The “MIS 11 paradox” and ocean circulation: Role of millennial scale events

Abstract

The role of millennial scale climate variability in supplementing the astronomical forcing of glacial–interglacial transitions remains a major unresolved question. Here we compare the occurrence and character of “terminal” ice rafting events in both the North and South Atlantic during the last deglaciation (Termination I, TI) and during the transition between Marine Isotope Stages (MIS) 12 and 11 (or Termination V, TV). We show that TV experienced a massive terminal ice rafting event in the North Atlantic that was more intense and longer lasting than Heinrich event 1 (H1) of the last deglaciation. This massive ice rafting event was linked to cold stadial conditions and reduced deep water formation in the North Atlantic, in parallel with warming at high southern latitudes, similar to the bipolar seesaw pattern exhibited during H1 over the last deglaciation. We propose that the particular intensity and duration of the TV ice rafting event resulted from the especially large volume of Northern Hemisphere ice sheets during MIS12. In turn, the unusually long duration and large amplitude of TV likely resulted from the exceptionally prolonged collapse of the AMOC during the TV Heinrich stadial, and from a subsequent transient AMOC “overshoot” with respect to later MIS11 interglacial circulation. Furthermore, we suggest that the intense Heinrich stadial of TV contributed to the deglaciation primarily via meridional heat transport anomalies that would have enhanced the incipient warming arising from relatively weak insolation forcing, and only secondarily via CO2 release.