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—i—i—i-.—l—i—i-1-1—l—i_i_i—i_I_i-1_i_i_l_i_i___._I_i_,_,_i_L_

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FIGURE 5.26 Electrical conductivity (ECM) in the GRIP Summit ice core (Greenland) between 15,500 and 10,500 and 10,000-40,000 calendar years B.P. Cold episodes such as the Younger Dryas are characterized by low ECM values, warmer episodes by higher values reflecting the relative change in atmospheric dust loading (Taylor et a/., 1993b).

—i—i—i-.—l—i—i-1-1—l—i_i_i—i_I_i-1_i_i_l_i_i___._I_i_,_,_i_L_

10 15 20 25 30 35 40

FIGURE 5.26 Electrical conductivity (ECM) in the GRIP Summit ice core (Greenland) between 15,500 and 10,500 and 10,000-40,000 calendar years B.P. Cold episodes such as the Younger Dryas are characterized by low ECM values, warmer episodes by higher values reflecting the relative change in atmospheric dust loading (Taylor et a/., 1993b).

and so the ice is more likely to have been subjected to shear (Alley et al., 1995). At both locations it is possible that "boudinage effects" (Staffelbach et al., 1988; Cunningham and Waddington, 1990) have caused some layers to be differentially thinned at great depths. Boudins are "pinch and swell" structures (Fig. 5.27) that can develop in materials where viscous layers are sandwiched between less viscous material. In such conditions, initially small surface irregularities in individual layers can become amplified as the layers thin. Because ice from the last interglacial period is relatively clean (like Holocene ice, it contains little wind-blown dust; Mayewski et al., 1993) whereas glacial age ice is quite dust-laden (and therefore less viscous than the interglacial ice), conditions appear to be favorable for flow boudinage, leading to "swelling" of layers in some locations relative to others; indeed some parts of a particular layer may have been pinched out altogether. Obviously in the narrow diameter of an ice core only a tiny sample of any layer is obtained and it is impossible to tell if a "pinch" or "swell" is being sampled. However, theoretically such flow boudinage could lead to sharp discontinuities in the 8lsO record.

As the two Summit cores correlate well with each other down to -2700 m, and with ice cores from Dye-3 and Camp Century, the abrupt changes in the last glacial

Core A

Core B

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