.org/): topography and bathymetry from Global Multi-Resolution Topography (GMRT, http://www.marine-geo.org/portals/gmrt/) synthesis by Ryan et al., 2009. a) Samples location, structural sketch map of the Adriatic and isobaths (structural data modified after Scisciani and Calamita, 2009); b) close up of the central part of the study area around the Bonaccia field, with location of seismic profiles and wells interpreted in this paper.

to shallow gases trapped in the Late Pleistocene and below the Holocene maximum flooding surface (Trincardi et al., 2004; García-García et al., 2007) whereas, in other cases, it might originate from more deeply buried Pliocene to Pleistocene foredeep succession (Mattavelli et al., 1983). Significant uncertainties still arise in the assessment of both the real source/reservoir systems that feed the different seeps widespread on the basin floor, and the mechanism of fluid migration in relation to the stratigraphic architecture and tectonics. The aim of this work is to investigate methane-derived carbonates on the basis of their petrographical, mineralogical and geochemical characteristics, to compare these occurrences to biogenic carbonate

concretions widespread in the Adriatic basin, hypothetically associated to methane seeps (Casellato and Stefanon, 2008). Furthermore, to shed light on the stratigraphic and structural setting associated to gas leakage. 2. Geologic framework The Adriatic Sea is an epicontinental elongated basin about 800 km long and 150–200 km wide, which extends from the gulf of Trieste to the Otranto strait that connects the Adriatic and the Ionian Seas (Artegiani et al., 1997). The northern-central part of the basin is

R. Capozzi et al. / Marine Geology 332–334 (2012) 174–188

45°36’00’’

14°21’00’’

14°22’00’’

14°21’30’’

a

b GABB 2

45°36’00’’

176

86

86 88

86

45°35’30’’

45°35’30’’

85

86

86

x

BGA_b

87

86 x

GABB_2

x

GABB_3

x

BGAf_3

86

x a

Carbonate build-up Sampling site

87

86

85

SSS/SB profile in Fig. 3

14°21’00’’

14°22’00’’

14°21’30’’

Fig. 2. Image showing: a) distribution of carbonate concretions and locations of cores recovered in the Bonaccia area; b) 3.5 kHz sub bottom profile, showing location of the GABB_2 core at the top of a mud volcano structure, in correspondence of gas accumulation below the seafloor (courtesy of ENI SpA).

c) 25 mt

BGAf_3

Carbonate concretion

8 mt

25 mt

10 cm

Gas charged sediments

a) b) BGA_b

5 cm Fig. 3. Carbonate concretions from the Bonaccia area: a) BGAf_3 shows a vuggy structure due to a complex set of tubular and spherical voids in fine-grained siliciclastics; b) BGA_b, slightly more massive with evident benthic assemblage; c) coupled side-scan sonar/sub bottom profile (dashed line in Fig. 2) crossing the concretion structure of Bgaf_3 and underlying gas charged sediments.

R. Capozzi et al. / Marine Geology 332–334 (2012) 174–188

MED_00

a)

Table 2 Bulk rock carbon and oxygen isotopic composition, inorganic and organic carbon contents of the samples analysed in this study and listed from north-central, southern and northern Adriatic basin.

10 cm

MED_03

b)

177

C and O stable isotopes composition

Carbon composition

Org-C isotopes composition

(‰VPDB) δ13C and δ18O

Weight %

(‰VPDB) δ13C

Sample

δ13C

δ18O

TIC

TOC

δ13C OC

BGAf_3_1 BGAf_3_2 BGA_b_1 BGA_b_2 GABB_2 GABB_3 MED_00 MED_03 MED_01 GIU

−29.45 −27.85 −18.92 −18.89 0.51 −0.55 2.43 3.24 2.89 1.57

1.66 2.11 2.41 2.56 0.24 −2.27 1.33 4.39 2.72 1.85

6.5 7.37 7.58 9.24 n.a. n.a. 11.18 7.57 n.a. 9.77

0.14 0.12 0.18 0.22 n.a. n.a. 0.50 0.99 n.a. 0.29

−28.02 −27.08 −26.34 −28.06 n.a. n.a. −19.96 n.a. n.a. −19.92

n.a.: not analysed.

5 cm 20 cm

c)

MED_01

10 cm Fig. 4. Front (left) and polished surface (right) views of other samples studied: a) MED_00 shows a vuggy biohermal macrostructure with widespread borings; b) MED_03, massive carbonate crust, consisting of biogenic components cemented by microcrystalline carbonate (biomicrite); c) MED_01 similar to MED_00.

characterized by a shallow and gently southeast dipping platform which reaches a depth of about 100 m in 350 km. A first deep occurs in the central Adriatic, the Mid-Adriatic Deep, where the maximum

depth is up to 260 m. A second deep is located in the southern part, between the Gargano peninsula and the Albanian coastline, with a maximum depth of 1251 m. The steep Otranto strait, around 800 m in depth, separates the Adriatic from the central Mediterranean Sea, where the Ionian Sea is the deepest part (over 5000 m deep). The geologic investigation of the Adriatic basin has been carried out for exploration purposes by ENI and other companies by means of seismic surveys and deep well drillings. Other significant seismic data have been provided by the CROP Project (Deep Seismic Exploration of the Central Mediterranean and Italy; Finetti, 2005) and a large number of papers deals with the evolution of this basin. The present Adriatic basin belongs to the Adria plate, which played a major role in the geologic evolution of central Mediterranean (Channel and Horvath, 1976) and in the deformation of the peri-Adriatic chains. The marine development of Mesozoic extensional events went on up to the Late Jurassic with the formation of faults detaching in the Triassic evaporites and leading to salt diapirism (Mattavelli et al., 1991; Merlini and Cippitelli, 2001). During the Paleocene-Early Eocene times, the Africa-Europe convergence started, marked by the first arrival of terrigenous supply, and it went on from the Oligocene to the Early Miocene (Mattavelli et al., 1991). The Adriatic plate became the foreland basin of the Dinaric/Albanian and the Apennine chains, which record different vergence and timing of deformation (Fantoni and Franciosi, 2008). The major flexuring of the Apennine-Adriatic foredeep occurred during the Messinian Early Pliocene and the progressive deformation of the foredeep deposits is marked by thrust formation (Argnani et al., 1991; Merlini

