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US ITASE Science Implementation

Science implementation for US ITASE is divided into four phases with nominal start dates and duration defined below:

Phase 1:Corridor/ground-based sampling planning 1999 -----------> 2003
Phase 2:Ground-based sampling 1999/2000 -------> 2003/2004
Phase 3:On-going corridor studies 2000/2001 -------------> 2005/2006
Phase 4: Interpretation and modeling 2000 -----------------------------------------> 2007

Phase 1 - Corridor/Ground-Based Sampling Planning Phase

Four study corridors were defined at the 1996 Baltimore meeting (Figure 1) based on a combination of optimized sample distribution, accessibility and specific science goals. These are:

1. Byrd region toward Siple Station

2. Byrd region toward Pine Island Bay toward the Executive Committee Range

3. Byrd region to Siple Dome

4. Byrd region to South Pole

It has been determined that these broad corridors are the best ground-based sampling sites. The following information has been collected in order to develop such a plan.

Published spatial distributions. Some published information describing spatial trends in stable isotopes (Figure 7) and chemistry (Figure 9) currently exists, although coverage over West Antarctica is generally fairly sparse. These data were summarized and interpreted prior to determining ground-based sampling sites.

Meteorological modeling. The potential exists for substantially increasing the spatial resolution of derived annual accumulation rates by using more advanced calculation techniques, and by exploiting reanalysis projects (e.g., Kallberg, 1996) that yield atmospheric analyses with a fixed data assimilation scheme. This work was done before major US ITASE traversing commenced in West Antarctica which provided guidance on the sampling strategy and helped to identify critical areas.

Satellite images. Satellite imagery were used to derive surface slopes by photoclinometry since slope may control wind and accumulation rate. Satellite passive microwave data (e.g., Nimbus-7 ESMR) were used to estimate the current accumulation rate patterns, and satellite infrared images (AVHRR) can be used to estimate surface temperature patterns prior to selecting the core sites.

Airborne radar - SOAR. A SOAR Twin Otter flew several lines and cross-lines through each US ITASE corridor to map bedrock, ice thickness and internal layers at scales of 10 to 25 ice thicknesses (25-50 km) on either side of the ground-based sampling routes to select appropriate sites for sampling. The SOAR Twin Otter Aerogeophysical survey aircraft has already covered some blocks in the US ITASE sampling corridors using ice-penetrating radar. The ice thickness was used to calculate expected thinning of annual layers in cores, and the internal layering is important for understanding spatial variability further back in time. Previous experience shows that spatial variations in accumulation are closely related to ice sheet topography, which in turn is related to bedrock topography and bedrock heat flux. Knowledge of bedrock patterns from radar will allow a better assessment of whether the spot measurements of accumulation at core sites, and their associated near-surface radar tie-ins, are representative of long-term regional accumulation patterns.

High resolution ice-penetrating radar. Airborne ice-penetrating radar surveys are being carried out using a system that can record high quality internal layering in the upper 50 to 100 meters. These data identify regions with good preservation of stratigraphy in the past 200 years, and are linked to the accumulation rate histories derived from the individual core sites. Multiple flight lines were flown in each corridor, with cross tie lines where possible. Even if such a high-resolution radar system does not detect bedrock, maps of the upper internal layering will be invaluable to the traverse planning and data interpretation.

If the high-resolution radar and the TUD radar in the SOAR aircraft can be flown simultaneously, then a comprehensive wide swath radar corridor (10-25 ice thicknesses) should be flown at this stage to recover ice surface, bedrock, and deep, intermediate and shallow internal layering. All these data will be valuable for interpreting various aspects of the past 200 years of climate.

Phase 2 - Ground-Based Sampling

Ice cores and surface snow sampling (including ground truth for remote sensing). Ice core recovery is nominally being conducted at 100 km intervals along ground-based sampling sites determined in Phase 1 of this study. Minimum 200-year ice core multivariate (Tables 1 and 2) records are retrieved requiring the penetration depths specified for the sample drill sites on Figure 1. Annually resolved records are developed from a suite of dating tools (Tables 1 and 2) sampled either continuously or at discrete sub-annual intervals (i.e., 6-10 samples/year). Surface sampling (fresh snow, snowpit) will be conducted at each ice core site and more frequently if needed. Atmospheric sampling is being conducted at selected ice core sites along with AWS units.

