Venting and seepage systems associated with mud volcanoes and mud diapirs in the southern Tyrrhenian Sea

Marine Geology 347 (2014) 153–171

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Venting and seepage systems associated with mud volcanoes and mud diapirs in the southern Tyrrhenian Sea Marzia Rovere a,⁎, Fabiano Gamberi a, Alessandra Mercorella a, Heba Rashed b, Andrea Gallerani a, Elisa Leidi a, Michael Marani a, Valerio Funari c, Gian Andrea Pini d a

Istituto di Scienze Marine, Consiglio Nazionale delle Ricerche, Via P. Gobetti 101, 40129 Bologna, Italy Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, 50121 Firenze, Italy Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Piazza di Porta San Donato 1, 40126 Bologna, Italy d Dipartimento di Matematica e Geoscienze, Università degli Studi di Trieste, Via Weiss 2, 34128 Trieste, Italy b c

a r t i c l e

i n f o

Article history: Received 15 October 2012 Received in revised form 15 November 2013 Accepted 20 November 2013 Available online 26 November 2013 Communicated by D.J.W. Piper Keywords: cold seep pockmark gas plume burrow authigenic carbonate siderite

a b s t r a c t High resolution swath bathymetry and backscatter data, seismic CHIRP profiles, multibeam water column acoustic measurements and sediment samples were collected on a cold seep province in the southeastern Tyrrhenian Sea, at a water depth of 500–1000 m. The mud volcanoes, characterized by a high backscatter signature, are the site of gas venting at the seafloor that formed a 630-m-high plume in the water column. The mud volcanoes feature a precipitation of iron-oxy-hydroxide crusts and pyritized and Sulfur burrows in the sub-surface and authigenic siderites, also cementing burrows, further down, showing a sharp transition from the oxic zone toward the sulfate-methanogenic zone. The mud flows are characterized by an intermediate backscatter seafloor and by the presence of gas in the sediment only 2 m below the seafloor. The mud flows consist of 1-m-thick drapes of water-rich mud extending downslope from the mud volcanoes. They act as sealing layers that prevent large fluxes of gas venting at the seafloor (low venting) and favor oxic conditions close to the sediment–water interface and the abundant precipitation of post-oxic siderites a few meters below the seafloor. The mud diapirs are characterized by a low backscatter seafloor and large fields of pockmarks. In coincidence with the normal faults, organogenic carbonate crusts form at or very close to the seafloor and are associated with chemosymbiontic bivalves (lucinids). The youngest shells are AMS radiocarbon dated 640–440 BP, suggesting that the seepage activity may have been clogged by the carbonates, only very recently. Similarities between the normal faults in the study area and the tectonic setting of the inland Calabrian Arc show that normal faults can control the location of fluid pathways and, probably, also the rising of the mud diapirs. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Subsurface circulation and surface emission of geofluids are well known processes occurring both on land and offshore. When they are associated with the rise of solid material, they can originate mud volcanism, of which terrestrial examples had already been found by the ancient Greeks. These were successively described in the late XVIII century by the early land geologists who witnessed mud eruption episodes in Europe (e.g. Spallanzani, 1793), in the Carpathians and in the remote territories of the Russian empire (see Kopf, 2002). On the contrary, submarine deep sediment remobilization and cold seeps are a more recent discovery, whose study and understanding have grown concomitantly with the increasing resolution of the marine exploration techniques, especially the sonar seabed mapping. As a consequence, the number of the newly discovered cold seeps, since when pockmarks ⁎ Corresponding author at: Istituto di Scienze Marine, Via P. Gobetti 101, 40129 Bologna, Italy. Tel.: +39 051 6398861; fax: +39 051 6398940. E-mail address: [email protected] (M. Rovere). 0025-3227/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.margeo.2013.11.013

were first imaged by King and MacLean (1970) on the Nova Scotia margin with side-scan sonar data, is continuously updated (Milkov, 2000; Judd and Hovland, 2007; Huuse et al., 2010; Anka et al., 2012). High resolution seafloor mapping has revealed fluid-rich structures also in the Central Mediterranean area: in the Adriatic Sea (Geletti et al., 2008), on the carbonate platform of the Malta plateau (Savini et al., 2009), in the eastern Sardinia continental slope (Dalla Valle and Gamberi, 2011). For many of the Mediterranean region examples, the relationships between mud tectonics and the distribution of the Messinian evaporites are supported by increasing evidence indicating that the mud diapirism is initiated in sediment lying below the evaporites (Camerlenghi and Pini, 2009) and favored by the presence of normal fault systems (Gamberi and Rovere, 2010; Capozzi et al., 2012a). Cold seeps are also hotspots for increased biological activity, where chemosynthetic communities rely on the consortia of methaneoxidizing archaea and sulfate-reducing bacteria (Paull et al., 2005). These biomediated reactions increase carbonate alkalinity in the form of dissolved inorganic carbon, which can lead to the precipitation of carbonate minerals and limit the methane flux toward the sediment–water

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interface (Borowski et al., 1996). Thus, when the fluid flow is vigorous it produces mud volcanoes and hydrocarbon plumes in the water column; however, when the flux is slow it forms authigenic carbonates near the seabed (Talukder, 2012). This paper outlines the advancements in the characterization of a cold seep province in the Paola Ridge, along the NW Calabrian margin (SE Tyrrhenian Sea), with respect to the results presented in Gamberi and Rovere (2010), that were based on bathymetry and backscatter data, on the single channel seismic profiles acquired in 1999 and on a few regional 30 kJ seismic sparker data acquired in the late 1960s. New high resolution bathymetry and backscatter data, CHIRP profiles, multibeam water column acoustics and direct seafloor samplings were acquired in 2011. By combining the interpretation of seafloor backscatter and the outcomes of the analysis on sediment samples, we distinguished among: (i) high backscatter areas where gas venting is active, (ii) areas of intermediate backscatter where the gas is not vigorously venting at the seafloor, and (iii) areas characterized by large fields of pockmarks and low backscatter, where ceased seepage activity is shown by the presence of authigenic carbonates associated with chemosymbiontic fauna, buried below hemipelagic mud. Finally, we highlight the possible relationships between the general geodynamic setting and the normal faults affecting the cold seep structures in the study area. 2. Regional setting The study area is located along the Paola Ridge, a NNW–SSE 60-kmelongated anticline that confines the Paola Basin westward (northwestern Calabrian margin), in the southeastern Tyrrhenian Sea (Fig. 1a). The Paola Basin area forms the rear of the Calabrian Arc, in the upper plate of the Ionian subducting slab (see Faccenna et al., 2011; Fig. 1b). On land,

