Days 3 and 4 of the final Leg of the 2017 OOI Regional Cabled Array O&M Cruise were filled with a wide variety of activities. Jason Dive J2-1007 visited the International District Hydrothermal Field in the caldera of the Axial Seamount at a water depth of ~1520 m. This 16 hour dive began at 2012 PDT on Aug 21st and included the recovery and deployment of a total of five instruments that monitor the temperature and chemistry of hydrothermal fluids associated with fluid-rock interactions in the subsurface where heating produces fluids at 350°C. These instruments included a mass spectrometer (MASSP) that measures the gas content of the fluids and transmits the data in real-time to shore via the cable. Two other instruments, RASFL and PPSDN, collect and preserve hydrothermal fluids and particulates, the timing of which can be controlled from shore to respond to volcanic and other events. These samples are recovered annually and the fluids are analyzed for gas content, major ions, and trace metals; and the particulates are processed for microbial DNA. At the end of the dive, the anchor for the 10 m high mooring containing the RASFL and the PPSDN instruments deployed last year was released. The mooring floated to the surface and was recovered using tag lines and the ship’s crane. After the mooring and instruments were brought aboard, quite a bit of sampling had to be performed rather quickly. The science team employed the help of many of the Leg 3 students to obtain and process the recovered samples.
While the scientists and students worked on the RASFL and PPSDN samples, engineers from the UW Applied Physics Laboratory Ocean Engineering Group began deploying the Deep Profiler mooring and instrumented wire crawler at Axial Base (~2600 m water depth). The instruments onboard the wire crawler provide critical information on the currents, chemistry, and energy budgets of the deep ocean. Before the deployment, 2500 m of single conductor cable was wound under tension onto the drum of an 18,000 lb Medium Lift Winch (MLW). During deployment, the cable, with a 4600 lb anchor attached to its end, was slowly unwound off the drum and put into the water off of the ships’s aft deck. A short distance above the anchor, a docking station provides charging of the wire crawler battery as well as the capability to download the data for transmission back to shore via the OOI CA fiber optic cable.
The operations required for the deployment are very demanding and require physical strength and stamina as well as precise coordination between the various members of the team including the line handlers, various winch operators, and those tending the descending cable and components on the mooring. The wire crawler was attached to the descending cable when 75% of its length had been deployed. Near the end of the deployment, a 5 ft diameter, 2600 lb orange float was attached to the top of the cable to keep the mooring cable properly tensioned and vertical in the water column. After 8.5 hours of continuous deck work, the deployment was completed shortly before midnight on Aug 22nd.
Jason then dove (Dive J2-1008) to provide the finishing touches to the Deep Profiler deployment. The ROV followed signals from the two transponder beacons, previously attached to the mooring float and docking station, to the base of the mooring where it connected the cable that provides power and communications to the docking station. Jason then retrieved the lower beacon and ascended to ~100 m below the surface to inspect the top float and recover the attached transponder before ascending to the surface and completing the dive. The rest of Day 4 was spent in transit to the Endurance Array Offshore Site where another such mooring will be recovered and redeployed in 600 m of water depth.