Determination of the surface and bed topography in central Greenland
Hodge, S. M., D. L. Wright, J. A. Bradley, R. W. Jacobel, N. Skou, and B. Vaughn
Journal of Glaciology, Vol. 36, No. 122, p. 17-30, 1990
Surface and bottom topography of the central Greenland ice sheet was determined from airborne ice-radar soundings over a 180 km by 180 km grid centered on the 1974 "Summit" site (lat. 72¡ 18' N, long. 37¡ 55' W), using the Technical University of Denmark 60 MHz ice radar. Over 6100 km of high-quality radar data were obtained, covering over 99% of the grid, along lines spaced 12.5 km apart in both north-south and east-west directions. Aircraft location was done with an inertial navigation system (INS) and a pressure altimeter, with control provided by periodically flying over a known point at the center of the grid. The ice radar was used to determine ice thickness; the surface topography was determined independently using height-above-terrain measurements from the aircraft's radar altimeter. The calculated surface topography is accurate to about ± 6 m, with this error arising mostly from radar altimeter errors. The ice thickness and bottom topography are accurate to about ±50 m, with this error dominated by the horizontal navigation uncertainties due to INS drift; this error increases to about ±125 m in areas of rough bottom relief (about 12% of the grid). The highest point on Greenland is at lat. 72¡34' N, long. 37¡38' W, at an altitude of 3233 ± 6 m a.s.l. The ice surface at this point divides into three sectors, one facing north, one east-south-east, and one west-south-west, with each having a roughly uniform slope. The ice divide between the last two sectors is a well-defined ridge running almost due south. The ice is about 3025 m think at the summit. Excluding the mountainous north-east corner of the grid, where the ice local reaches a thickness of about 3470 m and the bed dips to about 370 m below sea- level, the maximum ice thickness, approximately 3375 m, occurs about 97 km south-south-west of the summit. The average bed altitude over the entire grid is 180 m and the average ice thickness is 2975 ± 235 m. The ice in most of the south-west quadrant of the grid is over 3200 m thick, and overlies a relatively smooth, flat basin with altitudes mostly below sea-level. There is no predominant direction to the basal topography over most of the grid; it appears to be undulating, rolling terrain with no obvious ridge/valley structure. The summit of the ice sheet is above the eastern end of a relatively large, smooth, flat plateau, about 10-15 km wide and extending about 50 km to the west. If the basal topography were the sole criterion, then a site somewhere on this plateau or in the south-west basin would be suitable for the drilling of a new deep ice core.
A glaciochemical survey of the summit region, Greenland
Mayewski, P. A., M. J. Spencer, M. S. Twickler, and S. Whitlow
Annals of Glaciology, Vol.14, p. 186-190, 1990
Spatial representativeness and an understanding of controls on chemical species distribution are essential requirements of any significant ice core investigation. Snowpit studies provide an essential tool in this process. In preparation for the central Greenland deep drilling effort a series of snowpits was sampled in detail for oxygen isotopes, major anions, major cations, total acidity and radio nuclides. The results of this sampling program are used to define: (1) the chemical composition of the snow in the region, (2) the input timing and spatial distribution or major chemical species, (3) the potential dependence of species concentration on accumulation rate, and (4) the signal characteristics identifiable in the region over the last few years.
Accumulation rate variations around Summit, Greenland
Bolzan, J. F. and M. Strobel
Journal of Glaciology, Vol. 40, No. 134, p. 56-66, 1994
Shallow cores to about 17 m depth were recovered at nine sites distributed on a 150 x 150 km survey grid centered on Summit in central Greenland. Measurements of the stable oxygen-isotope ratio and the gross- beta activity as a function of depth enabled annual summer and winter horizons to be identified and dated. From these data, average accumulation rates were computed for 1959-86 with an uncertainty of about 5- 8%, based on a detailed error analysis. The contours of constant accumulation rate in the region suggest a primary moisture flux to the southwest of the grid. Similarly, contours of the average oxygen isotope ratio are consistent with the progressive isotopic depletion of a precipitating air mass moving from the southwest, except for the two eastern-most sites, which show anomalously enriched isotopic values. Accumulation rate time series and the corresponding annual total accumulation over the grid show no apparent persistent trend over the 40-year record spanned by the cores.
Chemical species spatial distribution and relationship to elevation and snow accumulation rate over the
Greenland ice sheet
Yang, Q., P. A. Mayewski, E. Linder, S. Whitlow, and M. Twickler
Journal of Geophysical Research, Vol. 101(D13), p. 18,629-18,637, 1996
Major chemical species (Cl -, NO3-, SO32-, Na+, K+, Mg2+, Ca2+) from 24 snowpits (sampled at a resolution of 3 cm, total 2995 samples) collected from north, central and south Greenland were used for this investigation. The annual and seasonal (winter and summer) concentration of each chemical species was calculated and used to study the spatial distribution of chemical species over the central portion of the Greenland Ice Sheet. A two-sided t-distribution test (a=0.05) results suggest that the mean annual concentrations of major chemical species in snow do not vary significantly over this portion of central Greenland. Seasonal concentration variations were also examined for central Greenland and were found not to significant.
The relationship between snow accumulation rate and concentration was investigated using two snowpits groups, snowpits from central Greenland, and north and south Greenland both show that chemical concentrations do not vary with snow accumulation rate. However, when integrating snowpits over northern, central and southern Greenland, the results suggest that NO3 - concentration decreases, while Na+, K+, Mg2+, and Cl - increase as snow accumulation rate increases. In addition, the relationship between elevation and chemical concentration shows that Cl -, Na+ and Mg2+ concentrations decrease, while NO3- concentration increases with increasing elevations over the Greenland ice sheet.