Greenland ice core studies

Abstract

Greenland deep ice cores represent continuous sequences of deposits spanning the last up to several hundred thousand years. Isotopes and impurities in the ice reveal a broad spectrum of information on past environmental conditions in the North Atlantic region, for example climatic changes in terms of surface temperature and annual precipitation; turbidity and chemical composition of the atmosphere, including its CO 2 concentration; volcanic activity and its climatic impact; and cosmic radiation flux changes in the past. One of the fundamental parameters in ice core studies is the isotopic composition of the ice, for example the 18O concentration (on the relative δ-scale), which varies in phase with the temperature of snow formation. The seasonal variations allow identification of annual layers in the core and, hence, absolute dating back to at least 8000 yrs. B.P. With some reservations, smoothed δ-records may be interpreted in terms of climatic temperature changes, cp. Fig.1, which shows δ-profiles along two deep ice cores from Dye 3 (southeast Greenland) and Camp Century (northwest Greenland). The latter spans all of the last glaciation and the preceding interglacial (Eem). The time scale in units of 10 3 yrs. B.P. has been derived by comparison of the most predominant °-features with their analogues in sea sediment cores (Dansgaard et al., 1982). The dramatic δ-shifts in times of full glacial severity seem to recur with a quasi-periodicity of 2550 yrs., perhaps caused by abrupt changes of the North Atlantic sea-ice cover. Spectral data analysis by the maximum entropy method shows similar, but greatly damped δ-oscillations in the well dated post-glacial part of the Camp Century record, in antiphase with independent estimates on world-wide degree of glaciation (Dansgaard et al., 1984). Figure 2 (left) shows the deepest 340 m of the Dye 3 δ-profile plotted on a linear depth scale. The very deepest 25 m of ice is silty, and the δ's are higher than in Holocene ice indicating a time of deposition with unusually high temperature of formation, probably the Eemian interglacial. However, the record is hardly continuous that far back in time. Fig.2 (middle) shows some of the δ-oscillations in detail, and Fig.2 (right) gives further details of three representative δ-shifts along with a dust concentration profile (increasing towards left). The dust is probably brought to Greenland mainly from distant areas covered by loss. The dust content changes less abrupt than δ, and not at exactly the same depth, which indicates that the measured δ shifts are not due to temporal discontinuities in the ice core. The general antiphase between dust content and δ suggests higher storminess in times of lower δ's. The circles are CO 2 concentrations in air inclusions in the ice, but it is still doubtful if they can be interpreted in terms of past atmospheric CO 2 concentrations (Hammer et al., 1980). Great volcanic eruptions inject acid gases into the stratosphere, particularly sulphuric gases, which are spread all over the hemisphere under oxydation. The strong acids re-enter the troposphere, and are subsequently washed out by precipitation causing acid layers in the ice sheets. Hence, acidity profiles along the Holocene part of the ice cores reveal violent volcanic activity in the Northern Hemisphere, and comparison with the δ profiles suggest a substantial cooling impact on the climate (Hammer et al., 1980).