Report of Activities on the RVIB N.B. Palmer Cruise 02-02

12 May 2002

 

On 12 May, the Palmer was back working in the central portion of Marguerite Bay under weather conditions that had changed some. The day was overcast and the mountains of Adelaide Island to our north were obscured, but clear skies eventually developed to the south giving us another wonderful view of the mountains of Alexander Island. The barometer continued its slow climb, which started yesterday, and reached a high of over 1007 mlb in the late evening. Winds were generally light (< 10 kts) out of the southwest to west for the entire day and temperatures varied between -8 and -9C. The ice in this portion of the Bay became less thick and was more newly formed, presenting no difficulties for doing towyoing or CTDs.

 

Late in the evening of 11 May, after completing the first pickup section with BIOMAPER-II on survey line 7, the Palmer steamed from station 50 over to station 43 to begin the next section. BIOMAPER-II was deployed there and then towyoed along survey line 6 from station 43 to 41. From there, the Palmer steamed to a location further in Marguerite Bay (68 15.783S; 68 59.683W) where a series of CTD casts were made to conduct studies of FRRF performance during a daylight period, and to obtain a nutrient profiles for comparison with previous measurements made inside the Bay on this cruise. We intended to do a 1-m Reeve Net tow to collect live krill for an APOP cast with freshly caught animals, but this proved impossible given the pack ice conditions. So an APOP calibration cast to 205 m was done instead. After this completing the work at this station, the Palmer steamed to the location of Station 28 where an ROV under ice survey was done under pancake ice slabs that were interspersed with open water areas. At the end of the ROV survey, BIOMAPER-II was deployed again for a towyo to station 27. This was another section that was not done during the survey, but deemed important to get because of the strategic location of the section relative to the coastal current running along the west coast of Adelaide Island. Seabird and marine mammal observations continued to be made along the tracklines during daylight and 2 sonobuoys were deployed, one between stations 43 and 41, and the other between the MBCTD station and station 28. Thus, the second day of the post-grid work proceeded as planned.

 

CTD Group report (John Klinck, Tim Boyer, Chris Mackay, Julian Ashford, Andres Sepulveda, Kristin Cobb)

On 12 May, the CTD group did five casts at a station in Marguerite Bay (dubbed MBCTD) at 68 15.77S; 68 59.71W (grid 314.020). The first four casts were to look at the behavior of the FRRF at different lowering speeds and to compare PAR and fluorescence readings between CTD and FRRF. CMiPS also benefitted by getting data at the same place with different lowering speeds to see if this changed the low frequency noise seen in previous casts. These casts may help establish a lowering protocol for CMiPS. All of these casts were to 100 m and no water samples were taken. The lowering speeds were 5, 20, 30, and 50 meters per minute. The 10 m/min test was done during the last cast. CTD casts with FRRF are normally done at 10 m/min for 50 m then 20 m/min to 100 m.

 

A final CTD cast was done to 200 m to obtain nutrient samples to test if the ammonium levels in Marguerite Bay had increased since we last visited stations in the area. This cast was lowered 10 m/min to 100 m then 50 m/min to 200 m. Bottles were closed during the upcast.

 

Station MBCTD (cast 101, 272 m) The surface layer had uniform conditions to 45 m (-1.65C, 33.4, 0.06 ug/l). A second density jump occurred at 100 m and the pycnocline started at 150 m. Some mild temperature and salinity jumps were seen between 50 and 150 m. There was no deep temperature maximum.

