IMMEDIATE SCIENCE REPORT
K043 : Raised Antarctic Beaches, Isostasy and Dating
Antarctica New Zealand December 1996 - January 1997
1. Popular Summary of Scientific Work Achieved
The aim of this event was to look at raised beaches along the Scott Coast from Cape Bernacchi to Dunlop Island. These features have developed since the last glacial maximum approximately 18,000 years ago when the ice sheets were at their greatest extent. The weight of the overlying ice depressed the land which has slowly rebounded after the ice melted. While rebounding a series of beach ridges were formed.
One part of the project is to date the beach ridges and associated rock platforms by three different methods and obtaining relative heights above sea level today. This will allow modelling of the volume and extent of the ice during the last glacial maximum. There has been approximately 120 m of sea level rise since the last glacial maximum, with the well constrained northern hemisphere ice sheets contributing about 90 m. The Antarctic contribution is less well constrained, so by this study combined with other similar studies from East Antarctica a better idea of the amount of ice in Antarctica during the last glaciation should be obtained.
To obtain a height above sea level today it is necessary to know where sea level was on the raised beach ridges. The second part of the project looks at the modern beach formation. Very little work has been done on Antarctic beaches and the processes which form them. Studying the modern beaches along the Scott Coast will add enormously to this knowledge. By linking processes found on the modern beaches to features in the raised beaches should give an accurate position of sea level on the raised beaches.
The third part of the event aims to identify the provenance of glacial moraines along the Scott Coast and to provide a chronology for retreat of ice from this area using surface exposure dating techniques.
The work was carried out from a series of four camps along the Scott Coast, one at Marble Point which covered the area from South Stream to Gneiss Point, one at Kolich Point, one at Spike Cape which covered both tombolos and the mainland and finally one at Dunlop Island.
There are well developed boulder beaches along most of the coast which are dominantly storm deposits with very little ice influence. A good correlation was able to be established between the size of ridges and materials to the energy of formation. Links were established between active processes and sediments found in the raised beaches, such as the evidence of a shallow tidal channel features on Marble Point. Well developed raised rock platforms between the beaches should provide the key dating tool (surface exposure dating). These will allow a better record of relative sea level fall in the area than has previously been available. Links have been established between the various dating techniques. Surveying of marine limits and beach profiles page break should be more accurate than in previous surveys due to better controls. Preliminary observations of volcanic erratics indicate the Ross Sea ice extended to an approximate altitude of 350 m on Hjorth Hill and covered Marble Point and Spike Cape. Moraine deposits imply a more recent advance of the Wilson Piedmont Glacier which may overrun earlier Ross Sea moraine.
2. Proposed Programme
|(1)||survey the height of the marine limit and provide profiles of the beach ridges and associated rock platforms.|
|(2)||discover what alternative dating methods are viable in this coastal area.|
|(3)||find a correlation between the different dating techniques.|
|(4)||collect datable material (radiocarbon, thermoluminescence or surface exposure) to date beach ridges, preferably in sequences.|
|(5)||describe the variation in beach forms along the coast.|
|(6)||describe the raised beaches from a geomorphologic point of view.|
|(7)||attempt to see the active beach forming and describe the processes.|
|(8)||discover what influence ice plays in the formation of the antarctic beach.|
|(9)||identify the provenance of glacial moraines along the Scott Coast.|
|(10)||provide a chronology for retreat of ice from this area using surface exposure dating techniques.|
3. Scientific Endeavours and Achievements
Surveying of Beach Ridges:
Four benchmarks were put in for the event by the surveyors (K191) which were linked to sea level via the tide gauge at Cape Roberts. These were a combination of existing marks and two new ones at Dunlop Island (iron rod on top, by the large cairn), Spike Cape (on a large boulder near the isthmus connecting the mainland to the two tombolos), Kolich Point (on the 4th raised ridge at the point) and at Marble Point (WALO). This gave us a good starting point to run a GPS base station from, as well as a point to tie the levelling to for good height control. The GPS run was a Trimble Pro-xl and was used to give approximate locations for each surface exposure sample site, sites of each hole dug and other significant point.
Levelling was done using a dumpy level to reduce problems with battery power and reliability in cold weather which may have occurred using an EDM (electronic distance meter). This did mean a lot more time was spent on surveying than may have otherwise been needed but gave us height locations within 10 cm. A line was run out from the survey marks along the beaches to page break pre-selected locations where profiles were to be done. From pegs at each preselected location a profile was run from the water's edge (or shore attached ice) up to the marine limit. Each profile was closed to the pegs which were in turn closed back to the survey mark.
When these data are fully reduced there will be profiles of the beach ridges extending laterally over varying energy conditions along the coast. This should give an idea of the relationship between energy and height, as well as starting to give an idea of any differential uplift occurring on the coast. Where the marine limit was previously poorly constrained these surveys should give the height with more accuracy.
Dating the Raised Beaches:
Some of the beaches in this area have been dated previously using radiocarbon from shells, bones, and other organic remains. There are difficulties in the Antarctic with calibration of these dates due to the long mixing time of Antarctic Ocean water. But they still remain a viable dating tool. This was one method used. Where shells were found to be in situ these have been taken. Two species were found to be dated, Laternula elliptica and Adamussium colbecki . These were only found in the lower energy, sandier beaches.
