THE MIDPLEISTOCENE CLIMATE TRANSITION - ONSET OF 100 KA CYCLE LAGS ICE VOLUME BUILDUP BY 280 KA

The Mid-Pleistocene Climate Transition (MPT) is the complex climatic c hange which brought the Late Pleistocene ice ages. We explore the MPT in the time and frequency domains by new methods of time series analys is. High-resolution oxygen isotope records reveal that the ice volume- related increase in delta(18)O mean (amplitude: 0.29 +/- 0.05 (1 - sig ma(mu),) parts per thousand, transition midpoint: 922 +/- 12 ka, durat ion: 40 +/- 9 ka) significantly preceded the abrupt increase in the am plitude of the similar to 100 ka cycle at 641 +/- 9 ka. This finding c an be quantitatively simulated using a simple ice-bedrock model in whi ch, due to the additional ice, the calving threshold is exceeded. The simulated calving events prior to similar to 650 ka are separated by s imilar to 77 ka, whereas after similar to 650 ka they occur pseudo-per iodically with a mean period of nearly 100 ka. The cause for the delay of the similar to 100 ka calving cycle was not a slow bedrock relaxat ion; rather, the coincidental combination of insolation, existing ice mass, and bedrock depression. (C) 1997 Elsevier Science B.V.