IMMEDIATE SCIENCE REPORT
K042 : Last Retreat of the Antarctic Ice Sheet in the Ross Region
Antarctica New Zealand November 1992 - December 1992. February 1993.
Abstract of Scientific Work Achieved
The objective of the sea ice based programme was to obtain cores from the sea floor in Granite Harbour. This part of the programme was not successful because of deployment and operational problems with our new vibracorer. The corer itself performed correctly in the 700 m deep water, the frame tilted on the soft sea floor and no useful core was recovered.
A ship based programme in February 1993 was very successful, with over 450 nautical miles of 3.5 kHz sea floor profiling data collected from the USCGC Polar Star off the South Victoria Land Coast. Data collected in Granite Harbour and off the Nordenskjold Ice Tongue are being used to map the extent and thickness of the Holocene mud blanket. The data collected off Cape Roberts will be used to characterise the sea floor and compile a high resolution bathymetric map for selecting drill sites in the area.
The project is to determine the timing and rate of retreat of the edge of the Antarctic icesheet across the Ross continental shelf since the last glacial maximum 20,000 years ago to resolve the present substantial differences of opinion. This involves coring from fast sea ice off the Victoria Land coast in Granite Harbour and northwards, and from a ship in the central Ross Sea.
The cores will penetrate the recent layer of mud, deposited under sea ice/open water conditions like today's, and into diamictite beneath, deposited when the shelf was covered by the extended ice sheet. The corer has been designed to penetrate and recover up to 6 m of both soft mud and diamictite in water depths to 1000 m. The timing of glacial retreat is obtained from carbon dating contemporaneous shell material and organic carbon in organic rich sediment just above the diamictite. Thorium 230 dating by mass spectrometry on suitable carbonate materials may also be attempted.
The first stage is to check the local retreat history from cores with that developed from terrestrial ages in the McMurdo Sound area. The second stage of this programme extends the study north of Granite Harbour and out into the Ross Sea using a ship and the sea ice where appropriate.
The volume and extent of the Antarctic ice sheet at the height of the last glaciation is still virtually unknown, as is the history of the retreat of its margin to its present position.The question is of interest to those studying the structure of the earth (Lambeck 1990) as well as those concerned with the Antarctic contribution to past and future sea level change (SCAR1989). There is a widely held view that the Antarctic ice sheet expanded to the edge of the continental shelf, increasing in volume by some 30% (Kellogg et al. 1979, Denton et al. 1989, 1991). This is based partly on numerical modelling and partly on the wide extent of over-compacted diamictite beneath a metre or so of mud on the continental shelf (Anderson and Molnia 1988).Others cite evidence of the old beaches on the Victoria Land coast and in Prudz Bay which are raised only a few metres to indicate past ice volumes were only a few percent larger than today (Calhoun et al. 1992).page 2
The only attempt that we know which uses this approach to establish the retreat history of the ice sheet anywhere on the Antarctic continental shelf has been by Domack et al. (1990, 1991) from the Oates Coast. They obtained ages from piston cores for the beginning of post-glacial mud accumulation there ranging from 9000 to 3000 years ago. However, there were problems with either the dating procedure or the coring as the pattern of ages with core depth was irregular and inconsistent. Piston corers are known to blow away the top few centimetres (and more at times) of surface sediment, and occasionally disturb sediment as it pierces the sea floor in a few seconds. Our vibracoring system has been designed to take undisturbed core by vibrating its way down over a period of 15 minutes. Thus we should from the outset have better quality core material to date.
The Ross continental shelf is an area of particular interest because of concern over the behaviour of the West Antarctic Ice Sheet (Bindschadler 1990). West Antarctic ice may have reached the edge of the continental shelf 20,000 years ago, though Drewry (1979) has argued for much more limited extent of the ice in the Ross Sea at that time, which he considers is more in keeping with the relatively small negative regional gravity anomaly over the Ross continental shelf.
In the last few years research by VUW personnel (Barrett et al. 1983; Ward, 1984; Ward et al. 1987; Macpherson 1987; Pyne et al. 1991.) has substantially improved our understanding of sedimentary processes along the Victoria Land coast, and has led to the development of techniques and expertise for sampling the sea floor from the fast ice. We are now interested in using these skills to acquire data on the retreat history of the ice margin since the last glacial maximum.
