Influence of hydroxyl-bearing minerals on the isotopic composition of ice from the basal zone of an ice sheet

Abstract

ONE of the important questions in ice-core studies is the extent to which isotope measurements made on ice loaded with bedrock debris can be trusted when interpreting the climate record from ice-core data. Here we show that the isotopic characteristics of basal ice containing debris from beneath the Greenland ice sheet can be understood only if isotope exchange between hydroxyl-bearing minerals and water is taken into account. Sub-glacial abrasion of clays and micas increases the number of broken bonds at the border of the mineral structure, promotes cleavage and so increases the rate at which these minerals exchange oxygen and hydrogen isotopes with water. One implication of our results is to call into question the recent suggestion¹ that the basal ice of the Greenland ice sheet is 'superimposed ice', and that therefore extensive melting must have taken place during the last inter-glacial. The enrichment in ¹⁸O of the basal ice, thought to be acquired during the formation of the superimposed ice, may in fact be the result of isotope exchange with hydroxyl-bearing minerals.