JC183 Blue Mining cruise to the TAG hydrothermal field, Mid Atlantic Ridge...by Paul Lusty

HyBIS RUV being recovered after an 8 hour dive on Southern Mound
in the TAG hydrothermal field.

We are now a few weeks into the RRS James Cook (JC138) cruise to the Trans-Atlantic Geotraverse (TAG) hydrothermal field of the Mid-Atlantic Ridge at 26oN, 45oW, with about 14 of the original 31 science days remaining. Hydrothermal activity and mineralisation were discovered here in the 1970s, at water depths in the range of 3400 to 3500 metres. The current cruise forms part of the ‘Blue Mining’ EC-funded project, which aims to bring sustainable deep sea mining a major step closer. In a duration of 48 months the Blue Mining project will address all aspects of the deep sea mining value chain, from resource discovery to resource assessment, and from exploitation technologies to the legal and regulatory framework.

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.

Selected images from the HyBIS dive on Southern Mound. Clockwise from top left: 1) Fault scarp showing carbonate ooze overlying oxidised sulphide-rich debris flow material with massive sulphide below, 2) Atacamite covered boulders on the seafloor, 3) A fish on Southern Mound captured during the HyBIS surveying.

The geophysical experiments have been interspersed with dives using the 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.

We have regularly deployed the RD2 drilling rig on Southern Mound, with mixed success, due to a combination of technical issues and the challenges of landing the rig on the uneven seafloor, which typically has a thick (>2 metre) cover of very soft sediment. The drill deployment involves positioning the ship above the recorded beacon location, lowering the 6 tonne rig to 30–50 metres above the seafloor (typically 3450 metres water depth, so it takes about 2 hours for the rig to reach this depth) at which point the flashing beacon generally becomes visible on the RD2 cameras. It is then sometimes necessary to make small changes to the rigs position to ensure that it is directly over the target location before landing it on the seafloor. Moving RD2 is only possible by making small changes (typically 5 metres) to the ships position, but small and accurate movements are very difficult due to the rig hanging about 3.5 km below the ship. This positioning accuracy is vital due to the small dimensions of the suitable landing sites we have identified with HyBIS (typically with a maximum size of 15 x 15 metres, compared to the 3 metre square footprint of RD2). Once the rig contacts the seafloor it is an anxious wait viewing the fluctuations on the pitch and roll sensor display to determine if the landing site is suitable for drilling. Due to the thick, soft sediment cover it has not been possible to make significant adjustments to the angle of the rig on the seafloor as the sediment offers no resistance and the legs simply dig in. Despite these challenges we have started drilling two holes, recovering some sample material from the upper part of Southern Mound, before technical problems have required recovery of the rig to the surface. We will persist with the drilling over the remainder of the cruise as the aim is to drill long holes into the cores of the sulphide mounds to understand their deep subsurface mineralogy and form.

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.