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April 9th, 2003; Leg 4, Day #6

Today’s update is provided by Dave Clague.  

Today was a quiet day on the ship as we transited from Guaymas Basin to the northernmost segment of the East Pacific Rise, due east of Cabo San Lucas. The transit will take us about 30 hours, and we will arrive later tonight in plenty of time to dive tomorrow at 6:30 in the morning. This segment of the East Pacific Rise, named the Alarcon Rise, will be our work area for the next two dives as we search for hydrothermal vents in the region. But I have gotten ahead of myself. The quiet day was used by most of the scientific party to catch up on samples in the lab, add to notes on samples and observations from the previous dives, do some laundry, read a book, get some exercise, even to try one’s luck at fishing off the stern of the R/V Western Flyer. At right, Joe and Darrell look on as Erik reels in a small yellowtail tuna. I suspect there will be more than one person with sunburn tonight, despite the cloudy day! 

The ROV team spent a good part of the day making repairs to the elevator recovered at Guaymas and reconfigured the ROV for tomorrow’s dive. Unfortunately, we scientists changed our minds about what exactly we wanted to be able to do tomorrow, making some extra work for our fellow shipmates. One of the difficult things about a cruise is being able to plan ahead, yet adjust to account for equipment that is not working as we had hoped or that gets damaged, or simply because you do not find what you expected to find. Each dive starts with high hopes and expectations that are gradually, or sometimes catastrophically, adjusted to the reality of the conditions when you get the ROV to the bottom. Only rarely does an exploration dive find exactly what even the most experienced scientist expects to find. It is this string of surprises and challenges that keeps us all coming back to sea—the opportunity to find the unexpected, the unimagined, and the undiscovered. Today, I will tell you what we plan to do tomorrow and what we hope to find and then tomorrow, I will tell you what actually happened and what we found.



Shana and Josh squeeze into
the science van to run some
sulfide samples.





A few years ago, a plume of water emitted from hydrothermal vents was detected above the Alarcon Rise during a cruise led by Dr. Peter Lonsdale at Scripps Institution of Oceanography. During their cruise, they did not locate the vents that were emitting that plume of hydrothermal water since they did not have an ROV or submersible. Armed with a bathymetric map of the region and the location where Lonsdale found the plume, which he generously provided to us, we are now going to see if we can locate the actual hydrothermal vents and sample the animals that live there. This requires a bit of wishful thinking: that the vents are still active today, that the plume was located close enough to the vents to serve as an exploration guide, and that the vents will be large enough for us to have a reasonable chance to locate them. 

It seems like it should be fairly simple, but the location where the plume was detected can be some distance from the actual vents as the hot water rises and then spreads out, only to be carried by unknown currents in unknown directions. In addition, the location where the plume was located is not that precise, since the plume was detected by a sensor towed behind a ship. It also makes it more challenging in that the ROV cameras can only “see” a short distance (15 meters or so, if the water is very clear) on the bottom, and the possible area is large. It is quite possible to drive by in the dark and miss the feature one is searching for.

To increase our chance of success, we have also considered where other hydrothermal vents are located on the ridge system. We have examined the bathymetry and tried to locate the axis of spreading on the ridge axis (the place where the latest lava flows should have erupted), and we have determined the region along the ridge axis where the ridge erupts most frequently, which is the shallowest point along the axis. It turns out that the plume is located within a few hundred meters of the ridge axis, but several kilometers north of the shallowest, most volcanically active part of the ridge. For these reasons, we have planned our initial survey to move to the south, back and forth across the axis of the ridge, working towards the shallowest section of the ridge axis. We will also look for increased animal populations that commonly surround the actual hydrothermal vent communities and for the presence of hydrothermal sediments. Using these types of clues, we hope to locate the vents on the sea floor.  

During this preliminary search, I will also be able to get a good look at the volcanic terrain along the ridge axis, sample some of the lava flows for later analysis, and attempt to determine the style of eruptions that have occurred in the region and shaped the ridge axis. One of my main interests is in determining if eruptions along the East Pacific Rise have a mildly explosive character, as we have found at the Gorda Ridge off northern California. To determine this, I will try to collect fine, glassy particles that settle out of the water from such eruptions. Many of these particles are delicate, thin, bubble-wall fragments that form when large bubbles of gas escape through the surface of the magma at the erupting vent. I will try to collect these small, sand-sized particles in sediments using short cores or to vacuum them directly from the surfaces of lava flows using a small suction sampler designed for this task. If our plan is sound and we are lucky, I will collect the samples I need to test the idea that eruptions along mid-ocean ridges are significantly more violent than usually thought, and we will have located the hydrothermal vents and their animals communities for sampling on the next dive and perhaps on the last dive at the end of the cruise.

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