Sunrise in the pack ice on a crystal clear and very cold (-12°C) winter morning on the Western Antarctic Continental Shelf is a spectacular event. This morning (28 July), there were gorgeous golds and reds of predawn light and a ghostly reflection of what looked like the sun above the horizon, which was yet to rise. To the north and east, there were massive icebergs scattered about in the pack ice silhouetted against the pre-sunrise light as was the outline of Adelaide Island's mountain crests some 30 to 40 nm away. Today, we have been working our way towards the Northern end of Adelaide Island and we are approaching the last station (#6) on the first transect line. Our current position at ~1700 is -66° 27.83S; -68° 04.64W. Winds are fairly light at 10 to 12 kts out of the southwest (235) and the air temperature is -9.4°C.
Yesterday (27 July), we arrived at station #1 in the early morning (~0600) and immediately set to work conducting the station work described below. In the course of the day, we completed work at stations #1 to 3 and began the work at station #4. BIOMAPER-II was towyo'd between the stations to collect biological and environmental data down to 250 m, and bird and mammal surveys took place along the transits between the stations during the day light hours.
Working in the pack ice has taken some of us, who green horns, some getting use to. The minimal ship motion is a big positive, but plowing through the ice and the leads results in quite variable ship speeds making the towing of gear more difficult. In addition, the width of the open water in the wake depends strongly on the nature of the ice pack and how fast the ship is moving. At slow speeds, 1.5 to 2.5 kts, the open wake area is thinner and the ice moves in to fill it sooner. Thus, towing wires for the nets, which may have an angle of 45° or more off the stern of the vessel, may hang up on ice chunks as they get in the way of the tow wire. More about this in tomorrow's report.
John Klinck has provided a detailed report of the physical work done by the CTD group at the first four stations on the survey grid that were occupied today. The first station was offshore of the shelf slope while the second was at the shelf break. The third and fourth stations were on the outer and mid-continental shelf, respectively.
Station #1 (3123 m depth, cast to 1000 m) had conditions that were transitional between a typical oceanic ACC station and a shelf station. The mixed layer was rather deep (180 m) with a very sharp (10 m) transition to the pycnocline. The mixed layer temperature was freezing (-1.82° C) with slightly warmer water near the bottom. The main pycnocline was about 100 m thick (200 m to 300 m). The O2 minimum was at 450 m while the temperature maximum was between 500 and 550 m with a temperature of 1.62°C. Typical ACC water (UCDW) has temperatures above 1.8°C. There is strong indication of layering between 180 to 300 m depth, with layer thicknesses of 5 to 10 m. Thinner layers might also exist, but detailed plots have not been made, at this point. Water at 1000 m was likely at the salinity maximum and thus LCDW, but lack of deeper data precludes a definitive statement.
Station #2 (758 m) is more typical of an outer shelf station, with a 90 m mixed layer with a sharp (10 m) transition at the base. A pycnocline (100 m thick) occurs below this (100 to 200 m). There is an oxygen minimum at 240 m and a temperature maximum at 250 m with a temperature of 1.85° indicating that relatively new ACC (UCDW) water is or has recently moved onto the shelf. There are distinct layers in the temperature record below about 100 m, with some layers being 10 to 15 m thick.
Station #3 (340 m) in on the outer shelf with a mixed layer about 95 m deep with a 10 m transition at the bottom. There is a weak oxygen minimum near the bottom (300 m). The temperature maximum (1.55° C) is at the bottom, which is just at the boundary between oceanic and shelf waters. There is some evidence of layering between 150 and 250 m, but there seem to be fewer layers compared to the other 3 stations.
Station #4 (346 m) has more structure near the surface with an 80 m thick mixed layer, but there is considerable density structure in the bottom 20 m of the mixed layer. There is a weak midwater oxygen minimum (240 m), but the bottom O2 is a little lower. The temperature maximum (1.65°C) occurs at 250 m,which is more oceanic than coastal in character. Layering is moderate in the main pycnocline, with thick layers with temperature reversals between 150 and 250 m.
Without over analyzing these four samples, this region seems to be a mixture of oceanic waters that intruded on the shelf at different, but recent, times and perhaps different places. The offshore station indicates that the ACC is meandered away from the shelf break. The unusual structure in the mixed layer at station #4 may indicate active mixing, entrainment or deepening. It would be interesting to see if other measurement differ here relative to other stations so far.
