Timing of the descent into the last Ice Age determined by the bipolar seesaw

We present planktonic foraminiferal fauna and isotope records from the SE Atlantic that highlight the nature of millennial‐scale variability over the last 100 kyr. We derive a hypothesis‐driven age model for our records based on the empirical link between variations in Greenland temperature, ocean circulation, and carbonate preservation in the deep SE Atlantic. Our results extend earlier findings of an antiphase (seesaw) relationship between north and south for the largest abrupt events of Marine Isotope Stage (MIS) 3–2 and the last deglaciation. In particular, we find that Heinrich Stadials were paralleled by inferred southward shifts of the thermal Subtropical Front. These were followed by pronounced rebounds of the front with the return to interstadial conditions in the north. Our results also shed light on the mechanism of glaciation. In contrast to the last deglaciation, which was a globally symmetric change superposed by interhemispheric asynchronicity, we find that the descent into full glacial conditions at the onset of MIS 4 (~70 ka) displayed interhemispheric synchrony. We suggest that this globally synchronous descent into glacial MIS 4 was preconditioned by orbital changes, but the timing was ultimately determined by abrupt changes in ocean/atmosphere circulation patterns i.e., the bipolar seesaw.