the Calabrian Arc is dissected by extensional rift basins such as the Crati Graben (Fig. 1a), where ≈ 1 mm/a E–W extension is registered by GPS measurements (D'Agostino et al., 2011). A set of normal faults run parallel to the coast and lower the Paola Basin with respect to the Coastal Range, that crops out in the coastal area (Fig. 1c). The Paola intraslope basin belongs to the peri-Tyrrhenian basins formed due to the extensional tectonics that led to the opening of the Tyrrhenian Sea, as a consequence of the roll-back of the Ionian subducting slab (Malinverno and Ryan, 1986). These basins are filled by up to 5 km of a succession consisting of Tortonian–Messinian Kastens et al. (1990) or Serravallian (Mattei et al., 2002) to Pleistocene sediments. Deep data documenting the structural setting of the Paola Basin are limited to some 30 kJ sparker profiles, that penetrate the sedimentary succession down to the “M-reflector” (about 1.5–2 s T.W.T.). This is recognized throughout the Mediterranean as an erosional unconformity that marks the base of the Plio-Pleistocene sequence, often referred to as the top of the Messinian evaporites in the Tyrrhenian Sea (Fabbri and Curzi, 1979). However, seismic correlations are made uncertain by the lack of boreholes in the immediate surroundings of the study area. At sea, the Paola Ridge is bounded to the north by an E–W-oriented ridge, belonging to the Glabro and Palinuro volcanic complexes (Fig. 1a). On land, the ridge is bordered by the Sangineto Line (Fig. 1a), a tectonic alignment with a poorly defined character, being interpreted either as a sinistral strike slip and a normal fault, bounding the southern Apennines to the south and which has been active since the Tortonian times (Amodio-Morelli et al., 1976; Scandone, 1982). On the basis of the seismic sparker data, the Paola Ridge was interpreted as the result of an early Pleistocene episode of tectonic contraction (Argnani and Trincardi, 1988), preceded by continental rifting and followed by limited extensional faulting. SISTER-11 (Pepe et al.,

Fig. 1. a) Bathymetric map of the Paola Ridge area in the Paola Basin. b) Regional sketch of the Calabrian Arc subduction zone (from Faccenna et al., 2011) with the extensional corridor (slash field) hypothesized in this paper. c) Topography–bathymetry profile of the northwestern Calabria margin.

M. Rovere et al. / Marine Geology 347 (2014) 153–171

2010) and the CROP M27 multichannel seismic lines (Fig. 1a), all striking approximately the same direction, perpendicular to the NW Calabrian margin, represent additional, more recent available information. Pepe et al. (2010) found no evidence of significant normal faults affecting the Messinian to Quaternary sediments in the NW Calabrian margin, which may have experienced continuous shortening and subsidence. Milia et al. (2009) interpreted the Paola Basin as being initially formed as an extensional basin and successively affected by the activity of NW–SE strike-slip faults. Gamberi and Rovere (2010) interpreted the Paola Ridge as the expression of a mobile mud belt

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(comprised of diapirs D1, D2, and D3 and mud volcanoes MMV and RMV) connected with a set of extensional faults trending NW–SE to NNW–SSE (Fig. 1a). 3. Methods During the MVP11cruise, carried out onboard the R/V Urania in the Paola Ridge area in the period from August 25th to September 8th 2011, a Kongsberg EM710 (70–100 kHz) swath bathymetry system was used. Bathymetry (Fig. 2a) and backscatter data (Fig. 2b) were

Fig. 2. a) 3D view of the new bathymetry acquired with the EM710 multibeam system. Dashed lines trace the high backscatter areas. b) Kongsberg EM710 and c) Simrad EM12 backscatter draped over the bathymetry.

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processed with the CARIS HIPS & SIPS suite (version 7.1) and sound velocity was corrected with different casts of a Seabird 911 conductivity, temperature, depth (CTD) profiler. A test of water column acoustic measurement was performed above the summit of the RMV mud volcano at a water depth of 730 m (Fig. 1a), using the EM710 to achieve 3D mapping of bubble plumes. Water column data were processed with the Fledermaus Midwater module (Quality Positioning Services B.V.). This tool shows water column data with their exact position in space and time, preserving relevant information, such as time-based navigation and attitude (roll, pitch and heave corrections). The time series were sub-sampled during format conversion, yet preserving the full resolution of the water column features. Seismic sub-bottom data were collected with the Teledyne BENTHOS III CHIRP system having a frequency modulation of 2– 20 kHz. The CHIRP systems allow a penetration of about 50–100 m in the sediment succession, with a theoretical vertical resolution of 10 cm. The highest backscatter areas were selected as high-priority seafloor sampling targets, other sampling stations were identified according to the onboard preliminary interpretation of the newly acquired multibeam data and CHIRP profiles. Onboard it was not possible to correctly process the new backscatter data with CARIS HIPS & SIPS version 7.0, that was available at the time. Therefore, we relied on the old Simrad EM12 backscatter data (Fig. 2c) to select the sampling stations. More than 30 sampling stations were performed (Table 1). For each of the sampling stations, both a gravity corer, capable of penetrating up to 12 m below a soft-sediment seafloor, and a box corer,

to collect the preserved water–sediment interface, were used (Fig. 1). The recovery of the coring device was video recorded in order to document eventual gas emission close to the sea surface or during its early onboard storage and initial observation and description (Table 1). 14 C AMS radiocarbon dating was performed on a number of benthic samples (lucinids) at the Beta Analytic Inc. Laboratories in Miami (U.S.). Ages were calibrated using the Calib 5.0.2 Radiocarbon Calibration Program (Stuiver and Reimer, 1993). The results were adjusted for local reservoir correction (ΔR = −18 ± 60). Bulk mineralogy of the carbonates was analyzed by X-ray diffraction on powdered samples, using the X-ray Philips PW 1050/37 diffractometer, with a Cu anode, a graphite monochromator and with 2°/min goniometry speed, in a scanning range of between 5° and 70°, applying 40 kV accelerating voltage and 20 mA current and the X'Pert PRO Philips acquisition system. Seismic images, from a set of 1 kJ sparker profiles (0.1–2 kHz) acquired in 1987 aboard the CNR R/V Bannock (Fig. 1a), were interpreted using the structural modeling software MOVE 2011 (Midland Valley Exploration Ltd.). The sparker data were available only on a small number of structures (D1, D2, R1MV, NFD3, NFD4, Fig. 1a). A 3D model was built through the reconstruction of several regional unconformities mapped in the seismic data. Two recent basin-wide unconformities were picked from Trincardi et al. (1995): the green drape which corresponds to a sediment drape (condensed section) and inferred to represent a trangressive to highstand deposit, tentatively dated to mid-Pleistocene; the red drape is a several-meter-thick high amplitude reflector, deposited during the sea level rise and high stand related to Isotopic Stage 5.

Table 1 Description of the coring sites during sampling operations. HB = high backscatter, WC = water column, CH = coring head. The seismic facies of the CHIRP profiles at coring stations are also listed. Long ° N

Lat ° E

Sampling station

15.6470 15.6470 15.6481

39.1766 39.1767 39.1787

GC01 GC02 GC03

15.6190 15.6250 15.6171 15.6293 15.6304 15.6466 15.5755 15.5902 15.4762 15.4908 15.7382 15.7330 15.7582 15.6469

39.1888 39.1352 39.1258 39.1431 39.1873 39.1743 39.1303 39.2504 39.3164 39.3918 39.1933 39.1963 39.1918 39.1766

15.6790 15.7196 15.7266 15.7368 15.6933 15.7879 15.8151 15.7239 15.5023

Depth (m)

Length (m)

