Constraints on the Origin of the Middle Pleistocene Transition From the Glacial Sedimentary Record of the North-Central U.S.

Long paleoclimate records preserved in deep-sea sediments show a change in the variability of global ice volume during the middle Pleistocene ~1.2 Ma, consisting of a transition from dominant 41-kyr ice sheet cycles to dominant 100-kyr ice sheet cycles during an interval of little change in orbital forcing. A number of hypotheses have been proposed to explain this climatic shift but a consensus has yet to emerge. Here we address this issue by testing an hypothesis suggesting that the cause of the middle Pleistocene transition (MPT) was related to a change from all soft-bedded ice sheets to mixed hard-soft bedded ice sheets through the erosion of a thick regolith by ice sheets that eventually uncovered fresh crystalline bedrock, thereby causing a fundamental change in ice sheet response to orbital forcing. For this purpose we sampled late Pliocene-Pleistocene glacial sedimentary sequences from the north-central U.S. The complex glacigenic sequences of this region consist of multiple tills interbedded with paleosols and volcanic ashes that indicate that the oldest glacial unit was deposited > 2.0 Ma. Samples from these sequences were examined for petrographic, mineralogic and geochemical indices in order to see whether changes in their composition support a change in basal-ice boundary conditions. The sedimentary sequences were constrained chronologically by paleomagnetic measurements on till units and by their stratigraphic relationship with three volcanic ashes derived from eruptions of the Yellowstone Caldera. The till compositional data indicate that compositional changes do occur in the glacial sedimentary sequences that span the MPT. The older groups of till units (reverse polarity-Matuyama Chron) contain a low amount of crystalline lithologies whereas the younger groups of till units (normal polarity-Brunhes Chron) are rich in crystalline rocks. The clay mineralogy of the older tills is characterized by lower abundance of expandable clays (illite/smectite) and by higher abundance of illite than the younger tills, and the younger tills are also characterized by the appearance of chlorite. Stronger contrasts are observed in the silt fraction where the younger tills are characterized by important increases in calcite, feldspars (albite) and kaolinite whereas the older tills show depletion in these mineral phases and a high content in quartz. Slight changes in the abundance of major and trace elements are observed between the older and younger groups of tills, which suggest the glacial erosion of an increasingly less weathered source rock with time. Because studies on ice flow direction in the study area do not support a provenance change, the compositional changes documented are more likely to reflect changes in the composition of the source rock being eroded through time. Therefore our results suggest that a change in the nature of the substrate underlying ice sheets may have contributed to the MPT, thereby underlying the importance of substrate-regulated ice sheets in the variability of the climate system.