Climate records from ice cores

Palaeoclimatic records can be obtained from ice cores drilled from glaciers and ice sheets, containing information on accumulation and atmospheric composition through time. In the upper part of ice cores, annual layers are commonly preserved as alternating bands of clear and bubbly ice. Deeper in the ice cores, however, annual layers are usually not discernible and dating is made indirectly. Short ice cores can be drilled manually, but long cores have to be retrieved by sophisticated mechanical equipment.

Annual layers in ice sheets and ice caps form as a response to winter accumulation and summer ablation. Near the glacier surface the winter layers are normally light in colour, while summer layers are darker due to partial melting and impurities. Deeper in the ice mass, however, the annual layers are difficult to detect due to thinning and distortion through pressure from the ice above and flow deformation. Analyses of ice cores from high altitudes and high latitudes, where little or no surface melting takes place during the ablation season, have demonstrated that these contain a great deal of palaeoenvironmental information. For example, annual layer thickness provides information about winter snowfall and degree of melting (determined by summer temperature). Aerosol and dust particles give information about the history of volcanic activity and dust storms in desert regions. Trace gases in the atmosphere are recorded in the composition of carbon dioxide and methane trapped in the englacial air bubbles. Stable isotopes (mainly oxygen isotopes) are proxies for climate change and act as correlation tools between terrestrial and

11,000 12,000 13,000 14,000 15,000 16,000 17,000 18,000 19,000 20,000 21,000 22,000 23,000

Holocene

GS-2a

Greenland Stadial I GS-1

Greenland Interstadial I Gl-1

Holocene

GS-2a

Bored Schoo Quots

Greenland Stadial I GS-1

Greenland Interstadial I Gl-1

Greenland Stadial 2 GS-2

Greenland Interstadial 2 GI-2

Oxygen isotope ratios (<i>IB0, %o)

Figure 3.5 The ¿'l80 record from the GRIP ice core (Dansgaard et al, 1993) between 11,000 and 23,000 years bp, with proposed stadials and interstadials. (From Bjorck et al., 1998).

Greenland Stadial 2 GS-2

Greenland Interstadial 2 GI-2

Oxygen isotope ratios (<i>IB0, %o)

Figure 3.5 The ¿'l80 record from the GRIP ice core (Dansgaard et al, 1993) between 11,000 and 23,000 years bp, with proposed stadials and interstadials. (From Bjorck et al., 1998).

marine records. Natural and artificial radioactive isotopes provide the possibility of dating ice cores. The wide range of data obtained from ice cores therefore makes them one of the most important archives of Late Cenozoic palaeoenvironmental data.

Deep cores from Greenland (Fig. 3.6) and Antarctica and shorter cores from minor ice caps and glaciers have documented annual and decadal climate change beyond the last interglacial. The first ice cores were obtained from Camp Century (NW Greenland) in 1966, Dye 3 (south Greenland) in 1981, Renland (east Greenland) in 1988, and from Devon Island (North West Territories) in 1976. On the Antarctic ice sheet, cores were retrieved from Byrd Station (1968), Dome C (1979) and Vostok Station (1985). In the early 1990s, two cores were obtained from the summit of the

Greenland ice sheet. The Greenland Ice Core Project (GRIP) reached bedrock at 3029 m in 1992, while the North American Greenland Ice Sheet Project Two (GISP2) drilled c. 30 km away from the GRIP location, and reached bedrock at a depth of 3053 m in 1993. The GISP2 and GRIP ice cores, going back approximately 250,000 years, unequivocally demonstrated the presence of rapid climate-change events.

Initial interpretations of the GRIP ice core indicated that the rapid climate shifts present during the last ice age also persisted through the previous interglacial (the Eemian, Sanga-monian, or isotope stage 5e). The GISP2 ice core also showed significant oscillations through the same period. The timing and character of the fluctuations were, however, different. Detailed analyses showed large structural disturbances caused by ice flow at

Figure 3.6 Six deep drilling sites in Greenland: Camp Century (US Army Cold Regions Research and Engineering Laboratory, 1966); Dye 3 (GISP, 1981); Renland (Nordic Council of Ministers, 1988); Summit (GRIP, 1992); the USA camp (GISP2, 1993); Hans Tausen (Nordic Environmental Research Programme, 1995); and the NGRIP started in 1996. (Adapted from Johnsen et al, 1997)

Figure 3.6 Six deep drilling sites in Greenland: Camp Century (US Army Cold Regions Research and Engineering Laboratory, 1966); Dye 3 (GISP, 1981); Renland (Nordic Council of Ministers, 1988); Summit (GRIP, 1992); the USA camp (GISP2, 1993); Hans Tausen (Nordic Environmental Research Programme, 1995); and the NGRIP started in 1996. (Adapted from Johnsen et al, 1997)

about 2800 m depth in the cores, corresponding to an age of ca. 110,000 years (Alley et al., 1997a). In addition, comparison with the undisturbed Eemian sequence in the Vostok ice core from Antarctica indicated that the sequence of ice layers older than 110,000 years are disturbed in both the GISP2 and GRIP ice cores (Chappelaz et al., 1997).

In the GISP2 and GRIP cores it was possible to count annual layers into the glacial period (>10,000 years ago) and possibly down to

110,000 years, or for about 90 per cent of the core lengths (Fig. 3.7). (For visual-stratigraphic dating of the GISP2 ice core, see Alley et al, 1997b.)

The age differences between dates of the cores and independent age markers are about 1 per cent in the Holocene and 5-20 per cent through most of the glacial period (Meese et al, 1997). The GISP2 and GRIP cores show an almost perfect match back to 110,000 years ago. Volcanic markers and atmospheric

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