Science and Implementation Plan: Chapter 5
Science Plan
In this revised Science and Implementation Plan, the key scientific questions identified in the original plan (NASA Conference Publication 3115, Volume 1) are reexamined. Within some disciplines, the questions have been revised to reflect recent discoveries. As before, broad tasks required to answer those key questions and specific investigations to carry out those tasks are identified.
The numerous feedbacks and interactions that are part of the Antarctic environment will necessitate many multidisciplinary investigations. Assignment of such investigations here to any single discipline is arbitrary and not intended to preclude interdisciplinary collaborations. In some cases, joint investigations are mentioned in more than one discipline.
5.1. GLACIOLOGY
5.1.1. Ice Dynamics
Ice flow determines the rate at which snow that falls on an ice sheet is returned to the oceans. Major changes in mass balance can only occur by changes in ice flow. Flow by internally deformed ice is reasonably well understood, but flow complexities introduced by rapid basal sliding and anisotropic crystal orientations are not. Both occur in West Antarctica where numerous ice streams, whose flow is controlled by these factors, dominate the dynamics of the ice sheet. Rapid ice flow makes ice-sheet collapse possible. It also can complicate interpretation of ice-core paleoclimate records.
1) What is the present distribution of surface-elevation change, and what is the net mass balance of the West Antarctic ice sheet?
Net mass balance, the difference between mass gain and mass loss, is the most basic measure of ice-sheet health. Local net mass balances indicate the pattern of mass redistribution and can assist in interpreting the cause of any imbalances. There are two principal techniques used to measure mass balance: direct measurement of elevation change, and calculation of thickness change based on measured volume fluxes.
Very precise repeat measurements of elevation can be done at isolated points with either GPS positioning or conventional surveying (where fixed points are visible), while satellite altimetry provides the broad spatial coverage necessary to assess the general behavior of large ice sheets. GPS control sites have been established on the West Antarctic ice sheet at Byrd Station and on ice stream B. Coverage of West Antarctica with a radar altimeter began in 1992 with the ERS-1 satellite but data are limited to the area north of 81.5¡S. Thus, only one control site is now in satellite view. Reduction and analysis of the altimetry data in the U.S. will most probably be funded by NASA. Because of the large footprint of the radar altimeter and failure of the precise orbit determination system, it is expected that at least a 10-year data set will be required before reliable indications of regional elevation change are obtained. This means the first return from these data will not arrive until well into the next century. Radar altimeter data will be compared with thickness changes at Byrd Station and calculated in limited regions by other methods (see below). In many areas of West Antarctica, these data will provide new measurements and will be useful in directing future WAIS research.
Laser altimeters provide the same capability to directly measure elevation change, but their smaller footprint and expected greater precision in satellite position offer a significant enhancement to this technique. Laser altimetry has been accomplished in Antarctica from a Twin Otter. NSF/OPP has made this facility (SOAR) available to USAP investigators. Precision of repeat elevation measurements is limited until precise GPS positioning can be used to navigate the aircraft in real-time. A scanning laser altimeter avoids this limitation and has been successfully operated in Greenland, however the aircraft is not suitable for Antarctic operation at this time. NASA's Geoscience Laser Altimeter System (GLAS) is scheduled to be launched into space in 2001, and will provide dense, continuous coverage to 87¡S. Acceleration of the launch schedule could make GLAS data relevant for WAIS.
A precaution that must be observed with elevation measurements is that they are not a direct measure of mass change. Changes in firn density, or changes in bed elevation (due to either changing till thickness or tectonic motions) also cause surface elevation change. Thus, geophysical, geological and glaciological studies must accompany surface elevation measurements to interpret the degree to which such changes represent changes in ice mass.
The second method for calculating net mass balance sums mass fluxes into and out of any given region to calculate the rate of change of ice mass contained within the region. This method avoids the difficulty with elevation measurements just mentioned, but introduces other problems-primarily an increased reliance on field data which often are difficult to collect. This method has already been applied in many areas of West Antarctica and provides the basis for all that we currently know about thickening and thinning of its ice sheet.
The basic data required are ice thickness and velocity across vertical sections in the ice, accumulation at the upper surface and melt or accretion at the base. Each parameter is discussed below.
Velocity data can be obtained by a variety of approaches: conventional surveys; satellite surveying techniques (e.g., GPS); aerial photogrammetry; and, most recently, repeat satellite imagery. Each approach has limitations. The first three require a surface field party. In addition, conventional surveys require fixed points (which are rare) and photogrammetry requires that a network of control photo-targets be established. Satellite imagery techniques have proven to be as accurate as traditional methods and provide detailed data sets in regions where surface features are plentiful. Fortuitously, these tend to be the regions of active flow with crevasses that would hamper the mobility of surface field groups. Thus, there is a complementary match between regions requiring field work and regions where surface motion can be measured from space. Landsat and SPOT have been the primary sources of satellite imagery. Near the turn of the century, ASTER will also become available.
SAR interferometry holds the potential for remotely measuring surface motion in any area. The technique produces a detailed field of relative motion (in the direction to the satellite). Offsetting the power of this technique are restrictions including satellite coverage and spatial and temporal phase coherence of the radar backscatter. Currently, the coverage of the ERS-1 SAR is limited to north of 78¡S. Radarsat, scheduled for launch in late 1995, will extend routine coverage to 79.5¡S with a brief period when the entire Antarctic will be viewed.
The major error source when applying the mass balance method to ice stream B is the uncertainty in the accumulation rate. The majority of surface accumulation data come from analysis of shallow surface cores where peaks in radioactivity indicate stratigraphic horizons corresponding to the years 1955 and 1964, when there was an increase in the testing of nuclear bombs in the atmosphere. Even the sparse measurements of accumulation rates available reveal that snow accumulation is highest along the coast, but important spatial variations are missed by any in-situ sampling strategy. Accumulation rates in most West Antarctic areas are known only roughly. Large local variations discovered in the past make it imperative that improvements be made to this data set. Renewed interest in the utility of passive microwave imagery, combined with ground truth and satellite temperature data, may significantly reduce the errors of accumulation rate determinations over most West Antarctic regions. These analyses, however, must conform to the new meteorological understandings of the interannual and spatial characteristics of storm patterns which determine when, where, and how much moisture is delivered to the ice sheet. A fuller discussion of this aspect appears in the Meteorology section (Section 5.2).
Ice thickness data are collected by radar soundings. Airborne surveys are the most time- and cost-efficient means of collecting field data over large areas, but often these rapid surveys are not capable of resolving internal layers that are proving to be extremely valuable indicators of past dynamic changes in ice flow. The utilization of smaller aircraft based remotely in Antarctica has further increased the ability to cover remote areas. Recent field seasons have seen a Twin Otter aircraft log over 25,000 flight kilometers of radar sounding lines. No method of obtaining these data from satellites seems feasible at this time.
The basal mass gain or loss is concentrated under the ice shelves, where it can be substantially positive or negative. This is a primary concern of the WAIS oceanographers and is discussed more in the Oceanography section (Section 5.3).
There is no single preferred method for determining the net mass balance of the entire West Antarctic ice sheet. Remote sensing data can provide many, but not all of the necessary parameters. A cross-checking strategy employing both the elevation change method and the mass flux method should be pursued. A few surface control points should be maintained with repeated GPS measurements to confirm the satellite altimetry results. Landsat TM images should be acquired as early as possible to determine velocities along the entire perimeter of West Antarctica. The USGS already holds many of the necessary visible images as part of an image acquisition and analysis program of major Antarctic outlet glaciers. Additions to this archive may be discovered from a pre-1972 collection of declassified photographs.