Site

Sampling description

Seismic facies

727 727 726

1 3.54 3.96

RMV HB RMV HB RMV HB

Transparent Transparent Transparent

GC04 GC05 GC06 GC07 GC08 GC09 GC10 GC11 GC12 GC13 GC14 GC15 GC16 GC17

808 728 729 709 760 728 860 922 1055 717 530 557 521 728

4.00 3.34 4.00 5.72 5.12 3.80 5.00 5.85 5.85 5.86 4.90 5.27 0.20 6.00

RMV out of HB MMV HB MMV out of HB MMV HB RMV mudflow RMV HB RMV mudflow Propeller ridge R1MV mudflow R1MV HB D1 D1 D1 fault scarp RMV HB

39.2090 39.1149 39.0985 39.0945 39.0597 39.0405 39.0492 39.0917 39.4096

GC18 GC19 GC20 GC21 GC22 GC23 GC24 GC25 GC26

750 731 653 634 677 680 679 694 576

4.97 5.76 0.60 4.43 5 5.79 5.80 1.00 5.79

D1 SW scarp D2 D2 fault scarp D2 D3 NFD1 NFD1 D2 fault scarp R1MV HB

15.5122 15.5348

39.3965 39.3723

GC27 GC28

570 552

5.78 3.12

R1MV HB R1MV HB

15.5173 15.4926 15.5069

39.3533 39.3319 39.3956

GC29 GC30 GC31

707 957 617

5.52 5.40 5.79

R1MV mudflow R1MV mudflow R1MV HB

Degassing in WC, mud expulsion from CH, H2S smelling Degassing in WC, mud expulsion from CH, H2S smelling Degassing in WC, mud expulsion from CH, H2S smelling, carbonates No degassing, no smelling Degassing in WC, mud expulsion from CH, H2S smelling No degassing, no smelling Degassing after retrieval on deck, authigenic carbonates Degassing after retrieval on deck, H2S smelling Degassing in WC, mud expulsion from CH, H2S smelling Degassing after retrieval on deck, carbonates No degassing, no smelling Degassing after retrieval on deck, carbonates Degassing after retrieval on deck, H2S smelling No degassing, no smelling No degassing, no smelling No degassing, authigenic carbonates Degassing in WC, mud expulsion from CH, H2S smelling, carbonates No degassing, no smelling No degassing, no smelling No degassing, authigenic carbonates, lucinids No degassing, no smelling No degassing, no smelling No degassing, no smelling No degassing, no smelling No degassing, authigenic carbonates, lucinids Gas bubbles at the sea surface, mud expulsion from CH, H2S smelling, authigenic carbonates No degassing, no smelling Gas bubbles at the sea surface, mud expulsion from CH, H2S smelling No degassing, H2S smelling Degassing after retrieval on deck, authigenic carbonates Degassing after retrieval on deck, H2S smelling

Transparent draped by reflections Transparent Transparent draped by reflections Transparent draped by reflections Reflective with ascending transparent masking Transparent Reflections with ascending transparent masking Reflective Reflective with ascending transparent masking Transparent thin drape Hummocky Hummocky Hummocky Transparent Hummocky Hummocky Hummocky Hummocky Hummocky Hummocky Hummocky Hummocky Transparent thin drape Reflective Transparent Transparent thin drape Transparent thin drape Reflective

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4. Results 4.1. Mud volcanoes 4.1.1. RMV and MMV The RMV and MMV mud volcanoes are connected by a common area of high backscatter comprised between − 20 and + 5 dB (Fig. 2b, c). Pockmarks (diameter b 100 m) are rare on the MMV, and almost absent on top of the RMV (Fig. 3a). A concentration of pockmarks is present at the base of the southern slope of MMV, in coincidence with the headwalls of shallow-seated slides (Fig. 3a). A CHIRP profile crossing the two mud volcanoes (Fig. 3b) shows that they are characterized by a deep acoustic transparent zone that interrupts the sub-bottom reflections of the structures' flanks. The transparent facies is overlain by a few reflections in the MMV, whereas it reaches the seafloor at the top of the RMV (Fig. 3b). Several sites have been sampled over the highest backscatter area on top of the RMV and MMV mud volcanoes (GC01-02-03-05-07-09-17, Table 1). During core recovery, all the gravity cores were observed to strongly degas in the water column, when close to the sea surface. The top section of the gravity cores was thrown out, before reaching the sea surface. The presence of gas within the sediment was also directly evidenced by a strong H2S smell. When retrieved to the deck, the box cores still contained gas and long-lasting intense bubbling was present during the sub-sampling operations. Cracks due to gas expansion can be found at different levels within the cores (Figs. 3c, l). In some cases, a completely fluidized mud was found in some sections of the cores (Fig. 3c). The preservation of the water–sediment interface in the box cores displayed the presence of iron oxy-hydroxide crusts at the seafloor and in the upper part of the sedimentary column (a few centimeters below the seafloor). The iron oxy-hydroxide crusts were very abundant in all the box cores (Fig. 3d) and sometimes showed fresh burrows (Fig. 3e). Abundant pyrite crusts and pavements with disseminated crystalline Sulfur (Fig. 3f) and small pyritized burrows (Fig. 3g) were also collected within the box cores over the top of the RMV structure, 10 to 50 cm below the seafloor. Tubular authigenic carbonates (siderite, FeCO3) were found further down within the core sections or in the core catcher, about 6 m below the seafloor (Fig. 3h, i). The few cores and box cores performed on top of the MMV registered a lower degree of oxidization at the water–sediment interface with respect to the RMV, no oxy-hydroxide crusts or pyritized burrows were found. Post-processing of the multibeam water column acoustic data revealed a gas flare (Fig. 4a) that almost reached the sea surface above the summit of the RMV mud volcano (Fig. 4b). The associated CTD cast showed mixing in the water temperature in the first 100 m of water depth with values rapidly changing from 26 to 14 °C, before registering a normal trend (Fig. 4c). Furthermore, density values remained almost constant until 100 m water depth; above this depth, values changed rapidly from 1 to 1.16 t/m3 (Fig. 4d). 4.1.2. R1MV The presence of a further mud volcano, here termed R1MV (Fig. 2a), was discovered with the data acquired during the MVP11 cruise. R1MV is a flat-top structure and rises about 500 m above the adjacent seafloor (Fig. 2a). High backscatter of the seafloor is present only in the western slope of R1MV (Fig. 2b, c). Pockmark fields are almost absent on the R1MV, except for a 100-m-wide depression (Fig. 5a). Several cores were collected in this area (GC13-26-27-28-31, Fig. 5a). During core recovery, mud was expelled from the coring head in several test sites (Table 1). Moreover, a strong H2S odor was smelt when the cores were taken onboard. The upper part of the GC28 core (Fig. 5a, b) consists of a completely disrupted section of dark gray mud, intercalated with deformed tephra layers (Fig. 5b). The smell of the dark mud very closely

