Sea exploration with the RV Southern Surveyor
11 January – 1 February 2008
08 reports from the
the Deep Sea Learn
about the voyage objectives, meet the
the Autonomous Benthic Explorer (ABE) >more
Report No 2: Surviving
the "Zone of Death"
By Ron Thresher
The last few days have seen
our first spell of good weather (defined as winds more
or less consistently less than 20 knots) and we have
used the time to do three ABE deployments.
The first launch, in mid-day and good weather, was beautiful,
with the vehicle slipping easily into the water after being
released from the crane that hoisted it over the side.
Deployment takes about 8 people, most holding lines to
keep the vehicle from swinging about while in mid-air.
The conditions were good, and deployment went smoothly,
despite the ship's crew having no previous experience with
the vehicle. Its recovery, however, was much more complicated.
In normal operations, the ABE is recovered by bringing
the ship about a hundred meters from it, after ABE surfaces
(which takes about an hour from 1000 m). Then the ABE's
motors are used to swim it to the stationary ship, after
which a line is slipped over a cleat on the top of the
ABE, more lines are secured fore and aft on the vehicle,
and ABE is hoisted out of the water, swung onto the deck,
and lowered into its cradle. Not quite so simple on the
Southern Ocean. Winds had been increasing throughout the
day, as had the height of the waves. By the time recovery
was started, the winds were gusting to more than 35 knots
and a 4-6 m swell and steep chop was buffeting the ship.
We tried the standard approach, but quickly gave it up.
ABE's motors, though adequate in the relatively quiet water
1000 m below the ship, could make no progress in the heavy
seas, and so the effort was abandoned. For the next attempt,
the ship's master very slowly moved Southern
to and up-wind of the ABE, providing it a small pocket
of sheltered water in which the recovery could be attempted.
As the ABE drew into the still pocket, its motors kicked
on and it was steered towards the waiting crewmen, who
successfully got a line on it, and then several more. The
rocking ship yanked the vehicle in and out of the water,
and as it was lifted clear, it began swinging wildly back
and forth, pulling lines free and hammering against the
hull. Quickly, ABE was dropped back into the water, to
stop the swing, and a second attempt made, this time with
even more lines and people holding them. This time success,
as the ABE's gyrations were slowed, and the vehicle dropped
solidly into its cradle, into which it was quickly lashed.
It took most of the next morning to repair the damage
to the vehicle, most of which fortunately was superficial.
A few replacement hull plates and a few braces straightened
out, and it was ready to go in again.
We've now done three missions
with the vehicle. The first, described above, was a brief
dive to test our systems. The second, intended as a long
working mission along the top ridge of the Sisters seamounts,
quickly ran into problems. As ABE tried to swim through
the valley between the first two peaks, it suddenly stopped
moving, prompting fears it had tangled in lost fishing
gear (the Sister's Hills have been heavily trawled in
the past). If fishing ropes were tangled in the ABE's
propellers, our study would be over. ABE would be stuck
on the bottom, forever, unless another deep diving vehicle
could be sent down to recover it. After a tense 40 minutes
and some manoeuvring by the vehicle (turn left, turn
right, all done by the ABE's on-board computer following
its emergency programming), the vehicle again began to
move, prompting a lot of relief and speculations ranging
from amorous giant squids to complex bottom topography
that might have confused ABE's navigation systems. The
truth proved more interesting. Having cleared the valley
once, ABE had to traverse it again on its return trip.
Hitting the "zone of death", ABE was suddenly
shoved more than 100 m off-course, to the far sides of
the pinnacles, apparently by a previously unknown strong
current running through the valley. This current had been
what stopped the vehicle the first time, as its weak electric
motors could not make progress against the strong currents.
Hopelessly off its programmed course, ABE nonetheless spent
the next eight hours photographing the far side of the
hills, before surfacing and being recovered.
Location of the Southern Surveyor, 11:42 pm 20 January 2008
The final mission was more "textbook".
Sent down to a deep pinnacle and ridge from 1250-1850
m depth, ABE spent a more or less uneventful 23 hours
on the bottom photographing and surveying the ridge and
Results so far? The second dive, on the shallow (850 m)
peaks of the Sisters seamounts, was disappointing. The
mission confirmed that the tops of the pinnacles are barren
rock, with the occasional piece of broken coral lying on
the bottom, a few live corals in cracks and crevices, and
the occasional orange roughy swimming by. Several years
of fish trawling has apparently largely denuded the tops
of the seamounts. It had been suggested that we could find
healthy reefs in the valleys between the pinnacles, which
were thought to have been lightly fished, but even there
coral was very sparse. The third mission, to the deepest
part of the Sisters, was also informative. Unlike the pinnacles
of the shallower seamounts, the deeper ones were blanketed
by extensive fields of deep-sea coral, from about 1250
to 1400 m. Mixed into the corals was a range of echinoderms,
hydroids, seastars and crinoids, as well as the occasional
fish and crustacean. Below this zone, life was much sparser,
with large areas of apparently barren rock and sediment
slides along steep cliff faces. We see the occasional stalked
crinoid, urchin or seastar, but none of the reef present
only a few hundred meters farther up the slope. However,
even deeper, at 1850 m, we again found coral. A steep cliff
face about 50 meters high ran along the base of the ridge.
At its bottom were a few solitary corals, relatively sparse,
some fossil, but also a few apparently live, hundreds of
meters deeper than we'd expected.
We are now en route to our second survey area, a series
of underwater ridges and mountains near the Tasman Fracture
Zone. The next area will test how deep the coral, and other
reef animals, can be found, as we hope to reach depths
of 3 km on the next set of ABE missions.
report] | [next
report] | [home]