COMPILED REPORTS OF THE
U.S. ICE CORE RESEARCH WORKSHOP
3.1 ANTARCTIC DEEP CORE: OBJECTIVES AND SITES
(Moderator: C. Bentley)
There are two primary interrelated purposes for deep core drilling in Antarctica: to examine the record of global paleo-environmental changes as viewed from the Antarctic, and to study changes in the Antarctic ice sheet itself, together with its more immediate surroundings. It is important to emphasize that changes in the volume of the Antarctic ice sheet have direct global consequences through changes in sea level. In particular, the potential exists for sea level rise as rapid as a meter per century if the flow of the inland West Antarctic ice sheet into the surrounding ocean should accelerate, as it theoretically is capable of doing.
In citing two primary purposes, we should make it clear that they are strongly intertwined-ice sheet changes are both a consequence of, and a contributor to, global environmental change. Furthermore any deep core drilling should, and inevitably will, contribute to both aspects; nevertheless, different primary objectives for different deep coring projects will result in different criteria for site selection and thus in different drilling sites.
It was the consensus of the Workshop that the primary emphasis for the U.S. deep ice core program should be on West Antarctica. The dynamics of this ice sheet are presently poorly understood but are important to understand. Because of its responsiveness, the West Antarctic ice sheet is particularly important in climatic and, especially seal level changes. The record in West Antarctica is probably complicated because of elevation variations caused by changes in the thickness of the ice sheet through time, perhaps including disappearance altogether. East Antarctica records provide a necessary base for comparison in decoding the elevational influences since the East Antarctic ice sheet has been significantly more stable through time and should have been subject to much smaller elevation influences. Thus, studies should continue in East Antarctica. Additionally, longer time records can be obtained from East Antarctica, certain paleoatmospheric/paleometeorology problems are better solved there, and records from East Antarctic cores from sites close to West Antarctica may show perturbations due to changes in the West Antarctic ice sheet.
Specific objectives of the U.S. program may be listed as follows. Of course, any drilling plan that would address more than one objective at one site would be particularly valuable.
- Determine the history of the West Antarctic ice sheet over the last 20,000 years. Understanding the past history of the ice sheet, together with understanding its dynamics, is crucial to being able to predict its future, and thus to predicting sea level change. This objective complements the ongoing Siple Coast Project, which currently is a major focus of the U.S. program.
- Obtain a high-resolution paleo-environmental record over the last 20,000 years. This is needed to correlate events in Antarctica with those in Greenland and at lower latitudes in order to determine what signals are truly global, and to search for north-south differences in the timing of events. This work has a time priority in that it is necessary to modeling and interpreting global climatic changes.
- Determine the history of the West Antarctic ice sheet on the 200,000 year time scale to integrate with geological evidence on changes in glacial extent, particularly in the Ross Embayment. Of particular interest is the period of the last interglacial, 100,000 to 125,000 years ago, when the West Antarctic ice sheet may have been mostly or entirely absent.
- Determine the third dimension of glacier dynamics. Studies of West Antarctic ice dynamics are essential to predicting the future of the ice sheet, but are hampered by the unavailability of any information about processes in, and characteristics of, the ice throughout its thickness. Core holes are needed to study bottom processes, to examine deformation through the ice sheet, and to determine the mechanical and other physical characteristics of the ice, particularly near the bed.
- Examine the paleo-environmental record over the last 200,000 years or so. T'his is essentially the same objective as for GISP III. The reasons for moving it below other objectives for the U.S. Antarctic program is that it is being addressed elsewhere, both in Greenland and Antarctica, and that there may be no appropriate site in West Antarctica, where we propose the main U.S. activity should be concentrated. Nevertheless, this objective remains highly important and should be addressed to the extent possible in any deep core hole.
- Study atmospheric processes that address the relationship between the physical and chemical properties of the atmosphere and the ice core. This research should be coordinated with complementary investigation of stratosphere/troposphere exchange, the sources and pathways of contaminants reaching Antarctica, and the processes by which contaminants in the atmosphere are deposited to the ice surface. These complementary studies can be accomplished by measuring airborne contaminants, cloud and precipitation microphysical and chemical characteristics, and changes in contaminant distributions in the aging snowpack via snowpit sampling.
