ORIGIN OF THE MIDDLE PLEISTOCENE TRANSITION BY ICE-SHEET EROSION OF REGOLITH

The transition in the middle Pleistocene (similar to 0.9 Ma) seen in d elta(18)O deep-sea-core records from relatively low-amplitude, high-fr equency (41 kyr) to high-amplitude, low-frequency (100 kyr) ice volume variations under essentially the same orbital forcing can be attribut ed to a change from an all soft-bedded to a mixed hard-soft bedded Lau rentide ice sheet through glacial erosion of a thick regolith and resu lting exposure of unweathered crystalline bedrock. A one-dimensional i ce sheet and bedrock model which includes transport of sediment and ic e by subglacial sediment deformation demonstrates that a widespread de forming sediment layer maintains thin ice sheets before the transition which respond linearly to the dominant (23 and 41 kyr) orbital forcin g. Progressive removal of the sediment layer eventually causes a trans ition to thicker ice sheets whose dominant timescale of change (100 ky r) reflects nonlinear deglaciation processes. In model simulations ove r the last 3 Ma initialized with no ice and a uniform 50 m sediment la yer the time series of ice volume and extent agree in several importan t aspects with the observed records.