Table 1 Location, mineralogy and Mg molar contents of the carbonates of the samples analysed in this study and listed from north-central, southern and northern Adriatic basin. Location

Mineralogy

Mg (mol%)

(WGS84)

Weight % of carbonates on bulk sample and relative % of each phase

mol % of Mg in differents carbonates

Sample

N

E

depth (m)

Tot Carbonates

Cal

Mg-cal

Dol

Arag

Cal

Mg-cal

Dol

BGAf_3 BGA_b MED_00 MED_03 start end MED_01 start end GIU

43.58937° 43.59147° 42.11167°

14.36389° 14.35078° 16.80383°

85 85 222

61.25 85.55 96.72

26.31 21.58 0.00

50.84 66.61 96.18

22.86 6.23 0.00

0.00 5.58 3.82

2.01 2.35 -

11.41 14.43 14.09

50.00 50.00 -

42.40683° 42.42150°

18.05667° 17.98900°

331 360

93.37

0.00

98.42

1.25

0.32

-

7.49

48.21

45.01000° 45.03110° 45.24578°

13.29755° 13.27085° 12.77091°

37 36 28

89.91

5.18

88.99

1.74

4.09

0.00

12.21

48.88

92.03

16.64

78.98

2.43

1.95

0.00

14.09

50.34

178

R. Capozzi et al. / Marine Geology 332–334 (2012) 174–188

and Cippitelli, 2001). Focusing on the northern-central Adriatic, offshore the Ancona promontory (Fig. 1) the deformation front in the northern part is the closest to the coastline with the main detachment developing in the Mesozoic layers during the Early Pliocene. Southward, the formation of more distal fronts, during the Pliocene, is due to the inversion of Mesozoic normal faults (Argnani et al., 1991; Geletti et al., 2008; Fantoni and Franciosi, 2010). Some evidences suggest that thrusting and folding ended in the Early Pleistocene (Di Bucci and Mazzoli, 2002). The sedimentary infill of the Po Plain-Adriatic foredeep consists of a thick (thousands of meters) succession of turbidites of Messinian to Pleistocene age (Ghielmi et al., 2010). Since the Middle Pleistocene, the main source for turbidites was the Po River delta. The deposition advanced along the foredeep axis, coupled with the shifting of the depocentres toward southeast, reaching the Central Adriatic area, SE of Ancona (Ghielmi et al., 2010), also recording the impact of glacial-interglacial cycles (Trincardi and Correggiari, 2000; Ridente and Trincardi, 2002; Ridente and Trincardi, 2005). The Bonaccia area is located about 70 km offshore the Ancona promontory, close to the eastern border of the Holocene mud wedge (Cattaneo and Trincardi, 1999; Correggiari et al., 2001; Cattaneo et al., 2003; Ridente and Trincardi, 2005) which shows shore-parallel depocentres, in response to the along-margin advection during the sea level rise and highstand. East of the Holocene wedge deposition, siliciclastic sand and silt, with minor carbonate and rare dolomitic clasts, constitute the terrigenous sediment, generally ascribed to relict sediments deposited during lowstand intervals (Servizio Geologico d'Italia, 2011).

a)

200 µm

b)

100 µm

3. Materials and methods We analysed different carbonate concretions recovered in the Adriatic Sea; their location is shown in Fig. 1. The samples from the Bonaccia area analysed in this study have been dredged in the proximity of the gas field, exploited by ENI, at about 85m water depth (Figs. 1 and 2). In this area pockmarks, mud volcanoes and fluid release have been documented (Judd and Hovland, 2007, and references therein) and are similar to those reported by Conti et al. (2002) in a wide area just southeast of the Bonaccia site. Two samples of carbonate build-ups have been analysed together with two cores. The first Bonaccia sample has been referred to as BGAf_3 (Fig. 3a) and shows a vuggy structure, due to tubular and spherical voids that develop in a fine-grained siliciclastic sediment that includes molluscs shells of the species Varicorbula gibba, Turritella communis, Aporrhais spp., Cardium spp., Nuculana commutate. This structure appears then colonized by Serpula vermicularis in association with Neopycnodonte cochlear. Turritella communis, Nuculana commutata and Varicorbula gibba are typical of muddy seafloor and tolerate high sedimentary rates (Scarponi, pers. comm.). Neopycnodonte cochlear is abundant within muddy seafloor over 50 m in depth, also where the sedimentary rate is moderately high. The sample referred to as BGA_b (Fig. 3b) appears slightly more massive, without elongated voids, and the biogenic assemblage is equivalent to the previous sample. In this case, also red algae and bryozoans encrust the surface of the carbonate build-up. A common characteristic of the BGAf _3 and BGA_b samples is the dark grey colour and, at the surface, a black coating of organic matter that strongly reacts to peroxide. To the west of the carbonate build-up of BGAf _3, a little mud cone has been revealed above a gas accumulation which can be observed few meters below the surface by a 3.5kHz sub-bottom profile (inset in Fig. 2, courtesy of ENI). A Kullenberg core about 1 m long has been recovered at the top of the mud cone and it is referred to as GABB_2 (Panieri, 2003). The core GABB_2 is made up by the typical lithofacies of the Adriatic shelf, mainly characterized by quartz and feldspars, also sampled in another core, referred to as GABB_3, located outside the seepage area (Fig. 2). The two cores contain a benthic foraminiferal assemblage characteristic of the Adriatic fauna. In GABB_2 core the

c)

