Multibeam backscatter-driven investigations reveal previously unknown cold seeps in the southeastern Tyrrhenian Sea

High resolution swath bathymetry data (EM 710), 3-7 kHz seismic CHIRP profiles, water column multibeam measurements, box core and gravity core samples were acquired during the 2011 R/V Urania MVP11 cruise in the Paola basin, southeastern Tyrrhenian Sea. The area was investigated with direct sampling techniques to preliminarily test a recent hypothesis, interpreting the study area as a mud mobile belt, located along an anticline at 700-1000 m water depth, due to intense hydrocarbon-seep activity. Different structures associated with fluid escape and/or seepage have been classified, based on their morphology, geophysical character and nature of the sediment deposits and byproducts, they include: (1) active venting sites (mud volcanoes), (2) low venting sites (mud flows), (3) inactive seepage sites (mud diapirs). Geological response at the seafloor to the hydrocarbon-rich fluids and gases is highly variable and dependent largely on rate and duration of delivery, as well as fluid and gas composition. In the study area three structures have been recognized as sites of rapid expulsion at the seafloor of fluids and fluidized sediment resulting in buildups of sediment in the form of mud volcanoes, with about 10-km-wide diameters and sheet-like mudflows, characterized by a high backscatter signature, extending from 10 to 14 km downslope. Associated to the mud volcanoes are: precipitation of iron-oxy-hydroxide crusts and pirityzed and sulphur burrows in the sub-surface and tubular authigenic siderites in the deeper section. The EM710 water column acoustic measurements at the top of one mud volcano revealed a nearly 700-m-high gas plume. The mud flows are characterized by the deposition of very thin drapes of water-rich mud downslope the mud volcanoes, which prevents active venting at the seafloor and favors oxic conditions of the near-surface sediment and precipitation of authigenic carbonates in the sub-seafloor. A large number of mud diapirs have been discovered in the area, but only three mud diapirs have been sampled, they are associated with large fields of pockmarks, they show variable sizes and planforms and have relief of hundreds of meters above the seafloor. Slow seepage promoting lithification of the seafloor through precipitation of organogenic carbonate crusts has been evidenced in one of the mud diapirs, where normal faulting exposed the sub-seafloor; associated to the carbonates and the faulted pathways, dead communities of chemosymbiontic organisms were found, suggesting a ceased gas seepage activity.