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resembled that of crude oil when the core was opened in the laboratory. In the same station, the uppermost sedimentary succession, preserved exclusively in the box corer, consists of burrows filled with pyrite (Fig. 5c) and crystalline Sulfur (Fig. 5d). The GC26 core (Fig. 5e), located further north (Fig. 5a), shows gray mud completely disrupted by expansion cracks, alternating with several oxidized levels and chemical alteration through the entire section. It is unclear whether a thin transparent drape or layered reflections are present at the seafloor in the CHIRP profile over the GC26 (Fig. 5f). However, in places these reflections are disrupted by a transparent facies, sometimes organized in vertical columns, possibly representing ascending gas (Fig. 5f). The GC13 core (Fig. 5g), located downslope of the GC26 (Fig. 5f), was only associated with gas expulsion once it was retrieved on deck. The CHIRP profile (Fig. 5f) shows a transparent thin drape at the seafloor. The upper section is comprised of apparently undisturbed gray cohesive mud, yet the typical upper hemipelagic soft brown mud is missing (Fig. 5g). Further down, evidence of expansion cracks left by the gas is present. The nearby GC27 and GC31 cores (Fig. 5a), collected where the CHIRP profile shows an undisturbed layer of reflections and far from a vertical transparent facies represented by probable ascending gas (Fig. 5i), show no expansion cracks and no upper hemipelagic soft brown mud (Fig. 5h, l). 4.2. Mudflows Two narrow bands of intermediate-to-high backscatter (− 40/ − 30 dB) extend downslope from both the RMV and the R1MV (Fig. 2b, c). While the feature descending from the western slope of the RMV had already been interpreted by Gamberi and Rovere (2010) as a mudflow, the feature downslope from R1MV was supposed to be due to mass-wasting processes. All the coring stations along the mudflows were not associated with degassing in the water column and none of them was associated with the expulsion of the coring head (Table 1). Gas started to be slowly expelled only after the cores were retrieved on deck, the sediment cores “self-extruded” during the core opening operations and sectioning and for several days afterwards, during their storage in the fridge cell at −20 °C. 4.2.1. RMV mudflow In the distal part of the RMV mudflow, CHIRP data evidence a transparent facies close to the seafloor (Fig. 6a), where the GC10 core (Fig. 6b) was recovered and where the backscatter is between − 40 and −30 dB (Fig. 6c). The GC10 core shows an upper undisturbed and brown oxidized part and a lower part with expansion cracks. Tephra layers show alterations and unusual green colors (Fig. 6b). In the core catcher of the GC10 core, tubular authigenic siderite was found (Fig. 6d), approximately 6 m below the seafloor. No cores are available along the proximal part of the RMV mudflow, where the backscatter intensity is lower, between −45 and −35 dB. A CHIRP profile shows that several transparent bodies of comparable thickness (less than 1 m) are present at the seafloor (Fig. 6e). 4.2.2. R1MV mudflow In the distal portion of the R1MV mudflow, CHIRP data show possible evidence of gas ascending toward the seafloor (Fig. 7a). Here the backscatter is also comprised between −40 and −30 dB (Fig. 7b) and the GC12 core was recovered (Fig. 7c). The GC12 core (Fig. 7c) and the GC30 core (Fig. 7e) are very similar to the GC10 (Fig. 6b). Expansion cracks start only 2 m below the seafloor. Strongly oxidized tephra layers and very thin fine sandy turbidites are intercalated within the cohesive gray mud. Toward the bottom of the recovered sections, tephra layers show their normal light yellow/gray colors (Fig. 7c). Tubular authigenic siderite was found approximately 6 m below the seafloor in the GC12 core catcher (Fig. 7d). In the core station GC29 (Fig. 5f), located further upslope where the intensity of the backscatter is lower (−45/−35 dB),

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Fig. 3. a) Close inset of the 20-m-resolution DTM and b) CHIRP profile over the MMV and RMV mud volcanoes. Location of the profile displayed also in Fig. 1a. c) Core GC17; d) iron oxyhydroxide crusts and e) pyritized burrow in slightly cemented mud, rich in iron oxyhydroxides, found in the box cores on top of RMV; f) pyrite crusts and pavements with Sulfur and g) small pyritized chimneys found in the box cores on top of RMV; h) authigenic carbonates found in the core catcher of GC17; i) tubular authigenic carbonate found in the coring head of GC17; l) core GC07.

sedimentary bodies are preserved at the seafloor (Fig. 7a). In the upper section of the GC29 core (Fig. 7f), a mixing of water-rich gray and brown mud is visible, no expansion cracks in the lower section of the core are present, siderites in the form of burrows were found 2 m below the seafloor (Fig. 7g). Several pyrite crusts were found in the lower section of the core (Fig. 7f).

4.3. Mud diapirs 4.3.1. D1 D1 is a WNW–ESE trending structure measuring 11 km in length and with a relief of 300 m (Fig. 2a). On the crest of the D1, normal pockmarks (sensu Hovland et al., 2010), having diameters of tens of meters,

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Fig. 4. a) Fan view showing the gas plume in the swath range; b) 3D Fledermaus scene of the bathymetry and gas plume in the water column; c) temperature and d) density profiles acquired during the CTD cast. Location of the water column measurement is given in Figs. 1a and 3b.

are aligned along the same trend of the structure. They are from 20 to 150-m-wide and 1 to 10-m-deep (Fig. 8a). N–S-elongated pockmarks are also present (Fig. 8a). Pockmarks are also present in the northwestern portion of the structure, where a steep escarpment, probably corresponding with a normal fault, is visible (Fig. 8a, b). The central part of the crest of D1, where pockmarks are concentrated, is characterized by a rough seafloor corresponding to a hummocky facies in the CHIRP profiles (Fig. 8b), underlain by bright reflections with a hyperbolic shape. Below, an acoustic transparent zone interrupts the sub-bottom reflections on both sides of the hummocky facies (Fig. 8b). The CHIRP profile also shows that the eastern flank of D1 is affected by a set of NNW–SSE trending normal faults (Fig. 8b). Sometimes these breach the seafloor and their maximum offset is about 10 m. On the central axial portion of D1, bioturbated sediments, bearing no evidence of gas and topped by brown soft hemipelagic mud, were sampled at station GC14 (Fig. 8c), on the floor of a large pockmark field (Fig. 8a). The GC15 core, located nearby (Fig. 8a), shows the same characteristics of the GC14 (Table 1). On the contrary, along a small N–S trending fault in the eastern axial portion of D1, only 1.7 km away from GC14, the corer device could not penetrate below a hard substrate consisting of authigenic limestones developed at the seafloor at coring station GC16 (Fig. 8a). The limestone crusts consist of laminated bioclastic foraminifera-rich packstones. Benthic organisms (Serpulidae) encrust some portions of the authigenic limestones (Fig. 8d). A thin surficial coating of red iron oxy-hydroxides is apparent in some portions of the carbonate crusts (Fig. 8e). The intensity of oxidation and the amount of benthic fauna colonization are indicative of seawater exposure (Magalhaes et al., 2012). An undisturbed sedimentary succession, consisting of green mud and chemical alterations on the thin tephra layers, was collected in the GC18 coring station (Fig. 8f). The GC18 is located over the steep escarpment that bounds the northwestern tip of D1 (Fig. 8a). About 2 m below the seafloor, fragments of chemosymbiontic (sulfate-oxidizing bacteria) bivalves (lucinids) and decapodan crustaceans (Calliax lobata) were found in GC18.