800 µm

Fig. 5. Thin section photomicrographs of samples from Bonaccia area: a) BGAf_3 sample, scale bar 200μm and b) BGAf_3 close up, scale bar 100μm; c) BGA_b sample, scale bar 800μm. Silt-sized grains of quartz and feldspars are interspersed within a micritic cement. Black spots are interpreted as iron sulphides. Black arrow in c) indicates a glauconite crystal.

microfaunal density is reduced, even more toward the top, in concordance with the increase of Bulimina marginata which is known to survive in stressed environments, as those induced by cold seeps and related oxygen depletion (Panieri, 2003). The other carbonate concretions (slabs and blocks), from southern and northern Adriatic sea, investigated in the current study, were recovered during three cruises by Prof. C. Piccinetti in the scope of the MEDITS project (International Bottom Trawl Survey in the Mediterranean). The analysed samples are shown in Fig. 4 (Fig. 1, for location).

R. Capozzi et al. / Marine Geology 332–334 (2012) 174–188

a)

179

b)

30 μm

c)

1.5 μm

d)

EDAX 3 Ca

6.0

EDAX 5

4.0 Ca

9.0

Mg Na

2.0

Cl

Fe

6.0 2.0

4.0

6.0

8.0

3.0

20 μm 2.0

e)

6.0

4.0

8.0

f)

Ca

Mg

10 μm

1.0

2.0

3.0

4.0

Fig. 6. SEM photomicrographs of BGA_b and BGAf_3 samples and related EDAX spectra; particular of the cement structure: a) globular shaped (arrows), cauliflower structures (circle); b) elongated bodies and cavities (arrows) are visible by scanning electron microscopy, suggesting a microbial influence in the formation of the samples. The white star represents the location of EDAX analysis reported in EDAX 3 (Fig. 6d); c) aragonite crystals in sample BGA_b. The white star represents the location of EDAX analysis reported in EDAX 5 (Fig. 6d); d) EDAX spectra: EDAX 3 related to Fig. 6b shows the main composition of the cement, made up of calcite/Mg-calcite with Fe (likely due to oxides), and some Na, Cl; EDAX 5 spectrum of the aragonite in Fig. 6c in BGA_b; e) SEM photomicrograph of BGAf_3 Mg-calcite cement. The white star represents the location of EDAX analysis reported in Fig. 6f ; f) EDAX spectrum of Mg-calcite in Fig. 6e.

Sample MED_00, dredged offshore the Gargano promontory at a depth of 222m, shows a small biohermal macrostructure up to 15cm in diameter widely interrupted by spherical or irregular voids of various dimension and with widespread boring at the surface (Fig. 4a). Serpulids, bryozoans, sponges and red algae are the main encrusting organisms that are typical of the Adriatic coralligenous assemblage (Casellato and Stefanon, 2008). However, the depth of the recovered sample is higher than that known for red algae colonization (Bosence, 1991). The cut face of the carbonate sample reveals contorted laminae (Fig. 4a) coated by black and orange layers. This cut face shows a thrombolitic macro-fabric, very similar to that described for samples from the Black Sea (Reitner et al., 2005). The southernmost analysed concretion MED_03, from offshore the south Croatian coast, has been recovered at a depth of about 330m (Fig. 1). The sample is a massive carbonate crust of 60x40x3 cm in

dimensions, with no internal structure, which shows small cavities and tubular voids, probably due to burrowing in unconsolidated sediments. The upper surface is colonized by benthic fauna as brachiopods, serpulids and bryozoans; it is light grey coloured with black coating, whereas the inner part is dark to light ochre (Fig. 4b). The sample MED_01 was recovered offshore the Istrian coasts, at a depth of 35 m (Fig. 4c, see Fig. 1 for location) and is similar to the MED_00 (Fig. 4a), displaying a vuggy structure and diffuse colonization of benthic organisms, mainly rhodolithes, serpulids, and sponges. The colour is light grey with black and orange coating bounding internal voids and external areas of the sample. The cut section shows a thrombolitic macro-fabric and locally a stromatolitic-like lamination. In the northern Adriatic the concretion GIU recovered at 28 m water depth at the proximity of the Giuditta gas field, has a fabric similar to the previous sample. In this case the encrusting benthic

180

R. Capozzi et al. / Marine Geology 332–334 (2012) 174–188

δ18 O ‰ (VPDB) -10 +20

-8

-6

-4

-2

+15

0

+2

+4

Fermentation cements

Warm-water carbonate sediments Marine dolomites

+10

δ 13 C ‰ (VPDB)

Burial Ooids dolomites +5 Burial cements Marine limestones 0

Oozes Evaporative dolomites Marine cements Warm-water skeletons Mixing-zone dolomites

-5 -10 -15 -20 -25

+6

Meteoric Soil cements calcites Freshwater limestones Early concretions

-30

Methane-derived cements

-35 -40 -45 BGA_b MED_03

BGA_f GIU

MED_00 GABB_2

MED_01 GABB_3

Fig. 7. Carbon and oxygen stable isotopes diagram, modified after Nelson and Smith (1996), based on generalized isotopic fields for a selection of different carbonates typologies (calcite, siderite, dolomite) from New Zealand. Isotopic values of carbonates and cores analysed in this study are plotted in the diagram (see text for further explanation).

organisms are widespread at the surface, with rhodolithes, bryozoans and boring sponges as the dominant phyla. Samples, analysed in this study, have been stored after recovering at 4 °C, at the Earth Science Department of the University of Bologna. The petrography, morphology and micro fabric of the carbonates have been examined in polished slabs and thin sections by means of standard petrographic techniques (e.g. Adams and MacKenzie, 1998).

