Other formats

    Adobe Portable Document Format file (facsimile images)   TEI XML file   ePub eBook file  


    mail icontwitter iconBlogspot iconrss icon

Victoria University Antarctic Research Expedition Science and Logistics Reports 1992-93: VUWAE 37

1992-93 Sea Ice Season

1992-93 Sea Ice Season.

In November and early December 1992 we attempted to recover sea floor cores from Granite Harbour to study the Holocene Retreat of MacKay Glacier- Antarctic Ice sheet in the Ross Sea. We were using new vibracoring equipment and winch mounted on the NZAP Nodwell/HIAB crane. The Vibracorer had successfully recovered sandy mud core from Petone wharf on a firm substrate (mussel bed) and had been successfully pressure tested to 500 m in Cook Strait. The winch had been load tested in Wellington but could not be operated to design depth until coupled to the Nodwell in Antarctica.

We set up in the inner basin of Granite Harbour to recover core from 700 m water depth approximately 4 km east of the MacKay Glacier Tongue terminus (Figure 1). A 2.5 m long gravity core had been recovered in 1989 at this site by colleagues from Rice University and showed that 0.5-1.0 m of soft mud (diatomaceous ooze) was present on the sea floor and that the marine-glacial transition was deeper than the 2.5 m base of this core.

Assembly of the equipment at Scott Base took 1-2 days longer than expected and also longer in the field to recheck the systems in Granite Harbour. Some minor problems were identified during assembly and after the 150 km cargo train transport to Granite Harbour which require minor modifications. We also had minor problems with the winch/HIAB systems which were overcome in the field by adjusting hydraulic settings and an increased familiarity with the equipment capabilities.

Two coring attempts were made in a depth of 700 m . The process which was controlled by a computer at the surface involved pushing and vibrating the core tube 5 m down below the vibracorer frame in two 2.5 m runs, and then retracting the core tube. On page 5 both occasions retraction of the core tube was not completed, leaving 2.5-3 m extending below the corer frame. When the corer returned to the surface we found that the 101 mm diameter steel core tube was bent between 30 to 45 degrees 2.5-3.0 m from the cutter end. The corer was also covered in mud on one side indicating it was lying on its side on the sea floor. This probably happened before the coring process began because no stiff mud was recovered, suggesting that the core tube had penetrated "horizontally" and not below 1-1.5 m depth where stiff mud is known to occur at this site. This also accounts for the incomplete retraction of the core tube which is not designed for "horizontal" operation. When the corer was then lifted off the sea floor it became upright and bent the core tube.

Figure 1. Ship track, 3.5 kHz profile lines from USCGC Polar Star and sea ice coring site 92-1 in Granite Harbour.

Figure 1. Ship track, 3.5 kHz profile lines from USCGC Polar Star and sea ice coring site 92-1 in Granite Harbour.

page 6

It is clear that the present three feet supporting the coring frame do not provide sufficient stability on a soft muddy sea floor. The computer/datalogger controlled operation however went very well. We are redesigning the feet and testing the corer in the muddy sea floor in the deeper parts of Wellington Harbour in June. For the 1993-94 antarctic field season we would also expect to carry out modifications to the winch and build a sledge/lifting frame to deploy the corer in conjunction with Nodwell/crane to increase the efficiency and safety of the operation from sea ice in Antarctica.

Several other small programmes where successfully completed during this sea ice season in the Cape Roberts area. Two counts of nesting Skuas at Cape Roberts where done on 19 November and 1 December to establish the nesting pattern for the Cape Roberts Project CEE. Approximately 45 pairs where distributed over the entire area of Cape Roberts with 9 pair in the south bay area.

A detailed topographic survey of the south bay area was also completed by the NZAP Surveyor to help planning of equipment storage for the Cape Roberts Project.

Data from the tide gauge at Cape Roberts was collected for the period 4 Dec. 91 to 3 Dec. 92. The wind speed and direction sensors were replaced and a new temperature sensor added to the instrument array in an attempt to recalibrate the original temperature sensor. The tide gauge was also levelled over a 25 hour period by the surveyor.

Sea ice thickness and bathymetry data were collected for the Cape Roberts project at four potential drillsites between 10 and 15 km offshore. Sea ice thickness ranged between 2.3 to 2.5m at the sites and the ice edge was 22 km offshore. This survey was carried out using 2 Alpine II skidoos, box sledges with echosounder and a Magellan NavPro 1000 GPS receiver.