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INSOLATION SIGNATURES AT 6VN DEVIATION FROM THE MEAN

FIGURE 2.19 (Continued)

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INSOLATION SIGNATURES AT 6VN DEVIATION FROM THE MEAN

FIGURE 2.19 (Continued)

SEASONAL PATTERNS OF GRADIENT DEVIATIONS (30°N - 90°N)

JFMAMJJASOND FIGURE 2.20 Variations in insolation gradients (monthly) expressed as departures from the last 150,000 yr averages for selected time periods. Periods of maximum ice growth (e.g., at 71,000 and 23,000 yr B.P.) correspond to periods of stronger than average insolation gradients in all months (left-hand diagram).Times of rapid ice decay (e.g., 128,000 and 11,000 yr B.P.) correspond to generally weaker than average gradients (right-hand diagram). Gradients calculated for Northern Hemisphere (30-90°N) (after Young and Bradley, 1984).

orbital frequencies. For example, Fig. 2.21 shows that the radiation gradient from 30-70° N in mid-July over the last 200 ka had a strong -40 ka periodicity in extraterrestrial radiation. However, because of differential effects on radiation attenuation in the atmosphere, and latitudinal differences in surface albedo, the high latitude obliquity signal is reduced, leading to a dominant -23 ka period in the latitudinal gradient of absorbed radiation (Tricot and Berger, 1988; Berger, 1988).

The astronomical theory of climatic change has tremendous implications for Quaternary paleoclimatology but there was little reliably dated field evidence to support or refute the idea until the mid-1970s. Since then many studies have demonstrated that variations in the Earth's orbital parameters are indeed fundamental factors in the growth and decay of continental ice sheets (e.g., Broecker et al., 1968; Mesolella et al., 1969; Hays et al, 1976; Ruddiman and Mclntyre, 1981a, 1984; Imbrie et al., 1992, 1993a). This evidence is discussed in more detail in Section 6.12 but the major issues are summarized here in Fig. 2.22. Variations of incoming June solar radiation at 65° N are broken down into their component parts (precession, obliquity, and eccentricity) and compared to the same bandpass filtered components of the marine 8lsO record of the last 400 ka (representing changes in continental ice volume). Clearly, the frequency bands associated with precession and obliquity are similar (and coherent with) the 8lsO ice volume signal, but the 100 ka radiation signal is completely inadequate to explain the strong 100 ka cycle in ice volume. Sev-

FIGURE 2.21 The gradient (from 30-70°N) of incoming solar radiation in mid-July over the last 200 ka (top) compared to the modeled gradient of radiation reaching the surface (middle) and of the gradient of radiation absorbed at the surface (bottom). Because of differential absorption and reflection with latitude, the dominant periodicity of radiation absorbed at the surface shifts from that of obliquity to that of precession, which is more characteristic of a lower latitude influence on the gradient (from Tricot and Berger, 1988).

FIGURE 2.21 The gradient (from 30-70°N) of incoming solar radiation in mid-July over the last 200 ka (top) compared to the modeled gradient of radiation reaching the surface (middle) and of the gradient of radiation absorbed at the surface (bottom). Because of differential absorption and reflection with latitude, the dominant periodicity of radiation absorbed at the surface shifts from that of obliquity to that of precession, which is more characteristic of a lower latitude influence on the gradient (from Tricot and Berger, 1988).

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OBLIQUITY BAND (-41 ka)

PRECESSION BAND (-23 ka -19 ka)

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OBLIQUITY BAND (-41 ka)

ECCENTRICITY BAND (-100 ka)

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100 ka ICE VOLUME CYCLE

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