Report of Activities on the RVIB N.B.
Palmer Cruise 02-04
The survey work in the central
sector of the Southern Ocean GLOBEC grid continued on 21 August with the vessel
finishing work at station 44, steaming to station 45, and starting work
there. Both of these stations are
furthest from the shore with station 44 at the end of survey line 6 and station
45 at the end of survey line 7. Both
were in the northeasterly flow of the Antarctic Circumpolar current.
A tow with the 10-m MOCNESS went
well except for a couple of snags of the towing wire on ice, but the catches
were good and Jose Torres was especially pleased to have caught a fish species
that he had never seen before and a couple of other rare ones. This tow ended around 0330 and a little after
0400 the ice collectors were out on a floe next to the ship. The ROV went in for a few minutes, but was
sidelined by thruster problems. A 2000 m CTD cast was completed around
0800. The 1-m MOCNESS tow, which was
last up at station 44, was aborted because of a problem with the A-frame. It was originally thought to be due to frozen
hydraulic lines, but it turned out to be a pump motor that had failed and had
to be replaced. The replacement took
several hours, delaying the deployment of BIOMAPER-II for the steam to station
45. Around 1330, the towed body was put into the water and was towyoed the remaining distance to the station. It was an excellent day for seabird and
marine mammal observations which took place throughout much of the daylight period.
The work at station 45 began at 1830 with the ice collectors being deployed onto a floe next to the ship, but the ROV was still under repair and was not deployed at the same time as had become the custom. This was followed by another 2000 m CTD cast and a Tucker trawl to collect live animals. Furcilia of Euphausia superba are in short supply this year and most stations now have Tucker trawls scheduled to try to collect enough for the shipboard experimental work.
The deep freeze that began on 19 August continued into a third day with temperature hovering around -25ºC. The barometric pressure settled in around 984 mb and the winds were out of the southwest about 15 to 20 kts throughout the day. About the time the CTD cast was being completed, the full moon was setting and the sun was rising. Its light was filtered into a wondrous array of colors and reflections by sea smoke coming up from the open leads spread throughout the pack ice. The exceptionally clear skies stayed the day. At the close of day, there were some wispy clouds near the horizon, which together with the sea smoke gave rise to the great sunset. The pack ice was mostly 10/10 and there were a number of frozen leads - large flat newly formed ice areas with no snow on them.
CTD Group report (Eileen Hofmann,
Bob Beardsley, Baris Salihoglu,
Chris MacKay, Francisco (
During 21 August, we completed
two CTD casts at stations 44 and 45, which are at the outer end of survey
transect 7. These stations were nominally
in 865 m and 2966 m of water, respectively.
The actual location of station 44 is on the outer shelf slope, but an
extended MOC-10 tow prior to the CTD cast resulted in the station being moved
off the shelf. As a result, this station
ended up in 3410 m of water. Station 45, which is intended to be in deep water,
was 2971 m deep. The consequence of moving station 44 off the shelf is that we
obtained two CTD casts in oceanic water on the offshore side of the southern
boundary of the Antarctic Circumpolar Current.
The CMiPS
instrument remained on the CTD for both casts, which meant that the casts only
went to 2000 m rather than the full depth because CMiPS
is not rated for depths greater than 2000 m.
This gives two CMiPS profiles in deep water.
The vertical temperature and salinity profiles at the two stations were similar. Both showed a well mixed layer just at the freezing point that extended from the surface to about 60 m to 80 m. Below this layer, temperature and salinity increased as Upper Circumpolar Deep Water was encountered. Both casts were deep enough to also include Lower Circumpolar Deep Water, which was seen at about 550 m to 600 m.
Water with temperature of 1.85ºC
was encountered at 400 m at station 44 and 1.81ºC water was encountered at
station 45 at 300 m. These temperatures
indicate that the southern boundary of the Antarctic Circumpolar Current was
crossed. This is consistent with
observations from other outer shelf stations that suggest that the southern
part of the Antarctic Circumpolar Current is flowing along the outer edge of
the west
Sea Birds (Chris Ribic and Erik Chapman)
Surveys were conducted on 21
August for over 7 hours as the ship traveled between stations 44 and 45. This transit was just off the continental
shelf and ran roughly perpendicular to the shelf break. Ice was mostly vast floe in 10/10ths ice
concentration. Temperatures were around
20ºC, and wind-chill dipped into the high 40's.
The transect crossed several large leads that
had recently frozen over and were now covered by a thin layer of new gray
ice. The ship's track snaked through
many large ice-bergs and second year sea-ice floes.
