May 22,
1998
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Staying optimistic in a hostile environment
We last surfaced two days ago on 20 May to affect repairs
to the balky laser line-scanner. As you may recall, we fixed the data recording
drive, only to submerge and find the shutters to the line-scanner had vibrated
shut during transit to the bottom. When we surface, the shutters were fixed,
but then the recording drive failed again. We spent the day on the surface
trying to figure out how to fix the drive and finally pulled one out of
Ian's PC computer on the Chouest. Mark installed the drive and got it working
and we submerged again about dinner time. When we got to the bottom, we
fired up the equipment only to find that the laser power supply kept tripping
the breaker.
Subsequently, we determined there was a short in system.
Evidently, either the electronics bottle flooded or there is a short in
the cable, perhaps at the penetrator where it enters the NR-1 hull. Sadly,
the laser line-scanner portion of our program was kaput. The problem with
being on the cutting edge is that sometimes you get bloodied. The laser
was a great idea, to make huge digital "aerial photos" that could
be used to monitor the seep sites. Maybe it will come together on a later
cruise. I feel badly for Ian, who worked for months to get this machine
and NR-1 together.
One thing I've learned in 21 years of going to sea is that
things break. The sea is a hostile environment for equipment. I always say
that you should kiss your equipment goodbye before you put it over the side
because there's a good chance you won't see it again. Anyway, Neptune decided
lasers are out this year. So when something major breaks, it's time for
plan B. We still have the X-Star profiler chirping away and we can still
cruise along and look out the viewports and shoot digital still photos-so
that's what we are doing now.
After submerging, we dove to the well-studied GC234 seep
site. The day before, our colleagues on the R/V Carolyn Chouest had laid
down a network of four acoustic transponders that would help navigate the
sub precisely. We found Ian's bubbleometer (a device to measure the volume
of gas seeping out of the seabed) sitting where he left it last year to
collect data. It will have to wait a few more months to be recovered because
we have no way to recover it easily. At GC234, we ran very closely-spaced
X-star acoustic reflection profiles in a grid around two sites known to
have gas hydrate mounds. Our goal was to image the subsurface layers and
make a 3D layer image of these sites. Those little surveys went well and
we moved on. Next we ran back to the south to site TAMU-10 to look at the
mound again and try to find the source of the feature that looks like a
huge sediment flow emanating southward from this mound. There wasn't much
atop the TAMU-10 mound-just a scattering of disarticulated clam shells and
some beggiatoa bacteria mats. Signs of a little bit of seeping, but not
enough to support a sizeable chemo community. We never found anything that
looked like a flow. In fact the seafloor to the south, where the flow appeared
to be, was covered with a thin layer of acoustically transparent sediment
that showed extensive surface burrowing. I think what we were seeing in
the sea surface side-scan sonar records was a buried flow that happened
some time in the past and has now been smoothed and burrowed by benthic
organisms. The side-scan sound waves typically penetrate the seabed by a
few meters or more, so that's why we were seeing a buried layer.
We turned from site TAMU-12 (a point on the flow) to TAMU-14,
another large mud mound. Like some of the other large mounds we have explored
already, this one showed a few signs of seepage, but no extensive chemo
communities. We saw a patches of beggiatoa bacteria mats, mussel and clam
shell beds, and a few small, stunted tubeworm bushes. From TAMU-14 we cruised
to Brine Pool NR-1, a small mud volcano with a brine lake about 40-m in
diameter at its center. The brine contains methane, so it is ringed by a
donut-shaped bed of chemosynthetic mussels that Ian and others have studied
for a number of years. The lake is called "Brine Pool NR-1" because
it was discovered by this very vessel during a 1989 cruise. Because of its
geometry, the mussel beds are good acoustic reflectors and so they can be
seen a long way off in the submarine's forward-looking sonar. That's how
they found them back then. Our survey here has consited of a grid of closely-space
profiler lines to determine the structure of the mound beneath the pool.
Our data shows a depression about 15-20 meters deep and 120 meters or so
across. It appears this depression was blown out by an explosive event,
perhaps caused by dissociating gas hydrate, and it was subsequently filled
by a mud volcano. Brine escaping up a fault made a vent here and filled
the depression.
As I write, we are finishing the Brine Pool survey and
will head for site TAMU-15. In the morning we surface and this message will
be sent. Despite the setbacks, morale is still high. One reason to surface
is to let Mark off and bring student Alice Pechahchy on. Since we cannot
talk to the surface ship, she doesn't yet know of this change in plans and
is in for an early morning surprise. Ian felt his hard-working grad students
deserve a ride. If things go as planned, we will do this again in a few
days and then it will be student Mike Peccini's turn.
That's all from 2087 feet below the sea this night. |
