Crystal size variations in Eemian-age ice from the GRIP ice core, Central Greenland

Continuous measurements of ice crystal size have been carried out on an 80 m sequence between 2790 and 2870 m depth in the GRIP ice core from Central Greenland. The ice in this interval is at present considered to orginate from the Eemian interglacial period. The record reveals that the crystal size in ice older than 100,000 yr is highly dependent on climatic conditions at the time of snowfall. This dependence shows up as a strong correlation between d18O values and crystal size throughout the Eemian, as well as a negative correlation between crystal size and several soluble and insoluble impurities. Although high-resolution impurity records are available from selected parts of the Eemian ice, the study is not conclusive on which impurities are most effective in slowing grain growth. It is shown that the normal grain-growth process, commonly observed in the upper few hundred metres of polar ice sheets, does not yield grain sizes compatible with observed ones at this depth in the ice sheet, even in those parts of the Eemian ice where impurity drag effects are not present. Polygonization of crystals within the ice sheet and the nucleation and rapid growth of new grains at relatively high temperatures in the lowest part probably play an important role in producing the observed grain-size variations. The relevance of possible flow disturbances of the GRIP Eemian climatic record for the results presented is discussed briefly.