In order to understand controls on deposition for snow and impurities, it is necessary to follow trajectories of water- and impurity-laden air masses. After deposition, many of the ice sheet properties of interest are carried along by ice sheet flow. As a consequence, cross-sections or profiles that follow air flow trajectories or ice flow lines are valuable and necessary complements to any ground-based sampling. These studies allow physical and chemical changes and processes to be tracked from atmospheric source regions to deposition sites, and then through the ice sheet to the ice core where the samples are recovered. Without these accompanying studies, interpretation of ground-based sampling would be difficult, processes controlling deposition would be unclear, and temporal gradients in deposition would be harder to separate from spatial gradients.

Coupled surface snow and snowpit measurements (e.g., accumulation rate, temperature, snow albedo, snow surface brightness, visible and solar-infrared observations, surface roughness, melt layers, snow grain size, hoar development, emissivity) are planned as verification of remotely sensed snow properties. Snowpit observations combined with visible/near-infrared spectral albedo and thermal infrared observations will also be useful for testing/validating radiative transfer modeling of the snowpack. Since modeling of this sort provides the direct link between the physical properties and the optical properties of the snow, model results represent a way to extend the usefulness of physical properties observations obtained in regions where, and at times when, surface-based albedos are not available.

Calibration and testing of models of snow-atmosphere transfer functions require knowledge of the timing of snow accumulation, as well as measurement of chemical concentrations in the accumulated snow/firn (McConnell et al., 1996). This can be achieved by sampling snowpits adjacent to AWS units that are equipped with automatic depth gauges to measure snow depth with time. Sampling should be done at the same time the AWSs are serviced.

Studies such as these lead to the concept of US ITASE as a "scientific glue" that is conducting multi-disciplinary research over spatially broad corridors. Geophysical experiments will be deployed on the ground traverses to assist in this issue.

Ice penetrating radar. Surface radar capable of detecting crevasses (Delaney and Arcone, 1995; Arcone et al., 1996) will be carried routinely on all over-surface traverses. In addition to providing crevasse detection, layering data derived from this high resolution radar will be useful for mapping accumulation patterns.

Downhole gamma detector and gross beta filtration. Downhole gamma ray detection will be used in every borehole to detect the depths to the atmospheric bomb test layers (1954-1965). Measurements of gross beta from Cesium 137 and other bomb products on cation filters from the recovered shallow cores also provide a tried-and-true determination of average accumulation rates over the past decades. The difference between the techniques is that the downhole gamma counter has the advantage that no dedicated core needs to be carried by the traverse vehicles, while the beta filtration method requires less time at the core site.

Both methods of detecting the bomb layers require good estimates of the overlying snow density in order to derive accurate accumulation rates. The drills used to recover cores for density measurements must be in good condition to recover cores of uniform diameter, and the actual diameters must be logged. The importance of obtaining reliable diameter measurements on the core that is weighed must be emphasized strongly.

Borehole temperatures. Borehole temperature profiles should be measured in every hole immediately after drilling in order to detect modern spatial patterns of mean annual air temperature. It is unlikely that old 10-meter firn temperature data from the IGY era will be adequate to compare with modern data to detect recent temperature trends because there are too many sources of error associated with residual seasonal cycle effects at 10 meters, interannual variability, calibration offsets and air convection in open holes.

However, it should be possible to directly measure temperature trends over the past 200 years by high-resolution (1omK) continuous temperature logging as carried out at GISP2 and Taylor Dome. Best results will be obtained by using fluid-filled pipes that can be emplaced in boreholes of about 300-400 meters depth. The holes are drilled and the pipes emplaced during the traverse. The fluid is an environmentally acceptable liquid that has known heat transfer properties. Measurements have been made successfully in DFA (arctic diesel) and in n-butyl acetate. Since the fluid is used to provide thermal contact, not to pressurize a hole, it is not necessary to keep the density around 0.9 Mg/m3; other fluids besides DFA and nBA (e.g., alcohols) could be possibilities.

Ice motion and strain - GPS. High accuracy positions are now be obtained in minutes using GPS receivers. Ice motion measurements are started during ground-based sampling, and the second measurement of position needed to get displacement and velocity will be carried out on visits in subsequent years.

Measurements of the ice speed and direction of flow for marker poles at each core site are initiated as the cores are drilled. The deformation rate is also known at each site. This requires at least a 3x3 array of markers with a spacing of an ice thickness surrounding the core site.

