Niels Bohr Institute University of Copenhagen Geophysical Department Juliane Maries Vej 30 DK 2100 Copenhagen Denmark

78.1 Introduction

The major ice sheets of Antarctica and Greenland play an important and special role in palaeoclimatic research. These kilometre-thick ice deposits owe their existence to precipitated snow. The snow compacts into glacier ice, which deforms during the flow of the ice sheet. The number of years covered by the cores depends on how deep and where the cores were drilled. Drill sites with low annual snow accumulation are also the sites where the oldest ice can be retrieved; the EPICA deep ice core from East Antarctica is presently the ice core that covers the most years back in time, i.e. 740 kyr (EPICA Community Members, 2004). Several other deep ice cores from East Antarctica add to the geographical coverage, i.e. the Dome Fujii, Vostok, Dronning Maudland and Dome C cores.

In West Antarctica the Byrd, Taylor Dome, South Pole and Siple station cores have contributed important palaeoclimatic information: owing to the higher annual snow accumulation in West Antarctica compared with the plateau of East Antarctica the information in West Antarctic ice cores is mainly limited to the last glacial cycle, but they offer a better seasonal resolution and hence a more precise dating by seasonal stratigraphy (see e.g. Hammer et al., 1994). Many more cores to shallow depth have been obtained from Antarctica in order to improve the geographical coverage of more recent times.

In Greenland the Camp Century, Dye 3, GRIP, GISP2 and North GRIP deep ice cores provide the information source for the last glacial cycle on the main ice sheet. Also in Greenland many more ice cores have been retrieved, but they are mainly cores drilled to shallow or intermediate depths.

Palaeoclimatic and environmental information from ice cores is not limited to the polar ice sheets and many important ice cores have been obtained from glaciers and ice caps. I have, however, in the following chosen to concentrate on the chronology of a few deep ice cores as they offer detailed information over the last glacial cycle. This also ensures that the chronology of abrupt climatic changes (DO events, Dansgaard et al., 1993) during the last glacial stage is included.

Before presenting and discussing some of the most up to date chronologies of the polar ice cores it is appropriate to consider briefly some general approaches to the dating of ice cores.

78.2 Dating of ice cores—choice of approach

In the more central parts of the polar ice sheets, except close to the bedrock, the ice flow has left the chronological order of the past snow deposition intact although the original annual surface snow accumulation has been thinned as the ice moved vertically downwards.

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