April 25th, 2003; Leg 5, Day #5

Today has centered around one ROV Tiburon dive that started early this morning and ended after dinner time. Like yesterday, the ROV collected samples along a transect across a local ridge crest in the Guaymas Basin. Sediment core samples (see right) and measurements from pH, methane, and radium sensors and a special heat flow temperature probe were taken at intervals along the transect line. The flanks of the ridge were coated in the loose mud that characterized yesterday's dive.

Excitement mounted in the control room by mid-afternoon as Tiburon ascended up the flank to the ridge crest. The vehicle's camera recorded the changing seafloor as it went from solid sediment cove to carbonate rock rubble. More benthic animals were observed in areas with rocky rubble versus the soft mud. Tiburon had crossed from the Pacific plate to the North American plate. And we hoped to find another gas vent along the summit.

Charlie Paull led the survey of the area using Tiburon's sonar, maps of the seafloor, and knowledge of the rock types and features. Though this search didn't find a geyser-like vent as hoped, it did find evidence of hydrocarbon gases when Tiburon landed on the seafloor and disturbed a bubble cloud out of the sediment. (At left, more benthic animals were observed in rocky rubble versus the soft mud.) 

Tomorrow's mission is focused on finding gas vents in this ridge crest area. Peter Brewer's group has added a new instrument to Tiburon for the dive, a specially-designed heated gas funnel. As we saw firsthand on Tuesday, hydrocarbon gas bubbles quickly freeze into ice-like hydrates when captured at deep-sea temperatures and pressures. With a heated element in the funnel, the instrument will melt the material back to its gaseous state. Three pressure-rated cylinders on the device will store the captured gas samples. The targeted ridge crest is about 1500 meters deep with a hydrostatic pressure of 150 atmospheres or 2250 pounds per square inch. At room temperature and pressure, the volume of each sample cylinder corresponds to 18 liters of material, enough gas to fill nine two-liter soda bottles. The cylinders will be returned with the R/V Western Flyer to MBARI to be processed.

– Debbie Meyer

Peter Brewer, Ed Peltzer, and Buzz Scott ready 
the heated gas funnel for ROV Tiburon.