Mud volcanism and fluid seepage at Venere mud volcano in the Calabrian Accretionary Prism (Central Mediterranean)

Abstract

Cold seeps are the natural expression of the release of gases, liquids, solids, or a combination of components, which are sourced from subsurface sediments and are emitted at temperatures comparable to surface values. Understanding the processes that generate, transport, and discharge fluids and solids at the geosphere-hydrosphere interface is important, since they are part of global material recycling, impact the development of submarine ecosystems, influence sediment dynamics and stability of the seafloor, and potentially point to the occurrence of energy resources. This work has the aim of investigating the processes that govern the activity and evolution of submarine mud volcanoes (MVs) and associated cold seeps by studying Venere MV, as an example. The presented results and conclusions are based on data from two research cruises (RV METEOR M112 and RV POSEIDON POS499). They were obtained by multiple methods, including: Hydroacoustic seafloor and water column measurements by ship and an autonomous underwater vehicle (AUV); visual observations, photo-mosaicking, and geological sampling by a remotely operated vehicle (ROV); sediment coring techniques including cores taken under in-situ pressures. In addition, various analyses (e.g. stable isotope analyses, XRF scans, WDS analyses, etc.) were carried out post-cruise on selected samples. Venere MV is located within a deep-sea canyon at 1600 m water depth in a forearc basin of the Calabrian accretionary prism. Two different but co-existing fluid discharge mechanisms can be characterized: 1) extrusion of mud breccia from a conduit at the summit, containing thermogenic methane and freshened pore waters indicative of a deep-source ( 3.5 km) and 2) hydrocarbon release at peripheral seeps hosting cold-seep ecosystems and authigenic carbonates along inward-dipping ring faults at the caldera edge. The interpretation of the seafloor morphologies, sediment deposits, and cold seep structures support the main conclusions that Venere MV experienced moderately extrusive but relatively continuous activity throughout the last centuries and that past sediment transport processes in the canyon affected its overall morphology. The peripheral seeps of Venere MV host sites of gas bubble release, flat carbonate pavements, mounded and ruptured carbonate domes, as well as crater-like collapse features. The observations indicate an evolution from plain to colonized seeps in soft sediments, to carbonate pavements that trap fluids, to ruptured but colonized structures, over decadal, centennial, and millennial timescales, respectively. This work presents previously unknown details on geological, geochemical, and biological processes that govern mudflow extrusions and fluid seepage not only relevant to the Calabrian accretionary prism, but also to MVs and cold-seep systems globally.