Major Glacial Cycle Change was not Driven by Removal of Slippery Sediment

Past sea level change informs our predictions of future sea level rise [1] and, more broadly, the large scale climate changes that drove ice sheet growth and decay [2]. Approximately one million years ago, the relationship between orbitally-driven insolation and glacial cycle sea level change shifted – small, 40 thousand year cycles ceded to large, 100 thousand year cycles[3]. A leading hypothesis for this Mid-Pleistocene Transition is that the North American ice sheet grew resilient to deglaciation by removing its bed of slippery regolith, exposing the high friction bedrock underneath[4]. Our data constrained numerical experiments show that the North American ice sheet exposed bedrock over an area similar to present day by 1.5 million years ago, well before the Mid-Pleistocene Transition began. Thicker pre-glacial regolith does not simply delay its removal and thus the transition – it limits ice-ocean interaction by infilling shallow marine areas while cooling via the elevation-lapse rate feedback. Topographic changes over previously glaciated domains play a critical role in early Pleistocene glaciation – enabling larger ice sheets during warmer periods. This highlights the important and under-appreciated role topographic change plays in past sea