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Oxygen-isotope analyses of foraminifera record the timing and magnitude of ice-growth events during ice ages. The timing of the major isotopic shifts during the late Pleistocene is relatively well determined, although small improvements continue to be made. In the SPECMAP marine oxygen-isotope time scale, the major isotopic transitions due to ice-growth events of the last glacial inception are centered at ca. 118–112 ka (substage 5e/5d), ca. 98–92 ka (substage 5c/5b), and ca. 75–68 ka (substage 5a/4). The ice volume at each isotopic event remains only partly known. This is due to difficulties in isolating local temperature effects on δ518O, changes in the isotopic composition of ice, and variations in ocean circulation and atmospheric vapor transport. Considering likely temperature, water-mass effects, and analytical errors, we can not discern any size difference in the part of the δ518O shifts due to ice growth at the substage 5e/5d and the stage 5/4 transitions. Both were roughly the same size, about 0.6‰. Assuming that the mean isotopic composition of glacier ice was −30‰ to −35‰, ice-volume changes are about 25 × 106 km3 across these transitions. The substage 5c/5d isotopic transition, the smallest of the three major ice-growth events of stage 5, was about a 0.3‰ shift, or 13 × 106 km3 of ice, using similar assumptions. In spite of the uncertainties in translating isotope measurements quantitatively into ice volumes, it is clear that the isotopic shifts between 120 and 65 ka require relatively large and rapid changes in global ice volume.

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