Ice thicknesses will need to be measured by airborne radar sounding at the locations where velocities are measured and as close to the grounding lines as possible. When measured inland of the grounding line, mass imbalance converts directly to sea-level change. Grounding lines can be determined from the imagery and are the most obvious place to measure discharge flux because surface velocities approximate the depth-averaged velocity and basal accumulation is minimal upstream of the grounding line. These radar sounding flights would cover approximately 3000 km and be located predominantly along the Amundsen and Bellingshausen coasts. Additional flights across the heads of all ice streams would permit separate calculations of the mass balances of the inland area and ice-stream area of West Antarctica. These flights would add an additional 3000 km of flightlines.
Significant additions to field measurements of accumulation rates of the West Antarctica ice sheet would require a major commitment of resources and is not advisable. More prudent is pursuit of the passive microwave data set meshed with meteorological studies. This approach may highlight specific areas where field data would be particularly helpful. Additions to the accumulation-rate data base along the highest ice divides of West Antarctica will come from ITASE cores (see Section 6.2.4.). A new mass flux calculation will permit a major improvement to the estimation of the net mass balance of West Antarctica.
Although WAIS is focused on the West Antarctic ice sheet, reconnaissance of other marine ice-sheet regions should not be overlooked. Wilkes Land in East Antarctica is such a region (see Map). Positioned farther north than most of West Antarctica, it is an excellent region for a program of monitoring surface elevation using satellite altimetry.
2) What are the physical controls on the motion and areal extent of ice streams?
In order to answer this question, four distinct aspects of ice-stream mechanics need to be understood:
- rapid basal motion and its effect on determining the positions of ice streams;
- shear-margin mechanics and its effect on the lateral extent of ice streams;
- onset of stream flow and its effect on the slower moving inland ice; and
- ice-stream buttressing by ice shelves and its effect on grounding line positions.
a) Basal Mechanics
Recent work has shown that physical conditions at the base of ice stream B are favorable for the occurrence of rapid basal motion either by basal sliding or by deformation of subsole till. The presence of deformable subsole till in a layer a few meters thick has been demonstrated. This till layer is spatially variable in thickness. The distribution of basal drag also varies spatially from near zero in well lubricated areas to near the plastic limit in other areas. Active till deformation accommodating ice-stream motion is highly probable.
To formulate the mechanics of rapid basal motion so that a numerical model can adequately predict ice-stream behavior, the following research tasks need to be accomplished. (1) The basal till flow law, the basal sliding law, and the individual contributions of basal till deformation and basal sliding to ice-stream motion need to be determined observationally at several representative locations in the ice-stream system. (2) The thickness of the till layer needs to be measured and mapped throughout West Antarctica to the extent feasible. (3) Hydraulic characteristics of the basal water system under the ice streams and in the transition area at the ice-stream head need to be determined and the physical control(s) on these characteristics must be ascertained. (4) The results of (1) through (3) need to be integrated into a quantitative model of the basal mechanics of ice streams.
Task (1) requires borehole geophysical work at the base of the ice streams, combined with laboratory testing of till samples obtained by coring. Also required at each site are measurements quantifying the current dynamics. These measurements must include the surface topography and velocity fields along with the internal layering. Task (2) requires high-resolution seismic profiling, combined with spot checks of till thickness at borehole sites. Task (3) addresses the key controlling variable in ice-stream mechanics, the basal water pressure; because this parameter is not detectable by remote-sensing methods, borehole geophysics is again required. Seismic monitoring of natural basal events provides clues to Task (3) and points to key study areas for Task (2). Task (4) is mainly interpretative and computational.
Tasks (1) and (3) have been essentially completed at one site. Remaining sites can be completed at the rate of one per year, so that seven sites can be completed by the end of the period discussed here. This should provide an adequate sampling of different conditions. Task (2) is best done by running seismic lines out from borehole sites. Additional geographic coverage can be obtained using initial results to guide the selection of sites. Tasks (1) through (3) require substantial LC-130 support. Once on-site, surface transport can be used to move the drill rig distances of up to 20 kilometers, as may be needed in the vicinity of some sites. Task (4) needs ongoing attention throughout the program, as field data accumulate.
b) Shear Margin Mechanics
It is now recognized that the resistive drag at the ice-stream margins is as important as the basal drag component. In areas with particularly well lubricated beds, the side-drag completely determines the ice stream velocity. Also, the mechanics of the marginal shear zones determines the positions of the lateral edges of the ice stream and, therefore, the ice-stream width, discharge and mass balance.
To get the information needed to quantify these aspects of shear zone mechanics and to incorporate them into an overall model of ice-stream behavior, the following investigations are needed: (1) mapping of shear zones by satellite imagery, comparative studies of different shear zones based on the imagery, and search for evidence of past or current changes in position of the zones; (2) search for relations between location of margins and features of basal topography; (3) ground-based study of selected shear margins to reveal detailed flow patterns, the continuity of internal layers across the margins, and whether the shear margins are currently migrating; (4) determination of the flow law of ice in the shear zones, which is thought to differ from the standard flow law (ice in the shear zone being weaker); (5) formulation of detailed marginal-shear-zone mechanics, including the patterns of crevassing and the physical mechanism by which the lateral position of the marginal shear zones is controlled.
Task (1) will use satellite imagery collected by Landsat (north of 82.6¡S) and SPOT, and eventually ASTER farther south. Lower resolution AVHRR imagery also is useful. Task (2) requires radar sounding in addition to imagery. Task (3) can be carried out with standard flow-velocity measuring techniques using strain grids plus GPS surveying at selected points. Ground-based radio-echo sounding is required to optimize the ability to follow internal layers into areas of disturbed ice. Task (4) requires ice coring to retrieve deep ice samples from the shear zones for mechanical testing (flow-law determination), and for structural and fabric studies aimed at revealing the mechanism of anomalous rheology. Task (5) involves computational interpretation and hopefully, the development of a quantitative theory of marginal shear zones.
The imagery required for (1) will require only a modest number of SPOT images (5-10) to augment the larger collection required for the net mass balance and basal mechanics studies already mentioned. For (2), more detailed radio-echo sounding data will be needed at locations where anomalous marginal features appear on imagery. A number of locations on ice streams D and E have already been identified and will be included in the radio-echo sounding flights already described. Tasks (3) and (4) require dedicated field camps at the margin. A minimum of two sites would be required-one site adjacent to ice stream B has already been completed.
c) Onset Mechanics
The initiation zone of ice streams must, by continuity, experience intense stretching due to the acceleration of ice as it makes a transition from slow flow to rapid flow. Thinning and headward migration of the onset area are probable. Understanding the conditions of this transition region are central to the ability to model the dynamic behavior of marine ice sheets. A basic challenge in studying this phenomenon is identifying its location. Satellite images reveal a spectrum of onset signatures. At one end are gradual transitions where flowbanding is well developed upstream of any tensile crevassing. At the other extreme is a sudden onset with long tensile crevasses extending across the entire stream width.
Specific tasks that must be undertaken to address the onset phenomenon are: (1) surface velocity measurements to identify the specific areas of acceleration; (2) characterization of the basal, lateral and internal ice upstream, within, and downstream of onset; (3) direct sampling of bed and water regimes in these areas; and (4) quantification of this process for numerical simulations.
Task (1) requires a broad-scale survey to determine the location(s) of maximum stretching. Visible imagery can guide the positioning of such a survey. Task (2) will begin with airborne radio-echo sounding, but must include ground-based surveys (both passive and active seismic and radio-echo sounding) once the critical onset areas are located. These studies will further refine the location where basal conditions (e.g., till thickness and dilatancy) occur. Drillers can then efficiently chose specific sites to undertake Task (3). Incorporation of all these field data into a physical model will complete Task (4).
