Immediate report of Victoria University of Wellington Antarctic Expedition 1988-89: VUWAE 33
ANTARCTIC BOTTOM CURRENTS (K042)
ANTARCTIC BOTTOM CURRENTS (K042)
A current meter mooring with eight small (400 cm2) sediment traps was deployed in 702 m of water near the snout of the McKay Glacier for 53 days between 15 November 1988 and 6 January 1989. The current velocities 1 m above sea floor average between 6.9 and 4.3 cm/s with intermittent speeds up to 10 cm/s. These speeds are considered capable of resuspending fine sediment to form a near bottom nephloid layer, and this is confirmed by much higher sediment fluxes below 520 m. The dominant flow direction at this site is towards the glacier tongue (240° – 270°T) but "daily" swings occur towards the south which are most likely related to the diurnal tidal cycle.
Tidal data were recorded continuously for fifty days (19 November – 8 January) from Cape Roberts, the first continuous tide record from the Antarctic continent in the Ross Sea sector. The Cape Roberts tide gauge is designed to record throughout the year and thus should provide data to establish a mean sea level bench mark as well as record relative sea level change over a longer period.
The purpose of this project is to investigate mud-carrying bottom currents in Granite Harbor. Such currents have been suggested as the explanation for the deposits of mud flooring the deeper parts of McMurdo Sound and the basins/harbors along the southern Victoria Land Coast, (Barrett et al. 1983, Kelly 1986, Macpherson 1987). There is no single obvious source or depositional process for the mud deposits although Anderson et al. (1984) suggested impinging geostrophic currents and subglacial meltwater as potential sources.
In the Granite Harbor sedimentary system there is good circumstantial evidence of mud transport in a sea floor nephloid layer Macpherson (1986), Dunbar et al. (1985), Dunbar & Leventer (1987). Macpherson (1987), suggests these currents might be density driven from under the glacier tongue.
This season's program was designed to check on bottom currents originating from beneath the Mackay Glacier Tongue in Granite Harbor. A single current meter/sediment trap mooring was to be set for about 2 months (November – January) at the snout of the Mackay Glacier Tongue. During the 1987–88 season a 14 hour deployment at this site recorded low velocity (<10 cm/s) flows which exhibit progressive changes in direction and duration that appear to be related to the diurnal tide cycle. A tide gauge was also to be established at Cape Roberts initially to record the tide cycle during the period of the current meter deployment and if successful remain to continue recording during the following year.
Scientific Endeavours and Achievements
1. Deployment of mooring and establishing the Cape Roberts Tide Gauge. (7 – 20 November, 1988).
Mackay Ice Tongue Site
A north-south bathymetry profile was measured across the north-eastern tip of the glacier tongue to determine the deepest part of the channel striking east from under the glacier tongue. Eight 250 mm diameter holes were drilled through 3 m thick second year sea ice and a 28 kHz echosounder transducer lowered through the holes to record water depth.page 3
The InterOcean S4 current meter was then set one metre off the bottom in about 702 m of water to check the mooring design and that the instrument was functioning correctly. Two days later on 15 November the current meter was recovered, checked, reprogrammed and redeployed on a mooring with 8 small (400 cm2) sediment traps (Figure 1).
Cape Roberts Tide Gauge
The fast ice on the eastern side of Cape Roberts forms a well developed ice foot each season with the tide crack between 8–12 m from the exposed rock on shore. Several sites were checked and the ice foot drilled to determine water depth profiles beneath the ice foot. We intended to install the tide gauge vibrating wire pressure transducer through the ice foot within a 2" galvanised pipe that would be bolted and possibly cemented to the shore rock face. Unfortunately a suitable site for cementing was not available and we had to be satisfied with terrier bolting 10 m of pipe to rock face (Figure 2). A 3 m deep hole was cut into the ice foot with a chain saw to install the transducer pipe, finally using a drill to break through to clear water below.
The data recording system consists of a Campbell CR10 data logger, 135 Ahr (C20) lead acid battery and 22W (GL125-M25) solar panel mounted on a tripod frame located about 10 m from the surface end of the transducer pipe. A tide gauge bench mark has been established by the NZARP surveyors and the height of this will be progressively refined as more tidal data is recorded.
We had also hoped to determine the tidal time lag between Cape Roberts and inner Granite Harbor by establishing a temporary tide station at Cuff Cape 22 km away. Unfortunately because of time constraints this was carried out during the neap part of the tide cycle (17–21 November), and the results have poor resolution.
Sea Ice and Surface Travel
This phase of our program was carried out using surface transport (ASV and trailer) on the sea ice. We were not subject to the poor flying weather this season and consequently the program went smoothly. The extent of fast sea ice along the Victoria Land Coast was much less this season than for the last 8 years but still suitable for safe surface travel to Granite Harbor from Scott Base. In Granite Harbor second year sea ice was present west of a line between First View Point to Cape Archer and this made travelling slower and more difficult than usual.
2. Recovery of mooring and Cape Roberts Tide Gauge data. (6–9 January, 1989).
Current meter mooring
The mooring was recovered during the evening of 6 January after flying by helo' to Granite Harbor from Scott Base. The mooring's sea ice anchor had melted about 30 cm into the sea ice and was partly refrozen. Melting of the sea ice around the site was less than expected and was probably due to a cloudier than normal summer in this area. Recovery of the mooring went smoothly once the subice line had been caught through a newly drilled 30 cm ice hole adjacent to the mooring line.
