Report of Activities on the RVIB N.B. Palmer Cruise 02-04

23 August 2002

The work at the off-shelf stations in the central sector of the Southern Ocean GLOBEC survey grid was completed on 23 August. BIOMAPER-II was deployed after station 46 was completed near midnight on 22 August and was towyo nearly all the way to station 64, the farthest offshore station on survey line 7. The towed body had to be pulled from the water several miles short of the station around 0700 because of heavy ice ridging that required repeated backing and ramming to get through. It took about 9 hours to transit the 30 nm between stations. Work at station 64 included ice collection and an ROV under-ice survey, a pair of CTD casts (one shallow and one to the sea floor - ~2950 m), and a Tucker Trawl to collect live animals. During the ROV dive, much of the under surface of the floe that the ice collections were made on was surveyed and essentially no krill furcilia or other planktonic animals were observed. This finding resulted in the cancellation of a SCUBA dive designed to collect these animals. The area of the station was dotted with icebergs large and small and the Tucker trawl began in a frozen lead that started at the base of one of the larger bergs, which towered above the Palmer’s bridge. During the tow, several Minke whales sounded within 100 m of the Palmer’s bow and then appeared later in the wake. BIOMAPER-II was again deployed in the late afternoon for the steam back toward the continental shelf. Because most of the work at station 64 took place during daylight hours, only a little seabird and marine mammal surveying was done.

The 12 nm run through thick pack ice to station 63 took about 5 hours. This station was also located in the deep offshore waters of the Antarctic circumpolar current. A CTD to the bottom (~2500) was completed in about two hours. Around midnight at the end of work at station 63, BIOMAPER-II was deployed and towyoed to station 62.

It was another very clear and cold day, and the first half was cloudless. In the afternoon, clouds appeared on the horizon and by sundown, the skies were overcast. Near midnight high cirrus clouds were overhead, but the full moon and some stars were still visible. The air temperature made a stab at becoming warmer on 23 August. At 0730, the temperature was -21.3ºC and at noon it was about -19ºC, but by midnight it was back down to -23ºC. The barometric pressure was somewhat higher than yesterday with readings around 993 mb most of the day. Winds out of the southwest were 10 to 15 kts in the morning and dropped a bit to 5 to 10 kts later in the day. All in all, it was another day to remember for its beauty.

CTD Group report (Eileen Hofmann, Bob Beardsley, Baris Salihoglu, Chris MacKay, Francisco (Chico) Viddi, Sue Beardsley)

During 23 August we completed CTD casts at stations 64 and 63 which are at the outer end of survey transect 8. Both stations were deep, being 2933 m and 2552 m, respectively. At station 64 two casts were done: the first to 100 m for FRRF sampling and the second to within a few meters of the bottom. Similar to our experiences from yesterday, the water in the tubing that connects the CTD sensors froze while the Rosette/CTD was brought back into the Baltic Room to remove the FRRF before the deeper cast. The water in the tubing was quickly thawed, the system flushed with seawater, and then the CTD was redeployed for the second cast. At station 65 only one cast was done, which went the full depth of the water column. No problems were encountered at this station with freezing water in tubing.

The stations along the outer portion of this transect are spaced at about 23 km intervals rather than the 40 km used for the rest of the stations in the survey grid. The closer spacing along the outer part of transect 8 is designed to provide better resolution of flow over a large bathymetric feature that extends seaward of the continental shelf edge at this location. This bathymetric feature is believed to produce instabilities, such as meanders, in the southern boundary of the Antarctic Circumpolar Current, which then bring oceanic water onto the west Antarctic Peninsula continental shelf. This flow scenario has been observed on SO GLOBEC cruise in 2001 and the earlier cruise in 2002 (NBP02-02).

The upper 50 m of the water column at the outer-most station (station 64) was just at the freezing point (-1.83ºC). Below this depth, the temperature quickly increased to 1.0ºC at 170 m. The maximum temperature of 1.84ºC was encountered at 350 m, after which the temperature decreased monotonically to 0.38ºC at 2927 m. Salinity in the upper water column was at 34.07 and then increased to 34.72 at about 450 m. Below this depth, salinity decreased in a monotonic manner to a value of 34.70 at 2927 m. Thus, this CTD cast sampled Upper and Lower Circumpolar Deep Water at the depths below about 200 m.

The vertical profiles of temperature and salinity at station 63 showed a pattern that was very similar to that observed at station 64. The only difference was the occurrence of fine scale structure in the temperature and salinity vertical profiles at about 200 m to 250 m. The observed structure suggested that mixing was ongoing at this location. This depth coincides with the temperature maxima that defines the southern boundary of the Antarctic Circumpolar Current. Thus, the structure seen in the vertical profiles may represent mixing across this boundary.

Sea Birds (Chris Ribic and Erik Chapman)

Surveys were conducted for just under an hour on 23 August as the ship moved between stations 64 and 63. Ice conditions were 9 to 10/10ths concentrations of vast floe with some large, recently frozen leads. No sightings of birds or seals were recorded during the survey.

BIOMAPER II group report (Gareth Lawson, Peter Wiebe, Scott Gallager, Phil Alatalo Dicky Allison, Alec Scott)

On August 23 we towyoed the BIOMAPER II from 1630 until 2100 through the deep off-shelf waters between stations 64 and 63. As an aside, towyo is a term coined by Peter Wiebe to describe the process of towing an instrument behind a moving ship while simultaneously lowering and raising it between the surface and some deeper depth. This up-and-down motion resembles the track of a yo-yo; hence the term towyo. During this particular towyo, we again observed diffuse scattering in the upper water column. At times only a single shallow scattering layer was present, centered at 75 m, while at other times a series of thinner sub-layers were evident, centered at 50, 75, and 100m. Often this shallow layer or series of layers were surrounded by weaker scattering extending from the surface to 200 m. VPR observations again suggested that these layers were composed of copepods, including Calanus, and small krill (furcilia or other larval stages): the krill appeared to be associated primarily with the thin sub-layers.

We also observed five very nice and dense patches of backscattering at approximately 300 m depth. These patches were large, ranging from 60 to 120 m in height and 350 to 500 m in width. The VPR captured images of various organisms at depths greater than 200 m, including calanoid copepods and gelatinous zooplankton, but despite one very near-miss, we never managed to get the VPR into one of the deep dense patches. Their composition thus remains unknown.

Current Position and Conditions

Recent satellite images have shown substantial leads cutting across the continental shelf pack ice, some running for 10s of miles. One such lead appeared to be running from near stations 61 and 62 over to station 48. We are now in that lead towing a Tucker trawl and are making significant progress moving to the inshore areas that have been impossible to get to until now. Our current position at 2315 on 24 August is -67º 54.865′S; -73º 00.51′W. The air temperature is -8.3ºC and the barometric pressure is 990.6 mb. Winds are around 7 kts out of the west-northwest (294). It is snowing and the upper unheated decks have ½ inch or more accumulation.

Cheers, Peter