Given the variety of onset areas seen in imagery, a "typical" site may not exist-a minimum of two sites should be studied. Landsat imagery has been used to plan the anticipated beginning of the first onset study on ice stream D. Airborne radar flights are also planned. SAR interferometry would be of particular advantage for determining the velocity field in a more crevassed onset area.
d) Buttressing Mechanics
The strength of the connection between the ocean and the ice sheet rests on how effectively resistance to ice-shelf flow is transmitted upstream to the grounded ice. For many years, the buttressing effect of ice shelves has been held to be the primary resistance to ice-sheet collapse. Detailed measurements now have been made that show that the mechanical effect of Crary Ice Rise, one of the largest ice-shelf features, is felt for only a relatively short distance up ice stream B. Nevertheless, the response behavior of the grounding line area is strongly dependent upon ice-shelf buttressing and it is in the grounding-line area that the marine-ice-sheet instability occurs. Evidence that current growth of Crary Ice Rise is having a large impact on the regional velocity field and may be reducing the discharge of ice stream B, supports this view. Further, satellite imagery has revealed contorted features that indicate strongly transient flow.
WAIS can accomplish an improved understanding of buttressing mechanics through the following investigations: (1) monitoring the changing deformation in this area; (2) developing mechanical models that adequately represent the flow of ice shelves past ice rises and over ice rumples; (3) mapping the full set of contorted flowbands providing a history of the flow on the ice shelf; and (4) incorporating the models of (2) into simulations that adequately describe the history indicated by (3).
Task (1) can be achieved primarily through periodic acquisition of SPOT imagery in critical areas and through surface field teams remeasuring existing stations left from previous years. Task (2) is an interpretative task building on the data of (1). Task (3) again relies on satellite imagery and can make good use of lower resolution AVHRR imagery, while (4) is computational in nature.
Crary Ice Rise, Steershead Crevasses, Roosevelt Island and Minna Bluff/Cape Crozier are the major sites of buttressing on the Ross Ice Shelf. Of these, only Crary Ice Rise has received intensive study. As monitoring of this location is continued, the other three sites, along with initial studies along the coastlines of the Amundsen and Bellingshausen Seas should be undertaken. SAR imagery might prove especially useful along the northern coasts, as cloud cover is pervasive.
3) How has the configuration of West Antarctica changed and how will the volume of grounded ice change over the next decades to several centuries?
The West Antarctic ice sheet is expected to change in the future because it has changed in the past. Some signatures and remnant effects of past behavior still exist in the ice. These features are seen in the imagery and within the internal layering of the ice sheet. Once identified and understood, they can be separated from the ice-mechanical investigations detailed above and used to strengthen the development of time-dependent numerical models.
The goal of these models is to adequately incorporate the physical processes and interactions identified by other WAIS investigations. Such models provide the only means to predict future behavior of the West Antarctic. The models should include heat flow to define material properties throughout the ice column and subglacial stratum, and meteorological calculations to account for varying accumulation rates as the ice-sheet configuration changes with time. No such completely coupled models yet exist.
These models must be validated by demonstrating that they can represent known past configurations as well as depict transient events identified by the analyses of field data. These data, in combination with satellite data will provide the initial conditions, the boundary conditions, and the ice-sheet configurations for such validation experiments. Field work supports the imagery-based hypothesis that older ice streams existed, following different courses. This discovery increases the challenge to both data collection and model performance. Appropriate input to the model might span the period from the last glacial maximum to the present. To be credible, any model must be able to accurately reproduce this period.
With a validated model, predictions about the future behavior of the ice sheet and its effects on sea level will be made. Again, initial conditions (the present configuration and net mass balance) and environmental data (accumulation rate, oceanic circulation) will be required. In the absence of a model capable of internally updating these environmental parameters, a series of scenarios might be provided by the relevant experts to bracket adequately the ranges of possible future situations. Model predictions for these various scenarios will encompass the range of possible future ice-sheet configurations and their effects on global sea level. Model runs that bracket likely ice-sheet response are desirable to address socio-political concerns centered on issues such as greenhouse warming, where even the best global climate models can only indicate a range of probable future climates.
This task is predominantly computational. Satellite imagery and radio-echo sounding provide the historical configurations. The development of suitably dynamic models is complex and should not be postponed while awaiting data collection and analysis. Enough is already known about possible ice dynamic mechanisms that fruitful model development is possible. Also, because of the eventual complexity of such a model and the variety of numerical methods that might be employed within the model, it is advisable that this task be undertaken by more than a single investigator or group.
5.1.2. Ice Cores
Ice cores provide unique records of climate and climate change. Long and high-resolution records of atmospheric composition and chemical loadings are key to understanding global climate change and biogeochemical cycles. Local records of snow accumulation and temperature provide "weather stations" for climate-change studies, as well as providing the boundary conditions for ice-flow and ice-stability models. The physical properties of ice cores and boreholes open a window into the mechanics of ice flow and the holes provide access to the bed beneath, further constraining ice-flow models. Ice coring can date internal layers detected by radio-echo sounding, providing a chronology that further enhances the use of these natural markers to determine the history of ice dynamics. In short, ice-dynamical studies and atmospheric studies are critical to the interpretation of ice cores, just as ice cores are critical to understanding atmospheric and ice-flow processes.
1) What is the history of ice-sheet forcing and response?
Occurrence of Eemian/Sangamonian/5e ice in the base of the ice core would resolve the long-standing debate about whether the West Antarctic ice sheet collapsed at that time and would be a major advance toward assessing the potential for future West Antarctic ice- sheet collapse. (Unfortunately, lack of such ice does not confirm collapse, as old ice may have been lost to basal melting or near-basal mixing processes.) Dating can be accomplished through counting annual layers, correlating isotopic or other records to dated sub-ice, continental shelf and deep-sea sedimentary records, through ice-flow modeling, or through newer methods utilizing cosmogenic isotopes. Spatial extension of the stratigraphy at such a location, through radar sounding of the surrounding area, would enlarge the region where ice could be inferred to have existed during the Sangamon. The temperature profile in the ice must be measured as well as the geothermal gradient and the thermal properties of the underlying bed to determine the melting rate of basal ice.
The ice core contains a record of snow accumulation (annual layer thicknesses, corrected for ice-flow thinning), and of temperature (stable-isotopic ratios, borehole temperature). Along with sea level, these are the major forcings of ice-sheet changes, which must be known over the most recent glacial cycle to allow modeling of the current and future ice-sheet behavior. The history of temperature/accumulation-rate covariation may provide important information on the likely accumulation-rate response to anthropogenic warming, and the likely effect on sea level.
Measurements of the total gas content of the air trapped in the cores provide a measure of atmospheric pressure that existed when the air cavities were sealed. Assuming stable atmospheric circulation patterns, this pressure can be converted to a surface elevation. Ambiguity associated with this assumption can be reduced by sampling the ice sheet at a number of locations. More sites provide a valuable measure of the pattern of elevation change as well as less ambiguity for each measurement. Sampling is desired in central regions and in more dynamic, marginal areas. A record of elevation since the last glacial maximum can be obtained by drilling to only a few hundred meters depth in some places, so it is feasible to collect several of these cores without expending major logistic resources. The ice-core record of former surface elevations will be compared with the geological record obtained from nunataks and coastal mountains (see Section 5.4.1).
2) What is the history of regional climatic conditions?
The chemical and particulate records in ice cores provide a multi-dimensional view of regional climate change. Variations in biogenic productivity in adjacent oceans (MSA, iodine), atmospheric processes (nitrate), wind speed (sea salt), continental aridity (crustal aerosol), etc. can be determined from ice cores. High-resolution data are especially valuable. An accumulation-rate record allows changes in atmospheric loading to be distinguished from changes in dilution by water flux, thus increasing the value of the chemical and particulate records. The regional and global record of volcanism is especially interesting, both as a climatic forcing and as a potential high-resolution tool for interhemispheric and ice-core/marine-core correlations. By identifying and characterizing the surrounding particulate source regions, the patterns and changes of atmospheric circulation can be inferred from the core analyses, making them useful for testing the predictions of atmospheric numerical models.
3) What is the high-resolution history of globally mixed gases?
Questions remain about the effect of high calcium levels in Greenlandic ice on the CO2 record recovered. The Antarctic record is not suspect, but existing deep Antarctic cores lack the high resolution of the Greenlandic records. A deep, high-resolution Antarctic core is needed to learn whether rapid fluctuations have occurred in CO2. Such a core also will allow confirmation of other Greenlandic measurements. The high-resolution records of CH4, delta18O of O2, or other gases will allow accurate correlation of the gas records of the cores between hemispheres.
