Figure 6

The last glacial-to-interglacial transition (11.6 to 14.7 kyr BP, Alley et al., 1993; Taylor et al., 1993) is indicated by d18O, electrical conductivity (Taylor), and calcium concentrations (Mayewski) in the GISP2 core from the Greenland summit.

a neutralization/titration of acidity. The accumulation over this period, determined from visual stratigraphy (Alley et al., 1993), closely resembles the ECM and the d18O records; it shows a drop to half its Holocene value in the Younger Dryas. Excellent agreement exists between the GISP2 records and the GRIP record (Johnsen et al., 1992a; Grootes et al., 1993).

Particularly interesting in all the records is the rapidity of the climatic changes. Transitions between the high (warm) and low (cold) ECM sections in the core are abrupt; they occur in less than 10 years, sometimes in as little as 3 years. Several transitions show an oscillatory behavior in which the ECM returns rapidly if briefly to pre-transition values (Taylor et al., 1993). Changes in accumulation (Alley et al., 1993) and in chemistry (Mayewski et al., 1993c) appear to be equally rapid. The isotopes have so far been measured only at 1 m (15- to 50-year) resolution, so they still lack this level of detail. Samples cut at 0.6 to 1.7 cm resolution are awaiting a detailed isotope study of these transitions.

The ice-core record of the glacial-to-interglacial transition thus shows an extremely dynamic climate system, with frequent alternations between warm and cold periods of varying length and with major changes in atmospheric circulation occurring over less than a decade (see Mayewski et al., 1993c). Recent work on high-sedimentation rate ocean cores in the North Atlantic (Lehman and Keigwin, 1992; Lehman, 1995, in this chapter; Ruddiman et al., 1977; Sarnthein et al., 1992; Vogelsang, 1990; Veum et al., 1992; Karpuz and Jansen, 1992; Broecker et al., 1990a; Eglinton et al., 1992) shows corresponding changes in the ocean.

The deeper part of the GISP2 and GRIP records (Johnsen et al., 1992a; Dansgaard et al., 1993; GRIP members, 1993; Grootes et al., 1993) shows that major, rapid climate fluctuations were not unique to the last glacial-interglacial transition, but occurred throughout the glacial and even interglacial periods. The pattern of interstadial and stadial episodes in the glacial part of the core provides evidence for frequent changes between cold glacial and nearly interglacial conditions during a time when the deep-sea isotope records (Martinson et al., 1987; Imbrie et al., 1984) indicate the existence of large continental ice masses. This pattern has been observed, although not in the same detail, in all long ice-core records in Greenland (Johnsen et al., 1992a). Major rapid changes during this time are also documented in Atlantic ocean cores (Broecker et al., 1990a; Sarnthein and Tiedeman, 1990), albeit at lesser resolution. A question still to be answered is whether corresponding climate changes exist in Antarctica.

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