Hydrogen and oxygen isotope studies on the McMurdo Ice Shelf, Antarctica

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Abstract
The abundances of deuterium and oxygen-18 in sections of firn core and samples of brine from drill holes in the McMurdo Ice Shelf have been measured. The isotopic abundances of the firn show an irregular variation that is predominantly seasonal. The isotopic variation in the firn is weak in comparison with the variation in surface snow and the δD versus δ18O ratio shows a small decrease that is attributed to evaporation of snow in summer. An average cloud temperature of -24°c is deduced from the average δ values of firn from site 207D by means of an empirical cloud temperature -snow δ value relationship from other authors. The isotopic data for brines from the eastern part of the ice shelf confirm their origin by lateral infiltration of sea water into the ice shelf from the seaward edge. Other modes of origin are suggested for brines from the southern part of the ice shelf. In the eastern part, lateral infiltration and cooling of the brine causes gradual removal of ice and mirabilite and thus depletion of the brine in deuterium and oxygen-18. At the same time, isotopic exchange acts to equilibrate the brine with the firn. The combination of these effects leads to a steady non-equilibrium isotopic fractionation between the brine and firn that decreases to the equilibrium fractionation at the greatest distance from the seaward edge of the ice shelf.