Leg 1 Logbook - Laser Raman Spectroscopy
Day 4 – If it is Friday, it is time to get to work ...
July 10, 2009
0900 hours – On Station at Hydrate Ridge, 50 nautical miles off the Oregon coast.
Latitude 44 degrees 34.2 minutes N
Longitude 125 degrees 8.9 minutes W
Last night, the R/V Western Flyer arrived on station at the southern summit of Hydrate Ridge. This site is well known among geochemists as an active deep water methane gas vent. The gas is more than 98 percent pure methane indicating that it is primarily biogenic in origin. This methane and the associated hydrogen sulfide support a local community of bacteria which form mats on the seafloor. Additionally, at these temperatures (approx 4 degrees C) and pressures (80 atmospheres), the methane reacts with seawater to form clathrate hydrates which are of great interest to the Brewer group and the subject of our expedition. Unfortunately, should the gas hydrates ever be exposed on the seafloor, they will quickly dissolve. It is only in the methane-rich sediments that they are stable. So our hunt begins by looking for either active gas vents (which we can see with the sonar on the ROV) or the bacteria mats on the seafloor.
Randy Prickett, Peter Walz, and Dan Benvenutti prepare the elevator for a dawn launch at Hydrate Ridge.
This morning everyone was up before dawn so that we could launch the elevator at first light (above). The elevator will carry several glass chambers where we will place the clathrate hydrates for inspection with our laser Raman spectrometer. Once the elevator is hung over the side by the A-frame and gently lowered into the water, a release line is pulled (see below) and it is allowed to free-fall 800 meters to the seafloor. A homing beacon is attached so that we can track it from the ship, and then find it on the seafloor. Later the ROV will move it into position near a hydrate collection site for our experiments.
MBARI’s vibra-core elevator is carefully lowered into the ocean from the A-frame on the stern of the R/V Western Flyer. When the release line is pulled, the elevator will free-fall to the seafloor 800 meters below. A homing beacon is attached so that we can track the elevator by sonar during its descent and then quickly locate it with the ROV on the seafloor.
Once the elevator is released, the moon-pool area is a flurry of activity as the ROV pilots ready the vehicle for launch.
Once the elevator was released, all hands turned to making the ROV ready for launch. The pilots were moving so fast, that not even high speed digital photography could “catch them on film” (above). Within less than a half hour, the ROV was ready for launch and our day’s dive could begin. We dove directly for the elevator as everyone was anxious to see if it landed upright and if the glass chambers had survived the free descent to the sea-floor. Following the sonar beacon, we quickly located the elevator and to everyone’s delight it had landed upright and all the equipment was intact (see below).
MBARI’s elevator has arrived safely and upright on the seafloor. Here it is located at 800 meters depth just slightly west of the southern summit of Hydrate Ridge. The orange color is the float pack made of syntactic foam that will allow the elevator to rise to the sea surface when the weights are released at the end of the experiment. One of the glass hydrate chambers is visible on the lower right of the elevator.
After exploring the seafloor for an hour, we located several bacteria mats which seemed likely candidates for harboring methane and hydrogen sulfide rich sediments below. Normally, core samples would be taken that would need to be carried to the sea surface and analyzed on-board. However, with our new pore-water attachment for our second generation deep-ocean Raman in situ spectrometer (DORISS2), we were hoping to conduct our analyses on the seafloor. So we quickly set-up the ROV relying on the skill of the pilots to carefully insert the probe straight-down into the sediment below the bacteria mats (see images below). Initial analysis of the spectra revealed the presence of methane and hydrogen sulfide just 20-30 centimeters below the seafloor. Encouraged by our early success we spent the day inspecting the sediments below the bacteria mats at a variety of sites near the southern summit of Hydrate Ridge.
The pore-water probe is carefully inserted into the sediment below a bacteria mat (white) with skillful manipulation of the ROV arm by the ROV pilot. Visible at left is a conventional push-core which was collected for comparison with the in-situ analyses.
A close-up view of the pore-water sediment probe. The scale on the side of the probe shows the depth of penetration (in this case 27.5 cm). A small rockfish lurks nearby keeping careful watch on our experiment.
Late in the day, we spotted several small gas vents on the ROV sonar and transited over to take a look. Unfortunately, before we could accomplish much, the wind had come up and with the sea surface getting rough we elected to recover the ROV and return another day.