underlain by a deeper wider area showing an acoustic transparent facies (Fig. 9b). In the core station GC20 (Fig. 9a, c), authigenic limestones and a fauna association comprising an extraordinary abundance of fragments of Calliax lobata and loose and disarticulated shells of lucinids were sampled below about 60 cm of red highly bioturbated mud (Fig. 9d). GC20 has been collected in coincidence of the NW–SE fault dissecting D2 (Fig. 9c). The limestone crusts are gray and consist of bioclastic wackestones and packstones (Fig. 9e), with fragments of disarticulated lucinids and C. lobata (Fig. 9f). Trace fossils are evident in some carbonates (Fig. 9e), probably corresponding to the burrows of the decapodan crustaceans (Richard Callow, pers. comm.). AMS radiocarbon datings performed on the lucinids, found in the box cores close to the seafloor, gave a calibrated age of AD 1310 to 1510 (cal 640 to 440 BP), while the ones retrieved about 60 cm below the seafloor were dated at 11760 to 11390 BC (cal 13710 to 13340 BP). The GC21 core located in the same limb of the fault with respect to the GC20 (Fig. 9a) shows a 30-cm-thick carbonate pavement at a depth of about 2 m below the seafloor and several other limestones disseminated in the lower section of the core (Fig. 9g). Fragments of lucinids and other chemosymbiontic fauna assemblages were found disseminated in the core section during post-cruise sub-sampling. The GC25 core, recovered on the other limb of the fault (Fig. 9a), was halted by authigenic carbonates cementing disarticulated lucinid shells (Fig. 9h) after having penetrated 1 m of organic-rich mud (Fig. 9i). In the same station, the BC21 box corer retrieved living individuals of Calliax lobata, organogenic limestones with a vuggy and spongy fabric (Fig. 9l) and loose and disarticulated shells of lucinids. AMS radiocarbon datings of the lucinids, retrieved 1 m below the seafloor and associated with the carbonates, gave the calibrated age of 8240 to 7850 BC (cal 10200 to 9800 BP). In the lower portion of the D2 edifice (Fig. 9a, b), at distance from the faults, the GC19 core station (Fig. 9a) recovered highly bioturbated sediments, but no carbonates and associated chemosymbiontic fauna were found (Fig. 9m).

4.3.2. D2 Mud diapir D2 has an average relief of 300 m, is elongated for 6 km in a NW–SE direction and is composed of two ridges separated by a saddle (Fig. 9a). The saddle is likely originated by a NW–SE oriented normal fault, almost parallel to the main trend of D2 (Fig. 9a). Pockmarks are aligned along the major axis of the structure. Densely distributed pockmarks, 20 to 50-m-wide and 0.5 to 1-m-deep, prevail in its northern edge (Fig. 9a). In the central part of the structure, 100-m-wide and 5m-deep pockmarks combine along N–S to NWN–ESE trending normal faults to form elongated depressions (Fig. 9a). Similarly to D1, in the area with the largest pockmarks concentration, the CHIRP profile shows a hummocky facies at the seafloor

4.3.3. D3 Diapir D3 has a circular dome shape, with a diameter of 7 km and a relief of 200 m (Fig. 2a). It is punctuated by pockmarks, especially concentrated in its western flank, toward the slope (Fig. 10a). Some of the pockmarks have dimensions in the order of 100 m in diameter and 10 m in depth. The CHIRP profile over D3 shows the hummocky facies in coincidence with the pockmark field (Fig. 10b). In the GC22 sampling station (Fig. 10a) collected in the center of a 100-m-wide pockmark on top of D3, a sedimentary succession consisting of bioturbated mud was found, but, further down, vertically elongated or circular traces are present (Fig. 10c). They cannot be ascribed to trace fossils (Richard Callow, pers. comm.) and they are most probably related to gas escapes.

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Fig. 5. a) Close inset of the 20-m-resolution DTM over the mud volcano R1MV. b) Core GC28; c) pyrite and d) Sulfur-filled burrows sampled in the box cores on top of R1MV. f) CHIRP profile over cores GC26 (e) and GC13 (g). i) CHIRP profile over cores GC27 (h) and GC31 (l). Location of the seismic profiles is displayed also in Fig. 1a.

No sulfate-related fauna assemblages or shell fragments were found within the core nor in the box core collected in the same station. NFD1 to NFD4 diapirs were discovered during the MVP11 cruise. They are dissected by N–S trending normal faults and studded with E–W aligned pockmarks (Fig. 2a). Pockmarks are on average 10 m deep and have a diameter in the order of 50 m. The normal faults are traceable in the sparker profiles to a depth of 200 m. Only two samplings are available over NFD1 (Fig. 1a), GC23 and GC24 cores (Table 1), where the upper soft

brown hemipelagic mud is missing and a sedimentary succession made up of bioturbated mud is present below. 4.4. Structural setting The CHIRP and the sparker profiles allowed the mapping of an arched belt of N–S to NNE–SSW trending normal faults in the Paola Ridge, that are subdivided into different sectors bounded by active

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Fig. 6. a) CHIRP profile showing the seismic facies of the distal part of the RMV mudflow. b) Core GC10. c) Backscatter image of the mudflow. d) Tubular authigenic carbonates found in the core catcher of core GC10. e) CHIRP profile of the proximal part of the mudflow. Location of the seismic profiles is displayed also in Fig. 1a.

orthogonal normal faults (Fig. 11). The normal arched faults have a trend similar to the normal faults located along the coast and dated as Pliocene and Pliocene–Quaternary (Ambrosetti et al., 1983; Fig. 11). The N–S to NNE–SSW-oriented faults are confined to the south by a NW–SE oriented normal fault running along the central Propeller ridge that dissects the Propeller basin into two graben-like sectors and continues eastward also cutting the diapir D1 (Figs. 11, 12a, b). The fault is also related to a 10-km-displacement of the basin borders (Fig. 11). This fault aligns with the Sellia–Decollatura Fault Zone (SDFZ, Tansi et al., 2007, Fig. 11) recognized on land. To the north, the faults are interrupted along a high angle WSW–ENE oriented fault that confines the R1MV structure to the south (Fig. 13) and may be related to an eastern system of en-echelon faults that appears to be connected to the east with a normal fault located offshore Cetraro (Fig. 11) and possibly with the Sangineto Line on land (Fig. 11). In the southern sector of the extensional belt, NNW–SSE trending extensional faults are also evident, particularly to the east of D3 (Fig. 11). The R1MV is confined to the east by a steeply-dipping, N–S striking normal fault that can be traced in the sparker profiles to at least a depth of 1.5 s T.W.T. (Fig. 14). The fault defines a depression east of the R1MV, which can be interpreted as a graben (Figs. 11, 14). Several unconformities were mapped in the study area. The magenta reflector is the deepest angular unconformity in the area and seems to mark the beginning of the deep mud mobilization (Fig. 12b), with the consequent uplift of the diapiric structures. Above the magenta reflector, significant upward wedging of the blue reflectors suggests an interval of growth strata (Fig. 12b). The orange reflector registers the last pronounced deformation, most probably related to the uplift of the structures, and the beginning of a decreased uplift rate (Fig. 12b). The

green reflector marks the base of the post-deformation sequence, characterized by the absence of significant upward wedging of the reflectors, pointing to the end of the uplift of the structures (Fig. 12b). The dark green reflector corresponds to the green drape and the red reflector to the red drape in Trincardi et al. (1995) (Fig. 12b). The base of the post-deformation sequence is close to the base of the green drape deposit, which is related to a period of relative sea-level rise in the midPleistocene, which also reflects episodes of deactivation of the sediment entry points into the Paola Basin (Trincardi et al., 1995). A similar succession of unconformities can be traced in the area eastward of the R1MV, where high-angle normal faults confine the graben (Fig. 14). The same stratigraphic succession is visible across the D2 saddle (Fig. 15). Here, a growth package of strata can also be coeval to the diapir rising. Deformed reflectors flanking the diapiric ridge, between the magenta and the orange reflectors, can be related to the uplift phase (Fig. 15). The post-deformation sequence, above the green reflector, shows a more uniform thickness over the diapir and in the adjoining basin area (Fig. 15). 5. Discussion 5.1. Mud volcanoes: high venting sites All the mud volcanoes have a high backscatter seafloor pattern (Figs. 2b, c) and are characterized by a deep acoustic transparent zone, indicating the possible presence of shallow gas migration. In the CHIRP profiles, the transparent facies is either buried beneath a thin drape (5–10 m) of layered reflectors (MMV in Fig. 3b) or reaches the seafloor (RMV in Fig. 3b), suggesting that gas-charged sediments are

