METEOR Fahrtbericht / Cruise Report M146, Henry Seamount Seepage Exploration (HESSE), Recife (Brazil) - Las Palmas de Gran Canaria (Spain), March 17 - April 16, 2018 (English)

The main objective of cruise M146 was the discovery and documentation of inferred fluid venting sites at Henry Seamount, an extinct 126 Ma old volcano located 40 km to the southeast of El Hierro island. Evidence of recent fluid discharge at this seamount was provided by rock and shell samples from a reconnaissance dredging campaign during METEOR cruise M66/1. Such discharge could reflect hydrothermal circulation of seawater through the old oceanic crust, a globally important process. This type of circulation requires basement outcrops through the impermeable sediment cover, which can be provided by seamounts and island flanks. To achieve the goals, we carried out a hydroacoustic survey of Henry Seamount using the ship's multibeam echosounder and PARASOUND system, and mapped an area of 31 km² in detail during seven AUV dives. About 600 km of reflection seismic profiles were shot across the seamount and its vicinity to reveal sub-bottom structures and basement outcrops. As a potential tracer for fluid discharge or recharge sites, the heat flow in the area was determined by more than 50 measurements along four profiles using a heat flow probe. During seven TV sled surveys we explored the seafloor on the top and flanks of Henry Seamount, and logged temperature, turbidity and geochemical potential of the bottom water. Five gravity cores and 13 grabs provided samples of sediment, shells and other biota, and revealed conspicuous basaltic ash and lapilli layers. Our data show that surprisingly large areas of the seamount are covered with Abyssogena clam shells, testifying widespread past fluid discharge, even though this is not reflected in the heat flow data. Oxydation-reduction potential (ORP) anomalies in the bottom water indicate that in some areas fluids are still discharging.