Scanning Electron Microscope (SEM) observations were carried out, coupled with energy dispersive spectrometer X-ray elementary analysis (EDAX), in fresh polished surfaces covered with carbon powder. SEM-EDAX analysis were carried out at the Department of Ceramic and Glass Engineering of the University of Aveiro, on a Hitachi S4100, SEM, operating at 15 kV and equipped with a backscattered electron detector (Rontec Energy Dispersive X-ray spectrometer system, series M) with a window for light elements. Part of the samples were previously analysed at the Department of Earth Sciences of the University of Bologna using a JEOL GSM 2400 (15–200,000× magnification) and a Philips 515B, equipped with EDAX reader Mod. DX4. Bulk mineralogy and relative abundances of carbonate phases were investigated by X-ray diffraction on powdered samples, using a Shintag X-ray diffractometer with Cu Kα radiation (1.5405 Å wavelength); scans were done from 4º to 70º 2θ at 0.02º/sec, using 40 kV accelerating voltage and 30 mA current. Peaks identification and minerals relative abundance estimates were performed by using Scintiag interpretation software and MacDiff® software package. Estimates on relative proportions of the minerals, based on the intensity of the diffraction pattern proportional to their concentrations in the samples, were obtained by measuring their relative peak areas. The peak areas were measured for the main peaks of the carbonate minerals aragonite (3.40 Å), calcite (b 8 mol% MgCO3, 3.036 to 3.012 Å), high-Mg calcite (8 to 30 mol% MgCO3, 3.012 to 2.946 Å), and dolomite (30 to 55 mol% MgCO3, 2.946 to 2.871 Å). The Mg/Ca ratio of the carbonate minerals was calculated from the shift of the d-spacing of the (104) reflection peak of calcite and dolomite from the stoichiometric peaks positions in the diffraction spectra (Goldsmith and Graf, 1958; Lumsden, 1979). The analytical results are reported in Table 1. Carbon and oxygen stable isotopes analysis were performed on bulk samples and were carried out with standard procedures with a VG Isotech PRISM mass spectrometer at the Godwin Laboratory, University of Cambridge (precision ±0.06‰ for 12 C/13 C and ±0.8‰ for 16O/18O). The results are reported in the conventional δ notation with reference to VPDB (Vienna Peedee Belemnite). The carbonate and the organic carbon contents were determined using a LECO CHNS-932 elemental analyser at ISMAR/CNR Bologna. The same set of samples was later subjected to combustion for 8 hours through a predefined stepwise increase in temperature up

a)

b)

150 µm

Fig. 8. Thin section photomicrographs of sample MED_00: a) biogenic structure, made up of red algae, bryozoans and sponges. Dissolution and bioerosion are responsible for the formation of small voids, partially filled by clotted lime mud. Dark bands bound biogenic component and have been ascribed to different episodes of dissolution and mineral formation; b) close up of dark rims associated with concentric bands, from orange to light grey in colour analysed in Figs. 10 and 11.

R. Capozzi et al. / Marine Geology 332–334 (2012) 174–188

181

videpi/. The location of seismic profiles and wells is reported in Fig. 1. The northern part (the first 1432 shot points) of M17-C has been processed at Marine Geophysics Laboratory of the University of Aveiro, Portugal. Deconvolution, Common Middle Point shorting, velocity analysis, Normal Move Out correction, stacking and migration were performed by using SPW TM by PARADIGMTM. 4. Results 4.1. Carbonates of the Bonaccia area

a)

b)

c)

500 µm

500 µm

4.1.1. Petrography and mineralogy The sample BGAf_3 shows very fine sand and silt-sized grains of quartz and feldspars, interspersed within a micritic cement that includes glauconite and iron sulphide (Fig. 5a-b).The terrigenous component is variable and ranges from 15% to 40% in weight, in different part of the sample. The BGA_b thin section (Fig. 5c) shows a very similar fabric, made up of mainly silt-sized grains together with foraminifera. Iron sulphide minerals are evidenced by black spots within brown to grey micritic cement. In the Bonaccia samples, the carbonate mineral phases present in the bulk samples are composed by high-Mg calcite (up to 66% in weight of total carbonates) with a significant amount of stoichiometric calcite (21% to 26%) and lesser amount of dolomite, except in the BGAf_3 sample (23%) (Table 1). Aragonite (~6%) is also present in the BGA_b sample. Detrital dolomites in this area (Ravaioli et al., 2003) could, however, enhance the weights of related carbonate phases. 4.1.2. SEM analysis SEM microfabric investigation coupled with EDAX analysis allows a better definition of the different components within the samples and the identification of various microbial-induced fabrics. SEM-EDAX investigation of the samples BGAf_3 and BGA_b from Bonaccia reveals different cements, made up of micritic to sparitic anhedral to euhedral calcite/ Mg-calcite and aragonite. Mg-calcite crystals appear as globular shaped with dimension ranging between 1 and 5μm (Fig. 6a, b) sometimes arranged in cauliflower structures (6a) of 10–15μm in size, and usually infilling pore spaces between silty detrital grains. Small rounded bodies (Fig. 6a), filaments and baton like structures (Fig. 6b) with dimension less than 2μm on length that occur in the clotted cement, can be ascribed to microbial activity. The EDAX diagram (Fig. 6d, EDAX 3) reveals the occurrence of minor Fe compounds, possibly linked to widely distributed framboids and pyrite crystals, seen in polished and thin sections. The occurrence of aragonite crystals is visible in the sample BGA_b (Fig. 6c, d-EDAX 5). In the sample BGAf_3 the cement has a high content in Mg (Fig. 6e, f), supporting the mineralogical analysis which shows a high percentage of high-Mg calcite and dolomite.