4) What is the physical nature of the ice and its substrate and have variations in these properties occurred?
Crystal size, fabric and temperature all influence the effective viscosity of the ice and represent the integrated deformation and temperature history of the ice. These data are useful in constraining flow models that attempt to reproduce the history of the ice sheet. Electrical properties are perhaps the best data to correlate with the existence of internal radar-reflecting layers which are believed to represent isochrones. The ability to date internal layers at a core site allows core dating to be extended to other regions if continuous internal layers can be followed.
Many of the most important data at a core site are collected once the core is removed, and many of these data will be directly relevant to other disciplines working within WAIS. Basal debris-laden ice or subglacial unconsolidated sediments may indicate greatly accelerated flow and also may provide key historical data through identification of microfossils, exposure-age dating of the rocks, or other indicators. Repeat inclinometry (for deformation and mass-balance calculations), sonic logging (for ice fabrics), and basal water pressure all should be measured. To assist the geologists, subglacial coring should be undertaken. Geothermal flux, which must be measured in the bed to obtain accurate results if the ice is wet-based or has been wet recently, is one of the important controls on ice flow, although one that is unlikely to change rapidly over time, except possibly close to volcanoes.
5) What is the history of solar-terrestrial and extraterrestrial interactions?
Cosmogenic isotopes including 14C, 10Be, in ice cores are largely controlled by processes beyond the Earth's atmosphere. High-resolution records of cosmogenic isotopic concentrations and snow accumulation allow calculation of fluxes, and partitioning of variations between changes in production (extraterrestrial controls) and deposition (Earth climate control). If time-series analyses of various ice-core parameters recover solar periodicities, these indicate the strength of solar-terrestrial interactions.
6) Precisely where should the drilling occur and what is the most cost-effective analysis plan?
Site-selection activities (aerogeophysics, ground glaciology and radio-echo sounding, shallow coring, automatic weather-station observations) have been started or are funded and will proceed soon. Vigorous pursuit of these activities is warranted, with expansion to include automatic weather stations at the likely deep-coring sites and initiation of long-term atmospheric-sampling and surface-glaciology studies there. This should lead to recovery of two deep cores, one inland and one on Siple Dome, beginning in 1997-98. These sites have been chosen to optimize the ability to answer the above questions. To conserve logistical capability, core processing should be concentrated in the National Ice Core Laboratory in Denver. It is worth noting that this is a much less expensive proposition than working in the field, but may involve more science dollars. (A meal in McMurdo or at a field camp is paid by logistics, but a meal in Denver comes from science dollars.) A full suite of isotopic, chemical, particulate, electrical, and physical studies of the core is warranted, as are borehole logging and sub-ice core recovery. Better development of fingerprinting techniques for volcanic ash should be pursued, to allow exceptionally high-resolution correlation with Northern Hemisphere ice cores through identification of global fallout of large eruptions. This must be tied to multi-parameter counting of annual layers, including chemical as well as electrical, optical and isotopic techniques. Further details are given in the WAISCORES document (U.S. Ice Core Working Group, 1992).
5.2. METEOROLOGY
The major meteorological controls on the ice sheet are accumulation rate and surface temperature. Thus, the broad goal of the meteorological component of WAIS is to understand the spatial patterns and temporal variability of precipitation (nearly equal to accumulation on large spatial scales) and temperature over the West Antarctic ice sheet as consequences of the coupling between the atmospheric circulation and the ice-sheet topography. These key environmental upper boundary conditions on the ice sheet will be described for both present and altered ice-sheet conditions for input into glaciological ice-flow models. Atmospheric numerical models will be important for understanding changes to the atmospheric circulation (and resulting accumulation and temperature patterns) through the glacial-interglacial cycle as it will be reconstructed by WAIS investigators, and for calculating future scenarios of atmospheric circulation used in predicting future ice-sheet behavior.
Interpretation of ice-core records will both require and contribute to understanding of temporal variations in snow accumulation. Annual oscillations in a number of tracers can allow layer counting and development of extremely detailed accumulation records. Longer term variations in these and other tracers in the ice reflect modification of the global climate system (changes in atmospheric circulation and their impact on biogeochemical cycles). Rigorous reconstruction of past climate and atmospheric composition requires better understanding of the links between the composition of the atmosphere and glacial ice records. Atmosphere-to-snow transfer is one poorly understood aspect of the overall transfer function between air and glacial ice. Improved understanding of this question demands investigation of atmospheric dynamics on smaller scales (e.g., boundary-layer/free troposphere exchange) in addition to the synoptic and mesoscale analyses outlined below.
1) What is the magnitude and interannual variability of moisture transport onto the West Antarctic ice sheet?
The atmospheric transport of water vapor across the coastline yields the snowfall that nourishes the ice sheet. Knowledge of the moisture transport in the West Antarctic sector is limited. Particular tasks that must be accomplished to answer the above question are: 1) analysis of past regional radiosonde records (particularly from Byrd Station) to extract moisture fluxes at the limited number of sampling sites; 2) development of methods to derive moisture fluxes from existing satellite remote-sensing data, particularly focusing upon the difficulties associated with underlying pack-ice and ice-sheet surfaces; 3) continuing analysis of broad-scale atmospheric numerical diagnoses (e.g., from the European Centre for Medium-Range Weather Forecasting) to ensure that all available observations are being incorporated; and 4) an expansion of the in-situ data-collection network including automatic weather stations (AWSs) on the ice sheet and offshore islands, and deployment of free-drifting buoys in the oceans.
Only task (4) requires field work. It is proposed to deploy two new AWS sites each year during the 5 years of Phase I, in addition to servicing previously established sites. This should be sufficient to establish a synoptic-scale array on the ice sheet. WAIS ice-coring activities could benefit significantly from an AWS placed at each coring site for year-round atmospheric observations. Collection of the traditional AWS variables could be augmented by measurements of snow accumulation from an array of acoustic snow-height sensors, and some simple, automated analyses of snow chemistry or at least automated collection of snow samples. At sites where air-snow investigations are conducted, networks of at least three AWS units are desirable to provide insight into vorticity and subsidence of regional-scale air-masses. Measurements at these sites should also include meteorological soundings (tethered and/or radiosondes) plus soundings of chemical tracers (e.g., O3 instruments can fly with the meteorological sensors) that would supplement the sparse regional sounding network. In addition, tower-based measurements of fluxes of heat, momentum and water vapor would provide local-scale information on vertical exchange between the snow surface and the atmosphere.
It is assumed that the World Climate Research Program's International Antarctic Drifting Buoy Project will succeed in monitoring the synoptic-scale atmospheric circulation over the ocean areas surrounding Antarctica. Additionally, it is anticipated that considerable assistance with the remote-sensing research will be obtained from groups already active.
2) What are the important synoptic processes that affect the moisture flux and what is the variability of each process?
Cyclonic activity controls the patterns and variability of moisture flux across the edge of the ice sheet, but very little is known about the basic processes. The long-term (15-20 year) climatology of synoptic systems (primarily cyclones) over high latitudes of the South Pacific Ocean should be established from synoptic analyses that have satisfactorily incorporated all available observations (e.g., the satellite measurements routinely being recorded at McMurdo and Palmer Stations). Particular emphasis should be placed upon establishing the patterns of variability and the associations between the sea-ice and cyclone occurrence. Satellite and available in-situ data need to be evaluated for the frequency and characteristics of these synoptic influences on the West Antarctic ice sheet.
It may be possible to supplement such investigations by tracing snow strata to map the amounts and extents of snowfall produced by major storms over the ice sheet. Continued investigations of mesoscale cyclone formation, tracks, etc., are needed on seasonal and interannual time scales to resolve their role in snowfall over the ice sheet. Once again, satellite and available in-situ observations will provide the key data.
