ROBERT A. BINDSCHADLER, National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, Maryland 20771
XIN CHEN and PATRICIA L. VORNBERGER, General Sciences Corporation, Laurel, Maryland 20727
Ice streams are the rapidly moving conveyors of ice that drain the majority of ice in West Antarctica. Their behavior holds the key to the ice sheet's future, yet it remains unknown why they occur where they do. Investigations focused on the crevassed margins, the lubricated bases, the entrances into the floating ice shelves as well as the trunk of the ice streams have greatly increased our understanding of how an ice stream moves and changes, but very little attention has been directed toward the areas where ice streams begin (Anandakrishnan et al. in press).
Landsat imagery shows flow features that suggest ice stream D begins in a relatively narrow region close to Byrd Surface Camp. This situation made it our preferred choice for a survey of the surface velocity and deformation field. Ice flow at Byrd was approximated to be 13 meters per year (m/yr) (Whillans 1979). We later measured it to be only 11 m/yr, and with repeat imagery, we tracked the motion of a crevasse 160 kilometers (km) downstream at 130 m/yr, so our survey grid was designed to cover this at a 5-km spacing and a width that varied from 60 km upstream to 30 km downstream. Our primary objective was to locate the onset region and characterize the ice flow and deformation in its vicinity.
Surface surveys were conducted November to December of 1995 and 1996 by traversing the area with a mobile camp supporting a field team of five. Snowmobiles were used to establish each site and global positioning system (GPS) receivers used to collect data at each site in both years. The grid was surveyed in a series of 11 blocks. A GPS base station at the mobile camp centered within each block provided a precise position from which each surrounding grid site could be located by differential baseline analysis of the GPS data. Data were processed daily to ensure data integrity, and when necessary, measurements were repeated. Multiple observations at sites shared at block boundaries demonstrated that survey precision was 8.5 centimeters horizontally and 15 centimeters vertically (Chen, Bindschadler, and Vornberger in press).
Figure 1 shows the measured velocity vectors. It demonstrates that the major flow enters the grid south of Byrd flowing westward and turns to the southwest as it accelerates. This direction follows a subglacial trough averaging roughly 950 meters below sea level indicating that subglacial relief exerts a major influence on the direction of streaming flow in this area (Bamber and Bindschadler in press).
Figure 2 shows the surface strain-rate field. The increasing shear at the margins of the developing ice stream is clearly represented. Not shown is the strain-rate field measured upstream of Byrd at sites first established as part of the Byrd Station Strain Network (BSSN) (Whillans 1979). Our strain measurements were identical to the BSSN measurements collected between 1963 and 1967 indicating no change in the deformation field over more than 30 years.
The region of shear near the grid's upstream end is particularly interesting because it occurs where a series of flowlines originates, which themselves run obliquely to the measured flow direction. This region is believed to be undergoing change.
Based on our observations and analysis of the driving forces exerted on the ice, we identify the onset of ice stream D as near the downstream end of our grid (at approximately the 125 km coordinate in figure 1).
This research was supported by National Science Foundation grant OPP 93-17627.
Anandakrishnan, S., D.D. Blankenship, R.B. Alley, and P.L. Stoffa. In press. Geologic constraints on the flow-margin of a west antarctic ice stream. Nature.
Bamber, J.L., and R.A. Bindschadler. In press. An improved elevation data set for climate and paleo-climatic modeling: Validation with satellite imagery. Annals of Glaciology.
Chen, X., R.A. Bindschadler, and P.L. Vornberger. In press. Determination of velocity field and strain-rate field in West Antarctica using high precision GPS measurements. GPS Journal .
Whillans, I.M. 1979. Ice flow along the Byrd Station Strain Network, Antarctica. Journal of Glaciology, 24(90), 15-28.