During the previous evening,
First Mate Mike Watson saw an Emperor Penguin at station 44. An Emperor was also seen during the survey alongside
a recently frozen lead. A steady stream
of Snow Petrels was observed flying directionally past the ship. All of the birds were flying in the same
general direction, moving from inshore toward the ice edge. Perhaps these birds are moving from leads
that have frozen over in the recent cold temperatures to feed in open water
along the ice edge. An adult Southern
Giant Petrel was also observed flying in the direction of the ice edge. No Crabeater Seals or Adélie Penguins were observed
in the survey period, though penguin and seal tracks were seen on the surface
of the ice.
A summary of the birds and marine mammals observed on 21 August (YD 233) during 7 hours, 14 minutes of survey time as the ship traveled between stations 44 and 45 is the following:
|
Species (common name) |
Species (scientific name) |
Number observed |
|
Snow Petrel |
Pagedroma nivea |
38 |
|
Emperor Penguin |
Aptenodytes forsteri |
1 |
|
Southern Giant Petrel |
Macronectes giganteus |
4 |
Marine Mammal report (
“Unpredictable, changing,
wonderful, and wild”, these are the words that come to mind when thinking of
Krill distribution,
physiology, and predation (Kendra Daly, Kerri Scolardi,
Emily Yam and Jason Zimmerman)
We completed two more Tucker
trawls at stations 44 and 45. The composition
of zooplankton in the upper 100 m in a very diffuse scattering layer was
similar at both sites. The euphausiid, Thysanoessa macrura, and the copepod, Calanus propinquus, were relatively abundant. In addition, we collected a few Euphausia superba, other copepods (e.g.,
Metridia, Paraeuchaeta),
some salps, a few ctenophores and amphipods, and a polychaete worm.
ROV report (Scott Gallager, Phil Alatalo, Alec
Scott)
ROV under-ice survey # 9 was conducted at station 43 on 20 August. This region was characterized by small, 1-m thick, re-frozen ice floes about 10 to 20 m in diameter with an extensive ridge field on the order of 1 to 2 m high. These characteristics proved difficult for the ROV to operate in because of the dangers of tether entanglement on the extensive 3-dimensional ice structure below. The bridge also had a difficult time finding stable floe on which the ice team could be deployed because cracks appeared quickly after the ship began to penetrate an area. After three attempts, a solid floe was located and we were off. First the ice team landed on the ice and then the ROV went into the water at 0727. Before the deployment, the bulb for the high intensity strobe on the 3D camera needed to be replaced. This required about 20 minutes of extra time on the deck allowing the temperature of the vehicle and cameras to reach the air temperature of 23ºC. Upon entering the water, ice crystals formed on the metal parts and roared to the surface in a turbulent plume. The glass camera windows iced over with an opaque layer which looked like someone had painted the windows black. The ROV was picked up out of the water, the camera windows sprayed with ethanol to dissolve the ice, and then returned to the water. The first hour of the dive was spectacular as the ROV ascended into cavernous under ice-domed structures constructed of rafted floes. As we moved through our transects, there were scattered furcilia up in the shallowest regions and small groups of 5 to 10 individuals located at 8 to 10 m depth immediately below the deepest ridges. Small numbers of ctenophores drifted by followed by the occasional fish larva. The under-ice surface of the floe being worked on by Frank Stewart and the ice team was smooth with the exception of pock marks 10 cm or so in diameter. A small number of single furcilia floated by with no signs of aggregations. The ridge line was clearly visible on the ROV mounted sonar system allowing good measurements of floe and ridged surfaces.
The ROV then moved into some
unconsolidated brash and bergy bits on order 2 to 10
m in diameter. Furcilia were present in low numbers
(approximately 2 to 10 individuals per cubic meter). The brash was moving around
a bit with a 1 kt current. Soon the ROV could not be moved up, down, or laterally
within this minefield of ice. The tether could be seen to have lodged into
several crevasses in front of the vehicle and additionally behind the vehicle
in its blind spot, keeping it on a short leash. We worked for several hours
maneuvering in and around the ice blocks and ridges trying to untangle the
trapped tether. A clump weight was dropped down the tether from the ship in an
attempt to lower the point of insertion and free at least one end. Several
times during this ordeal we gained some tether giving us the false impression
that we had been freed; however, as one region of the tether was freed,
additional entrapment points became evident. We were faced with two tenable options:
Send in the divers who might be able to free the tether, but possibly at the
same time jeopardize their own safety in this difficult environment. Another
thought, provided by Chief Scientist Peter Wiebe, was to very gently turn the
ship screws and get the water moving behind the ship just enough to cause the
brash to move and possibly free the tether without damaging the vehicle. We decided to try the second idea. Mate Mike Watson deftly engaged the screws
and put on 5º pitch at low rpm. After a minute, we could see from the cameras
on the ROV that the brash was moving. The deck crew continued to tug on the tether
and the ROV dove hard. Suddenly, we could see daylight off in the distance and
we knew we were free. The ROV was retrieved, but sustained some damage to a
thruster due to being jammed with ice. A spare thruster was swapped for the
damaged one and we were ready to go by the time we arrived at the next station.
