Venting and seepage systems associated with mud volcanoes and mud diapirs in the southern Tyrrhenian Sea
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 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 methane-oxidizing 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 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.
Section snippets
Regional setting
The study area is located along the Paola Ridge, a NNW–SSE 60-km-elongated 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
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 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
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 < 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
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 present at the seafloor or very close to it. Areas of shallow and deeper seated gas are
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
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
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