Feature ArticleExploring Ultradeep Hydrothermal Vents In the Cayman Trough by ROV
National Oceanography Centre
The location of the Mid-Cayman Spreading Centre (MCSC) within the Caribbean Sea (a) and the EM 120 multibeam bathymetry map of the MCSC showing the location of two different hydrothermal mounds (b). The 5,000-meter-deep Beeb site (c) and the 2,300-meter-deep Von Damm site (d) are also shown.
With their contrasting styles of fluid venting, sulfide mineralization, geological setting and host-rock interaction, the differences between these two sites indicate that depth and basement rock type may significantly affect the metal content of hydrothermal seafloor massive sulfide deposits.
Ultradeep hydrothermal vents hold a key to understanding the subsurface formation of mineral deposits on land. In 2010, an expedition led by the U.K.'s National Oceanography Centre to the MCSC at 6,000 meters depth aimed to locate, film and sample ultradeep hydrothermal vents. These vents are considered to be the highest pressure, hottest and most copper-rich seafloor hydrothermal systems known to date. It was predicted that this study would answer some critical questions about the evolution of hydrothermal fauna nourished by the chemicals in the vents.
The survey of the MCSC used a Simrad (Horten, Norway) EM 120 multibeam swath echosounder deployed from the RRS James Cook and the National Oceanography Centre's Towed Ocean Bottom Instrument (TOBI), a deep-towed 30-kilohertz side scan sonar equipped with an optical plume sensor to detect hydrothermal plumes. CTD casts verified a number of potential plume signals, two of which were in the general vicinity of hydrothermal water-column plumes reported in 2009 by a U.S. team from the Woods Hole Oceanographic Institution.
With two strong plume signals verified, a near-bottom survey was conducted by the National Oceanography Centre's Auto-Sub 6000 (AS6K), an AUV capable of diving to 6,000 meters and running a mission up to 48 hours at speeds of 7 knots. For the MCSC missions, the AS6K was deployed with a Simrad EM 2000 high-resolution multibeam swath echosounder for seafloor mapping and imaging, a three-component magnetometer, and a suite of optical and chemical sensors, including an Eh sensor to detect the source of hydrothermal venting.
The AS6K missions, each of which covered several square kilometers, found evidence for near-bottom hydrothermal activity at the two sites. The deepest one had strong Eh and temperature anomalies a few tens of meters above the seabed and had an approximately 100-meter-diameter circular region of almost zero magnetic intensity over the same part of the seafloor. The shallower site on the flank of Mount Dent had scattered Eh signals and weak temperature anomalies but little or no magnetic anomalies.
After locating each of the two different sources of hydrothermal venting to within a few hundred meters, the third and most crucial phase of the exploration began with visual observations and sampling. During its 2009 expedition, the U.S. team from Woods Hole had deployed the ultradeep diving hybrid ROV Nereus but failed to locate any hydrothermal vents on the floor of the MCSC. With this in mind, a different approach was adopted to meet the challenge. A traditional type of ROV would, at these depths, need expensive floatation, be heavy and cumbersome, and require a large operating team. Instead, the National Oceanography Centre team developed a smaller, lighter, modular and versatile system, HyBIS.
The HyBIS concept was developed to survey and interact with the deep-ocean floor without recourse to expensive and complex work-class ROV technology. Developed in collaboration with Hydro-Lek Ltd. (Finchampstead, England), HyBIS is a 6,000-meter-rated, fully modular, electrohydraulic ROV comprising a command-and-power unit fitted with three high-definition and secure-digital cameras; 1 kilowatt of lighting; a pan and tilt bar; a sensor suite including a sector-scanning sonar, an acoustic navigation beacon, hydraulic pumps, electric thrusters and fiber-optic telemetry; and a universal hydraulic and mechanical docking interface. The dock allows a variety of different tool modules to be interfaced with the command module, including a 0.5-cubic- meter hydraulic grab, a five-function manipulator arm and retractable sample tray, a passive winch to recover up to 3- tonne bottom landers and a module for the visually guided delivery of ocean-bottom seismometers to the seafloor.
Unlike a conventional ROV, HyBIS does not have any floatation; rather it is suspended directly from the ship by its umbilical cable, allowing it to recover or deploy heavy payloads with thrusters powerful enough to provide a radius of maneuverability between 2 and 5 percent of the water depth (e.g., 200 meters radius at 5,000 meters depth). Deployment from a ship using a standard 18-millimeter diameter electro-optic oceanographic cable minimizes heave. During the MCSC survey, HyBIS was able to maintain a constant survey speed of 0.5 knots, even at 5,000 meters, for at least eight hours.
As a result of its small footprint and relatively simple configuration, HyBIS requires two operators, making it both cost effective and readily accessible to the scientific community. Operation involves a pilot (usually a scientist) and winch driver (usually a ship's crew member) sitting side-by-side in front of screens relaying video feeds from the cameras, as well as position, depth, heading and other onboard data. Other members of the scientific team operate the high-definition camera and act as navigators.
The versatility of the HyBIS system was demonstrated during the MCSC expedition when it was fitted with a slurp gun for biological sampling, pumps and filters for sampling minerals in the hydrothermal plume, and watertight bottles for sampling hot vent fluids as they exited the seafloor.
The Ultradeep Beeb Vent Site
During its first dive to about 5,000 meters depth, and following a three-hour search pattern, HyBIS located and imaged the vent site named Beeb, which is surrounded by basaltic pillow lavas and forms a steep mound that is almost vertical in places. Near the top of the mound, both diffuse and mineral-rich, high-temperature 'black smoker' chimneys were found. To continue this article please click here.
Dr. Bramley Murton is a senior geologist at the National Oceanography Centre, Southampton, England. With more than 20 years in deep-ocean research, he is an expert on the processes of formation and evolution of the oceanic crust, hydrothermal systems and seafloor massive sulfide deposits. With Jo Garrad, he originated and developed the HyBIS concept, and led its exploration of the Cayman Trough.
Veit Hünerbach is a geologist and technologist at the National Oceanography Centre, Southampton, England. His expertise is in acoustic mapping of the deep seafloor, and the development and installation of seabed observatories for long-term monitoring. He operated and piloted the HyBIS ROV during its exploration of the Cayman Trough.
Jo Garrard is a design engineer at Hydro-Lek Ltd., a U.K.-based engineering company specializing in underwater robotic systems. He was the lead mechanical engineer during the design process for the HyBIS ROV.