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Fig. 7. a) CHIRP profile showing the seismic facies of the R1MV mudflow; b) backscatter image of the mudflow; c) core GC12; d) fragments of tubular authigenic carbonates found in the core catcher of core GC12; e) core GC30; f) GC29; g) tubular authigenic carbonate found in GC29. Location of the seismic profile is displayed also in Fig. 1a.

present at the seafloor or very close to it. Areas of shallow and deeper seated gas are very close to each other (Fig. 3), showing that the conditions of fluid circulation and ascent may vary with a spacing of only hundreds of meters. Direct evidence of gas-charged sediment derives from the abundant expulsion of gas observed during coring (Table 1) and box coring operations. In addition, a vertical 630-m-high gas plume emanating from the seafloor has been imaged through the acquisition of water column data on top of the RMV. The flare ends at about a water depth of 100 m (Fig. 4). Even higher plumes have been recently imaged by multibeam systems off the northern California margin (Gardner et al., 2009) and in the western Svalbard (Bünz et al., 2012). When there is bubble emission from the seafloor, there will be acoustically detectable columnar features in the water column (Hovland et al., 2012). Therefore, we interpret the gas plume in our study area to be related

to the presence of free gas venting at the seafloor. Mixing patterns of the temperature (Fig. 3c) and density profiles (Fig. 3d), which were measured during a CTD cast over the RMV summit, were observed in the shallowest 100 m of the downcast. When bubbles rise through the water column, they can cause turbulence and, if strong enough, this can draw water from the surrounding seawater column into the upward rising gas bubble stream, causing an upwelling of bottom water, which can give rise to temperature anomalies (Hovland et al., 2012). We suggest that this process can account for the temperature anomalies we observed in the water column during the CTD cast. On top of the mud volcanoes, pockmarks are less abundant with respect to the mud diapirs (Fig. 2a). We also related this aspect to the presence of fast and continuous venting, which probably prevents the preservation of very well defined pockmarks. In this paper, in fact, we

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Fig. 8. a) Close inset of the 20-m-resolution DTM and b) CHIRP profile over D1. c) Core GC14. d) Carbonate crust with Serpulidae and e) with iron oxyhydroxide coating found in core GC16; f) core GC18. Location of the seismic profile is displayed also in Fig. 1a.

use the term venting for the sites characterized by vigorous fluxes, and seepage for slow fluxes sites, according to Talukder (2012). In the box cores collected over the mud volcanoes, iron oxyhydroxide crusts were found at the seafloor indicating that oxic conditions prevail there. Pyrite crusts and pavements, pyritized bioturbational structures and Sulfur-lined burrows were found further down in the sedimentary

succession (20–50 cm below the seafloor). These findings indicate that organic matter degradation by sulfate-reducing bacteria (Jørgensen, 1982) is occurring a few centimeters below the seafloor and that the methane–sulfate transition zone is very shallow in the area. Authigenic carbonates were collected exclusively at depths of 5–6 m below the seafloor, but less abundantly if compared to the sampling

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Fig. 10. a) Close inset of the 20-m-resolution DTM with a 5-m-resolution DTM over the GC22 sampling station and b) CHIRP profile over D3; c) core GC22. Location of the seismic profile is displayed also in Fig. 1a. Not all the cores are displayed in their entire lengths in Figs. 3–10.

stations in the mudflows (Fig. 6e). All the carbonates in the mudflows are siderites (FeCO3). When the sampling operations preserve their original shape, the carbonates show a tubular morphology (Figs. 3i, 6d, 7g), but no internal hollow conduits are visible. Fine lineaments trending both perpendicular and parallel to the tubules elongation are interpreted as wall scratch marks along burrows, in analogy with Campbell et al. (2010). The carbonate tubules are therefore the result of sub-seafloor precipitation of carbonate within burrows. In some cases, more complex morphologies result when the cementation is

extended in the sediment surrounding the trace fossils (Zorn et al., 2007), forming a concretion that encompasses the burrow (Fig. 3h). 5.2. Mudflows: low venting sites The high backscatter signal at the summit of the mud volcanoes continues as downslope elongated narrow tongues of intermediate to high backscatter signal (Figs. 2b, c). In the CHIRP data, they correspond to thin transparent bodies draping the slopes of both RMV and R1MV,

Fig. 9. a) Close inset of the 20-m-resolution DTM and b) CHIRP profile over D2. c) CHIRP profile across the D2 fault scarp. d) Core GC20; e) carbonate crust with trace fossil; f) fauna assemblage recovered at GC20 station; g) core GC21; h) carbonates and associated fauna assemblage recovered at GC25 station; i) core GC25; l) organogenic carbonate with fauna association found in BC21; m) core GC19. Location of the seismic profiles is displayed also in Fig. 1a.

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Fig. 11. Structural sketch of the Paola Ridge. Neotectonic faults in marine areas are from Ambrosetti et al. (1983); regional fault zones are from Van Dijk et al. (2000) and Tansi et al. (2007).

especially in their proximal parts (Figs. 6e, 7a). In the proximal part of the mudflows, backscatter shows intermediate intensity in coincidence of the thin transparent layers at the seafloor (Figs. 6c, 7b). On the contrary, the intensity of the backscatter increases in the distal parts of the mudflows, where CHIRP profiles suggest that there may be gas ascending toward the seafloor (Figs. 6a, 7a) and cores retrieved there (GC10, GC12, GC30) vigorously self-extruded on deck (Table 1). The cores have expansion cracks only in their bottom sections, meaning that gas was trapped about 2 m below the seafloor (Figs. 6b, 7c). No sulfides were found in the cores and box cores and siderites were found at 5–6 m below the seafloor. The few cores collected in the proximal parts of the mudflows, where the intensity of the backscatter is lower, are furthermore characterized by the mixing of the upper hemipelagic brown mud with water-rich gray mud and the retrieval of iron carbonates at 2 m below the seafloor and sulfides further down in the sedimentary column. The precipitation of FeCO3 inside burrows occurs predominantly in the area of the mudflows, probably because they can prevent the free emission of fluids at the seafloor, favoring deeper trapping in the sub-seafloor and the consequent authigenesis. We thus interpret the mudflows as sites of low gas venting, where active flow features and actively venting gas streams are absent at the

seafloor; the gas remains trapped within the sub-surface sediment at variable depths, but can sometimes reach closer to the seafloor, as also suggested by CHIRP seismic images (Figs. 6a. 7a). The areas, where the trapping of the gas is more effective, are located on the upper flanks of the mud volcanoes, where the draping of the fluidized water-rich mud descending from the summits of the mud volcanoes (Figs. 6e) most probably acts as a seal. The GC29 core (Fig. 7f) shows an inversion in the appearance of the mineralization products: siderites precipitated in burrows 2 m below the seafloor and are found above the sulfides, which are 3–4 m below the seafloor. This is the opposite of what is observed in the mud volcanoes and in the GC10 and GC12 cores, where the siderites were found at 5–6 m below the seafloor. We relate this to the different settings of fluid circulation and fluxes, which influence the depth of the methane– sulfate transition zone. Probably, carbonates in the GC29 can be ascribed to siderite formed in the post-oxic zone (Berner, 1981), while siderites in the mud volcanoes precipitated in the methanogenic zone. 5.3. Mud diapirs: inactive seepage sites In the Paola Ridge, the diapirs have abundant pockmarks but sampling operations did not reveal any clear proof of gas-charged