This season's programme was to plot the retreat of grounded ice, firstly by coring from the sea ice in the area of Granite Harbour and secondly by coring from an ice breaker in McMurdo Sound and northward in the Victoria Land basin. It became clear during the year that the ice breaker support could not be guaranteed. At best we could only expect 1-2 days ship time during tanker refuelling and the details of the ship's coring and echo-sounding equipment were not forthcoming from NSF via NZAP for our planning. Consequently we cancelled the ship based programme. However an opportunity arose at short notice to use the ice breaker in early February 1993. Our revised programme objectives were to collect 3.5 kHz data (with the ice breaker's echosounder) in Granite Harbour and around the Nordenskjold Ice Tongue to determine the bathymetry and thickness of the "post glacial mud blanket". The second objective was to collect 3.5 Khz data offshore of Cape Roberts to determine detailed bathymetry in the area of proposed drill sites for the Cape Roberts Project.page 3
Anderson, J.B; Molnia, B. 1988. Glaciomarine sedimentation. SEPM Short Course Notes. 1988 GSA Annual Meeting, Denver.
Barrett, P.J.; Pyne, A.R. and Macpherson, A.J. 1983. Observations of the sea floor in McMurdo Sound, Antarctica. N.Z. Antarctic Record. 5: 16-21.
Barrett, P.J., Pyne, A.R. and Ward, B.L., 1983. Modern sedimentation in McMurdo Sound, Antarctica. In Oliver, R.L., James, P.R. and Jago, J.B. (eds.). Antarctic Earth Science. Australian Academy of Science, Canberra: 550-554.
Bindschadler, R.A. 1990. SeaRISE: a multidisciplinary research initiative to predict rapid changes in global sea level caused by collapse of marine ice sheets. NASA Conference Publication. Preprint, Results of workshop at College Park, Maryland, January 1990: 47 p.
Colhoun, E.A., Mabin, M.C.G., Adamson, D.A., and Kirk, R.M. 1992. Antarctic ice volume and contribution to sea level fall at 20,000 BP from raised beaches. Nature, vol. 358. pp. 316-319.
Denton, G.H.,Bockheim, J.G., Wilson, S.C.,Stuiver, M. 1989. Late Wisconsin and Early Holocene Glacial History, Inner Ross Embayment, Antarctica. Quaternary Research 31. 151-182.
Denton, G.H., Prentice, M.L., and Burckle, L.H. 1991. Cenozoic history of the Antarctic ice sheet. In R.J. Tingey (ed.), Geology of Antarctica. Oxford University Press
Drewry, D.J. 1979. Late Wisconsin reconstruction for the Ross Sea region, Antarctica. Journal of Geology. 24 : 231-244.
Domack, E. W., Jull, A.J.T., Anderson, J.B., Linick, T.W., Williams, C.R. 1989. Applications of tandem accelerator mass spectrometer dating to Late Pleistocene Holocene sediments of the East Antarctic continental shelf. Quaternary Research. 31: 277-287.
Domack, E.W., Jull, A.J.T., Anderson, J.B. and Linick, T.W. 1991. Mid-Holocene ice sheet recession from the WilkesLand continental shelf, East Antarctica. In Thomson, M.R.A., Crame, J. A. and Thomson, J.W. (eds.) Geological Evolution of Antarctica. Cambridge University Press, Cambridge: 693-698.
Kellogg, T.B., Truesdale, R.S., and Osterman, L.E., 1979, Late Quaternary extent of the West Antarctic Ice Sheet: new evidence from Ross Sea cores: Geology. 7: p. 249-253.
Lambeck, K. 1990. Ice-loading of Antarctica and its implications for mantle modelling. 10th Australian Geological Convention, Hobart, Tasmania, Abstracts.
Macpherson, A.J. 1986. Glaciological, oceanographic and sedimentological data from Mackay Glacier and Granite Harbour, Antarctica. Antarctic Data Series No. 12, Antarctic Research Centre, Victoria University of Wellington, 89 p.
