Leg 3 Logbook - Gas Hydrates
Day 2 — An eventful first dive
August 3, 2009
Latitude 44 degrees 34.079 minutes N
Longitude 125 degrees 9.18 minutes W
The day started out like most days on the Oregon coast—calm and foggy. At 9:00 a.m. sharp, we left the dock and steamed out the entrance channel, passing underneath the picturesque span of the Yaquina Bay Bridge. Both ship’s crew and science crew took photos, talking of past and future journeys, and thinking of our friends and family on shore. The ship’s crew has been at sea (with short breaks) since the beginning of July.
Second Mate Pat Duffy takes souvenir photos of Jim Boedecker and Dan Benvenuti as we leave Yaquina Bay.
Although the skies remained overcast, the winds stayed relatively light. We passed pairs of murres—adults and young swimming side by side—while albatrosses wheeled over the swells. According to the crew, the water has been unusually warm (up to 60 degrees Fahrenheit), and albacore tuna have been biting just offshore.
On the way out, we had our required security and safety briefing, which included donning bright red survival suits (and waving at the camera). It’s one of those little rituals that are part of going out to sea on a research cruise.
Tess, Craig, and Yirang model their survival suits during the security and safety meeting.
Around 2:00 in the afternoon, we reached our day’s dive spot, about 64 kilometers (40 miles) offshore. As we prepared to lower the ROV into the water, a pod of dolphins swam by, splashing and eyeing our strange looking craft. In addition to its usual cameras and manipulator arms, the ROV carried about 32 push cores and five long, aluminum vibracore tubes that we hoped to use to collect samples of seafloor sediment. It also carried four Niskin bottles for collecting seawater.
Brian Schaefer helps launch ROV Doc Ricketts during our first dive of the cruise. On the far right you can see the seven-foot-long aluminum core tubes on the side of the ROV that are used to collect samples of deep-sea sediment.
Today’s dive was in the southern portion of an area called Hydrate Ridge, but we didn’t see any methane hydrates during the dive. What we did see, at least on the sonar maps, was a strange, oval depression in the seafloor, several hundred meters across, with a 30-meter-high mound in the middle. In the exaggerated vertical scale of the sonar maps, this feature looked to me like the ruins of a medieval castle, complete with a surrounding moat.
We reached bottom on the flat seafloor outside the “moat.” Although the seafloor was covered with mud, we weren’t able to insert our core tubes more than half a meter, presumably because of some very hard layers within the mud. When we later examined a little of this mud in the lab, it was very hard, as if it had been buried a few tens of meters below the seafloor, then somehow ended up back near the sea surface again.
We descended into the “moat” and then began to climb up the central mound. Soon we came upon huge slabs of very hard, light-colored rock. Looming out of the darkness, these outcrops again reminded me of the ancient ruins of some massive fortress. They were split by large cracks, some of which were half a meter across and several meters deep.
These slabs are most likely a type of rock known as carbonate. Carbonate rocks often form where methane from beneath the seafloor comes in contact with chemicals such as sulfate in seawater. Some geologists who have studied Hydrate Ridge believe that the carbonate mounds in this area formed above the seafloor, growing upward around groundwater seeps like the weird tufa towers at Mono Lake.
Charlie Paull has an alternative hypothesis. He believes that these carbonates formed below the seafloor, within the mud, and were later lifted up by pressures from below. They could also have been exposed when the softer sediments around them were eroded away by currents. Charlie has studied similar carbonate structures off Southern California and in the Gulf of California, where he has seen similar evidence of uplift and erosion. One of his goals on this expedition is to find evidence for his hypothesis in the heavily studied areas of Hydrate Ridge and Barkley Canyon.
Unfortunately, not only were we unable to collect any deep core samples during our dive, but we had a significant equipment failure. A few hours into the dive, a rack that we used to carry the seven-foot-long vibracore tubes broke off the ROV and fell to the seafloor. It took us a few minutes to realize what had happened, but at that point there wasn’t much we could do.
This photo, taken by ROV Doc Ricketts, shows the massive slabs of rock that lie cracked and broken on the slopes of southern Hydrate Ridge. For scale, you can see the seven-foot-long vibracore tubes and the core rack that fell off the ROV during our dive.
Since we still had some time left to dive, we decided to collect video instead of cores. We spent the next hour or two flying slowly up the sides of the mound, videotaping the rugged slabs of carbonate rock. As Charlie pointed out, the cracks in the slabs looked a lot like the cracks that form on the crust of a loaf of bread, as it rises in the oven. This and other observations added support for his hypothesis that the rocks on Hydrate Ridge were pushed up from underneath.
In the end, the ROV pilots were able to bring the core rack back to the surface, but not the vibracores we had collected. After preserving the remaining push cores and water samples, most of the science team headed to bed.
The captain has turned the boat northward and is presently heading toward the hydrate outcrops off Vancouver Island. We will be steaming north all night and most of tomorrow. While we’re underway, the ROV pilots will repair the core rack and the science crew will figure out how to get the sediment samples that Charlie needs to understand some of these strange and impressive seafloor features.
Michael Riedel, from the Canadian Geological Survey, examines some of the short sediment cores that we collected during our dive.