Report of Activities
on the RVIB N.B. Palmer Cruise 02-02
The Southern Ocean GLOBEC survey, on 29 April was focused on
stations 55 and 56 at the near shore end of survey line 7 and the beginning of
line 8 within an ice pack filled region known as George VI sound. This sound, named after George VI, King of
England, is a major fault depression that is 300 miles long with several very
deep basins including those that compose the Marguerite trough, which runs
northwest/southeast through the middle of
The trackline took the Palmer on a 30 nm transit from station 54 across the entrance to George VI sound to station 55. We left open water and came into the ice pack about 8 nm before arriving on station. The ice never got very thick and the ship moved through it on only two engines. Another 30 nm transit on a southerly course down into the sound to station 56 took place in the late afternoon and evening. After pushing through relatively loose ice pack for several hours, the going got substantially tougher, the deeper into the sound we steamed. The ice floes thickened and were covered with a very thick blanket of snow. As we pushed through the mix on four engines, the snow and ice stuck to the hull of the ship, slowing our passage. Still about 10 nm from the station location, the Palmer began to back and ram to make forward progress. Eventually, some 7.7 nm from station, the ship came to a grinding halt. Very thick slabs of ice with a meter or more of snow and tightly packed blocked our way. At around 1900, after making about 1/4 nm in 40 minutes, the stopping point became the station location.
The snow of the night of 28 April continued into the early morning hours of the 29th, but the winds were light out of the north, the barometer remained relatively high (990 mlb at 0500), and the air temperature stayed around the freezing mark (-.04ºC). During the day, the visibility improved with the thinning of the clouds over head and the winds stayed in the 10 to 12 kts range. Sea surface temperature was -1.79ºC and salinity was 32.718 psu. Winds were close to zero during the nights work at station 56.
Work completed at two stations included 2 CTDs, ice collection at station 55, an ROV under-ice survey and an APOP cast at station 56. Seabird and marine mammal surveys took place during the daylight when visibility was adequate and BIOMAPER-II was towyoed most of the ways between stations 54 to 56, being recovered to the deck only when the backing and ramming became necessary in the heavy ice pack. A solo sonobuoy was deployed during a transit between stations.
CTD Group report (John Klinck, Tim Boyer, Chris Mackay,
Julian Ashford, Andres Sepulveda, Kristin Cobb)
The CTD group did two casts on 29 April in southern
Station 55 (cast 59, 507 m). There was no real surface layer; temperature and salinity increased to about 55 m where the rate of increase increased. Surface values were -1.8ºC, 33.0 psu and 0.5 μg/l chlorophyll. A few (5) prominent 10 m thick layers occurred above 150 m in a background of small-scale variability. Temperature and salinity were still increasing at the bottom (0.9ºC, 34.6).
Station 56 (cast 60). There was a uniform mixed layer to 30 m (-1.8ºC, 33.1 psu, 0.15 μg/l chlorophyll). Prominent 5 m thick layers occurred in the pycnocline to about 250 m. The deep temperature was nearly uniform below 350 m (1.2ºC).
Marine Mammal report (Debra Glasgow)
April 29 was a day in 10/10 ice. The marine mammal survey was maintained from 0910 until dark at 1652 while on transit. The visibility was not great in the morning, light snow showers and overcast conditions created a white out effect ahead. There were very few pools, cracks, or leads in the ice pack. Ice floes and brash was covered in snow. Some algae was evident in our wake as brown discoloration in the ice. The floes in the morning were >1 meter thick, 3-10 meters wide with some areas of thicker first year ice. Ice was sampled when we stopped at station 55 and found to be 253 cm thick with a 105 cm coating of snow, although this floe appeared to be slightly thicker than others around it.
Ice conditions were similar in the afternoon with thicker, larger floes more common. Two leopard seals were sighted, and several Adélie penguins and an Emperor penguin were recorded by the bird observers. From 1530, (69 03.97ºS; 60 08.15ºW) until 1608 (69 05.88ºS; 69 09.65ºW), six Weddell seals were sighted on the ice. Several seal breathing holes were also passed while in this area. No cetaceans were seen.
Sea Birds (Erik Chapman and Matthew Becker)
On 29 April, seabirds were surveyed between stations 54 and 56 in 10/10ths pack ice. The survey began about 3 nautical miles from the ice edge. Loose floes, about 10 m in diameter and separated by slush and brash, comprised the ice coverage. Heavy snowfall over the past several days also left about 1m of accumulation atop the ice. As we continued into King George VI Sound, the floes were packed increasingly tighter, until by late afternoon we were pushing through a single sheet of a vast floe.
