TEXTURAL CHARACTERISTICS AND IMPURITY CONTENT OF METEORIC AND MARINE ICE IN THE RONNE-ICE-SHELF, ANTARCTICA

The texture and physical properties of an ice core, recovered to 215 m depth from the Ronne Ice Shelf, Antarctica, have been studied with regard to formation and transformation of the ice. At a depth of 152.8 m, a sharp discontinuity marks the transition between meteoric ice accumulated from above and marine ice accreted from below, as testified by electrolytical conductivity and stable-isotope measurements as well as geophysical field surveys. Automated image analysis of thin sections indicates that the decrease in grain-boundary density and the increase in grain cross-sectional area with depth is commensurate with though not necessarily caused by thermodynamically driven grain growth down to 120m depth, corresponding to a vertical strain of roughly 65% as computed with a simple temperature-history, particle-path model. The observed increase of grain-boundary density (i.e. a decrease of grain-size) with age in the marine ice is in part explained by the thermal history of this layer. Sediment inclusions at the top of the marine-ice layer affect the observed grain-boundary density profile by inhibiting grain growth and dynamic recrystallization. This may allow some conclusions on the role of temperature, particulate inclusions, stress and strain rate in controlling the grain-size evolution of deforming ice, supplementing earlier laboratory experiments conducted at much shorter time-scales. Salinities (0.026 parts per thousand), brine volumes (0.09-0.2 parts per thousand) and solid-salt concentrations have been computed from electrolytical conductivity measurements (mean of 51.0 x 10(-6)S cm(-1)) for the marine ice. An assessment of salt incorporation and desalination rates shows that these low salinities can at present only be explained by a unique densification mechanism of under-water ice crystals at the base of the ice shelf.