As many ice velocity measurements as possible are obtained along traverse routes. These are detailed on three parallel lines separated by about one ice thickness. These data are useful to interpret the accumulation information contained in the internal layers mapped along the traverse routes. If ice velocities for the region can be obtained by radar interferometry, these GPS ground-based data can be used to verify the satellite-derived results.

Ice sheet thickening rate. When vertical velocity of the firn is accurately measured, the ice sheet thickening rate can be detected for the sea level experiments described earlier. A marker pole or other device should be frozen as deeply as possible in the firn at each ice core site and any other sites where practical, and the position of its upper end measured by GPS, following procedures established previously at other Antarctic and Greenland sites.

Meteorological measurements. Since AWS data from Byrd Station shows that the wind direction there is predominantly from the north and at Siple Station the predominant winds are from the south and west, additional stations were deployed to assess the importance of this large change in wind direction between the two stations. Further, some AWS units are needed to the east and south of the array in Figure 5 to monitor the spatial and temporal variations of the warm air advection pattern affecting the region. In addition to the suite of standard sensors, an acoustic depth gauge could be used to determine the resulting impact on the accumulation rate, which has a large time-averaged spatial gradient in this area (Giovinetto and Bentley, 1985). With appropriate sensors, a full surface energy calculation can be performed (Bintanja et al., 1996) to understand the factors governing this spatial potential temperature pattern. In conjunction with the AWS measurements, investigations of cloud properties over West Antarctica using ground-based or satellite remote sensing would provide a major advance in understanding the generation of this warm air advection signature. A Fourier-transform interferometer sited at Byrd Station, for example, could be used to infer West Antarctic cloud optical properties (base height, optical depth, and spectral emissivity) during the summer. A systematic survey of the snow surface albedo as a function of snow microstructure could be done along the ITASE traverse route to determine this critical aspect of the summer surface energy balance. In addition, at several of the ground sampling sites it was also valuable to deploy data-logger-based AWS measuring wind, air and snow temperatures and visible radiation to supplement satellite-based AWS. These studies are done in conjunction with the ITASE traverses, and would provide critical information for the ITASE sampling as well as interpretation of the temporal variations. The temporally and spatially varying warm air advection should be studied in relation to the broadscale atmospheric circulation to determine the dynamics controlling its inland propagation.

Phase 3 - Ongoing Studies in the Corridors

Some limited number of sites sampled during Phase 2 will require reoccupation to complete experiments. These include:

Ice motion measurements. The ice motion markers installed and positioned during Phase 2 should be revisited and the displacements measured.

Vertical motion. All markers set in the firn to record absolute vertical ice motion must be resurveyed and vertical displacements calculated.

AWS.The weather stations will need servicing trips and will periodically need to be reset at the current surface level. If datalogger-based AWS units are deployed, data will need to be collected at intervals of a few years.

Reoccupation of AWS sites also offers the opportunity to deploy and then collect aerosol sampling units that can give a time history of atmospheric chemistry.

Airborne radar - SOAR. SOAR data gathering was accomplished prior to ground-based sampling. Logistics and science planning may not always allow this sequence, in which case SOAR flights would be requested over regions previously sampled by US ITASE.

High-resolution borehole temperature logging. High-resolution continuous temperature logs will be obtained in up to six 300-400 meter boreholes. Ideally, the holes can be cased to the surface and fluid-filled. Results will provide a direct thermal record of temperature changes at each site over the past 200 years, and delineate the region that had a "cold" Little Ice Age (as seen at South Pole) from the region that had a "warm" Little Ice Age (as seen at Siple Station). In order to correct the measured temperatures for downward advection of cold ice, the ice velocity field will be measured and modeled carefully.

Ice dynamics in support of key cores and temperature logging. At key locations where ice layer thinning must be calculated precisely, where older ice and climate records are taken, and where high-precision deep temperature logs are carried out, it will be important to understand the local ice flow in more detail. Individual proposals to measure ice flow (GPS) and ice depth and internal layering (radar) may be submitted later to study key sites.

Crustal geophysics. It is possible that geophysicists studying crustal structure and tectonic processes will also be interested in ITASE. Traverse routes may have common locations (e.g., the ANTLITH (Antarctic Lithosphere) seismic line across WAIS to South Pole) could share logistical support and holes.

