PIXE analysis as a tool for dating of ice cores from the Greenland ice sheet

Abstract

Sections from the 2037 m long Dye 3 ice core drilled in 1979–1981 in the ice sheet of Southern Greenland were analysed with PIXE. The seven selected sections were from depths between 1778 and 1813 m, which corresponds to a time interval between about 8500 and 10 000 years B.C. at the end of the last Ice Age. During this time period, fast climatic changes of several degrees centigrade per century are known to have taken place. The exact time scales of these changes need yet to be verified by renewed measurements using nonconventional stratigraphie dating techniques such as PIXE. The problem is highly relevant for the prediction of climatic changes in our present age.

A new sample preparation technique was developed which enables the determination of annual thicknesses of the parts of the ice core representing 10 000–40 000 years before present, where the thickness of the annual ice layers are believed to be less than 2.5 cm. More commonly used techniques of dating, such as measurements of oxygen and hydrogen isotopes δ¹⁸O and δD, nitrate, acidity or conductivity all have difficulties in resolving annual cycles in thicknesses of less than about 2 cm. The new technique involves sublimation of 18 cm long ice sections, after which the material contained in the ice is deposited on a thin backing. In this way, the material to be analysed is preconcentrated through the removal of the H₂O, while still retaining the spatial distribution pattern of the various water soluble and insoluble components along the ice core. The resulting spatial resolution of the sublimation technique is estimated to be ±1 mm.

A PIXE analysis was performed in contiguous millimeter steps across the sublimated ice sections. Estimations of annual ice layer thicknesses were based on the patterns of seasonal variation along the ice sections for several major and minor elements quantified with PIXE. For the seven ice sections studied, annual ice layer thicknesses between 1.2 and 2.5 cm were found.