Results

Although the final field season for the first phase of US ITASE has just been completed and there remains much data processing and laboratory work to do, initial results from US ITASE are impressive. A few examples are given below:

Shallow radar

Variable accumulation rates and ice movement can deform stratigraphy and thus affect the vertical density distribution of annual layers. However, field measurements show that slow variation of amplitude in these parameters (eg., Figure 3) along the US ITASE traverse routes allow single ice cores to be representative well beyond local scales. Internal stratigraphy in both shallow and deep (discussed below) radio echo sounding records represent isochronal events. This has been validated by sub-annual scale dating of ice cores penetrating these layers, providing therefore a record of depositional and ice flow history along the traverse.

Horizons detected by shallow, short-pulse radar transmitting near 400 MHz are particularly continuous along or near ice divides, are continuous within 2 m of the surface, get stronger into the firn, and still persist beneath the firn, although they weaken with greater depth (Arcone, 2002). Away from ice divides, they may plunge up to 35 meters yet maintain their amplitude. For this latter reason, and since they are continuous where density is highly variable (near-surface), and where there should be no density contrasts (beneath the firn), they do not appear to be responses solely to density contrasts, as is the current paradigm for firn. Comparisons between radar horizons and ice core chemical series are in progress.

Deep radar

The bedrock topography and geothermal flux beneath West Antarctica strongly influence ice flow and the overall stability of the ice sheet. Continuous ground-based low frequency radar (eg., Figure 3) characterizes bedrock topography, internal ice stratigraphy, and provides the potential to determine regions of cold and warm based ice. Ice thickness measurements (up to 3200 m depth) contribute to ice flow models that characterize the dynamics in ice core sites.

As with the shallow radar, internal ice reflectors represent isochronal regions, but due to the longer wavelength, are probably longer duration events. Internal ice stratigraphy is mapped along the traverse route with layers as deep as 2200 m.

Ice coring

US ITASE utilizes spatially distributed (Figures 3 and 4), multi-parameter ice core measurements to develop climate proxy records (Figure 5, next page). A depth-age scale for each core is produced using the multi-parameter procedure developed for the Greenland Ice Sheet Project 2 (Meese et al, 1997).

Parameters containing an annual signal include: visible stratigraphy, major ions, oxygen isotopes, and hydrogen peroxide. The accuracy of the dating within each core and between cores is determined using sulfate peaks from known volcanic eruptions. The presence of several major events in all of the U.S. ITASE ice cores allows precise calibration of annual layer counting between cores (eg., Meese and Gow, 2002).

Temperature dependence of crystal growth rates investigated thus far is consistent with previous measurements across Antarctica and Greenland. However, there is a large gap between –300C to –500C in previous studies that we will be able to address with US ITASE data. Snow and firn physical characteristics show large site-to-site variations both on the surface and with depth (Leeman and Albert, 2002). A feedback exists between air-snow vapor transport processes and the physical properties of snow that can induce large site-to-site differences in metamorphism (Albert, 2002). Variations in permeability and microstructure can cause differences in the dynamics of the air-snow transport processes between sites for reversible chemical species. Sites where the air-snow exchange is dominated by ventilation will likely show much greater post-depositional change in reactive species concentrations, while sites where interstitial transport is dominated by diffusion will show better preservation of chemical records. In addition, changes in climate at a given site could make the snow and firn more or less likely to retain the climate signal for reversible species As illustrated in Figure 5, stable isotopes are poorly preserved in the highly permeable firn from a site visited in 1999-2000, but are well preserved elsewhere.