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Fig. 12. a) 30 kJ sparker profile across the fault zone of the central Propeller ridge. b) 1 kJ sparker profile across the fault zone of the central Propeller ridge. Location of the seismic profiles is given in Fig. 11.

sediments. However, strong bioturbation, authigenic carbonates and chemosymbiotic bivalves, ascribed to the genus Lucinoma, were found in the first 1–2 m below the seafloor in coincidence with fault scarps that dissect D1 and D2 (GC20, GC25 and GC16 coring stations, Figs. 2a, 8a, 9a).

The hydrogen sulfide in the lucinids' habitat is produced by anaerobic methane oxidation coupled with a sulfate-reducing consortia of archaea and bacteria (Borowski et al., 1996; Greinert et al., 2001), responsible for the precipitation of the authigenic carbonates that represent the substrate associated with the lucinids (Fig. 9h). The

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Fig. 13. 1 kJ sparker profile over the fault bounding to the south the R1MV structure. Location of the seismic profile in Fig. 11.

precipitation of authigenic carbonates associated with the seepage of methane-rich fluids is a well known and documented process (Greinert et al., 2001; Meister et al., 2011; Capozzi et al., 2012b; Magalhaes et al., 2012; Vanneste et al., 2012). The methane may be oxidized to CO and CO2 in the water column or may be precipitated to authigenic carbonate crusts on the seafloor or in the sub-seafloor (Suess et al., 2001), as studies in modern and ancient examples of cold seep sites have shown (Campbell et al., 2002; Conti et al., 2004). All the diapirs of the study area show abundant pockmarks. Pockmarks are commonly located in areas where gas is accumulated in nearsurface sediments (Judd and Hovland, 2007). Gas leakage through pockmarks often supports specific bacterial communities (see Nickel et al., 2012) and causes carbonate precipitation (Paull et al., 2005; Bayon et al., 2007; Campbell et al., 2010; Roberts et al., 2010; Capozzi et al., 2012b). In D1, the limestone crusts (Fig. 8d, e), cored in station GC16 in coincidence with a normal fault (Fig. 8a), form a pavement at the seafloor that inhibited core penetration in the sub-seafloor. The limestone crusts are coated by a layer of Mn and Fe precipitates, Serpulids encrustations (Fig. 6a) and coral stems, indicating that, after their precipitation probably due to gas seepage, the crusts were coated by Mn/Fe oxides and colonized by benthic organisms and thus exposed at the seafloor. In D2, carbonate crusts have been found buried below a variable thickness of hemipelagic organic matter-rich red mud (60 cm to 1 m). They are associated with fragments of disarticulated lucinid shells and decapod crustaceans. None of the lucinids was found alive. The youngest organisms are radiocarbon dated 640–440 BP. Ancient examples of fossiliferous wackestones with densely packed disarticulated bivalves, similar to the crusts of D2, are found in the fossil cold seeps of Miocene age of the Northern Apennines. They are

Fig. 14. 1 kJ sparker profile over the graben located eastward of R1MV. Location of the seismic profile in Fig. 11.

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Fig. 15. Intersection of two 1 kJ sparker profiles in the saddle of the D2 diapir (view from NW). Location of the seismic profiles in Fig. 11.

interpreted by Conti and Fontana (2005) as a specific facies indicative of diffuse methane-rich fluid venting. In their example, evidence of dissolution of the skeletal carbonate near the sediment/water interface suggested that, as a consequence of the slowly upward migration of the oxic/anoxic interface, sediment passed through the sulfate-reducing zone, where the associated authigenic carbonates precipitated incorporating the dead lucinid shells. In our examples, no signs of dissolution are present and the youngest shells are dated 640–440 BP. We therefore presume that the burial of the lucinids has been a quick and relatively recent process, probably due to a change in the conditions of constant fluid seepage. A possible explanation is that, due to the increased precipitation of carbonate cement in the sub-seafloor, crusts have grown to a thickness capable of clogging the upflow of methane. A similar mechanism has been already invoked for other examples of cold seep environments (Conti and Fontana, 2005; Naudts et al., 2008). In the diapiric structures D1 and D2, the authigenic crusts were sampled exclusively in coincidence with tectonic escarpments, suggesting that, as already seen in other contexts (Olu-Le Roy et al., 2004; Law et al., 2010), faults are exploited as primary conduits for upward fluid migration within the diapirs. The lack of any high seafloor backscatter on the diapirs also shows that carbonate precipitation outcropping at the seafloor occurs in laterally restricted areas. Occasionally, in the vicinity of deep-water seeps, also species that are not unique of seep sites exhibit enhanced densities (Levin, 2005).

This is also observed in D2, where a very high concentration of living decapod crustaceans was found. We presume that their density is related to the high concentration of organic matter due to the presence of the lucinids. 5.4. Normal faults: possible pathways for fluids and diapiric rise Several similarities between the Calabrian Arc structural setting on land and the offshore study area were observed (Figs. 1a, 11). For example, the system of high angle normal faults, that originate the graben east of the R1MV (Fig. 14) and also affect the diapirs to the east (Fig. 11), is consistent with a similar system that originates the Crati Graben on land (Tansi et al., 2007), and interpreted as a transtensional Quaternary fault system active since the early Pleistocene (Tortorici et al., 1995). Further south, N–S-oriented normal faults terminate against a series of approximately WNW–ESE trending orthogonal faults, that can have the same structural significance of the SLFZ and the SDFZ fault zones, that contribute on land to the development of the Catanzaro Graben (Tortorici et al., 1995; Tansi et al., 2007). The spiral planform of the Propeller basin and the 10-km-displacement of the basin borders may be the result of bending and “shift” of the two sectors of the basin, caused by the fault dissecting the central area of the basin, probably acting as a strike-slip fault (Fig. 11). This fault can be tentatively interpreted as the offshore prolongation of the SLFZ zone (Soverato Lamezia Fault Zone, after Van Dijk et al., 2000).