Macpherson, A.J. 1987. The Mackay Glacier/Granite Harbour system (Ross Dependency, Antarctica) - a study in nearshore glacial marine sedimentation. Ph.D. thesis, Victoria University of Wellington library: 173 pp.
Pyne, A.R., Ward, B.L., Macpherson, A.J. and Barrett, P.J. 1985. McMurdo Sound Bathymetry, 1st Edition. NZ Oceanographic Institute. Miscellaneous Series 62: 1:250,000.
Pyne, A.R., Barrett, P.J., Dunbar, R.B. and Macpherson, A.J. 1991. Sedimentation around a polar glacier tongue, Mackay Glacier, Antarctica. Proceedings of the 6th Antarctic Earth Science Symposium, Saitamo, Japan, September, 1991.
SCAR Steering Committee for the IGBP 1989. The role of Antarctica in Global change, Scientific priorities for the International Geosphere- Biosphere Program (IGBP). ICSU Press/SCAR, University Printing Services, Cambridge, England: 28 p.
Ward, B.L. 1984. Distribution of modern benthic foraminifera of McMurdoSound, Antarctica. Ph.D thesis Victoria University of Wellington: 187p.
Ward, B.L., Barrett, P.J. and Vella, P. 1987. Distribution and ecology of benthic foraminifera in McMurdo Sound, Antarctica. Palaeogeography. Palaeoclimatology. Palaeoecology. 58: 139-153.
Scientific Endeavours and Achievements
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.page 7
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).
The profiles in Granite Harbour and at the Nordenskjold Ice Tongue will be used to identify sites for future sea floor sediment coring (vibracoring).
The 3.5 Khz data will be processed and a map compiled of the area offshore of Cape Roberts to plan future drillsites. This data will be published in conjunction with results from 1993-94 programmes planned by US and Italian investigators. The data in Granite Harbour will be analysed to identify coring sites for the 1993-94 season.
The sea ice operation made no significant or lasting impact. Human waste and grey water was consigned to the sea via holes in the sea ice. All other wastes (burnable and nonburnable) were returned to Scott Base for disposal as per NZAP policies. A NZAP Environmental Return is appended to this report.
1993-94 Proposal Revision.
- 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.
- 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.
- Ship -based programme. Depends primarily on availability of a suitable ship and successful sea ice operation.
Management of Science in the Ross Dependency
Planning the original ship based coring programme did not proceed easily. The difficulties appeared to occur because NSF would only programme a very short time window for NZAP use of the ice-breaker even though previous years experience indicated that the ice-breaker could be available in the Mcmurdo area for several weeks during normal tanker and cargo ship support. Our technical enquiries via NZAP regarding ice-breaker coring and 3.5 Khz equipment also were not passed on to the Coast Guard. This made planning very difficult and ultimately caused us to cancel the ship request. We hope with improved communications between NZAP and NSF-OPP in the future that these difficulties can be overcome.
Our experience with the ice-breaker in February has shown us that we would need to build some special deployment equipment for the vibracorer if we used the ice-breaker in the future. The ice-breaker's inability to stay precisely on station in open water remains a major problem in utilising this ship for open water coring but it is a very suitable platform in pack and fast ice.
We are grateful to the following people and groups who have contributed to the development of the new coring equipment: VUW Mechanical Workshop (especially Alan Rennie and Mike Turner),Eric Broughton (VUW Geophysics Institute), VUW Works and Services Carpentry Workshop, Peter Dennet (Capital Hydraulics), Steve Mercer (MAF).
Garth Varcoe is fondly remembered for his confidence and support for the project, and during the abysmal testing conditions at Petone Wharf in early October 1992. Also Ron Rodgers (NZAP) and Ross Major (Works Consultancy) were responsible for design and refitting the NZAP Nodwell.
The Scott Base summer staff led by Dave Comber provided much appreciated support during the season and special thanks to Peter Grube (Bear) our field plant operator. Scott Base staff, Paul Chaplin, Dave Comber, Brian Green, Greg Harris, Mike Mahon, Dave Milne, stood watches on the ice-breaker cruise. Lastly we thank Captain Hagstrom and the crew of the USCGC Polar Star for their efforts to complete the 3.5 kHz survey programme.page break page break page break page break