Fluid flow in and around a mud volcano field seaward of the Barbados accretionary wedge: Results from Manon cruise

A field of mud diapirs and mud volcanoes situated in the Barbados trench at 13 degrees 50'N and extending along an old oceanic fracture zone (Mercurus) was investigated during the Manon cruise using both surface ship and Nautile submersible sampling and measurements. The entire zone from 13 degrees 50'N up to 14 degrees 20'N has an anomalously high heat flow which implies that fluids are drained into it from a segment of the accretionary wedge a few hundred kilometers wide. Two structures interpreted as diatremes, Atalante and Cyclops, expell large amounts of water and methane. We propose that they were formed from the release of a light fluid when gas hydrates were dissociated in the sediment as the result of the circulation of warm fluid in the area. However they expell only a small fraction of the incoming fluid, implying that disperse flow is the dominant mode of expulsion in this area. The chemoautotrophic communities on the surface of the structures rely mostly on sulfides. Submersible observations, temperature measurements in the sediment, and the chemistry of the pore fluid indicate that convection of seawater occurs within the first few meters of sediment through high-permeability channels, such as cemented carbonate conduits. We propose that this convection is driven by the density difference between the pore fluid and seawater, but fresh water released by the dissolution of shallow hydrates may also contribute. This shallow convection may be a frequent process in cold seep environments.