Leg 3 Logbook - Gas Hydrates
Day 6 — A Tale of Two Dives
August 7, 2009
Latitude 48 degrees 40.26 minutes N
Longitude 128 degrees 50.27 minutes W
Today was another two-dive day, but the two dives were very different
from one another. The first dive exemplified the repetitive, systematic,
and sometimes tedious effort that goes into collecting scientific
data—the kind of data that forms the basis for a respectable scientific
paper. The second dive was full of excitement and dramatic discoveries.
During the morning’s dive, Charlie collected a series of samples and
measurements starting within Bullseye Vent and extending out into the
surrounding flat, muddy seafloor. As in yesterday’s dives, we had plenty
of time to watch brittle stars crawl around on the mud while we
collected vibracores and heat-flow measurements. We only traveled about
50 meters between each measurement, and it was visually hard to tell one
site from another.
Craig and I have developed a new, more
efficient method for removing excess water from the vibracore tubes.
The goal of all this repetitive sampling was to find out if there were
any differences in the sediment chemistry or heat flow between the
“vent” and the surrounding area. If the sediments near the vent were
very different from those farther away, it might imply that the
vent was still active. Conversely, if there were very few differences
between the vent and surrounding seafloor, it would imply that the
Bullseye Vent site might have been active in the past, but is not
currently active. We won’t know which of these alternatives are true until
the samples are analyzed in the lab. But everything we have seen over
the last two days suggests that not much is happening in Bullseye Vent
No less than eight people swarm around ROV Doc Ricketts
during one of our quick “pit stops” between dives. Since the ROV can
only carry five vibracores (the vertical aluminum tubes on the right
side of the image), we often come up to the surface as soon as we have
used up all the cores on the vehicle. This allows us to collect up to 10
vibracores a day. We need these two-meter-long cores to obtain
samples of seafloor sediment at depths where methane in the sediment is
interacting with seawater.
After several hours of systematic sampling, we brought the ROV back to
the surface for a quick “pit stop” and core removal. Then we headed back
down, this time to explore an underwater slope a few hundred meters east
of Bullseye vent. The excitement started during the long descent, when
several large Humbolt squid followed us down through the water, hunting
small fish that were attracted to our lights. These squid are typically
found in tropical waters, but apparently followed a patch of warm water
all the way north to Canadian waters.
When we were about 150 meters above the bottom, the real excitement
started. The sonar screen on the ROV (which is looks like a color
version of the classic circular sonar screens in old submarine movies)
started showing strange reflective patches in the water around us.
Normally we don’t see anything on the sonar until we reach the seafloor.
We flew over toward one of these patches and discovered that it was a
stream of little bubbles rising up through the water. It felt as if the
ROV had suddenly flown into a glass of champagne. However, instead of
carbon dioxide, we knew that these bubbles probably contained methane,
rising up from vents on the seafloor.
We tried to follow the bubbles down to the seafloor to find an active
methane vent, but they mysteriously disappeared. So we wandered the
seafloor looking for the source of the bubbles. At first we were
frustrated in our search, but then we made the second exciting discovery
of the dive—we stumbled upon the bones of a dead whale. The whale’s
vertebrae formed a neat line about eight meters (26 feet) long across the seafloor.
They were coated with white bacteria like bread mold. The whale’s
two-meter-wide skull loomed in the darkness nearby, creating a macabre
but fascinating scene.
We were fascinated to find this backbone of a dead whale
on the seafloor. From the size of the head and vertebrae, we figure it
was either a humpback or a blue whale.
Such “whalefalls” provide a huge banquet for deep-sea animals, which can
last for years or even decades. They also support many amazing and
unusual animals. For example, Bob Vrijenhoek, one of our fellow
researchers at MBARI, discovered a family of worms that live almost
entirely on the bones of whale bones. These worms have no eyes, legs,
mouths, or stomachs, but they do have some internal bacteria that let
them live off the fat in whale bones.
We wanted to collect a piece of the dead whale’s backbone, but none of
its vertebrae would fit in our sample boxes. So we collected dozens of
high-resolution photographs, which we hope will provide hours of
entertainment for Bob Vrijenhoek and his research team.
After taking the photos, we reluctantly left the whale carcass behind
and returned to our previous exercise—chasing bubbles. This led us to
our next discovery—a patch of seafloor where huge, flat slabs of rock
had buckled upward and cracked like a concrete driveway being forced
upward by tree roots. Charlie jumped out of his chair in excitement and
exclaimed, “This is exactly what I hoped to see!” The buckled rocks
demonstrated that something related to the methane vents was forcing the
seafloor up from underneath. Charlie believes that this same process
created many of the mysterious seafloor features he has seen around
After taking yet more photographs, we left the buckled slabs and
followed the sonar until we finally found what we had been looking
for—an active methane vent. It was even marked by an “X” on the seafloor
(the “X” had formed where two cracks crossed an outcrop of rocks and
mud). From within a three centimeter wide crack in the seafloor, a continuous
curtain of bubbles rose up into the water, creating an almost magical scene.
A beautiful curtain of bubbles streamed out of this crack in
the seafloor—our first active methane vent!
After taking still more photographs, we finally got down to the first
planned scientific task of the dive—installing Laura Lapham’s second
osmosampler on the seafloor. Laura was elated to find a methane vent
where she could place her chemical sample collector right next to the
stream of bubbles. Of course, by placing the apparatus on the seafloor
next to the vent, we improve the visual aesthetics of the site. But we
consoled ourselves in the knowledge that this seafloor area was
continually being reworked by the gas percolating up from underneath.
After successfully installing Laura’s sampler, the rest of the dive was
almost anticlimactic. We went back into “systematic fieldwork” mode,
collecting a few more vibracores and heat-flow measurements before
heading back to the surface. With no push cores and only a few
vibracores to process, the wet lab was deserted by 6:30 p.m. Quite a
change from previous evenings! Instead of processing cores, we sat
around the dry lab talking for hours about the things we had seen
during our dive (as well as everything else under the sun).
Tomorrow (Saturday) will be our last day working around Bullseye Vent.
On Sunday, a cable-laying ship will be laying an undersea research cable
in this area and another scientific ROV will be diving nearby at the
same time. But what we discovered today suggests that more discoveries
have yet to be made in this part of the ocean.