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The presence of NW–SE trending, strike-slip faulting in the study area had already been stressed in Milia et al. (2009). They suggest that the Propeller basin, which they call a minor basin, is a releasing structure formed during the right lateral activity of NW–SE oriented transcurrent faults. Besides fault activity, withdrawal subsidence and the rise of the adjacent diapiric structures contribute to originate the depressed structure of the Propeller basin (Gamberi and Rovere, 2010). In Gamberi and Rovere (2010) a pre-Messinian overpressured mud sequence was hypothesized as the source area of the deep plumbing system. The new data did not image deeper in the sub-seafloor, but permitted a more detailed mapping of the extensional faults confining the cold seep structures. They also allowed the mapping of other minor faults that seem to control the seepage distribution and activity, such as the shape and the distribution of the pockmarks, and the location and the precipitation of authigenic carbonates associated with chemosymbiontic fauna. Pockmarks are sometimes related to faults (Chand et al., 2008) and pockmark fields can be constrained by deep regional normal faults (Chand et al., 2009; Ostanin et al., 2012, 2013). Chemosymbiontic communities have already been found in coincidence of fault scarps (Olu-Le Roy et al., 2004). Unfortunately our seismic data do not perfectly image the link between the pockmarks and the deeper fault, because of a resolution/penetration gap between the CHIRP and the sparker systems, but faults in the study area are likely focused conduits for fluid flow. A regional unconformity of uncertain age is related to the initiation of diapirism in the study area, a syn-deformation sequence is present above this in the seismic profiles. The degree of deformation is high until the orange reflector (Figs. 12b, 14, 15), that thus probably registers a significant decrease in the diapiric uplift. After this, a post-deformation sequence is present above the green reflector (Figs. 12b, 14, 15), suggesting that the activity related to the diapirism has now ceased. The green drape and the red drapes belong to the post-deformation sequence. All the above observations suggest that normal faults are the most probable mechanism of emplacement of the rising structures, especially the mud diapirs and that they are favorable seepage pathways and control the location and distribution of the pockmarks, that are, in fact, particularly abundant in the mud diapirs. This is also in agreement with similar conclusions advanced in the inner forearc basin of the Calabrian Arc (Capozzi et al., 2012a), where normal faults, due to a generalized extensional regime active since the Messinian, provide the main pathways and boundaries for the rising diapiric structures. An attempt to link offshore and onshore structures has also been proposed in the Squillace Gulf (Ionian Sea), which is interpreted to be the marine prolongation of the Catanzaro Graben (Capozzi et al., 2012a). Our study area is located at the rear of the Calabrian Arc on the Tyrrhenian side of the Catanzaro Graben and can be interpreted as the prolongation of the same corridor of extensional tectonics that dissect the Calabrian Arc from SE to NW, along the SLFZ and the SDFZ fault zones (Figs. 1a, b). 6. Conclusions New high resolution bathymetric, backscatter and seismic data, coring and box coring samples were collected during the MVP11 R/V Urania cruise in deep-water cold seeps (500–1000 m) of the Paola Basin, southeastern Tyrrhenian Sea. The structures associated with the presence of fluids were classified as: (1) high venting sites (mud volcanoes); (2) low venting sites on the flanks of the mud volcanoes (mud flows); and (3) inactive seepage sites (mud diapirs). The mud volcanoes are sites of vigorous venting at the seafloor, as shown by: gas bubble emission during sampling and expansion cracks in the sediments cores; a 630-m-high gas plume registered in the water column; the high backscatter of the seafloor. Byproducts related to the chemistry of the sulfate reduction and methane oxidation zones consist of: pyritized burrows; pavements and crusts in the shallowest

sediment; and siderites precipitated inside or near burrows, a few meters below the seafloor. The mudflows are areas of intermediate backscatter seafloor and low gas venting. The gas is trapped in the sub-seafloor, as shown by the slow emission of gas when cores self-extruded once retrieved onboard and by the expansion cracks trapped 2 m below the seafloor. Most probably the fluidized mud descending from the flanks of the mud volcanoes acts as a seal for the gas, preventing its free and continuous venting at the seafloor. The sediment cores show a more preserved shallow oxic zone, abundant siderite precipitates inside burrows, 5–6 m below the seafloor, while sulfides are absent or below the siderites, probably indicating a different distribution and different depths of the geochemical zones, compared to the mud volcanoes. The mud diapirs are studded with pockmarks and dissected by normal faults; along these, organogenic carbonate crusts and fauna assemblages related to methane seepage (lucinids), buried below a 1-m-drape of soft mud, were found. The youngest chemosynthetic communities are dated 640 to 440 BP. The present day level of fluid activity within these structures must be lower than it was in the recent past and no active gas seepage through the pockmarks is taking place at the present time. The normal fault system described in the study area is comparable to the tectonic setting of the inland Calabrian Arc. Normal faults are associated with the cold seep structures indicating that: on a large scale, tectonic structures control the rising of the diapirs and the mud volcanoes; and on a smaller scale, faults control the location of the pockmarks and their shape and distribution. Acknowledgments Special thanks go to the crew of the R/V Urania during the campaign carried out in August–September 2011 and to the students onboard: Silvia Pace, Stefania Fuggetta, Aurora Giorgi, Camilla Rota, Fabrizio Memma, and Giorgia Mensali. We would like to thank Fabio Trincardi and Anna Correggiari for providing the additional sparker data used in the 3D modeling. We are very thankful for the constructive comments made by Editor David J.W. Piper, and reviewers Christian Berndt and Zahie Anka. Dr Anka's careful reading of the original manuscript and thoughtful comments improved the quality of the paper. This research was supported by the Ritmare Project (www.ritmare.it). Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.margeo.2013.11.013. These data include Google maps of the most important areas described in this article. References Ambrosetti, P., Bosi, C., Carraro, F., Ciaranfi, N., Panizza, M., Papani, G., Vezzani, L., Zanferrari, A., 1983. Neotectonic map of Italy, scale 1:500.000. Sheets 4, 6. Progetto Finalizzato Geodinamica, sottoprogetto Neotettonica, CNR, Italy. Amodio-Morelli, L., Bonardi, G., Colonna, V., Dietrich, D., Giunta, G., Ippolito, F., Liguori, V., Lorenzoni, S., Paglionico, A., Perrone, V., Piccarreta, G., Russo, M., Scandone, P., Zanetti-Lorenzoni, E., Zuppetta, A., 1976. L'Arco Calabro-Peloritano nell'orogene appenninico-maghrebide. Memorie della Societa Geologica Italiana 17, 1–60. Anka, Z., Berndt, C., Gay, A., 2012. Hydrocarbon leakage through focused fluid flow systems in continental margins. In: Anka, Z., Berndt, C., Gay, A. (Eds.), Hydrocarbon leakage through focused fluid flow systems in continental margins. Marine Geology, 332–334, pp. 1–3. Argnani, A., Trincardi, F., 1988. Paola slope basin: evidence of regional contraction on the eastern Tyrrhenian margin. Memorie della Societa Geologica Italiana 44, 93–105. Bayon, G., Pierre, C., Etoubleau, J., Voisset, M., Cauquil, E., Marsset, T., Sultan, N., Le Drezen, E., Fouquet, Y., 2007. Sr/Ca and Mg/Ca ratios in Niger Delta sediments: implications for authigenic carbonate genesis in cold seep environments. Marine Geology 241, 93–109. Berner, R.A., 1981. A new geochemical classification of sedimentary environments. Journal of Sedimentary Petrology 51, 359–365. Borowski, W.S., Paull, C.K., Ussler III, W., 1996. Marine pore water sulfate profiles indicate in situ methane flux from underlying gas hydrate. Geology 24 (7), 655–658.

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