NPB-9507 Weekly Cruise Report #3 ...
NBP-9507 Cruise Report
Week Three: November 10-16, 1995
During the third week of NBP9507, we worked in Bransfield Strait and
the southwest Scotia Sea. We acquired about 1,000 nautical miles of
swath bathymetry, 67 miles of 48-channel seismic data, lowered the ZAPS
sled seven times, acquired three water sample rosettes, retrieved one
sediment core, took two dredge hauls, and made two camera lowerings.
Bransfield Basin
At the end of last week we had just rendezvoused with RV Polar Duke and
started our first ZAPS sled drift. After we finished the sled tow, we
slalomed our way around the central Bransfield Basin dodging icebergs,
bergy bits, brash ice and some healthy pack ice. Sometimes our course
looked like a drunken sailor's meander back to his bunk. We then
approached the submarine volcano we had previously imaged (see the
report from week 1) and lowered the ZAPS sled and a rosette into the
caldera.
On two previous Seabeam survey runs we passed very close to what should
be a nearly symmetrical volcano on the steep northern margin of central
basin. Once again ice covered the feature and we did not get near
enough to image it. We then headed off to what is now being called Fish
Ridge for a ZAPS drift. The drift lasted eight hours and covered about
4.25 n.m. in a straight line along a sawtooth course created through
deft adjustments by the bridge officers to keep us atop the linear
ridge.
Southwest Scotia Sea
After Sled Drift #20 we headed north into the Scotia Sea in hopes of
collecting multi-channel seismic data. We spent the next two days
surveying the southwestern corner of the Scotia Sea northwest of
Elephant Island. On the second day, we crossed an east-west trending
ridge up to 600 m high with a steep (up to 45 degrees) south face.
Lawver interprets it to be the equivalent of the San Bernadino
Mountains where the structure is dominated by compressive forces
resulting from a bend in the San Andreas fault. In this area, the
compressive stresses could be the result of a bend in the recently
reorganized Scotia-Antarctica plate boundary. We also found a linear
fault trace along the east side of the Shackleton Ridge with rotated
fault blocks where the fault trend changes.
Seismic Work
On November 13th we deployed the seismic streamer. It took about three
hours to deploy the streamer and airguns. Gun 5 was still leaking so it
was shut off. We then headed north for about four hours and recorded
MCS data across the equivalent of the San Bernadino Mountains, turned
west for about an hour and then ran a parallel course to the south. As
we crossed the southwest Scotia Sea we saw a lot of structure in the
sediments which ranged from 1 to 2 seconds thick. Basement was
well-imaged but there was no sub- basement structure apparent on the
single channel monitor record. Toward the southern corner of the basin
we detected what may be a major normal fault. In all, we collected 13
hours of MCS data and about 67 n.m. at the rate of just over 5 knots.
The data look good.
Fish Ridge
We headed due south to take a ZAPS sled lowering along the ridge in the
eastern Bransfield Basin. The eastern Basin was now covered with a lot
of pack ice where previously it had been clear. We then returned to
Fish Ridge, passing to the south of Bridgeman Island once again. We did
the second ZAPS drift in the Fish Ridge area. The survey was confined
to a small box less than 2 km by 1 km. In the process of dodging
icebergs within this small box, the sled detected an anomaly. Continued
drifting was undertaken to try to determine the extent of this plume.
Dredge work
We next undertook Dredge #1. The dredge team consisted of Carol Chin on
the bridge advising the bridge of position, Jay Simpkins on Simrad
watch to relay depths to us, and Larry Lawver in aft control telling
the winch operator to lower and raise the winch. We laid the dredge on
the bottom at about 1450 meters of wire out, got the ship underway,
passed over the target area and then pulled in the dredge. We got a
series of four groups of significant pulls with the last being the
greatest at 5,000 lbs. of tension. The rocks weighed about 40 kg and
included two large pieces of unaltered vesicular basalt with many small
attached organisms and numerous smaller rocks including a few glacial
erratics. We also collected three fish, one slimey yucky worm-like
creature, numerous brittle stars, and a small arthropod. The largest
fish was a pouty-lipped Scotia sea Gapehead (Chaenocephalus aceratus)
about 47 cm long. Since the fish were wedged into the dredge basket
with the rocks we infer they were collected near the bottom. The
Gapehead is a bottom dweller although it has previously only been
collected in water depths of 5 to 439 m (Miller, 1993). This one was
collected somewhere around 1200 m.
Underwater Pictures!
Immediately after the first dredge we lowered the Benthos underwater
camera loaded with Polaroid black and white 35 mm film. We found this
camera in the ASA warehouse in Punta Arenas in 1993. During NBP9301, we
pressure tested the housing and fired the camera in the lab but did not
get an opportunity to deploy it. The camera is triggered by bouncing a
dangling trip weight on the bottom. Thirty-five bounces over Fish Ridge
produced about 20 successful photos. Most were of a flat muddy bottom
with an abundance of brittle stars, including over thirty in a field of
view estimated to be only two meters across! A couple of images showed
basalt outcrops along with various other benthic creatures such as
seapens.
Side trip to slope volcano and a core
While giving the ZAPS crew time to recuperate, we made a short swath
survey in which we finally successfully surveyed the volcanic edifice
perched on the northern slope of the central basin. We recovered our
second kastenlot core near where a German core contained higher order
hydrocarbons a number of years ago. We retrieved a full three meters of
mud, with more intriguing black- olive mud than in the first core.
Additional surveying was done in the northeast corner of central basin
and we finally returned to Fish Ridge (so-named for the Gapefish
collected in the first dredge).
Return to Fish Ridge
A second ZAPS drift was made along Fish Ridge in a similar sized box
next to the original one. Again two very large anomalies were found. A
second dredge and a second camera run were made. The proscribed dredge
course into the wind resulted in dredging along a nearly flat bottom.
One very fresh looking basalt rock was recovered, again with an
abundance of benthic flora and fauna. The rock was about 25 cm by 15 cm
by 15 cm and weighed about 6 kg. It appeared to have been part of a
pillow lava. The second camera run experienced initial difficulties
when the second pinger available on the ship turned out not to be an
oceanographic pinger but rather a location beacon. After worrying about
what to do, it was decided we should try to see if we could operate the
camera without the use of a pinger utilizing the tensionmeter. The
total package including camera and pinger with about 1100 m of wire out
weighed 750 lbs. When the camera hit bottom the tensionmeter fell off
to about 600 lbs. So, we were able to control the camera along the
bottom by observing the tensionmeter and raising and lowering the
camera about 5 to 10 meters between "hits". Twenty five such lowerings
get to be very boring. In the picture above, the bottom is muddy, with
a pillow basalt outcrop to the left (vent?) and a white brittle star
and seapen nearby. After the camera run we left the central Bransfield
basin and headed for the eastern Bransfield Basin where we began a ZAPS
drift along the central of what we call the Three Sisters. This ended
what we considered to be a highly successful and satisfying week.
Next Week Tune in for more seismic, coring, and ZAPS, a trip to the Chilean Base Frei, and Thanksgiving aboard the Palmer!
Reference
Miller, R.G., 1993, History and atlas of the fishes of the Antarctic
Ocean, Foresta Institute for Ocean and Mountain Studies, Carson City,
Nevada, 795 p.
Lawver, Klinkhammer, and the Shipboard Scientific Party
Jackson School of Geophysics»
Note: The photos and journals
contained in this website were prepared by Steven Stevenoski, Science
teacher at Lincoln High School in Wisconsin Rapids, Wisconsin