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Victoria University Antarctic Research Expedition Science and Logistics Reports 1982-83: VUWAE 27

McMURDO SOUND SEDIMENT STUDIES (K5) PART II. SHIPBOARD PROGRAMME - P.J. Barrett

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McMURDO SOUND SEDIMENT STUDIES (K5) PART II. SHIPBOARD PROGRAMME - P.J. Barrett.

Narrative:

The VUWAE programme for GLACIER CRUISE IV had three main aims:
1.To take several deep water samples not accessible from the sea ice.
2.To sample off (and if possible in) Granite Harbour.
3.To obtain continuous depth records from previously uncharted areas in western McMurdo Sound.

An outline of the proposed programme was submitted to RDRC in September 1981, and a more detailed cruise plan passed to Antarctic Division in July 1982. A revised programme was drawn up at Scott Base in the days before the cruise, after consulting with Dr. Mabin, University of Auckland, who had travelled down on the ship. The ultimate success of the cruise for VUWAE was in no small measure due to discussions with Dr. Mabin, who prepared us well to use fully the potential of the ship.

Barrett, Macpherson and Pyne flew to Scott Base on February 4 to find that the cruise had been delayed until February 12. Three days were spent organising a University store in the hangar, and the rest of the time planning and checking equipment sent down earlier. A meeting of all cruise participants was held at the NSF chalet on February 6; and included a 6-man USARP team of biologists to carry out midwater trawling, Dr. Mabin to look at beaches, and Messrs. Mitchell and Hunt from NZ Oceanographic Institute to sample the upper 200m of the water column. The biologists had to leave the ship about February 15, so we agreed their programme should have priority for that period, with Dr. Robinson as Chief Scientist. Dr. Barrett was named Chief Scientist for the second part of the cruise.

On February 12 Dr. Barrett arranged to go on board early to talk with Dr. Anderson, Rice University, who was a scientist on Cruise III. This was extremely useful in finding out how to work in with "the bridge" so that station time was minimised. We also learnt of Dr. Demaster's programme to determine sedimentation rates from radiometric dating, and of Dr. Dunbar's study of modern sedimentation using sediment traps. We took up the offer of his last undeployed trap and later successfully set it in 800m of water in Granite Harbour.

The ship departed McMurdo at 1700 on February 12 and we occupied two stations for grab samples while the biologists were organising their equipment. During this time we came to terms with the satellite navigation system, and the fact that there was no automatic system for recording the ship's position. During the first part of the cruise we persuaded the quartermaster on duty to mark the position on the depth recorder chart every 30 minutes, but realised that this was insufficient for good navigational control. For the second part page 44 (February 16-19) we set a watch using the marine science technicians to mark the chart every 10 minutes and at speed and course changes; tedious but necessary.

Sampling strategy was to use first the grab to take a bulk sample and determine seafloor character, for if there were many stones the sphincter corer could be damaged. Stations 1 and 2 were gravelly and no core was attempted. The next twelve hours were committed to trawling up and down Bird Basin, which brought us on to Station 3 at about 1500 on February 13. At Station 3 the wind and drift rate were high. The grab was successful with few stones but with the high wind (30 knts) and drift we decided to forego our first sphincter core attempt. The ship then trawled to Station 5, another deep water site just east of the northern tip of Ross Island, where both grab and sphincter cores were successful, though the corer had penetrated only 10cm. At the next station (6) the grab was successful but the trigger on the corer did not release. This was the pattern at the next 3 sites (Stations 8, 7 and 4), despite several hours of thought and effort in the ship's engineering shop. By the end of the first part of the cruise, however, modifications had been completed which we were fairly sure would work and in fact this proved to be so. Evidently, the fine tolerances in the trigger mechanism that were quite satisfactory for coring from a stable sea ice platform, were unsatisfactory for a shipboard operation where the platform was rolling and heaving. The failure to collect good quality cores with the water-sediment interface preserved was to some degree mitigated by the excellent grab recovery, which yielded in most cases chunks of the seafloor to a depth of 15cm with the surface identifiable and more or less intact.

The second part of the cruise, with its main objective Granite Harbour, began near midnight on February 15. After a grab in 620m of water off Scott Base (Station 26), we followed the edge of the McMurdo Ice Shelf to within 2km of the Victoria Land coast and took 3 grab samples, all sand with varying amounts of gravel. We then headed north for Granite Harbour, making a slight detour to core near station 3. This was also the first opportunity to test the corer, which to our great relief, worked well.