ROV deployment 10 on 21 August at
Station 44 was cut short because of the current overload indicator lights on
two thruster motors. The starboard and
lateral motors showed that a water leak had probably occurred and needed
immediate attention. The ROV was retrieved 20 minutes after entering the water,
but not before making observations on furcilia
abundance close to the ship. Approximately 1 to 2 individuals were present most
of the time the vehicle was close enough to the surface to have the 3D cameras
in focus. No observations were made at depth
because we were making preparations for immediate retrieval. Following retrieval, the two offending
thruster motors were removed, cleaned, and refilled with a special oil to keep
water from entering the shaft housing.
BIOMAPER II group report
(Gareth Lawson, Peter Wiebe, Scott Gallager, Phil Alatalo Dicky Allison, Alec
Scott)
At 1330 on 21 August, we deployed
the BIOMAPER-II for a four hour towyo through the
deep waters between stations 44 and 45. The only acoustic feature present was a
diffuse shallow scattering layer that was as much as 80 m thick and generally
associated with the pycnocline. Near the start of the
tow, this layer was centered at about 110 m, but by 1730 appeared to have moved
to a more shallow distribution centered at 50 m, possibly indicative of a
diurnal vertical migration. Images captured by the VPR suggest that this layer
was composed of copepods, including large calanoids.
Interestingly, we also captured images of numerous diatom chains, large single
cell diatoms, and large radiolarians. The cameras on the VPR have an image
volume of approximately 1.6 ml for the high magnification camera and 20 ml for
the low magnification camera. The imaging rate is 60 images per second giving
volumes sampled of 6 liters per minute and 72 liters per minute, respectively.
As such, for us to catch an image of a planktonic
organism they must be relatively abundant, typically greater than about 20 per
cubic meter. In dense patches or aggregations of copepods or krill, the VPR
captures images at a rate suggesting their nearest neighbor distance to be on
the order of 1 cm or less. We have found such densities to be fairly rare this
year compared with previous cruises in the fall and winter months.
In order to contribute
substantially to our measurements of acoustic backscatter, organisms must
either be quite large relative to the wavelength of the incident sound wave, or
must occur at extremely high densities. As such, the small diatoms and copepods
captured by the VPR may not be responsible for all of the backscatter we are
observing at our currently operating frequencies of 120 and 200 kHz, unless
they are present at very high densities. What might also be contributing to the
remaining backscatter remains unclear, but we hope that the MOCNESS net samples
will shed some light on this question.
In the past few BIOMAPER-II towyos, we have been plagued by problems with the tow-body
not flying properly. In order to collect the best data possible, we like the
body to remain perfectly level, but recently it has been flying at as much as a
22º angle nose-upward and 22º to port. Our initial attempts at solving this
problem involved angling the adjustable aileron on the tail fin slightly
downwards and adding horizontal stabilizers mid-way up the fin. Since these
measures failed to solve the problem, most recently we have replaced the
plastic tailfin used since the start of the cruise with a plywood
one made last winter. The plastic fin had a strong curve to starboard, whereas the
woods one is much straighter, so we are hoping this will solve the problem.
Problems with electrical noise on the VPR video signals have been solved for
the most part as we become poised to sample the offshore stations of 45, 46 and
64.
Current Position and
Conditions
The broad-scale survey work for
the past couple of days on the Palmer
has been focusing on doing the work at the off-shore stations in the central
sector of the Southern Ocean GLOBEC grid.
This is because we have been able to effectively work in the pack ice in
this area. We are currently steaming
from station 46 to station 64 while towyoing BIOMAPER-II.
Our position at 0017 on 22 August is -67º 30.098′S; -74º 20.048′W. The air temperature is -26.7ºC and the
barometric pressure is 989.7 mb. Winds are around 15 kts out of the southwest (230). Skies are clear and the
pack ice is mostly 10/10.
Cheers, Peter