The Zannone Giant Pockmark. First evidence of a giant complex seeping structure in shallow-water, central Mediterranean Sea, Italy

An active giant pockmark located offshore Zannone Island (central Tyrrhenian Sea, Italy), is analyzed by very high resolution multibeam bathymetry, high resolution seismic profiles, ROV video observations, sediment and water sampling. The active fluid emission area is located on the outer shelf, between 110 and 130 m water depth, and affects the Late Quaternary lowstand and highstand deposits resting on rocky bedrock. A variety of fluid-escape features characterizes the area, including the Zannone giant pockmark, several smaller pockmarks, hummocky terrains and areas of positive relief. Ground-truth video data show active seepages, bacterial communities, widespread lithified pavements, mounds, and cone-shaped structures. Evidence of active seepage includes both continuous and intermittent bubble release from the seafloor and a well-defined plume rising 70 m above the seafloor. The Zannone giant pockmark is about 900 × 500 m (surface of some 0.5 km2). It represents the first evidence of an active shallow-water seepage area in the central Tyrrhenian Sea (Italy) and the first record of a morphologically complex giant pockmark in the entire Mediterranean Sea. Some speculations on processes originating the observed features are explored, including possible occurrence of multiple eruption events, processes of fluidization–liquefaction and minor slides that may have modified the original morphology. Factors peculiar to the study area – shallow depth, thin sedimentary cover resting on a faulted rocky basement, seeping occurring through non-cohesive sandy sediments appear to have been key to the formation and morphology of the Zannone giant pockmark.