No additional field work is required beyond the expansion of the AWS and ocean-buoy network described in the previous section. It is expected that analyses of these data will mature as the data base expands.
3) What are the patterns of large-scale variability in the atmospheric circulation?
Synoptic processes express broader scale atmospheric, oceanic and ice-sheet conditions. An understanding is needed of the dominant, large-scale patterns of variability and how these processes interact. Examples of this variability on weekly and seasonal time scales are atmospheric blocking, and links with tropical convection and sea-ice variations. Emphasis is placed here on the interannual, global-scale El Ni–o Southern Oscillation (ENSO) events, which, as studies have indicated, may be strongly manifested in West Antarctica. Synoptic analyses can interact effectively with ice-core studies of ENSO-related depositional events. The South Pacific Cloud Band (SPCB) should be studied as a possible major candidate for transmitting the ENSO and higher frequency tropical impulses to high latitudes of the South Pacific Ocean.
Satellite and synoptic analyses are the primary sources of data. Analyses of existing and new data are required but field work is not needed.
4) How would the important atmospheric processes change under altered boundary conditions?
This question must be answered to provide ice-flow models with scenarios of past and future atmospheric forcings on the ice sheet. A coordinated observational and modeling effort is required with the purpose of understanding the relationship between the nature and variability of moisture fluxes and extratropical cyclones. The modeling studies require as input the topography of the ice sheet, the atmospheric composition and the oceanic surface conditions (sea-surface temperature and sea-ice distributions), illustrating once again the close links between the various disciplines.
Currently, two modeling strategies exist for simulating atmospheric processes. Regional (or mesoscale) models use relatively short time integrations (a few days to approximately 1 week) to understand fine-scale details of atmospheric processes, such as the evolution of cyclones and related synoptic and mesoscale processes. General Circulation Models (GCMs) are used for longer term integrations of a month or so to applications involving geologic time scales. Both modeling strategies must be employed in WAIS. Regional modeling efforts should be focused on the synoptic-scale environment, assessing the patterns of moisture transport on a case-study basis using data from available observational networks. These case studies then will be used to refine the parameterization schemes in GCM simulations.
In addition, physical processes not currently represented in the GCM simulations need to be incorporated. As an example, existing GCMs resolve the katabatic wind regime only crudely, yet recent studies have indicated that these winds may play a critical role in the evolution of the circumpolar vortex which, in turn, affects the flux of moisture onto the continent through the interaction of the circumpolar vortex with extratropical cyclones.
No field work is required for the modeling activities. The work will depend on refined GCMs with full coupling of atmospheric and oceanic simulations. Substantial opportunities exist for interaction with other WAIS activities. Predictive GCMs hold the promise of understanding the nature of the glacial-interglacial cycle.
5) What are the mechanisms by which the snow is actually generated?
This process is important in understanding how the atmosphere is affected by the ice sheet, but has never been studied observationally. Investigation of the detailed atmospheric dynamics associated with invasions of moist air into West Antarctica is needed, along with an evaluation of the accompanying cloud microphysical and chemical processes that govern cloud and precipitation formation. These studies will feed into theoretical and modeling studies of topographically forced snowfall.
Ice-core interpretation will be considerably enhanced by improved knowledge of the physics governing the parameters measured in ice cores, particularly stable isotopes and chemical constituents. Opportunities for cooperation with the ice-core drilling should include simultaneous measurements in the air and on the ice sheet. Air-snow exchange investigations were integrated with the recent deep drilling programs at Summit, Greenland. This experience has resulted in development of sampling, analytical and modeling approaches that will allow early determination of the important processes at each new polar coring site, so that these can be focused on in intensive field and laboratory investigations. (See the WAISCORES science plan for a more detailed discussion of air-snow exchange issues.)
A well-instrumented research aircraft, like the NCAR Electra or equivalent, is needed for these investigations. To minimize the transit time from the base airfield and to maximize the measurement time of the aircraft, it probably will be necessary to establish a suitable airstrip in West Antarctica, and to operate the aircraft from this remote location at least part of the time. The success of using blue-ice runways for other aircraft elsewhere in Antarctica may open the door for this logistic requirement. It is recommended that these aircraft campaigns be conducted during the first three field seasons of WAIS.
5.3. OCEANOGRAPHY
The principal direct link between the ocean and the ice sheet rests with the water that circulates under the large ice shelves of West Antarctica. An increase in the amount of heat being transported beneath and transferred to these ice shelves leading to a reduction in their ability to buttress the inland ice is often suggested as the likely mechanism by which a climatic warming could trigger an ice-sheet collapse. Assessing the validity of this hypothesis requires a better understanding of how sub-ice water circulation and heat transport work today, and an understanding of how warmer oceanic waters would be incorporated into the sub-shelf circulation. The oceanographic studies of WAIS will focus on these coupled questions and will provide numerous opportunities for interdisciplinary investigations.
1) What are the connections between the waters of the open ocean and those on the continental shelf?
It is through the exchange of water at the continental shelf break, and its subsequent modification on the continental shelf, that the global ocean directly influences the ice-sheet environment. The edge of the Antarctic continental shelf is one of the primary ventilation sites in the world ocean. There, deep water evolves into surface and shelf waters which interact with the atmosphere and sea ice, as well as with the ice margin and the undersides of the ice shelves. Heat and salt are vigorously exchanged in coastal polynyas and by deep vertical convection near the continental shelf break. Bottom waters formed near Antarctica account for more than half of the abyssal ocean volume.
To understand the transport of fresh water and salt across the continental shelf requires oceanographic measurements of salinity, temperature, chemical tracers, and currents on appropriate temporal and spatial scales at representative locations. Time-series observations should extend over several annual cycles, encompassing the probable residence time for waters on the continental shelf. Satellite measurements of sea-ice distribution and movement will help oceanographers to understand the forcings and interannual variability and to identify sites where air-sea interactions are highest. Efforts to model the ocean circulation on the continental shelf should be cognizant of regional differences in and sensitivity to changing wind fields and fresh water, salt and temperature fluxes.
2) What is the spatial pattern and temporal variability of ocean interaction with the ice shelves of West Antarctica?
Most of the ice in West Antarctica feeds large ice shelves that float over warmer sea water. An increase in sub-ice-shelf melting has long been believed to be a likely trigger for marine ice-sheet collapse (see Section 3). Modeling experiments have shown that changes in ocean thermohaline characteristics could produce shifts in the strength of the sub-ice circulations, which now appear to cause relatively high melt rates near the ice-shelf grounding lines. Because the seawater freezing point falls with increasing pressure, melting rates are highest where the ice-sheet drafts and grounding lines are deepest. These melting rates could increase if the grounding lines retreated into deeper basins, but basal sills that restrict the circulation, and ponding of dense seawater could create negative feedbacks.
Order-of-magnitude differences have been reported between the basal melting and freezing rates beneath different ice shelves. Where the sub-ice-shelf circulation is dominated by cold, dense, high-salinity shelf water as in the Ross and Weddell Seas, average basal melt rates appear to be in the range of 20 - 60 cm/yr. Where "warm" circumpolar deep water floods the continental shelf, as in the Bellingshausen and Amundsen Seas, basal melt rates can exceed 200 cm/yr. It has yet to be determined how quickly future climate change might alter the ocean characteristics or circulation near Antarctica, possibly shifting the present distribution of high-melting and low-melting regimes.
Field measurements and modeling efforts should focus upon these two different regimes, beginning in the Ross Sea which is better surveyed and has more extensive oceanographic data bases. A transect along the axis of the major Ice Shelf Water plume could extend from the ice-shelf grounding line to the deep ocean. Along this transect, a series of holes drilled through the Ross Ice Shelf would provide access to the sub-ice cavity in order to obtain water samples and deploy instruments to record currents and basal melting or accretion. These holes could also be used for collection of sub-ice sediments, whose analysis can provide information on the history of the ice sheet (see Section 5.4.2) and for biological studies of this unusual region.
