Precessionally-paced advance and retreat of Northern Hemisphere ice sheets during MIS 6 indicated by NW Iberian speleothem records

While there is widespread evidence for the cyclic advance and retreat of Northern Hemisphere ice sheets throughout the Late Pleistocene, the scarcity of absolute dated records has made it challenging to determine the precise relationship between high latitude insolation cycles and cryosphere response. Because the 18O of stalagmites from Northwest Iberian coastal caves tracks the 18O of the North Atlantic Ocean, they can provide new constraints on periods of surface ocean freshening due to meltwater from retreating ice sheets. Our new, absolute dated stalagmite 18O record from 216 to 111 ka reveals two precessionally-paced phases of Northern Hemisphere ice sheet retreat and regrowth between the MIS 6 glacial inception and the glacial termination. This variability in Northern Hemisphere ice sheets is simulated in some intermediate complexity coupled climate-ice sheet models, but has not been evident in benthic 18O stacks or sea level curves derived from them. One explanation is that antiphased Antarctic ice sheet expansion compensated for the Northern Hemisphere ice volume in the global average ocean 18O, implying a more dynamic Northern and Southern Hemisphere cryosphere in response to high amplitude precessional cycles in MIS 6.