 

Nutrients Status Report (Rob Masserini)

Nutrient analysis continued in the hydrolab on the N.B. Palmer. Since the last report, we have run and processed the data for an additional 412 seawater samples from 21 CTD casts. This brings us to a total of 1904 samples to date, resulting in 9520 separate analysis for nitrate, nitrite, ammonia, phosphate, and silicic acid. Also, we reached the end of the sampling grid with the completion of transects 9 through 13. Preliminary cross-section plots generated by Yulia Serevennikova for these transects indicate that the general hydrographic structure below the mixed layer for the shelf is consistent with earlier transects (please see earlier reports for concentration particulars). The offshore subsurface nitrite maximum at the base of the mixed layer was seen again on transects 9, 10, 11, and 13 approximately 20 nautical miles inshore from the furthest offshore station. However, on transect 12, this feature was seen much further inshore approximately 60 nautical miles inshore from the furthest offshore station. Two specific experiments that incorporate the use of nutrients, are planned within the remaining time of SO GLOBEC III. A station was selected that should be just outside of the gyre within Marguerite Bay and this should allow us to see if the ammonia concentration within the mixed layer of the bay was lower than on SO GLOBEC I because ammonification had not started (this region was sampled about two weeks earlier than last year). The second experiment is a transect along the deep trench that crosses the continental shelf and ends in the Marguerite Bay region. A higher resolution deep water sampling scheme is planned to hopefully elucidate the source of high silicic acid values (110 micromolar) found at approximately 300 meters depth on the shelf. It is thought that only means for silicic acid to be this high at this shallow of a depth, well inshore on the continental shelf, is for there to be upwelling of high silicic acid water from one of the trenches.

 

Marine Mammal report (Debra Glasgow)

May 12th was an overcast day, but with good visibility, light winds, and a cool -9C. The Palmer traveled through 7-9/10 young ice all day. It mostly consisted of nilas, consolidating pancake and young white/grey ice. There were also many small thin floes often with seals on board, or tracks and scats of seals on them. Thirty-three fur seals - most on ice floes, 36 crabeater seals - on the ice and in the water, 1 Weddell seal - on ice, and 2 leopard seals (by John Higdon and Karen Reiner) were recorded. Many other seals were also seen with binoculars on ice floes in the distance, but were too far away to positively identify. Many of the scats were bright orange suggesting the seals were dining on krill. One male fur seal was photographed on an ice floe having conveniently provided a fresh, bright orange scat sample on the ice beside him as he passed within 20 meters of the starboard beam.

 

At 1036, while checking with my binoculars on the I.D. of a group of approximately 7 seals swimming in the water, charging back and forth looking agitated, a male orca's dorsal fin surfaced on the edge of the ice about 1 nautical mile behind the seals. Two to four other orca's surfaced shortly after in the same place - all with small falcate dorsal fins as far as I could tell. The pod was 2.5 nautical miles away at 070 swimming to 320 at 68 13.22S; 69 48.69W. This pod was re-sighted 3-4 times swimming along the same ice edge after they turned into the ice a couple of times. There were groups of seals both in the water and on floes within 1 nautical mile of the pod. The seals in the water were obviously nervous, attempting to climb onto floes, changing their minds and trying other floes repeatedly. Ana Sirovic deployed a sonobuoy, but no sound was detected.

 

The marine mammal survey began at 0949 in the beginnings of another colorful sunrise and ended at 1414 when the Palmer stopped for a series of CTD's and an APOP cast. Darkness fell before the survey could resume.

 

Marine Mammal passive listening report (Ana Sirovic)

In the first 11 days of May, 18 sonobuoys were deployed. They were all omnidirectional buoys. Three of them failed upon deployment and the wire of one possibly got wrapped around BIOMAPER cable, broke, and was thus short-lived.

 

A humpback song was heard on 5 of the sonobuoys, most of these were also coupled with visual sightings of the animals. Blue whales were heard on 2 sonobuoys deployed on the 2nd and 3rd of May while we were working on the shelf break. Another blue whale was heard on a sonobuoy deployed on the 10 May, as the Palmer steamed north after finishing the grid stations. A set of unidentified calls were also heard on that buoy, and they most likely belonged to a seal, my guess would be a Weddell. Sonobuoys that were deployed after minke sightings yielded no minke calls, but on one of them clicks and whistles were heard that were definitely an odontocete, most likely killer whale sounds.

