The Physical and Structural Properties of the Siple Dome Ice Cores
A.J. Gow and D.A. Meese
The crystal structure and fabrics were studied in detail including the analysis of more than 2500 crystallographic c-axes conducted on 50 thin sections from the main PICO core. In the upper levels progressive increase in the size of crystals was accompanied by a reorienting of the crystallographic c-axes, initially favoring a broad clustering of axes about the vertical consistent with a deformational process dominated by rotation of the c-axes towards the axis of vertical compression. From approximately 360 m to 700 m, c-axis orientation changed to one consistent with a change in rheology dominated by longitudinal extension. Occasionally, the fabric transitioned from one more typical of annealing recrystallizaton. Coincidental with peak fallout of volcanic dust at around 700 m the axes become tightly clustered about the vertical, consistent with the sudden increase in p-wave velocity observed downhole between 690 and 800 m. This change is fabric, also accompanied by a very substantial decrease in crystal size, can probably be attributed to deformation dominated by strong horizontal shear related possibly to widespread incorporation in the ice of volcanic dust. Below 800 m c-axis fabrics become entirely a multi-maxima type composed of crystals with dimensions often exceeding the diameter of the core. Zones of very large crystals were observed below 900 m including several crystals growing side by side with the largest one measuring at least 15 cm x 13 cm.
In the upper section of ice in the Siple Dome core there is much evidence of reverse dips and slightly inclined layers between 438 m and 596 m. Steeply inclined layering was observed at 780 m and less steeply inclined and occasionally distorted layers between 759 m and 780 m. There was not much evidence of layer-deformation below 800 m because of the general absence of layer structure in the ice. This situation was further exacerbated by the badly fractured condition of virtually all core below 780 m.
Abundant layers of volcanic tephra were observed principally in the depth range 650 to 820 m. Close proximity to major volcanic centers in Marie Byrd Land points to these volcanoes as the likely sources of tephra fallout at Siple Dome. Sporadic deposition of volcanic ash was first observed at around 500 m and continued to 650 m before the onset of a major period of ash and dust fallout beginning at around 675 m and picking up substantially at around 700 m. Peak fallout was observed at 711-713 m with additional pulses down to 730 m followed by sporadic fallout to just below 800 m. The dating and distribution pattern of volcanic ash and dust fallout at Siple Dome are in excellent agreement with the volcanic debris fallout recorded in the Byrd Core. There appears also to be significant decrease in accumulation rate associated with peak dust fallout between 16,000 and 18,000 years BP.
Cores obtained by hot water drilling at Siple Dome in 1997-1998 underwent appreciably more relaxation than cores obtained with the PICO electro-mechanical drill. Also the PICO cores have remained very brittle since they were retrieved in 1999-2000. By contrast, relaxation of the hot water cores occurred very rapidly, allowing the brittle ice to be sawed or sectioned with ease a few days after they were drilled. Such rapid stabilization of the relaxation process has not been observed previously in other cores from Antarctica or Greenland. Continuous circulation of hot water during coring has likely contributed to the rapidity of relaxation. In addition, the fact that cores were allowed to remain at the surface at elevated temperature for several days likely promoted the onset of rapid relaxation.
It would appear that the basal ice at Siple Dome is frozen to its bed since the temperature measured at the bed is about 1C colder than the pressure melting point estimated on the basis of a 1004m thick ice sheet at the PICO drilling site. The drill failed to penetrate the bed. The basal ice was examined and photographed in some detail during routine processing of the core. The first appearance of debris in the ice was at 1001.82m, about 3 m above the bed. The ice encapsulating this debris was as bubbly as the debris-free ice above it. The remainder of the basal ice contained widely dispersed debris, sometimes stratified, and occasionally concentrated into layers several centimeters thick. Debris grains included a few pebble-sized particles but were mostly restricted to sand-sized grains and smaller particles. Overall, the basal ice contained varying concentrations of bubbles, though bubble-free layers of variable thickness were observed throughout the 3 meters of debris-bearing basal ice. The need now is to sample both bubble-rich & bubble-free basal ice to measure air content in order to determine the extent to which melt-refreeze, diffusive or shearing-in processes have contributed to the incorporation of the dispersed debris. Absence of air in the ice, for example, is generally attributed to a process of basal ice melting followed by slow freezing and simultaneous incorporation of debris from the bed.
As a result of these findings, we would like to further pursue examination of the volcanic layers, fabrics in and near the areas where inclined layers and other deformational-type structures were observed, the basal ice, and the annual layer structure for dating.