BIOMAPER-II indicated potential krill swarms under the
ice. Despite the presence of such prey
items, species adapted to foraging in open water, such as
A summary of the species and number of individuals of birds and seals within the 600 m transect (the quadrant between 600 m off the bow to 600 m off the port beam) during 5 hours, 8 minutes of daytime surveys between consecutive stations 54 and 56 is the following:
|
Species (common name) |
Species (scientific name) |
Number observed |
|
Snow Petrel |
Pagodroma nivea |
9 |
|
Adélie Penguin |
Pygoscelis adeliae |
25 |
|
Emperor Penguin |
Aptenodytes forsteri |
1 |
|
Kelp Gull |
Larus dominicanus |
4 |
|
Weddell Seal |
Leptonychotes weddelli |
4 |
Material Properties of Zooplankton Report (Dezang Chu, Peter Wiebe)
On 29 April, shipboard measurements were made on more than 70 juvenile krill (E. superba) that Kendra Daly sent us about a week ago, with a mean length of 25 mm and a standard deviation of 5 mm. The APOP bucket was suspended in a bigger container with running surface seawater to keep the temperature in the APOP bucket as constant as possible. The measured sound speed contrast of these juvenile krill was 1.032, higher than those obtained from the APOP casts for larger juveniles, but lower than that from the shipboard measurement (see the cruise report for 26 April). The density measurement was relatively easy. Since the ship was in the pack ice, the ship motion was small, although not always (see below). The motion compensated balance readings converged much faster and were more stable. The measured density contrast was 1.023, slightly smaller than almost all of the previous measurements, but larger than the one made with similar sized juvenile E. superba.
Also on 29 April around 2230, an APOP cast was made near station 56 (69 09.562ºS; 69 13.970ºW). In contrast to the previous casts, a different species of krill (E. crystallorophorus) were used in the cast, with mean length of 32 and standard deviation of 3 mm, respectively. The APOP cast procedure was modified for this cast. Instead of deploying the APOP right after the animals were transferred into the animal compartment, a shipboard sound speed measurement was made just before the APOP was deployed. Thus, animals had been in the chamber for about 40 minutes before they were put into the ocean.
The mean value of the sound speed contrast from the cast for these juveniles was 1.025 and the standard deviation was 0.004. Compared with the shipboard sound speed measurement (mean of 1.0265), the difference was not significant. The overall mean value of the sound speed contrast was within the variation range for E. superba from the previous APOP casts. The difference of the mean sound speed contrasts between the down- and up-casts was about 0.006 (1.028 and 1.022, respectively). Although the difference was not abnormal, the patterns of depth dependence were different. For the down-cast, there were some variations in sound speed contrast as a function of depth, but in general, they fluctuated around the mean. For the up-cast, however, there was a smoothed depth dependence pattern, a negative gradient (decrease in sound speed contrast) above 80 m and a positive gradient below 80 m, a pattern that has not been observed so far. It was not clear whether this unique pattern of depth dependence was due to the difference in animal species, or due to the fact that they were put into the chamber for a while before the APOP was deployed, or the combination of the two, or even due to other reasons. More measurements were needed to find out an answer to this riddle.
For the density measurement, there were some difficulties at the beginning. The measurement was interrupted by the severe shaking of the ship because the ship was breaking the ice and trying to make a way to station 57. The electrical balances and density meter didn’t function correctly, kept producing wrong and inconsistent readings. After about 40 minutes, the ship finally was on her way to station 57 and finally the performance of the instruments was back to normal. The density contrast of these juvenile E. crystallorophias was 1.009, which was low compared with most of the values for E. superba, but similar to that for slightly smaller juvenile E. superba (1.007, see the cruise daily report on April 19).
Zooplankton (MOCNESS/BIOMAPER-II) report (Carin Ashjian, Peter Wiebe)
There was no MOCNESS tow taken on 29 April. The tow scheduled for station 56 could not be done because of the thickness of the pack ice in that station area prevented towing without backing and ramming to make headway.
BIOMAPER-II was towyoed between stations 54 and 55 running from open water into a loose pack ice a couple of hours before reaching station 55. Although the noise problems persisted on some of the frequencies, the patterns of acoustic backscattering could still be observed. On the transit from station 54 to 55 in the late night, scattering was moderate near the surface with occasional patches of more intense targets between 40 and 60 m below the surface. Before first light (0630 to 0730), a thick scattering layer was observed at 70 to 75 meters. Evidence for vertical migration occurred around 0915 when the surface scattering layer significantly decreased and subsurface layers intensified. For much of the transits between stations the backscattering was higher than seen further offshore, but no dramatic acoustical structures were observed.
Cheers, Peter