A challenge to the Regolith Hypothesis for the MPT from present day sediment distribution and coupled climate-ice-sediment physics

Abstract

The mid-Pleistocene Transition (MPT) from 41 kyr to 100 kyr glacial cycles occurred in the absence of a change in orbital forcing. This presents a challenge for the Milankovitch theory of glacial cycles. A change from a low to high friction bed under the North American Ice Complex through the removal of pre-glacial regolith is hypothesized to play a critical role in crossing the threshold to longer and stronger glaciations. However, testing this Regolith Hypothesis requires constraint on currently unknown pre-glacial regolith cover as well as assessing whether glacial sediment processes remove the appropriate amount of regolith to enable glacial system change consistent with the MPT. Pleistocene regolith removal has not yet been simulated for a realistic, 3D North American ice sheet fully considering basal processes. Constraints on pre-glacial bed elevation and sediment thickness are sparse and the bounds are wide. What limits on pre-glacial regolith thickness in North America can be inferred from our current understanding of glacial processes and the present-day distribution of unconsolidated sediment? How does pre-glacial sediment thickness influence the evolution of Pleistocene glacial cycles? We answer these questions with an ensemble of whole-Pleistocene simulations with high-variance parametrizations and range of pre-glacial regolith thicknesses. We use the 3D Glacial Systems Model which incorporates the relevant glacial processes: 3D thermomechanically coupled hybrid SIA/SSA ice physics, fully coupled sediment production and transport, subglacial linked-cavity and tunnel hydrology, isostatic adjustment from dynamic loading and erosion, and climate from a 2D non-linear energy balance model and glacial index. This fully coupled system is driven only by atmospheric CO2 and insolation. The model captures the Pleistocene evolution of North American glaciation: 41 to 100 kyr glacial cycles shift, similar latitudinal extent in the early and late Pleistocene, LGM ice volume, deglacial ice margin chronology, and the broad present-day sediment distribution within the parametric and observational uncertainty. Constrained by large scale reconstructions of present-day surface sediment distribution, regional sediment distribution estimates, and regional bedrock erosion estimates, these results bound the mean pre-glacial sediment thickness. Our results suggest thin (<40 m) regolith and its removal occurring in advance of the MPT -- a challenge to the regolith hypothesis. In the case of very thin regolith cover, sufficient physical weathering via glacial processes occurs to increase the soft bed distribution during the course of the Pleistocene.