800 µm

Fig. 9. Thin section photomicrographs of samples: a) MED_01, originated by benthic organisms; cavities are often filled with botryoidal aragonite; b) MED_03 crust, made up of detrital and biogenic grains, mainly foraminifer tests, cemented by micritic carbonates; c) GIU sample: sponges and corallinaceous algae are dominant. Voids due to dissolution or bioerosion show dark rims and are filled by micritic cement and botryoidal aragonite.

to 400 °C, to remove organic carbon, and re-analysed for inorganic carbon. The Total Organic Carbon content (TOC) was determined by the difference between total carbon and the inorganic carbon concentration. Results are presented in weight percent (wt%). The relative precision of repeated measurements of both samples and standards was 0.03 wt%. The analytical results are reported in Table 2. The geologic setting of the Bonaccia area has been defined by means of the interpretation of the Deep Crust Seismic Profile M-17 C (CROP Project) and of industrial seismic profiles and deep wells available for public purposes on http://unmig.sviluppoeconomico.gov.it/

4.1.3. Carbon and oxygen stable isotope composition Carbon and oxygen stable isotopes of samples collected in the Bonaccia area were measured on bulk samples. The results (Table 2) show depleted δ13C values, ranging between −18.89‰ and −29.45‰, and moderately negative value of −0.55‰ for the sample GABB_3. δ 18O values are positive for all samples, except for GABB_3, which has δ18O value of −2.24‰. However, for GABB_2 and GABB_3 the δ 18O values likely represent the carbonate component of clastic sediment. Isotopes results have been reported on a diagram modified after Nelson and Smith (1996) (Fig. 7). Samples BGAf_3 and BGA_b fall into Methane-derived cements and Mixing-zone dolomites fields respectively. The more negative values of BGAf_33 are considered as the result of methane oxidation processes, whereas the less negative values of BGA_b could be interpreted as derived by sulphate reduction (Nelson and Smith, 1996). A variability of isotopic signature can be alternatively interpreted as due to mollusc shells and other biogenic fractions in the samples.

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Fig. 10. SEM photomicrographs and EDAX spectra of the portion from MED_00 sample characterized by dark rims shown in Fig. 8b: a) rounded bodies of about 1 micron in diameter and interpreted as calcified sulphate-reducing bacteria cells, black star indicating the location of analysis reported in EDAX 9 (Fig. 10d); b) sample MED_00: overview of the different parts of the sample; darker and lighter areas (low to high backscatter on SEM photomicrograph) correspond to white and black portions respectively in Fig. 8b; c) elongated structures on smooth sheet substrate, corresponding to an area between dark rims and orange portion of MED_00 in Fig. 8b; black star indicates the location of EDAX 11 (Fig. 10d); white star corresponds to EDAX 10 (Fig. 10d); d) EDAX spectra.

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Fig. 11. SEM photomicrographs and EDAX spectra of the passage to white portion of the rim in Fig. 8b of MED_00 sample. a) passage from a mucous sheet with a cylindrical body (c) on its top (1 micron in length) to an area characterized by globular and spherical bodies (cauliflower-like shape on the right); b) passage form darker to lighter part of the sample in Fig. 8b: cylindrical bodies (c), resembling ANME-1 cells (in the inset) depart from a smooth layer (ms); c) passage to the lighter parts of the sample, characterized by small spherical bodies (sb) of 1–2 microns in diameter: their surface is covered by wrinkled filaments (see inset), resembling bacterial mat or Mn-oxides (as their by-products), sometimes associated with calcite crystals; black star (in the inset of Fig. 11c) indicating the location of EDAX 6 in Fig. 11d; Mg-calcite crystals with lower Mn content border the spherical bodies (white star) (Fig. 11d, EDAX 7); d) EDAX spectra.

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a)

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Fig. 12. SEM photomicrographs: a) MED_03 showing widespread bacterial mats; b) biogenic structure of the sample MED_01, resembling an encrusting algae cellular wall, with dissolved borders and possible re-crystallization. In some cases the dissolution rims are covered by a coating that includes spherical structure; c) close up of spherical wrinkled bodies that cover the dissolution rims in Fig. 12b.

4.2. Carbonates of the southern Adriatic area 4.2.1. Petrography and mineralogy The sample MED_00 reveals a clotted mesostructures (sensu Shapiro, 2000) where large and small irregular voids are partially or totally filled by clotted cement. Its microfabric (Fig. 8a) shows a biogenic structure, made up of red algae, bryozoans and sponges. The pronounced dissolution and bioerosion are responsible for the formation of small voids that are filled by clotted lime mud, whereas larger macroscopic cavities are not completely filled. The sample is characterized by dark bands, extensively bounding the dissolution rims of the biogenic components. Fig. 8b shows dark rims associated with concentric bands, from orange to light grey in colour.

