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.

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.

Ice Breaker Support, 1-5 February 1993.

The USCGC Polar Star was used to collect approximately 450 nautical miles of 3.5 Khz bottom profile data off the south Victoria Land coast during 1-5 February 1993 (figure 1). This data was collected for two purposes.

Offshore Cape Roberts 11 east-west lines between 14 and 17 nautical miles long and spaced 1and 2 nautical miles apart were run in the area of future drilling (Cape Roberts Project). A detailed bathymetry map will be prepared from this data to better define potential drill sites.Over most of the area of potential drill sites very little sub sea floor structure was imaged indicating that no soft (gravel?) sediment is present and in many areas "hard sea floor" reflections indicate that the sedimentary basement outcrops at the sea floor.

The second purpose was to look in the deep basins of Granite Harbour and the Nordenskjold Ice Tongue for Holocene sediments deposited by ice retreat since the Last Glaciation (Figure 2). In Granite Harbour the presence of fast ice and time constraints restricted the planned survey and a line to the MacKay Glacier Tongue was not completed. Good data was recovered from the previously known basin of Avalanche Bay about 800 m deep where several metres of soft sediments drape over hummocky sea floor.

Offshore of the Nordenskjold Ice Tongue a (granitic?) basement high (570 m) has a "hard" sea floor reflection and a "glaciated" shape probably cut by northwards moving ice. Closer towards the ice tongue a high area averaging 500 m deep and about the same width (10 km) as the present tongue is present. This structure in contrast has a hummocky "soft" surface that is interpreted to be grounding zone sediments deposited beneath the ice tongue probably when sea level was 120 m lower during the last glaciation. On the south side this structure a east-west trending basin up to 1050 m deep is present and contains layered sediments several tens of metres thick (figure 2).

Figure 2. Block Diagram showing 3.5 kHz profiles offshore of the Nordenskjold Ice Tongue.

The profiles in Granite Harbour and at the Nordenskjold Ice Tongue will be used to identify sites for future sea floor sediment coring (vibracoring).

1993-94 Proposal Revision.

Programme (1993-94)

1. Complete the coring programme in Granite Harbour that was unsuccessful in 1992-93. Also core at Blue Glacier in conjunction with Powell's ROV study (NSF proposal) if funded.
2. Bathymetry around the Nordenskjold Ice Tongue (Skidoo and helo only). A helo reconnaissance this season showed suitable fast ice was present along the coast to travel and operate around the ice tongue. We did not have time to travel by skidoo to the Nordenskjold for the bathymetry as planned this season. Based on this seasons reconnaissance it would probably take up to 4 days to travel by bulldozer from Scott Base to the Nordenskjold. This is clearly too long to then give sufficient working time in the area within our sea ice operating window. Operating at the Nordenskjold would however be practical if faster vehicles (Challenger and Nodwell) were used or if the vehicles were staged and returned to Cape Roberts in a future season. In this case the one way trip would be only 95 km and possible in 1 to 1.5 days travel.
3. Ship -based programme. Depends primarily on availability of a suitable ship and successful sea ice operation.