Structure and timing of the last deglaciation: Oxygen-isotope evidence

Abstract

Foraminiferal oxygen-isotope data from 24 tropical Atlantic sediment cores, constrained by 77 14C dates, are stacked to form a composite record of isotopic Termination 1.. This record indicates that most of the isotopic transition at the end of the last ice age occurred between 14 ka BP and 6 ka BP. Minor isotopic expression of deglaciation is permitted as early as 16 ka BP, but the most rapid rate of change occurred between 14 ka BP and 12 ka BP. Three ‘steps’ of maximum change are present. Although they are close to the statistical limits of detection in the composite record, the clear presence of the steps in individual records suggests that they are real. We estimate their timing at 14-12 ka BP (Termination 1-a), 10-9 ka BP (Termination 1-b), and 8-6 ka BP (Termination 1-c). Centering of the termination near 11 ka BP is consistent with the ‘Milankovitch’ hypothesis that high summer insolation caused deglaciation. In detail, however, maximum rates of change prior to the 11 ka BP insolation extreme, and the inferred steps require additional mechanisms controlling the tempo of glacial-interglacial climate change. Steps equivalent to those in δ 18O have not been detected in ice-margin retreat data. Steps in the isotopic transition, if real, may record thinning of the ice sheets not accompanied by loss of area. Alternation between near-equilibrium and near-stagnant ice-sheet profiles during deglaciation is hypothesized, perhaps due to calving and unstable ‘down-draw’ of the ice sheets followed by partial re-equilibration. Significant problems remain. The effects of temperature on the isotope record are only partially constrained. Presently available data allow only semi-quantitative intercalibration of ice volume, sea level, and isotopic estimates of glaciation.