|HyBIS RUV being recovered after an 8 hour dive on Southern Mound |
in the TAG hydrothermal field.
This cruise focuses on the exploration and resource assessment phases of the project, with the aim of characterising the seafloor environment across the TAG field, with a particular emphasis on mapping and sampling the extinct Seafloor Massive Sulphide (SMS) deposits.
In an attempt to develop new tools for exploration and subsurface imaging of extinct SMS deposits (which unlike active deposits cannot be easily traced through physical and chemical anomalies in the water column), we are testing a range of Controlled Source Electro-Magnetic (CSEM), magnetic and seismic geophysical techniques. We have deployed the University of Southampton’s Deep-towed Active Source Instrument (DASI), and the three-axis electric field receiver, Vulcan, which will provide an image of the seafloor resistivity structure. ‘Sputnik’ is a novel and futuristic looking CSEM transmitter system, with shiny balls for high voltage seafloor electrical transmission has been used over a 24 hour period. This device is lowered to the seafloor where the arms are unfolded and it transmits electromagnetic waves. It is then lifted about 50 metres above the seafloor and the ship is moved to the next sample station where it is lowered for another transmission.
National Oceanography Centre’s robotic underwater vehicle (HyBIS), deployment of the British Geological Survey’s latest generation of remotely operated sub-sea rockdrill (RD2), and seafloor sediment coring. HyBIS has made five dives, undertaking sampling using its manipulator arm and making video traverses across the sulphide mounds. The dives have provided highly informative images of the expression of the seafloor mineralisation, its relationship to faulting, the basaltic host rocks, and the extent and thickness of hydrothermal precipitates and sediment cover. A highlight was a HyBIS dive which went across a major seafloor feature, probably a ‘lava lake’ (discovered on the high resolution bathymetry obtained on the sister Blue Mining cruise M127), culminating at the spectacular active black smoker complex of the TAG mound. Notably, this dive located two funnels (to allow the drill string to re-enter the holes during the drilling process) installed during the 1994 Ocean Drilling Programme: Leg 158. We have also captured impressive images of the deep-ocean seafloor fauna (e.g. fish, anenomies, crabs, sea slugs etc.) and bacterial mats. We have also seen the enigmatic honeycomb-like Paleodictyon structures on the seafloor, which are postulated to represent burrows arising from a creature possibly cultivating bacteria.
HyBIS has proved invaluable for surveying potential drill sites for the RD2 rig. During the dives we have been placing flashing LED beacons on the suitable locations identified to aid subsequent positioning of the rig on the seafloor. Even with the new GEOMAR high resolution (50 cm grid) autonomous underwater vehicle (AUV) acquired bathymetry data and HyBIS surveying it has been very challenging to locate sites which are of a suitable size and have a low enough slope angle to accommodate RD2.
|Deployment of BGS RD2 to drill the seafloor|
massive sulphide deposit at Southern Mound.
Sediment cores obtained using gravity and mega coring are providing new insights into the nature of the seafloor sediments (up to 2 metres below the seafloor) across the TAG field. The sediment cores record a dynamic seafloor environment, typically consisting of dark red, oxidised layers probably representing mass wasting of sulphide-rich material from the mounds, intercalated with carbonate ooze, and darker layers likely to originate from hydrothermal plume fallout.