Chris Ribic and Erik Chapman reported that on 27 July they surveyed for 4 hours and 42 minutes in heavy first year pack ice. Ice varied between 8 and 10/10ths coverage and consisted of cakes, floes, and new gray ice. A scattering of Snow Petrels were seen, mainly in association with leads. They saw a single Antarctic Petrel.
A summary of the counts is the following:
During the night, they surveyed for an hour from the bridge using night vision goggles. Visibility was excellent in the ice; a single Snow Petrel was seen during the survey.
Ari Friedlaender reports that marine mammal sighting effort began this morning at 0930 en route to station #2. Skies were partly cloudy, winds were 20 knots out of the SSW, and nearly complete ice cover surrounded the ship. The ice was mainly cakes of grey ice and consolidated smaller pancakes, with occasional small floes and open leads. It appeared that many of what had been leads previously were now covered in sheets of new ice. Only a scattering of open leads remained. Effort continued until reaching station 2 at 1200 and then commenced again at 1340 en route to station #3. Sighting effort ended at 1600 due to fading light and deteriorating sighting conditions. No whales were seen today, although one unidentified seal was sighted in a small open pool. A single Adelie penguin was also sighted near the edge of a series of leads.
Ana Sirovic deployed a total of 3 sonabouys - 2 omnis and 1 difar. It was another day in which no biological noise was heard even though the difar was deployed near station #4 and sent out signals for most of the night. Omnis sonabuoys (buoys that cover broader band of frequencies) seem to be transmitting for a very short time. It may be that the heavy ice affects their transmission. In the future, she plans to limit their deployment to either low ice conditions or after the presence of odontocetes.is visually confirmed.
On this cruise, in addition to the broad-scale mapping of animal distrbutions, there is considerable experimental work to be undertaken to measure the feeding, growth, and other physiological properties of various zooplankton species and fish. This work is just getting underway and is being done by two groups lead by Kendra Daly and Jose Torres. Kendra Daly reports that early this morning, they deployed a Tucker trawl [a frame with a square mouth 5' (1.5 m) x 5' and a net with graded mesh - 1/4" (0.635 mm) - at the front leading to a 0.5 m ring net with 0.707 mm mesh at the rear] to collect zooplankton for rate measurement experiments. About 50 Euphausia superba larvae were collected in the net along with a dozen small ctenophores, about 3 salps, a few naked pteropods, a predatory copepod, Paraeuchaeta sp., some small ostracods, and an amphipod. This is a significantly smaller haul compared to what we collected in this region during the April-May cruise. We completed one larval krill ingestion rate experiment (and three ping editing files).
At the completion of station #1 on the 27th, the towed body BIOMAPER-II was deployed. BIOMAPER-II is also known as the BIo-Optical Multi-frequency Acoustical and Physical Environmental Recorder. The II is because this is a second generation vehicle with much more capability than the first.
This towed body is a recently developed towed system capable of conducting both quantitative surveys of the spatial distribution of coastal and oceanic plankton/nekton. On this cruise, the system consists of a multi-frequency sonar (up-looking and down-looking pairs of transducers operating at 5 frequencies: 43, 120, 200, 420, and 1000 kHz), a video plankton recorder system (VPR) with 2 cameras, and an environmental sensor system (CTD, fluorometer, transmissometer). The lower four acoustical frequencies involve split beam technology and are able to make target strength and echo integration measurements. Also included are an electro-optic tow cable, a winch with slip rings, a control van (modified 20' shipping container), which holds the electronic equipment for towing operations, and real-time data processing and analysis, and a second van for doing maintenance and repairs on the towed body on the deck. The vehicle is normally operated to a depth of 250 meters at 4 to 6 knots in deep water and conservatively, to above the bottom in shallow water. The system can be operated in a surface towed down-looking mode, in a vertical oscillatory "towyo" mode, or in a sub-surface up/down looking horizontal mode.
Since the grid survey began, BIOMAPER-II has been deployed for towyoing between all of the stations occupied thus far. The acoustic backscattering has revealed a sparse plankton population for a good portion of the outer continental shelf along the first transect line. Small dense patches of scatterers have been seen just below the ice pack and in a layer between 40 and 80 meters below the surface. The video plankton recorder images suggest these patches are adolescent or larval krill. The VPR also showed that the subsurface layer was populated with many small copepods as well as other zooplankton forms. In some areas a moderately dense layer of small scatterers were present within 25 m of the bottom and distributed up to 100 meters above the bottom were isolated individual targets of larger size.