 

Sea Birds (Erik Chapman and Matthew Becker)

Very few birds were observed on 12 May at the mouth of Marguerite Bay near station 41. Surveys were conducted for nearly 3.5 hours with good visibility under cloudy skies. Ice conditions varied between 4 and 10/10ths coverage with loose pancake ice as the primary ice type. BIOMAPER-II indicated little scattering near the surface or deeper in the water column in this area. Bird abundance and species assemblage reflected the biology and ice conditions during the survey. Overall, bird abundance was relatively low and Snow Petrels were the most common birds observed. A few Kelp Gulls were also observed that were probably associated with the Kirkwood Islands around 10 nautical miles south of the ship.

 

A summary of the species and number of individuals of birds within the 300 m transect during 3 hours and 27 minutes of daytime surveys near station 41 is the following:

 

Species (common name)

Species (scientific name)

Number observed

Snow Petrel

Pagodroma nivea

15

Antarctic Petrel

Thalassoica Antarctica

1

Kelp Gull

Laru dominicanus

8

 

 

Material properties report (Dezang Chu and Peter Wiebe)

On May 12, an APOP calibration cast was performed in the mouth of Marguerite Bay between Alexander Island and Adelaide Island (68 15.787S; 68 59.534W). The bottom depth was about 220 m, just enough for a 205 m APOP cast. The objective of the calibration was to compare the differences in travel times between the two sets of transducer pairs that make up the APOP system. One set of transducers are used for the primary acoustic chamber, which is filled with animals during a normal cast, and the other pair are associated with the reference chamber, which is kept empty during a cast. However, during the calibration cast, the compartments of both chambers were empty.

 

It was observed from the calibration that the mean arrival time difference between the primary and the reference chambers varied from 17.8 ns to 75.9 ns, with a mean value of 47.6 ns. The influence of such differences on the sound speed contrast depends on the fraction of animal volume in the experimental chamber i.e. the ratio of the total volume of the animals to that of the animal compartment. The larger this ratio, the smaller the estimated error in sound speed contrast. The estimated error in sound speed contrast can be obtained based on the volume fractions used from the current cruise and the arrival time difference obtained from the calibration. In the worst case when the smallest volume fraction and the maximum arrival time difference are combined, the resultant error in sound speed contrast would be 0.012, while if the largest volume fraction and the smallest arrival time difference are used, the error would reduce to less than 0.001. If the mean volume fraction (0.26) and the mean time difference (47.6 ns) are used, the mean estimated error in sound speed contrast would be 0.0042, a reasonable number when compared with the standard deviations of sound speed contrast obtained from the previous APOP casts.

 

Theoretically speaking, if the calibration curve were absolutely correct, by applying the calibration results to the data from the previous APOP casts, the errors in sound speed contrast estimates would be corrected. However, direct application of the calibration results to the data might result in bias since the difference in arrival time is a function of depth and temperature. In other words, different temperature profiles may result in different calibration curves. It is desirable to do more calibrations during the cruise, if possible. In spite of uncertainties in calibration results, the error analysis will still stand.

 

Zooplankton (MOCNESS/BIOMAPER-II) report (Carin Ashjian, Peter Wiebe)

The second BIOMAPER-II pickup run from stations 43 to 41 was done from late night to noon on 12 May. While the towed body was parked in the garage prior to this run, Peter Martin did some more sleuthing to try and find the source of noise displayed on the 43 kHz echograms. This time the transducers themselves were tested by injecting signals at key locations in the pre-amp circuitry, but both transducers checked out OK.

 

This transit began on the western side of Marguerite Trough, a deep channel that meanders across the continental shelf and leads into the George VI Sound fault depression, and ended up on the east side of the trough. Maximum water depths across this section were over 900 m. Initially, volume backscattering in the trough was very low throughout most of the water column. There was some light to moderate backscattering in the mixed layer and a few thin layers in the pycnocline, but mid-water depths had very little backscattering and the deep scattering layer was light as well. Mid-way along the transit, the deep scattering layer increased in intensity some and so did the backscattering at the base of the mixed layer. During the approach to station 41, surface scattering was again weak, there were thin layers in the pycnocline, and the deep scattering near the bottom intensified more. Isolated intense, but small targets also were present in the pycnocline at the end of the run.

 

 

Cheers, Peter