- Criteria for Site Selection
- History of West Antarctic ice sheet, last 20,000 years:
- A region of simple flow, past as well as present, and so preferably with a local source of ice (i.e., an ice dome, ice rise, or ice divide).
- A region sensitive to changes in extent of the West Antarctic ice sheet.
- High-resolution, 20,000 year core:
- A region with a high snow accumulation rate at the surface so that the record will be expanded and counting of annual layers can be carried back in time as far as possible.
- Partly counter to criterion 1 -- a combination of deep enough ice and low enough surface accumulation rate that the 20,000 year level win be reached well above the bottom of the ice sheet.
- Expectation of high-quality core over critical time periods.
- Reasonable stability of ice flow over the last 20,000 years, i.e., no major changes in ice thickness or flow direction.
- History of the West Antarctic ice sheet over the last 200,000 years:
- A continuous stratigraphic record over the entire time range. This may mean actually drilling in East Antarctica, since there may be no place in West Antarctica where we can be sure to find ice any older than Wisconsinan. On the other hand, if drilling for this purpose is to be carried out in West Antarctica, then an alternate criterion is a site that would maximize the likelihood of finding marine sediments of last interglacial age, if ice that age is missing, so that paleontological dating might be possible.
- A location likely to show large changes in response to changes in the West Antarctic ice sheet.
- An expected flow pattern history simple enough that the record is likely to be interpretable.
- Glacier dynamics:
- A region where more than one hole can be placed along one flow line of interest.
- A region that will address a specific ice-dynamic problem, such as an ice stream, or a transition zone between a "catchment area" and an ice stream, or between an ice stream and an ice shelf.
- The 200,000 year paleo-environmental record (cf. site-selection criteria for GISP II):
- Oldest possible ice.
- Best resolution (thick annual layers).
- Most reliable dating.
- Atmospheric processes:
- High resolution capability (snowpits and cores)
- Regions that will address specific atmospheric problems (e.g., troposphere/ stratosphere exchange, marine influence)
- Area should not be dominated by strong local emissions if a global record desired.
- Regions that have remained geographically stable (e.g., no elevation changes)
- Specific Site Suggestions, by objective
- 20,000 year West Antarctic history:
- Siple Ice Dome or Ridge BC (ice thickness H = 1000 m, snow accumulation rate a° = 80 mm/yr).
- McMurdo Ice Dome (H = 800 m, a°=?).
- Roosevelt Island (H = 800 m, a° = 100 mm/yr).
- Berimer Island (H = 1000 m, a° = 200 mm/yr).
- High-resolution paleo-environment:
- Ellsworth Land (H = 200 m, a° = 2\500 mm/yr).
- South Pole (H = 2800 m, a° = 70 mm/yr).
- 200,000 year West Antarctic history:
- Vicinity of "Hercules Dome" (86S, 105'W) in East Antarctica. (H = 2500 . a° = 150 mm/yr).
- Central West Antarctic divide between Whitmore Mountains and ML Woollard. (H = 2500 m, a° = 250 mnx/yr, bed 1000 m below sea level).
- Deepest West Antarctic ice (Bentley Subglacial Trench 80-1/2'S, 1100, H = 4500 m, a° = 200 mm/yr, bed 2500 m below sea level).
- Byrd Basin 77-1/2°S, 97°W, H = 35000 m, a° = 300 mm/yr, bed 200 m below sea level).
- Glacier dynamics:
- Catchment - ice stream B - ice plain flow line (H = 800-2500 m, a° = 50-200 mm/yr).
- 200,000 year paleo-environment:
- East Antarctic site cooperative with other SCAR country or countries.
- South polar plateau (H = 3000 m, a° = 50 mm/yr).
- Same as C2.
- Same as C3.
- Same as C4.
- Atmospheric processes:
- Interior of East Antarctica (S. Pole) since near an atmospheric record).
- Traverses inland from the coast (W. and E. Antarctica)
- Circumnavigation of Antarctica to assess differences in coastal regions.
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