Measurements near the grounding lines in both deep-water and shallow-water environments should attempt to differentiate between meltwaters derived from the ice shelf and from the grounded ice sheet. Coordinated time-series measurements near the ice fronts would quantify the effects that cause high melt rates in that region, and that affect the variability of Ice Shelf Water (ISW) outflow. Present-generation oceanographic equipment must be tested in the sub-ice environment, where the near-freezing temperatures may present significant problems. Narrow ice-holes can be completed at lower energy cost but that may, in turn, require miniaturization of standard oceanographic equipment. Continued development of drilling technology should allow the rapid retrieval of crystallographically and stratigraphically unaltered ice cores from the basal regions of an ice shelf.
In the Amundsen-Bellingshausen Seas, relatively warm water floods the deeper portions of the continental shelf. This region may be an area of recent ice-shelf retreat, as marine geologists have identified sites where larger ice shelves probably existed within the past few thousand years. A survey of this region was carried out in early 1994 from the N.B. Palmer. Preliminary results show relatively warm water beneath all the fringing ice shelves, no well-developed oceanic front near the continental shelf break, major uncharted topographic features on the shelf, myriads of icebergs, and a highly diverse sea-ice cover.
3) What will be the spatial and temporal patterns of climatically induced changes in the sea ice and ocean of the Southern Hemisphere?
If the ocean near Antarctica becomes warmer and fresher, this could lead to decreased vertical heat flux from the deep water to the atmosphere and increased heat transport onto the continental shelf. If sea-ice production also decreases near Antarctica, then less-dense shelf water may be formed, facilitating access of that warmer water to the ice shelves. Some General Circulation Models yield less warming than earlier predicted in the Southern Hemisphere atmosphere, but more warming in the Southern Hemisphere ocean over the next several decades. Alternatively, a weaker thermohaline circulation, resulting from less sea-ice formation, could slow shelf circulation and reduce oceanic heat transport.
Investigating these questions will necessitate measuring, modeling and monitoring the spatial and temporal variability of the continental margin sea-ice cover and ocean circulation. Models that couple the deep-ocean, continental-shelf and sub-ice-shelf circulations must be constrained by the observed ranges of water properties, current velocities, sea-ice distributions and basal mass balances. Oceanographic data bases and paleoclimatic indicators need to be assembled and analyzed in the context of present-day seasonal, interannual and regional variability. Methods developed for the Arctic to extract sea-ice extent, thickness and motion from satellite data must be refined for Antarctic use. These parameters can be verified by moored and drifting instruments, sea-ice cores, and shipboard observations. Satellite and geochemical techniques should aid in the identification of coastal regions of high salt or meltwater flux or potentially high sensitivity to climatic perturbations.
4) What are the relationships between the northern limit of sea ice or between the leads and polynyas in the sea ice and regional accumulation rates on the ice sheet?
The ocean indirectly affects the ice sheet by providing the moisture which the atmosphere delivers as snow. Much of this accumulation is apparently delivered across hundreds of kilometers of sea ice, but the zonal range of source areas is not well defined. Leads and polynyas in the otherwise-solid sea-ice cover are also important local sources of moisture, increasing the flux of heat and aerosols to the atmosphere, and potentially raising accumulation rates over the ice sheet. Parameterization of this effect will be necessary in full-scale numerical models of the West Antarctic environment.
To determine the seminal relationships between these interactions, data of sea-ice concentrations, weather patterns, and accumulation rates are required. Extended time series of sea-ice concentrations are now available from satellite-based passive microwave sensors but the accuracy of concentration estimates needs to be improved. Weather patterns can be derived from satellite image data but supplemental data from automatic weather stations (AWSs) installed near the locations of perennial coastal polynyas would provide much-needed detail. These AWSs should be instrumented to measure accumulation to augment a record of past accumulation rates and their spatial pattern obtained from a set of shallow firn cores acquired at typical coastal sites.
The oceanographic projects that are essential to the realization of WAIS objectives offer numerous opportunities for interdisciplinary cooperation. These include interactions with: 1) modelers simulating the global-scale coupled ocean-atmosphere circulations and the ice-sheet response to global change; 2) meteorologists concerned with the sources of moisture to the ice sheet, the annual cycles of ocean surface temperature and the siting of AWSs; 3) remote-sensing specialists documenting spatial and temporal changes in the ice-sheet perimeter or sea-ice distribution and movement; 4) glaciologists interested in regional variability in ice-shelf basal mass balance and crevasse distribution; 5) geologists seeking environmental data to interpret sediment cores, seismic data or bathymetric features; and 6) biologists studying the dark and remote environments beneath ice shelves, the impact of changing sea-ice cover on productivity, or the role of DMS in the marine environment and in the atmosphere.
5.4. GEOLOGY AND GEOPHYSICS
The following three key questions are linked by the fact that the data to answer them consist of dated marginal positions of the ice sheet-be they on land, on the sea floor, or subglacial. Each topic is treated separately below.
1) What was the configuration of the West Antarctic ice sheet during and following the last glacial maximum?
2) What is the configuration of the West Antarctic ice sheet during an extreme glacial minimum?
3) Has the West Antarctic ice sheet undergone rapid, episodic mass loss in the recent geologic past?
5.4.1. Terrestrial Geology
Drift sheets, including end moraines and erratic boulders, bedrock striae and molded erosional forms, glacial fluvial and lacustrine features, trimlines of weathering, and emerged marine strand features provide reliable evidence upon which to reconstruct past configurations of the West Antarctic ice sheet. From these features, the boundaries of the ice sheet, both areal and vertical, and the flow direction can be determined. When tied to specific times by relative and absolute dating methods (e.g., AMS and conventional radiocarbon methods and cosmogenic surface exposure methods), these data will allow the development of chronologies defining ice-sheet growth and decay. In addition, they will provide bounds for the determination of rates of advance and retreat. Time resolution of suitable numerical dating methods for events since the last glacial maximum (LGM) is on the order of 50 to 100 years.
The most promising geographic study areas for these investigations are: the east flank of the Transantarctic Mountains; the nunataks and coastal mountains of Marie Byrd Land and Ellsworth Land; and the Pensacola, Thiel, Horlick and Whitmore Mountains. The Transantarctic Mountains have already received a great deal of study, but gaps must be filled and the results integrated with existing data. These gaps are included in the regions from Cape Adare to Coulman Island, Coulman Island to McMurdo Sound, and McMurdo Sound south to Darwin Glacier. Investigations should also be extended farther southward into the Reedy Glacier area. In this area, the record can be directly integrated with the past and present activity of ice stream A of the West Antarctic ice sheet and its tributaries draining the East Antarctic ice sheet. In addition, the former height of the ice sheet should be determined on nunataks of the Thiel Mountains and in the area of the Foundation Ice Stream. Foundation Ice Stream discharges into the Ronne Ice Shelf and is similar to ice stream A in that its drainage is of predominantly West Antarctic ice yet its tributary, Academy Glacier, drains a portion of the East Antarctic ice sheet. Previous studies in the southern Transantarctic Mountains and nunataks of west Antarctica until now have been hampered by lack of a reliable dating method applicable for the time since the LGM. Continuing improvements in the cosmogenic surface exposure dating method hold the promise of being able to provide absolute chronologies of events.
The glacial history of the Antarctic Peninsula, although not physically a part of the West Antarctic ice sheet, must also be determined. This record, from the LGM to present, will form the connecting link between the record of WAIS proper and the rapidly developing record south of 45¡S, in the Andes of southern South America, and the Southern Alps of New Zealand. The record from the peninsula will provide the basis for comparison of events both north and south of the Antarctic Convergence and hence, a composite record essential for documenting and understanding the role of Antarctica in global climate and sea-level change.
