Ice cores are a sedimentary record like many others, but comprise snow and ice rather than terrigenous particles, with layers deposited sequentially, although unlike other records the layers thin with depth due to ice flow. They have the benefit that they record the atmosphere (including sampling precipitation, aerosol and trace gases) more directly than any other archive, without biological mediation. Highly resolved records, potentially datable by counting annual layers, are recovered at sites with high snow accumulation rates. The longest records extend back hundreds of thousands of years. However, at these sites annual layers cannot be resolved and far more difficult exercises are needed in order to derive a reliable age scale (e.g. Parrenin et al., 2007a). A particular strength of ice cores is that so many aspects of climate forcing and response are recorded in one core, while a weakness (less obvious for this book that concentrates on Antarctic climate) is that most ice core records come from the polar regions (particularly Greenland and Antarctica).
The ice core record is held in three distinct forms. Firstly, the water molecules themselves, through their isotopic content, contain a record of the temperature at the time the snow fell. The ratio of Hi8O/Hi6O, or of HDO/ H2O (where D is deuterium, 2H) is generally considered to be a good proxy for site temperature, and the primary record of most ice core studies. This arises from the fact that, after water vapour has been evaporated from a warm ocean, it steadily loses water as it cools on the way to the poles, with the heavier isotopes having a lower vapour pressure and, therefore, condensing more readily, which leaves an airmass depleted in heavy isotopes to continue towards the poles. Although the real relationship is of course more complicated (Jouzel et al., 2003), in essence this leads to more negative isotopic values (expressed as per mil deviations from the composition of ocean water) in colder climates. As an aside, it is worth noting that it is the buildup of ice with negative isotope values in the ice sheets that leads to positive isotopic excursions in the oceans during colder (more glaciated times).
When snow falls, it takes with it aerosol particles and some gases that have an affinity for water. More are added just after deposition. It is in this part of the record, of material trapped in surface snow, that we find for example the spikes of sulphuric acid that are found in snow for a few years after major volcanic eruptions, as well as components of sea salt and terrestrial dust.
Finally, snow turns to solid ice in the cold polar regions only under the pressure of overlying ice that sinters the snow grains together. This is the process that seals samples of atmospheric air in air bubbles, typically at
60-100 m depth, and it is in these bubbles that all the stable trace gases, including carbon dioxide and methane, are found.
As already discussed, most ice cores come from the polar regions. In Greenland, the oldest continuous ice core so far is from North GRIP (North Greenland Ice Core Project Members, 2004), at 123 kyr, reaching just into the last interglacial. Several other cores, spanning the latitudes of Greenland, cover a slightly shorter period continuously. In Antarctica (Fig. 11.1), the oldest ice cores, spanning several G-I cycles, come from the East Antarctic plateau sites with very low snow accumulation rates. For many years, Vostok, spanning four glacial cycles and 420 kyr (Petit et al., 1999), was the oldest ice core, and it remains the longest in terms of depth. A slightly shorter period has so far been published from Dome Fuji (Watanabe et al., 2003), although a new core has recently extended the age span towards that of Dome C. The European EPICA ice core at Dome C now contains the longest (in age) continuous sequence, at just over 800 kyr (EPICA Community Members, 2004; Jouzel et al., 2007). A number of other cores have been drilled in the coastal regions of Antarctica that reach beyond the LGM (e.g. Law Dome, Siple Dome, Berkner Island); these are important because they hold the potential to derive records of climate and ice sheet size in the outlet regions of the continent. Finally, central West Antarctica is so far represented only by the Byrd core, but a new effort from the US Antarctic Programme (WAIS Divide Project) promises a new high-quality core in the next few years.
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