The Cape Roberts tide gauge was checked the following day after the helo move from the mooring site to Cape Roberts. The previous 50 days data was downloaded using a Zenith lap top computer at the tide gauge site. The data logger was then returned to the Cape Roberts hut where new power supply protection was installed and a new recording program downloaded into the logger. The following 36 hours were spent periodically levelling the sea surface to recalibrate the reinstalled logger. A swell surge with a range of up to 8 cm was measured with the ice edge still about 3 km to the east.page 4 page 5
The tide gauge has now been programmed to record through the winter months to November 1989. The instrumentation is rated to −55°C and should theoretically continue to operate during the winter. However, we are less confident that the transducer pipe will remain it the ice foot floats off. By 9 January the bottom part of the pipe was still frozen solidly into the base of the ice and there was little evidence of basal ice melting around the pipe. We had expected basal melting would occur, thereby freeing the pipe before icefoot breakout. The life of the lead acid battery power supply is also of some concern during the winter months even though theoretically it should not freeze if it remains sufficiently charged during the winter.
Fifty three days of continuously recorded data (15 November – 7 January), was recovered from S4 current meter at the Mackay Glacier Tongue site. Current velocities are low as expected with 10 day averages between 6.9 and 4.3 cm/s and intermittent speeds slightly in excess of 10 cm/s. The dominant flow direction is towards the glacier tongue (240°-270° true) with "daily" swings towards the south that appear to correlate with the main diurnal tide cycle.
At Cape Roberts 50 days of continuous tide data (19 November – 8 January) was recovered giving 48 days of data common between tide gauge and current meter. The tide data shows a dominant diurnal cycle with a maximum measured range of 1.192 m. Neap tides occur approximately every 14 days and are semidiurnal for about two days.
More detailed comparison between the tide record and current meter data is yet to be done but a preliminary comparison suggests a correlation between spring/neap tide cycles and current velocity/direction. However this correlation is out of step by about 2–3 days which is greater than expected and suggests a more complex coupling of tide and circulation within Granite Harbor than first thought.
The eight sediment trap samples showed a much greater "apparent flux" in the bottom four traps below 520 m than in the upper four traps. This result is similar to previous trap data from the inner basin in Granite Harbor and supports the contention that fine-grained sediment is resuspended from near the sea floor in this basin. The current velocities measured this season are certainly capable of maintaining this sediment in suspension. The trap samples will be analysed for opal content and organic carbon by Dr R. Dunbar at Rice University as part of our cooperative program.
The results of this year's program will be analysed during 1989 and written up jointly with R. Dunbar (Rice University) in this year.
Part of our program submitted for the 1989/90 season is to repeat this study at a site towards the mouth of Granite Harbor away from the influence of the Mackay Glacier. Measurement of bottom current velocities and water column sediment flux will help us to quantify the sediment contribution flowing into the inner basin of Granite Harbour from the Ross Sea. This seasons results have proven the value of collecting data with automatically recording instruments in the Antarctic. We intend to continue recording tidal data at Cape Roberts. The ability of the tide gauge to record data throughout the year is still to be proven but we anticipate that some refurbishing and modification of the tide gauge will be necessary next season to achieve the goals of measuring long term tidal change from the continent in this area of the Ross Sea.
This season's program went very smoothly and we thank the staff of Antarctic Division and all the staff at Scott Base for efficiently implementing our field program. We are especially grateful to John Alexander (Scott Base Operations Manager) the Scott base mechanics, Store Personnel, Garth Varcoe and his group who moved some of our equipment to Cape Roberts early in the season.
NZARP Surveyors, Garth Falloon and Pat Sole, provided invaluable survey assistance and helped establish the Cape Roberts tide gauge.
The VUW Mechanical Workshop maintained and built new equipment for this program which included the tide gauge frame and mooring recovery equipment. Eric Broughton (VUW RSES) and Peter Issacs (NZ Meteorological Office), helped with the development and programming of the tide gauge and NZ Oceanographic Institute (DSIR) maintained the S4 current meter.
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Barrett, P.J., Pyne, A.R. and Ward, B.L. 1983. Modern sedimentation in McMurdo Sound. In R.L. Oliver, P.R. James, and J.B. Jago (eds.), Antarctic Earth Sciences, Australian Academy of Science, Canberra: 550-554.
Dunbar, R.B., Leventer, A.R. and Marty, R.C. 1985. Vertical sediment flux beneath annual sea ice, McMurdo Sound, Antarctica. Antarctic Journal of the United States, 20:
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Dunbar, R.B. and Leventer A. 1987b. Sediment fluxes beneath fast ice: October 1986 through February 1987. Antarctic Journal of the United States, 22(5), 112-115.
Kelly, D.N. 1986. Sedimentation in Ferrar Fjord, western McMurdo Sound. B.Sc (Hons) project, Victoria University of Wellington Library: 50 pp.
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Macpherson, A.J. 1987. The Mackay Glacier/Granite Harbor system (Ross Dependency, Antarctica) - a study in nearshore glacial marine sedimentation. PhD thesis, Victoria University of Wellington Library: 173 pp.