The chronology of ice surface level and marginal position change, integrated with data developed from marine geology studies of the glacial record preserved on the continental shelf, will reveal whether episodes of rapid and episodic ice wastage occurred during deglacial cycles. The data documenting such episodes can then be compared with the global sea-level record. Ultimately, these combined data will provide the critical historical record of the transient behavior of the ice sheet essential for the control and calibration of ice-sheet numerical models. In addition, the chronologies will be useful in testing the consistency between interpretations derived from the ice core, ice dynamics, and marine geology components of WAIS and with the expanding records from southern South America and New Zealand just north of the Antarctic Convergence.
5.4.2. Marine Geology and Geophysics
The geologic record also extends to the ocean-covered areas of the continental shelf where the ice sheets deposit material and rework this material when they advance over it. The structure and stratigraphy of this sediment furnish a record of glacial and glacial-marine conditions at the time of deposition and possible clues to the roles that subglacial water, deforming till, bedrock geology and heat flow may play in the dynamics of ice-stream motion. By dating the core material using geochemical and paleontological techniques and by evaluating the relative composition and character of the sediments, the age of key units, and therefore the rates and patterns of ice-sheet retreat, can be inferred. Furthermore, the stratigraphic progression on the continental shelf may provide a direct record of the sequence of subglacial events that led to ice-sheet retreat from the shelf.
Paleontological techniques also provide information on the bottom- water characteristics (e.g., temperature, bathymetry, and proximity to grounded ice), upper water mass characteristics (e.g., presence or absence of glacial ice, salinity, light, mixing, and primary productivity), and terrestrial palynology (e.g., vegetation characteristics, spores and pollen).
The specific tasks that must be accomplished to answer the above questions are: 1) an analysis of existing high-resolution seismic data and piston cores from the Ross Sea; 2) a mapping of the sea-floor stratigraphy using multi-beam bottom imaging instruments; 3) a refinement of the location and dating sequence of significant geologic features found in (2); and 4) an overall synthesis of data and interpretation.
Task (1) will be undertaken initially to assess these existing data and to define better those areas where field studies should be conducted. Seismic analyses will concentrate on defining those areas where seismic facies and morphological features imply former grounding-line positions.
Many piston cores have been acquired from the Ross Sea continental shelf, mainly during Eltanin, Glacier and, most recently, R/V Palmer cruises. These cores contain a valuable record of the late Pleistocene-Holocene glacial setting on the shelf. Detailed sedimentological studies of these cores will help to locate former grounding-line positions and the paleodrainage divides of former ice sheets, and to study the nature of ice-sheet retreat from the shelf. Studies are being conducted to evaluate methods of age-dating these sediments so that the actual timing of ice-sheet retreat from the continental shelf can be established and to extract information of water mass characteristics and terrestrial palynology listed above. One or more planned transects will extend from the continental slope to the ice-sheet edge and along paleodrainage divides of specific ice streams (based on provenance studies of tills conducted during the previous two years' studies). Age dating of the contact between subglacial and glacial marine sediments in these cores will rely on the most current radiometric dating methods (i.e., Tandem Accelerator Mass Spectrometry: TAMS) and may provide a record of the timing and rate of ice-sheet retreat.
During the 1995 field season, mapping of the sea floor using multi-beam bottom imaging instruments (task 2) will provide a wealth of information about the subglacial environment during the last glacial maximum. This work, done in conjunction with high-resolution seismic profiling, will result in a three-dimensional reconstruction of the glacial sediments and the thin veneer of Holocene sediments that blanket the Ross Sea continental shelf. These sediments record the most recent expansion of the West Antarctic ice sheet and its retreat from the shelf.
Information gained during the first 3 years of the project will be used to reconstruct the geological setting (e.g., bedforms, subglacial meltwater channels, sediment properties) associated with former grounding-line positions on the shelf, and to examine the history of ice-sheet retreat from the shelf. The fourth year will be devoted to task (4), which is exclusively interpretive. If there have been problems in any of the prior data-collection seasons, repeated field work in the above areas may be required.
Longer cores are required to test the hypothesis that bed deformation led to rapid ice-sheet retreat. Marine geological and geophysical work accomplished to date has shown that the sedimentary unit deposited during the last glacial maximum is thicker than was anticipated. Piston cores from the continental shelf penetrated diatomaceous muds, which reflect a glacial marine setting similar to that of the present, with the ice sheet resting on glacial marine sediments that were deposited in a position more proximate to the grounding zone of the ice sheet. These cores bottom-out in the top of a unit that has tentatively been interpreted as deformation till. Thus, the recovered stratigraphic progression of bed deformation and ice-sheet retreat is incomplete. Shallow drilling is needed to accomplish this objective.
The modern analog for the depositional environment of an extended West Antarctic ice sheet is located underneath the Ross Ice Shelf at the grounding line of the ice streams. Sediment cores recovered from beneath a series of access holes drilled through the ice shelf will provide an unprecedented view of how the sediment structures evolve. Seismic work at each drill site will give the stratigraphic context of each core assisting in the generation of a single, continuous record from the grounding line to the edge of the continental shelf. This data set will enable a better interpretation of the dynamics and history of the ice sheet, the depositional mechanism and the role of water in the formation of these deposits. Logistic requirements of this project will be reduced by coordination with WAIS oceanographic investigators also working on the ice shelf.
Additional areas that must be studied include Pine Island Bay and the Wilkes Land continental shelf. Pine Island Bay is an area of particular interest due to the absence of ice shelves in the mouths of the two major outlet ice streams. Marine geologic and geophysical studies of the bay combined with the oceanographic studies recommended (see Section 5.3) should provide insight into the history of ice-sheet advance and retreat in this unique setting and possible causes. It is anticipated that one field season will be sufficient to collect the seismic data and cores needed to address the questions concerning the history of each area during and since the last glacial maximum. The timing of this study would be coordinated with oceanographic and meteorological interests in the region.
The continental shelf along portions of the Wilkes Land margin represents the terminal drainage for two large marine-based ice drainage systems which appear to be relatively stable. This contrast to the West Antarctic condition would provide information on those factors which contribute to this stability. The precise timing (within a few hundred years) of recession of the ice sheet can be determined because of the presence of: a well-defined terminal moraine system; a high-resolution post-glacial marine sequence; and a relict subglacial morphology, fluted surfaces, etc. The age, composition, and distribution of these features need to be evaluated through a program of high-resolution seismic profiling, side-scan sonar surveys, and coring.
5.4.3. Subglacial Geology and Geophysics
To understand the minimum extent of the ice sheet (question 1b, above), data must be collected from beneath the present-day ice sheet. Holes through the ice can be drilled rapidly using hot-water systems, permitting frequent access to the bed either underneath the ice shelf or underneath the grounded ice sheet. Collected cores will be examined in detail for sediment physical properties, composition and texture. Sedimentological, micropaleontological and geochemical analyses will be performed to constrain the environment of deposition, to establish the age of these deposits, and to help determine the southernmost extent of the grounding line during past interglacials. These data will be correlated with similar information derived from marine sediments collected on the continental shelf or under the ice shelves to provide a broader view of the history of the ice margin. Cores acquired within the present zone of ice-sheet grounding will be used to test models relating bed conditions to ice-sheet stability. The actual timing involved in drilling at these sites will rely on coordination with glaciological and oceanographic studies.
Subglacial geophysical investigations will also be directed at answering a much different question than the trio of questions listed at the beginning of this section.
4) What are the geologic controls on the flow of a marine ice sheet?
The answer to this question requires geophysical techniques to study the geologic setting of the ice sheet. For this study, the subglacial lithology, distribution of geothermal flux, and tectonic activity all must be considered. There is evidence that the positions of the ice streams are controlled by the subglacial lithology and perhaps by the thermal regime. If their rapid motion is caused by a deforming subglacial till made weak by pressurized subglacial water, then the ice streams must be located near a rapidly erodible source of sediment as well as in a region characterized by basal melting. It is known that the West Antarctic lithosphere is characterized by recent volcanism and associated high geothermal flux, but few details exist on the spatial pattern of these characteristics.