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The carbonate crust MED_03 is made up of detrital and biogenic grains, mainly foraminifer tests, cemented by carbonates. Some areas show homogeneous structure with brown carbonate cement and minor detrital components together with large bioclasts; other areas are less cemented, with major detrital and small biogenic components (Fig. 9a). Red and black spots are diffused in the entire sample, possibly due to oxides. These samples are very poor in terrigenous components; a grey-silty facies is present as secondary infilling of fractures and cavities. The main carbonate phase of the southern Adriatic samples is represented by high-Mg calcite with 96- 98% of the total carbonate weight, whereas calcite is absent (Table 1). The amount of high-Mg calcite can be partly ascribed to the occurrence of coralline algae (Bosence, 1991). 4.2.2. SEM analysis The MED_00 sample is the most interesting carbonate recovered away from the Bonaccia area. SEM micrographs allow to better define the microfabric of the bundles shown in the close up of Fig. 8b. A black rim, has a high backscatter in the SEM micrograph (Fig. 10b); it is in large part dominated by well defined bodies of about 1 μm in size with smoothed surfaces (Fig. 10a), whose composition, shown in the EDAX 9 (Fig. 10d), is mainly calcium coupled with the occurrence of some sulphur. These small structures are similar to mineralization associated with sulphate-reducing bacteria seen in cultures analysed by Van Lith et al. (2003), They morphologically resemble sulphate-reducing bacteria (Desulfosarcina, Desulfococcus groups) which have been observed in epifluorescence micrographs in marine sediments (Ravenschlag et al., 2000). Fig. 10c shows a close up of the passage to the darker part of the SEM micrograph (orange to lighter portions between two rims in Fig. 8b), where fossilized elongated structures, with 1 μm of maximum length, occur over a smooth substrate. The elongated structures are widespread in the sample and very similar to the aragonite batons found, as an example, in ooid cortex growth within mucous sheets (Folk and Lynch, 2001) and associated to nannobacterial occurrences. Furthermore, this kind of aragonite batons, ball-capped batons and nanograins on microbial biofilms has been observed in stromatolithic layers in MDAC (e.g. Magalhaes, et al., 2006). In the EDAX 11 (Fig. 10d) their composition is characterized mainly by Ca and Sr, supporting the hypothesis that it is aragonite, even though this latter corresponds to about 4% of the bulk composition of the sample (Table 1). EDAX on the surrounding crystals shows also S, Mn and Fe components (EDAX 10 in Fig. 10d). Other occurrences associated to a smooth substrate, partially covering detrital or biogenic grains, are cylinder-shaped bodies of up to 1μm long (Fig. 11a, b), which suggest a similarity in shape and size with ANME-1 cells (e.g. Orphan et al., 2002; Knittel et al., 2005; Reitner et al., 2005). There is a third layer observed in SEM/EDAX analysis which is in the transition zone to the low backscatter area (Fig. 10b, light portion in Fig. 8b)). In Fig. 11c, in this area, it is worth to note the occurrence of spheres of 1–2μm in diameter, whose composition is Mg-calcite containing up to 7% of Mn (Fig. 11d, EDAX 6). Possibly, there could be Mn oxy-hydroxides representing by-products of particular species of bacteria that oxidize Mn (Douglas et al., 2008). Mn enrichment have been also observed in microbialites (e.g. Reitner et al., 2005). Those spherical bodies are in close association with Mg-calcite crystals with lower Mn content (1.5%) (Fig. 11d, EDAX 7). The more homogeneous crust MED_03 in the SEM observation shows a diffused mucous film (Fig. 12a), that is very similar to carbonate precipitated during experimental growth of cyanobacterial mats (Ushatinskaya et al., 2006). MED_03 has been recovered from over 300 m water depth, well below the photic zone. In some experiments, cyanobacterial growth in dark condition is responsible for the production of Extracellular Polymeric Substances (EPS) and for the precipitation of a wide range of magnesium calcite (Zaitseva et al.,

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Fig. 13. (Top) CROP M17-C seismic profile (location in Fig. 1b); (bottom) CROP M17-C line drawing: the structural arrangement, from NNW to SSE, reveals a general inversion of extensional tectonic structures of Mesozoic age, interpreted as mainly developed during the Late Miocene and Early Pliocene. A wide unconformity at the top of the deformed units is dated to the G. puncticulata zone. East of the Corinna well projection, a significant ramp, verging to the north, is possibly related to a compressive stress direction toward NW, generating major shortening along previous lateral ramp. Gas accumulation has been evidenced in the Pleistocene succession providing the reservoir for the Bonaccia field, above the structural highs.

sediments with low organic matter content, as those from the southern Adriatic Sea (Table 2).

2006). This could support the interpretation that Mg-calcite of MED_03 can be the result of cyanobacterial activity. However, lowto high-magnesium calcite can precipitate, in normal saline condition in presence of sulphate-reducing bacterial activity (Douglas, 2005) or methanogenesis (Mazzullo, 2000).

4.3. Carbonates of the northern Adriatic area

4.2.3. Carbon and oxygen stable isotope composition Samples from southern Adriatic basin show small positive values of δ13C and δ 18O (Table 2) and have been classified (Fig. 7) in the diagram of Nelson and Smith (1996) close to the fields which represent the isotopic fingerprint of ooze and skeletons and interpreted as precipitated in equilibrium with ambient water (e.g. Allouc, 1990). However, the range of values determined for δ13C can reflect changes of dissolved CO2 in presence of sulphate reduction and/or methanogenesis. In this case the positive δ13C signal can be ascribed to a major contribution of HCO3- from sea water, diluting that from sulphate reduction (Wright and Wacey, 2005). Recorded stable isotopic composition can also be the result of methanogenesis (Mazzullo, 2000) occurring within

4.3.1. Petrography and mineralogy The sample MED_01 has been recovered in the vicinity of the Istrian coast and is dominated by benthic organisms (Fig. 9b). Silt-sized clastic component is interspersed within grey to brown clotted calcite and brown micrite, suggesting different phases and typologies of cementation. Cavities are often filled with botryoidal aragonite which likely is the last formed cement. Dark bands around voids and algae fragments are possibly due to the occurrence of oxides. The sample recovered near the Giuditta gas field (GIU) is very similar to the MED_00 sample (Fig. 9c). In this sample encrusting sponges, corallinaceous algae and serpulids are dominant. Voids due to dissolution or bioerosion show dark rims and are filled by micritic cement

Fig. 14. a) BP4-83-01 and BP4-83-05 seismic profiles; b) line drawing of BP4-83-01 and BP4-83-05 seismic profiles, which reveal gas accumulation above inherited structural highs in the region of the Bonaccia field: the gas accumulates in response to different compaction in the Pleistocene intervals locally seeping at the seafloor.

