Dynamic precessional forcing of NH ice sheets during MIS 6

The prolonged minimum in Antarctic δD and CO₂ and extended interval of high benthic δ¹⁸O between 190 and 134 ka (MIS6)  is  often interpreted as a prolonged stable glacial period with ice sheets insensitive to the large amplitude precessional variations in high latitude insolation.  Here, we evaluate the evidence for Northern Hemisphere (NH) ice sheet stability from indicators of North Atlantic surface ocean δ¹⁸O. Since the North Atlantic receives nearly all meltwater from NH ice sheets, its surface δ¹⁸O is highly sensitive to NH ice sheet mass balance.  The δ¹⁸O from speleothems in coastal caves in NW Spain is dominantly driven by the surface ocean δ¹⁸O signal from  a broad area of the North Atlantic Ocean which comprises the moisture source for rainfall above the caves. Here we present a new speleothem δ¹⁸O sequence from absolutely dated speleothem covering 216 to 111 ka. Our new speleothem δ¹⁸O shows evidence for large amplitude, precessionally-paced  variations in the d18O of the North Atlantic.  The magnitude of abrupt freshening during rising insolation at 183-178  ka is comparable to the positive δ¹⁸O shift marking ice buildup between 195-184 ka.  A significant freshening also occurs during rising insolation at157-151 ka.  Lower resolution δ¹⁸Osw records from the Southern Iberian Margin, derived from paired Mg/Ca and δ¹⁸O of planktic foraminifera G. bulloides, confirm these precessional cycles. The absolute speleothem chronology confirms that over all 4 examined precessional cycles, peak freshening occurs within a similar narrow range 65N summer insolation, but across a very broad range of 65N caloric summer insolation.  The prolonged stable δ¹⁸O in benthic stacks, despite significant variation in NH ice sheets, may imply significant antiphased precessional variation in the Antarctic ice sheet.  A more dynamic NH ice sheet history during MIS 6 also has implications for the GIA models used to infer sea level during the last interglacial.