Variations in Mid-Pleistocene glacial cycles: New insights from osmium isotopes

Reconstruction of Pleistocene glacial cycles from oxygen isotope records identifies dramatic changes in both global climate state and ice sheet size. The Mid-Pleistocene Transition (MPT) is an interval during which global ice sheets underwent a major transition from temporally symmetrical 41-thousand-year (kyr) cycles in the Early to Middle Pleistocene to asymmetrical 100-kyr cycles in the Middle to Late Pleistocene. The proposed timing of this transition and potential triggers remains contentious with possible mechanisms ranging from cryosphere dynamics to variations in Earth's energy balance. One suggested driver of the MPT involves the “Regolith Hypothesis”, which proposes that the Laurentide Ice Sheet (LIS) underwent fluctuations in ice sheet stability due to changes in subglacial substrate lithology. To empirically investigate this hypothesis, we use variations in seawater osmium (Os) isotope values from IODP sites U1302 and U1308 over an 800 kyr interval to evaluate changes in basal substrate across this transition. Our Os data identify a lithological shift beneath the LIS during the MPT that resulted in a marked change in the chemical weathering flux delivered to the ocean. We identify a pulse of glacially weathered material reaching the central North Atlantic, likely during the first Heinrich event at Marine Isotope Stage (MIS) 16, in response to increased ice sheet size post MPT. After MIS 16, the Os signal becomes less radiogenic which may correspond to a less extensive “missing glacial” which would have reduced overall weathering fluxes. Our Pleistocene Os isotope record (at ∼15 kyr resolution) contributes to our understanding of subglacial weathering of the LIS across the MPT and helps further constrain the flux of Os to the ocean during glacial-interglacial cycles.