Abstract
Submarine mud volcanoes release sediments and gas-rich fluids at the seafloor via deeply-rooted plumbing systems that remain poorly understood. Here the functioning of Venere mud volcano, on the Calabrian accretionary prism in ~1,600 m water depth is investigated, based on multi-parameter hydroacoustic and visual seafloor data obtained using ship-borne methods, ROVs, and AUVs. Two seepage domains are recognized: mud breccia extrusion from a summit, and hydrocarbon venting from peripheral sites, hosting chemosynthetic ecosystems and authigenic carbonates indicative of long-term seepage. Pore fluids in freshly extruded mud breccia (up to 13 °C warmer than background sediments) contained methane concentrations exceeding saturation by 2.7 times and chloride concentrations up to five times lower than ambient seawater. Gas analyses indicate an underlying thermogenic hydrocarbon source with potential admixture of microbial methane during migration along ring faults to the peripheral sites. The gas and pore water analyses point to fluids sourced deep (>3 km) below Venere mud volcano. An upward-branching plumbing system is proposed to account for co-existing mud breccia extrusion and gas seepage via multiple surface vents that influence the distribution of seafloor ecosystems. This model of mud volcanism implies that methane-rich fluids may be released during prolonged phases of moderate activity.
Highlights
Mud volcanoes (MVs) are geological structures created by the extrusion of sediments, water, and volatiles from subsurface plumbing systems that may extend to depths of kilometres[1,2,3,4]
Major MV provinces have been identified on the Calabrian accretionary prism (CAP), the Mediterranean Ridge, the Anaximander Mountains, and the Nile deep sea fan (Fig. 1a18)
In the Anaximander Mountains region, gas release at MVs supports the formation of shallow gas hydrates[22], which have been found to be prone to decomposition during eruptive phases of mud volcanism[15]
Summary
The present-day CAP in the central Mediterranean Sea (Fig. 1a) developed from the NW subduction of the African plate (an Ionian lithospheric slab) below the Eurasian plate during the Neogene[28,29]. Seismic reflection profiles of the offshore forearc basins indicate up to 2 km of sediment, overlying thrust structures (Fig. 1d31,32,37,38) These sequences are inferred to date back to the Tortonian and to contain Messinian deposits[30,31,32,36]. Bathymetric observations reveal elongate but smooth lobes (Fig. 2), consistent with less recent extrusive activity This is supported by a sediment core from the eastern summit (GeoB19234-1; Fig. 2), which contained mud breccia draped by several decimetres of hemipelagic sediment (see Loher et al.[27])
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