Most of the subglacial and englacial geophysical data of the West Antarctic ice sheet and its underlying bed that are needed for WAIS will be collected either by projects already planned, such as Antarctic Geophysical Initiative (AGI) (see Section 6.1.2), or by collaborations with WAIS investigators in ice dynamics, ice coring, or marine geology. This method of providing ancillary data for glaciological studies in the Siple Coast Project (Section 6.1.1) proved effective and established the broad geophysical data base upon which subsequent measurements during WAIS will be able to build.
Early in the WAIS program, a regional mapping of the ice-sheet configuration and bed-type needs to be accomplished in the region of the catchments of ice streams AÐE. Understanding the correlation of lithospheric boundaries and subglacial morphology with changes in the dynamic regime between the ice divide and the ice streams of this portion of the West Antarctic ice sheet is critical to understanding its evolution, but insufficient data exist. Airborne radar soundings combined with airborne gravity and magnetics should be flown in cooperation with transects being undertaken by the AGI program.
A detailed aerogeophysical survey also will be required in the vicinity of the proposed deep ice-drilling site on the divide between the Pine Island and Ross Embayments. The surface, bed and internal-layer morphology supplied by these studies are necessary boundary conditions for the ice-flow modeling of the age-depth relation needed to understand the history of the West Antarctic ice sheet and to date any ice cores from the ice sheet. Additionally, any indications of lithospheric properties that might coincide with high geothermal flux and a shortened core record can be evaluated before drilling begins. These data can be obtained in conjunction with the aerogeophysical efforts in support of ice dynamics and subglacial geology in the catchments of ice streams D and E and over the Byrd subglacial basin.
Understanding the water and sediment budget as well as the ice rheology in the region at the onset of ice streaming flow is critical to understanding the dynamics of the ice sheet and our ability to model it. Because of this, geophysical investigations of the bed character, bed morphology and ice fabric using both ground radar and seismological techniques need to be undertaken in the area near the onset of fast flow. These studies should begin immediately after completion of the aerogeophysical program described above.
Determining the current state of the West Antarctic ice sheet will require measuring the ice thickness of the outlet glaciers along the coast of Marie Byrd Land. Satellite-image-derived velocities can be combined with these data to determine the current mass outflow in these coastal areas to compare with independently derived estimates of the iceberg calving rate. Reconnaissance work of a similar nature in the marine portions of Wilkes Land is also advisable.
5.5. BIOLOGY
Biological phenomena in Antarctic terrestrial and marine systems are directly influenced by ice-volume changes associated with the advance and retreat of the West Antarctic ice sheet. Lakes in the Dry Valleys, for example, have blue-green algae which have been used for dating paleo-shorelines and sedimentary deposits that reflect changes in the local hydrology since the Last Glacial Maximum (LGM). Marine fossils (including algae, invertebrates and vertebrates) in emerged beaches and marine sediments, from the Ross Sea to the Antarctic Peninsula, have been used for interpreting the regional history of ice-sheet retreat. Emerged marine records also place constraints on the extent of glacial isostatic uplift, which is a key to modeling Antarctic ice-volume changes that have affected sea level since the LGM. While biological phenomena may not directly influence the West Antarctic ice sheet, they clearly are central to interpreting its complex dynamics.
1) How have WAIS margins and the coastal environmental features (such as temperature, glacial-meltwater runoff or sea-ice extent) in the transition zone between the WAIS and the ocean, changed since the LGM?
Biological phenomena that are impacted by the West Antarcitc ice sheet occur within individual organisms as well as their populations and communities. Individual organisms contain geochemical data (such as radiocarbon and stable isotopes in shells) that reflect the hydrochemical conditions which existed when they were alive. Over larger scales, changes in the ice-sheet margins directly affect the distribution and abundance of species in adjacent populations and communities. Together, these biological phenomena may provide sensitive indicators of the climate/sea-level feedbacks which impact the stability of the West Antarctic ice sheet. Specifically, biological phenomena address the two following multidisciplinary questions which are related to the environmental history and interactions associated with West Antarctic ice dynamics since the LGM:
2) What are the modern analogs and experiments which are necessary to establish baselines for interpreting biological phenomena in relation to WAIS dynamics in the past?
General criteria for selecting the key biological phenomena should be based on their fossil abundance, distribution and preservation. The first priority should be given to bio-markers, such as diatoms and foraminifera or other invertebrates, which complement ongoing glacial-geologic and oceanographic studies. These bio-markers provide two relevant types of information: chronological (based principally on their radiocarbon ages) and environmental (based on species growth, geochemistry and assemblages). In addition to enhancing the interdisciplinary framework of the WAIS program, coordination among biological, glacio-geologic and oceanographic studies will minimize logistics.
Chronological information based on the radiocarbon ages of Antarctic marine organisms is complicated by the upwelling of "old" deep water, which dominates the radiocarbon reservoir in the Southern Ocean. Prior to the anthropogenic impacts of nuclear bombs and fossil fuel combustion this century, the age of the Antarctic marine radiocarbon reservoir was around 1300 years. However, this radiocarbon reservoir may have been altered in the past by changes in the flux of glacial meltwater and the transport of North Atlantic Deep Water into the Southern Ocean. Unlike atmospheric radiocarbon variability, which has been interpreted from tree-ring and coral records since the LGM, variations in the radiocarbon reservoir of the Southern Ocean are unknown during this period.
Assuming that the radiocarbon reservoir in the Southern Ocean has been constant since the LGM is reasonable as a first approximation. However, additional research on the radiocarbon reservoir in the Southern Ocean is necessary for reconstructing the glacial history of the West Antarctic ice sheet to interpret the relation between "rapid" climate and ice-sheet changes. This research should be conducted across environmental gradients (such as meltwater plumes) that represent hydrochemical changes that may have impacted the Antarctic marine radiocarbon reservoir over time. In addition, the focus of this radiocarbon research should be on calcareous species, which account for the vast majority of Antarctic marine fossils.
Environmental information is recorded in the growth and geochemistry of marine species' skeletons. Species growth records reflect seasonal, interannual and decadal environmental variability - depending on the lifespans of the species that are being studied. Growth records are complemented by geochemical analyses (stable isotopes and trace elements), which can be used for interpreting productivity, salinity and temperature variations in the Antarctic marine environments. Together, the growth and geochemistry of modern marine species provide experimental analogs for interpreting the environmental variability, which is recorded in the skeletons of their fossils.
Environmental variability in the Southern Ocean also is reflected by the distribution, abundance and composition of marine species assemblages. For example, marine benthic species' diversities vary between eutrophic and oligotrophic environments. Changes in the abundance of selected marine species have been related to decadal oceanographic climate shifts in the Southern Ocean. There also is an association between the composition of marine diatom assemblages and sea-ice extent. To interpret the paleoecology of marine fossil assemblages adjacent to the West Antarctic ice sheet, environmental variations which affect their extant marine analogs should be experimentally investigated across comparable habitat gradients (e.g., depth and latitude).
3) How have the distribution, abundance and geochemistry of West Antarctic marine and terrestrial species been influenced by ice-sheet marginal phenomena (e.g. ice shelves, ice tongues and piedmont glaciers) since the LGM?
To interpret "rapid" changes in the dynamics of the WAIS, those species assemblages should be identified which have persisted over centuries and millennia with the ability to respond to environmental changes over years and decades. In this context, extreme environmental changes which eliminate these species assemblages would be highlighted. Moreover, subtle environmental changes that alter the dynamics of these assemblages could be calibrated against long-term baselines. An additional criterion for identifying these assemblages would be their relationships to phenomena which directly impact WAIS dynamics, such as ice-shelf or meteorological processes.
Appropriate paleo-assemblages might include algal mats in the Dry Valley lakes, benthic marine invertebrate populations in sheltered coves and harbors, or seal and penguin rookeries. After being identified, the distribution, abundance and geochemistry of the paleo-assemblages should be analyzed in relation to their modern analogs.