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April 22nd, 2003; Leg 5, Day #2

We arrived on station early this morning. By 7:30 a.m., ROV Tiburon was launched into the water and began a descent to the seafloor to about 1700 meters depth. During the transit, the ROV pilots carefully monitored several sensors for areas that had been recently fixed to verify that everything was functioning properly.  

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Charlie Paull checks instruments before ROV Tiburon's dive.

 

The science team watched output from various ROV sensors during the descent, starting with the profile display from the conductivity-temperature-depth (CTD) and oxygen sensors. The surface waters in the Gulf of California are warmer (~21 degrees C), saltier (35 PSU), and have less oxygen (~5 ml/l) than Monterey Bay. Concentrations of methane were measured with a special sensor, helping us with our discovery of a bubbling vent site later in the day.  

This first dive was cut short when the pilots noticed some leakage from the ROV. Before recovering it, we were able to take heat flow probe samples, a vibracore sample, and three push cores of the sediment.  

The pilots made their fixes quickly, and Tiburon was back in the water after lunchtime. During the descent, the ROV's sonar system imaged what appeared to be bubbles in the water column, evidence of the gas venting we had hoped to discover during our dives at this location. The ROV's cameras soon imaged small gas bubbles rising to the surface. The methane sensor jumped significantly and was beyond its upper limits of detection when in the gas plume. Values of 20 micromolar were recorded, nearly three orders of magnitude greater than normal seawater values of 0.04 micromolar.  

Eureka! We found a hydrocarbon gas plume on the seafloor, a beautiful geyser of gas bubbles emanating from a small hole, about the size of a tennis ball (see below left). White bacterial mats surrounded the gas vent with aggregations of small gastropod snails (see below right). Numerous clamshells were scattered over much of the ridge flank, but we did not find any live clams. Slabs and mounds of authigenic carbonates studded the surface. In fact, we couldn't take push core samples next to the plume because of the hard substrate. We had to search for the few spots with sediment cover to gather the samples that we did bring back to the ship.  

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Hydrocarbon gas plume venting from the seafloor.

 

 

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Close-up of a bacterial mat with loads of gastropod snails nearby.

 

 

This dive was unfortunately ended when Tiburon experienced a failure in its hydraulic system. The pilots are still working late into the evening to fix the problem after a long day of effort. Their talents at troubleshooting, rebuilding, and reworking the ROV from the smallest connector to the most complicated computer systems are truly amazing.  

Debbie Meyer

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