Dome Concordia ice microstructure: impurities effect on grain growth

We present a detailed analysis of the microstructure in the shallow part (100-580 m) of the European Project for Ice Coring in Antarctica (EPICA) ice core at Dome Concordia. In the Holocene ice, the average grain-size increases with depth. This is the normal grain-growth process driven by a reduction of the total grain-boundary energy. Deeper, associated with the Holocene-Last Glacial Maximum (LGM) climatic transition, a sharp decrease of the average grain-size is observed.Toexplain modifications to the microstructure with climatic change, we discuss the role of soluble and insoluble (microparticles) impurities in the grain-growth process of Antarctic ice, coupled with an analysis of the pinning of grain boundaries by microparticles. Our data indicate that high soluble impurity content does not necessarily imply a slowing-down of grain-growth kinetics, whereas the pinning of grain boundaries by dust particles located along the boundaries does explain modifications to the microstructure (small grain-sizes; change in grain-size distributions, etc.) observed in volcanic ash layers or dusty LGM ice. Moreover, classical mean-field models of grain-boundary pinning are in good quantitative agreement with the evolution of grain-size along the EPICA ice core. This suggests a major role for dust in the modification of shallow polar ice microstructure