High-frequency seismic reflections can also show internal structures in the ice such as ice fabric changes. This information may assist interpretation of ice core data by providing additional information about ice flow.

Phase 4 - Interpretation and Modeling

As discussed elsewhere in this document, ice core time-series contain a rich and varied record of climate forcing and response. However, the relation between a global, hemispheric or even regional atmospheric concentration and an eventual ice core concentration depends on transport from source regions, depositional processes, and post-depositional changes in the snow and firn. For example, Hogan (in review) notes that the warm air advection across West Antarctica intermittently delivers heat, moisture and particulates to South Pole throughout the year. Thus the chemical composition of the near-surface snow in the region between West Antarctica and South Pole is strongly influenced by the trajectory and intensity of this flow pattern, and significant gradients are expected for some species, particularly those of marine origin. Significant input of chemical species also occurs due to subsidence of stratospheric air over the pole and possibly through direct precipitation from the stratosphere. The latter mechanism may be particularly important for cosmogenic isotopes (Steig et al., 1996a), which are produced predominantly in the stratosphere, and for nitrate, which may be a proxy for polar stratospheric clouds (Mayewski and Legrand, 1990).

One goal of meteorological modeling wthin ITASE is to support predicitve modeling of the spatial distributions of chemical constituents in near-surface snow. Concentrations differences for some chemical species will be determined largely by atmospheric circulation. For others, some degree of chemical modeling and possibly explicit consideration of varying depositional processes will be needed. For example, the gradient of sea salt concentrations from coastal areas towards the pole should change over the course of El Nino-type oscillations. A complementary approach is to relate spatially distributed glaciochemical time-series collected by ITASE to observed meteorological fields. The resulting transfer functions can be used to infer aspects of the atmospheric circulation for the period before meteorological observations were routinely collected in Antarctic latitudes. On the other hand, it is generally assumed that the atmospheric concentration of cosmogenic isotopes such as 10Be is spatially invariant across much of Antarctica (Raisbeck and Yiou, 1988; Steig et al., 1996b). This assumption is probably not strictly valid; Steig et al. (1996a) suggest that, because of the longer stratospheric residence time of cosmogenic 36Cl, the 36Cl/10Be may be sensitive to the rate of stratospheric subsidence. The magnitude of these effects can only be estimated through the explicit calculation of air mass trajectories. In contrast, modeling spatial distributions of species involved in atmospheric photochemical reactions will need to consider variations in temperature, moisture, actinic flux, and other chemical species in a tropospheric air mass.

GCMs can be used to provide an estimate of source region influence on climate in an area such as Antarctica. This can be done by using a version of the code which "follows" moisture from its source to sink location (e.g., Koster et al., 1986). The Some GCMs include separate code for advecting any tracer; thus the same procedure can be applied to dust, marine sulfate sources, etc. The procedure can be used for a variety of past climates, including, for example, simulations of the Little Ice Age (Rind and Overpeck, 1993). In addition, some GCM versions have been modified to calculate the isotopic balance for water molecules (18O, D) (Jouzel et al., 1987, 1991). This capability is of obvious use in climate change and climate forcing experiments, allowing for direct comparison with ice core observations.

Of particular interest are the rapid climate variations seen in ice core records. GCMs have already been used to investigate interannual climate variability for some of these events during past climates (e.g., Rind, 1991; Fawcett et al., 1996), and the same procedure can be used to evaluate the ice core response. A more fundamental question, though, is how much of the rapid variations seen in ice core paleoclimate indicators is due to source region changes, regional differences (e.g., distance to the coast), local phenomena (e.g., wind redistribution and temperature differences), and how much represents a true global response.

In areas of complex terrain such as West Antarctica, state-of-the-art GCMs do not have the spatial resolution to depict variables on a scale suitable for direct comparison with the surface-based observations that will be collected by ITASE. The ice divide area near Byrd, where there is a sharp transition from marine to continental environments and where the warm air advection is active (Figure 5), is likely to be particularly problematic. One approach that will complement the above GCM simulations is to embed a mesoscale model within the GCM by specifying the initial and boundary conditions for the mesoscale model from the GCM output (e.g., Hines et al., in review). This approach applies the large-scale forcing simulated by the GCM to the West Antarctic region, but gives a much more accurate depiction of the regional topographic forcing than the GCM and provides model output on a length scale of 30 km or less.