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and, in some cases, by silt-sized sediment. Compared to other samples the thin sections does not show evidence of stromatolithic-like lamination. They exhibit a cavernous framework partially filled with sediment and intergrowth of botryoidal aragonitic cement. The main carbonate phase is represented by high-Mg calcite for both MED_01 and GIU samples together with different amounts of calcite (Table 1), which is absent in the deeper samples from the southern part of the Adriatic basin. The amount of high-Mg calcite can be partially ascribed to the occurrence of coralline algae (Bosence, 1991). 4.3.2. SEM analysis The SEM photomicrograph of MED_01 (Fig. 12b) displays the structure of an encrusting algae whose cell walls underwent to partial dissolution. These carbonates are mainly composed of Mg- calcite that frequently shows, at the dissolution rims, a coating that includes spherical structures (Fig. 12c) similar to those observed in the MED_00 sample. EDAX analysis (data not shown) confirms their mineralogical composition as high Mg-calcite coupled with Mn, and minor Fe and K. likely related to the composition of the wrinkled filaments. 4.3.3. Carbon and oxygen stable isotope composition Samples from the northern Adriatic basin show slightly positive values as for δ 13C as for δ 18O, which are mainly grouped between oozes and marine dolomites (Nelson and Smith, 1996). In general, carbonates forming in presence of methane seeps have negative δ 13C, as also described for crusts recovered in the Mediterranean (e. g. Gontharet et al., 2007). In the case of the northern Adriatic samples the stable isotopes composition suggests that carbonates formed in equilibrium with ambient water. However, this record cannot univocally exclude the isotopic fractionation deriving from carbonate reduction during methanogenesis (Wright and Wacey, 2005). In fact, in the northern Adriatic basin biogenic methane seeps through recent sediments have been documented (e.g. Panieri, 2006). 5. Seismic lines interpretation The carbonates studied in the Bonaccia site are widespread in an area where an important gas field is exploited. In this study, seismic reflection lines have been interpreted to shed light on the relationships between tectonic and sedimentary settings and possible gas leakage from the reservoir, leading to MDAC formation. The deep seismic line CROP M17-C (Deep Crust, Seismic Italian Project, Finetti, 2005) has been interpreted down to 4 sec TWT, and correlated with BP4-83 industrial seismic profiles. In Fig. 13, the M17-C profile ends north of the Bonaccia gas field and is oriented NW-SE, parallel to the strike of the foredeep structures. The stratigraphic intervals, down to the Triassic units, have been calibrated by the correlation of deep exploration wells Alessandra-01 and Corinna-01. Carlo-01 well, to the south, has been drilled in the Tertiary sequence and reaches the Upper Cretaceus. The Pliocene starts with the Early Pliocene (G. Puncticulata zone) in the Alessandra-01 and Carlo-01 wells and is clearly evidenced by the related seismic unconformity. The base of the Late Pliocene-Pleistocene succession is located at a depth from about 1 to 1.5sec. TWT. The extensional tectonics, generating Mesozoic highs and intervening basins, has experienced a general inversion since the Paleogene. The compressional events developed up to the Late Miocene and Early Pliocene and the Pliocene unconformity indicates the end of the deformation (Figs. 13, 14). In the M17-C profile, a significant thrust ramp occurs just south the Corinna-01 well. This ramp is verging to the north and can be interpreted as the reactivation of former lateral ramp of E-NE verging thrusts. Similar reactivations have been evidenced also for Late Neogene thrusts deforming the foredeep in the Po Plain (e.g. Capozzi and Picotti, 2010). The BP4-83 profiles cross the area connecting the M17-C profile and the Bonaccia site and are useful to shed light on the structural