The cruise track to Granite Harbour took us to a point east of Cape Roberts before heading on to Cape Archer. On the way north Messrs. Pyne and Macpherson carried out a sea ice reconnaissance of Granite Harbour by helo. They found it virtually ice-free, and we immediately worked to change our plans to take advantage of this. We decided firstly to carry out a bathymetric survey and then to sample on the basis of the new bathymetric data. Nevertheless, we did not expect to change our distribution of stations much, apart from moving them further into the Harbour.

The bathymetric survey took less than twenty hours, and revealed a rather less regular bottom topography than expected. During this period Pyne and Macpherson with helo support put in place a sediment trap on the seafloor at the tip of the Mackay Glacier Tongue for recovery next season. The first attempt by Zodiac failed because of carburettor icing.

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The day following the bathymetric survey was spent sampling and yielded 6 grab samples and three sphincter cores, all 50 to 60cm long. Our only problem was in getting the ship into the right position, taking into account drift, so that the ship was over the right spot when the sampler reached the seafloor (20 minutes or up to 1km after the ship was stopped and due to the time required to lower the sampler). This was most difficult in sampling the west-facing scarp off the Harbour entrance (Station 11) but also delayed us at Station 16, where the deepest part of the Harbour was cored.

During the later part of the sampling programme, it became clear that the ship would be required at McMurdo Station early on February 19 for the departure of the "Southern cross", though the cruise had been planned through to February 20. We therefore had no time to obtain the sphincter cores from Bird Basin, and instead plotted a track back across the Western Shelf that would give useful bathymetric data and also accurately locate a deep narrow valley off Butter Point. During the survey, samples and equipment were packed for transit to Wellington. The survey used up all of the remaining 12 hours of ship time and we arrived off Hut Point at 0730, February 19.

Transport:

Transport to and from Antarctica was by C-130 Hercules. However, the programme was carried out aboard the "Glacier", except that helos were used from the ship on two occasions (for ice surveillance and to set the sediment trap) and a Zodiac was used once (unsuccessfully).

The major difference between shipboard work and our traditional sea ice operation was in getting used to having so much assistance and so little time to think. We were fortunate in the first part of the cruise to share the ship with a team of USARP biologists so that we could work out a satisfactory scheme for recording navigation and find out the strengths and constraints of shipboard operation. McMurdo Sound is a small area for a ship that can, and on occasions did, steam at 12 knots. Also our team of three was small for an operation to utilise the ship fully 24 hours a day.

The chief problem was with navigation, as the printer for NAVSAT data was not working, and the data had to be recorded by hand. Also, because the ship's speed was manually entered into the system, the ship's location between fixes (which were obtained hourly on average) is subject to some error, but this will be reduced as we work on the track of the cruise with information from the ship's log. Nevertheless, these difficulties were far outweighed by the advantages of shipboard work which included:
1.An ability to sample throughout McMurdo Sound.
2.Continuous bathymetry.
3.Heavy duty hydrographic winch for deep water work.
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Weather:

The weather during our period at sea was overcast in the Sound, with winds up to 30 knts from the southeast in the first 2 days. In contrast, Granite Harbour most of the time had less than 4/8 cloud, and little wind.

Field equipment:

Our orange peel grab generally performed well in sediment ranging from soft mud to a coarse gravel. However, it did not trigger on several occasions for one of two reasons, insufficient line out (with a strong drift we found it necessary to pay out as much as 1100m of wire to reach bottom in 800m of water), or the release hook jammed in a shackle. Both problems were easily overcome once identified. The trigger mechanism on the sphincter corer also gave trouble, and required several attempts before it was modified to work satisfactorily. Nevertheless, it worked extremely well for the second part of the cruise.

The hydrographic winches on "Glacier" were ideally suited to our work. Continuous display of wire speed and load was particularly useful, as we soon found the optimal working speeds for each piece of equipment. The grab could be sent down at 80m/minute but if it came up much faster than 25m/minute the sample was washed. The sphincter corer could go down at 80 speeding up to 150m/ minute over the lower 70m, then up at 60. Faster retrieval risked the butterfly valve and the core. Operating speeds were important to establish not only for the most satisfactory operation of the equipment, but also because the engine room required 30 minutes notice to provide power for the ship to get underway.