An Investigation of Impurities in Greenland Ice Cores

Dartmouth College, Hanover NH

Investigators

Abstract

Baker 0221120 This proposal will be jointly funded by the Arctic Natural Sciences Program and Antarctic Glaciology Program. Several Greenland and Antarctic ice cores have revealed considerable information about the concentration and depth distribution of particles, trace gases and impurities. In order to accurately model phenomena such as diffusion and grain growth, and the electrical and mechanical properties of natural polycrystalline ice, including the impacts of temperature changes, it is necessary to know the concentrations of these impurities and their microstructural location. For example, understanding if the impurities are in the grain boundaries, triple junctions, or the lattice will enable the electrical conductivity to be modeled accurately with a clear physical basis. Thus far, the microstructural location of impurities in natural ice has received little attention. Recently, the Principal Investigator developed a new technique, using a low-vacuum scanning electron microscope, energy-dispersive x-ray microanalysis, a cold stage and controlled sublimation, to determine the microstructural location of impurities in polycrystalline ice. Preliminary work on ice cores from the Greenland Ice Sheet Project 2 (GISP2) showed that sodium chloride was located in the grain boundaries while most of the sulfate was in the lattice. Other impurities such as magnesium, calcium, aluminum, potassium, and silicon, were occasionally observed in either location. In contrast, examinations of ice core specimens from both Byrd Station and Vostok Station, Antarctica do not have sodium chloride at the grain boundaries, although magnesium sulfate has sometimes been observed there. The purpose of the research is to apply this novel technique to determine how the microstructural location of impurities changes with depth/age and impurity content in ice cores from three sites (Dye 3, Camp Century and GISP2) in the Greenland ice sheet and three sites in Antarctica (Siple Dome, Vostok and Taylor Dome). Comparison between the different sites is important because they are in different locations, at different elevations, have different snow accumulation rates, and are at different temperatures. Electrical conductivity measurements and simple mechanical tests will also be performed on specific grain boundaries from the cores prior to their microstructural characterization in the scanning electron microscope.