geometry under the gas field (Fig. 14a and b). The line BP4-83-01 shows gentle deformation due to compressional structures with opposite vergences (Fig. 14b). These structures can be interpreted as a tectonic inversion of Mesozoic extensional structures. The line BP4-83-05 also shows a thrust, verging to NW (Fig. 14b), which can confirm the deformation highlighted in the M17-C profile. The Bonaccia wells, cutting through the Plio-Pleistocene and the gas field (Fig. 14a, b) have evidenced that gas accumulation occurs above the previously described structural inversion. Gas is trapped above the structural highs, where the inherited geometry led to different compaction triggering fluid migration within the Pleistocene succession. The isotopic composition of the gas in the Bonaccia field indicates a biogenic origin (Lindquist, 1999) and leakage from deeper gas sources up to the Plio-Pleistocene is not evident. 6. Discussion This study deals with the characterization of some seafloor carbonates in the Adriatic Sea, to compare the MDAC recovered in the proximity of the Bonaccia gas field with other concretions widely distributed in the basin and tentatively associated to gas seeping. The different methodologies applied in this study have revealed that it is possible to distinguish the occurrence of MDAC on the basis of their mineralogical and isotopic characteristics. In particular, among the samples investigated, true MDAC likely belong only to the Bonaccia site in the central Adriatic, whose samples are made up by siliciclastics cemented by carbonates with distinctive negative δ 13C values (Table 2). This indicates that significant amounts of methane-derived carbon had contributed to the precipitated cement. The fine-grained sediments were cemented below the sea floor within the Sulphate Methane Transition Zone (SMTZ), where methane is anaerobically oxidized by microbially-mediated processes (Paull and Ussler, 2008). This interpretation can be supported by the occurrence of gas charged sediments at a depth likely corresponding to the present SMTZ, just few meters below the outcropping MDAC of the sample BGAf_3, (inset in Fig. 3). The studied carbonate concretions could have experienced different cycles of MDAC formation and exhumation, related to the Late Pleistocene sea level oscillations, because the Bonaccia site was subaerially exposed during lowstand periods. The present precipitation of carbonates can be shifted down within the sedimentary pile, in response to the new highstand equilibrium (Paull and Ussler, 2008). This is likely supported by the absence of Chemosynthetic Biological Communities (CBC) that need methane flux to the sea floor. Among the evidences that can confirm the role of the methanederived carbon in the formation of the Bonaccia carbonates with negative δ 13C, there is the black coating due to organic matter coupled with the presence of glauconite and iron sulphides, which form in reducing conditions. The different compaction of the Pleistocene succession, above the structural highs formed during the last compressive events from the Late Miocene to the Early Pliocene, is likely responsible for gas accumulation, mainly at a depth of 1.2 sec (TWT) as evidenced in the seismic profiles (Figs. 13,14). The gas field shows numerous leakages that reach the seafloor likely depending on geometry of the sedimentary layers, grain-size distribution and/or minor surficial faulting (Fig. 13, bottom). This is confirmed in reserved 3D seismic lines (ENI S.p.A.) acquired in the area of the gas field. The other carbonates recovered from the northern and southern sectors of the Adriatic basin, cannot be strictly correlated to gas seeping. There is no evidence of methane-derived carbon in the carbonates, which exhibit δ 18O and δ13C values characteristic of marine cements (Nelson and Smith, 1996). These carbonates are mainly composed by organisms typically recovered in the Adriatic coralligenous assemblages and their mineralogical composition indicates a dominance of high Mg-calcite which is a common phase in these assemblages (Bosence, 1991). However, the carbonates of the southern Adriatic basin have

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been recovered at over 200 m water depth, which cannot receive the necessary amount of irradiance for the growth of coralline algae. During the last glacial maximum the sea level was about120 m lower than present, possibly allowing the algal colonization also in this part of the basin. SEM observation has revealed the occurrence of bacterial activity likely performed by consortia that have been widely described in the formation of biogenic carbonates, microbialites and ooids (e.g. Folk and Lynch, 2001) under normal marine condition. A particular case is the MED_03 crust recovered at over 300 m water depth in the southern Adriatic, mainly composed by microcrystalline carbonates. SEM analysis on this sample has revealed diffused mucous film (Fig. 11c) similar to bacterial Extracellular Polymeric Substances (EPS) (Ushatinskaya et al., 2006; Zaitseva et al., 2006). Carbonates in the Mediterranean that show these characteristics have been found also at water depth of more than 500 m and associated with mud volcanoes and pockmarks (Gontharet et al., 2007). Furthermore, positive δ 13C have been interpreted as the signature of carbonates precipitated in equilibrium with bottom seawater (Allouc, 1990; Aloisi et al., 2000). However, positive δ13C can be recorded in low to high-magnesium calcite precipitating in presence of sulphate-reducing bacteria activity and in normal marine salinity ambient (Douglas, 2005). The occurrence of methanogenesis can lead to the same isotopic fingerprint (Mazzullo, 2000). In the Adriatic carbonates mineralized cell-like bodies and mucous sheets have been interpreted as of bacterial origin supporting the hypothesis that their formation can be associated with methanogenesis and/or sulphate-reduction within the upper sulphate reducing zone just below the sediment-water interface.

7. Conclusion Carbonate concretions have been recovered in the Adriatic basin during different research cruises in the past years. The Bonaccia site is an important gas field located in the northern central Adriatic Sea where there are information about the present geologic setting, by means of the available seismic lines and deep exploration wells. Seismic profiles interpretation reveals that carbonate concretions and active gas leakage occur mainly in correspondence at the sea floor above the reservoir, which is located at a depth of 1.2sec (TWT) in average. Samples collected in the Bonaccia site are made up of fine-grained sediments cemented by Methane-Derived Authigenic Carbonates (MDAC). Their formation can be ascribed to carbonate precipitation occurring within the Sulphate-Methane Transition Zone (SMTZ) and possibly developed in different phases during the Late Pleistocene sea-level oscillations. In fact, the Bonaccia site was subaerially exposed during periods of glacial low sea-level. None of the other Adriatic carbonate samples give evidence of contribution of methane-derived carbon, due to their isotopic signature, and are mainly of biogenic origin. However, micro-morphological and compositional evidences, like those for MED_00 sample, suggest a contribute by microbiological activity to their formation in presence of “local” sulphate reduction and methanogenesis, developing in sediments with high contribution of normal marine carbon source and low organic carbon.

Acknowledgements We wish to thank Prof. Luis Menezes Pinheiro and Dr. Vitor Hugo Magalhães for the fruitful discussion on the topic of this paper and for supporting the re-processing of the deep crustal seismic line of the CROP Project and the SEM-EDAX analysis, at Marine Geophysics Laboratory of the University of Aveiro, Portugal. We are indebted with ENI Spa (Italy) for the permission to use the offshore seismic data. Funding provided by the PRIN 2009 Project (University of Bologna, research grants to Prof. R. Capozzi).

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