BARS Recovery: A Great Success!
On July 9, 2011, we recovered the Benthic and Resistivity Sensor
(BARS), a cruise objective because of its failed hydrogen sensor and an isolation fault that disrupted its communication to the Main Endeavour Vent Field (MEF) instrument platform. The isolation fault was detected on the instrument on October 20, 2010 before the MEF junction box stopped communicating, just one month after installation. During inspection of the BARS cable, scientists discovered that the reason behind the isolation fault was the growth of a vent chimney directly on the implicated cable, melting the hose and exposing the inside wires directly to the environment! Gastropods had also started growing on the cable.
What the BARS Instrument Measures
In September 2010, the BARS instrument was installed at a depth of 2189.0 metres at the MEF Grotto site. It has sensors to measure resistivity, temperature, eH (oxidizable nature of seawater components) and hydrogen within black smoker vents, where temperatures of the emerging effluent can reach 350°C. BARS uses a probe inserted directly into the hydrothermal vent in order to measure the pure vent fluid as it comes out, before diluting in sea water. BARS data are available via our Data Search and Plotting Utility. See summary of current BARS research by Olson.
The scientific events detectable by the BARS instrument include several scales, from a single earthquake to a swarm of earthquakes, a dike intrusion, or a seafloor eruption. An event could be indicated by a small sudden temperature change (+ 3°C). Changes in the resistivity sensor (i.e. the mixing ratio among vapor, brine, and seawater) or in the hydrogen sensor (i.e. concentration) could be caused by an earthquake or intrusive lava event. The eH sensor would respond to changes in the concentration of reduced chemical species in the fluid (e.g. hydrogen sulfide).
Recovery Operation
Any operation is delicate when dealing with temperatures as high as 320°C, but especially so when your ceramic probe is super sensitive to rapid temperature change and lodged inside a vent plume over two kilometers beneath the waves. The ROPOS crew had to be extraordinarily careful and patient not to break the BARS probe during recovery. Principle Investigators, Dr. Marv Lilley and Eric Olson (both from the University of Washington), gave special instructions to mitigate the chance of breaking their instrument during the recovery operation at the Endeavour Grotto site.
Luckily, the ROPOS pilots were able to grab and gently pull the probe out of the 320°C vent plume without causing damage. In order to minimize heat shock to the ceramic part of the instrument, ROPOS pilots delicately removed the sensor tip from the black smoker fluid. The slow cooling process of holding the tip one foot from the black smoker orifice for a full minute, and retreating from the heat source at the same rate three times was a deemed as a reasonable approach by the PIs. We are happy to report it was a smooth operation.
The instrument will be brought to the University of Washington to be refurbished. Plans are to redeploy the BARS instrument at MEF in September 2011 along with a second BARS instrument at Endeavour Mothra Vent Field.