Lower energy beaches are also ideal sites for the second dating method attempted, that of thermoluminescence dating of sediments. Thermo-luminescence (TL) dating requires sandy material (preferably quartz rich) that has seen sunlight then been buried, such as in a beach ridge. Samples were taken in the sandy ridges and in one site where there were shells, a TL sample was taken alongside. Having the two dating methods in the single place should allow a correlation between the dates obtained. The method for collecting TL samples was changes very little to adapt to Antarctic conditions. Site selection was important as it was impossible to drive a tube to collect sediment into permafrost. A block of wood to place over the end of the tube that could be pounded into slightly frozen ground was useful. We found that if a hole was dug that was suitable, yet frozen, often waiting a day to allow some thawing of the surface layer was enough to get a tube into the sediment. When taking dose rate readings with the gamma spectrometer, to get the probe far enough into the sediment required the use of a small portable hand auger.
The main method of dating used was surface exposure dating using cosmogenic isotopes in the rock platforms associated with the beaches. Cosmogenic isotopes record the build up of cosmogenic radiation from hitting the Earth's surface. If a site has been eroded then exposed at the surface it will start building up the isotopes from the time it was exposed. By collecting surface samples of rock (preferably calcite or potassium rich rocks) a page break time since exposure can be obtained. This part of the coast was ideal for attempting this method as there are rock platforms extending from sea level to the marine limit. Collecting the rock is a relatively simple task of removing a 1 - 2 kg sample from a piece of flat lying rock with chisels and hammers.
No results have been obtained so far on any of the dating methods as the samples are not back from Antarctica at the time of writing, but the surface exposure dating appears to be a promising tool for recording uplift.
Geomorphology of Beaches:
Detailed descriptions were made from sections, using evidence at the surface as well as digging holes in strategic locations. It would have been nice to have trenches through the ridges, as may be possible in a temperate environment, but frozen ground did not make this possible. An essential tool in many holes, where digging below the permafrost level was desirable, was the use of a poinjar. This worked effectively to remove material, otherwise various shovels, spades and picks were employed. Along this part of the coast the permafrost level is about 0.5 m, but is extremely variable with water being struck in one hole. Information from the holes included the structure of the beach ridge, pebble counts to work out maximum energy and development of the beach from imbrication. Indications that there was little, if any, ice influence in most of the beaches could be seen by a lack of ice features in the holes.
Where holes were not dug surface pebble counts gave approximations to the same things. Holes were not able to be dug where the whole beach ridge was boulders. Other large scale measurements were made such as cusp sizes. Profiles of high energy sites and lower energy sites were made for comparison.
Measurements of the same features on the active beaches provided a basis for comparison to the raised beaches. After the sea ice broke out there were opportunities to watch the beach processes in action with waves and ice. Earlier, it was attempted to use an underwater camera to observe any processes occurring while there was still shore attached ice. This was not particularly successful but it is believed that the ground is covered in ice at this stage and not active. Future work, next season, will include a lot more process observations and measurements of the open water conditions.
Observations of the glacial moraines were done by mapping the extent of the various rock types that made up the moraines by a combination of noting where volcanic erratics were found with line counts and area counts. Surface page break exposure dating is to be done using cosmogenic isotopes from large erratics dropped out of the glaciers as they retreated.
Preliminary observations from the extent of volcanic erratics (derived from Ross Island and further south) indicate that the Ross Sea ice mass extended up to ~350 m altitude on Hjorth Hill above Cape Bernacchi, it also covered Marble Pt and Spike Cape. These observations place the limit of Ross Sea ice a minimum of 20 km further north than that previously reported (Hall and Denton, 1994. Antarctic Journal pp 20 - 22).
Moraine deposited by the Ross Sea ice appears to be significantly older, based on weathering and appearance of striations, than moraine deposited by the Wilson Piedmont Glacier. This implies a more recent advance of the Wilson Piedmont which may have overrun earlier Ross Sea moraine.
Future work intends to extend this work further along the coast, which will give an overall picture of the way the coast is rebounding with distance from the Ross Ice Shelf. Studying the beaches on Ross Island, and possibly Beaufort and Franklin Islands will enable a test of the source of the ice causing the isostatic rebound. There may be an isostatic response to expanding local ice on the Victoria Land Coast as well as to grounded ice in the Ross Sea. The beaches on Ross Island have a longer ice free season than elsewhere so will not only provide a comparison on levels of development, but also allow process measurements to be made over a length of time.
There are intentions to publish results from this work. Below are the topics that should become papers.
- Presenting results that show the extent of the volcanic erratics found along the coast from Hjorth Hill to Spike Cape and the relationship to the Wilson Piedmont Glacier.
- Presenting a description of the boulder beaches and how they are formed.
- Presenting the results of trying thermoluminescence and cosmogenic isotope dating on beaches where traditionally only radiocarbon has been used.
Further publications will result as this seasons and next seasons work are combined.
This event was supported by funding from The Australian National University in collaboration with Victoria University of Wellington. Edward Butler is being supported by a New Zealand Post Antarctic Science Scholarship. The support of event K191, the survey team from Terralink NZ Ltd were very helpful in placing benchmarks. The Quaternary Dating Research Centre at the Australian National University lent their gamma spectrometer for use with the thermoluminescence dating. The Antarctic Research Centre at Victoria University of Wellington lent their GPS equipment and underwater camera.