Shallow-ice microstructure at Dome Concordia, Antarctica

The shallow-ice microstructure at Dome Concordia, Antarctica, has been studied between 100 m and 580 m. An original digital-image-processing technique has been specially developed to extract ice microstructure (grain boundaries) from thin sections prepared during the two first scientific EPICA field seasons (1997/98 and 1998/99). Using this, not only the mean crystal size, but also crystal-size distributions and shape anisotropy, were determined. The mean crystal-size profile as well as crystal-size distributions reveal normal grain growth up to 430 m. Between 430 m and 500 m, a marked decrease of crystal size is observed and compared with a similar trend obtained in the "old" Dome C ice core formerly associated with the Holocene/Last Glacial transition (Duval and Lorius, 1980). This seems to indicate a slightly lower accumulation rate (by <10%) at Dome C. The shapes of the crystal-size distributions, though very similar, do evolve with depth and seem to be sensitive to climatic changes. An increasing flattening of crystal shape is observed with depth. This allowed estimation of the vertical strain rate in the shallow part of the ice sheet.