Limited Agulhas Leakage as a potential trigger for reduced AMOC intensity before the onset of Heinrich events

Interocean exchange of heat and salt around South Africa - the so called 'Agulhas Leakage' - is thought to be a key link in the maintenance of the Atlantic meridional overturning circulation (AMOC). It takes place at the Agulhas Retroflection, largely by the intermittent shedding of enormous rings that penetrate into the South Atlantic Ocean. Recent palaeoceanographic studies suggest that variability in the latitudinal position of the subtropical front (STF) in the Southern Ocean, acts as a gatekeeper for the Agulhas retroflection and moreover, that a variable northward migration of the STF potentially modulated the severity of glacial periods by altering the amount of Agulhas leakage with consequences for the AMOC. Here we present a high-resolution record of ice rafted debris (IRD) from the southern Agulhas Plateau (sediment core MD02-2588, 41'19,90 S and 25'49,70 E, 2907 m water depth) covering the last 350,000 years. We find distinct millennial-scale events with high abundances of IRD. These IRD events are indicators for a northward shift of the Southern Ocean frontal system, thereby allowing sufficient cooling and iceberg survivability as far north as the Agulhas Plateau. Our proxy record suggests significant millennial scale variability of the frontal movements throughout the last three glacial cycles. Largest IRD peaks occur during marine isotope stage 8 (~300,000 years BP) and hence during a period for which an extreme northward shift in the STF has been identified previously. We compare our IRD record with records of millennial scale climate variability in the North Atlantic after careful synchronization of individual age models using benthic oxygen isotopes. In general, IRD peaks recorded on the Agulhas Plateau occur during globally cold conditions but in anti-phase with coldest events (Heinrich events) in the North Atlantic, which systematically occur at the culmination of large reductions in AMOC. This observation is in line with the concept of a bipolar seesaw behaviour of the glacial Atlantic. As IRD peaks recorded in MD02-2588 tend to precede IRD peaks in the North Atlantic they lend credence to the emerging viewwe speculate that the events in the South may have been active in triggering a reduced AMOC intensity that has been observed to occur before the onset of ice rafting events in the North. A reduced salt export into the Atlantic ocean associated with the southern IRD events may have augmented the destabilization of AMOC activity in the North Atlantic triggering feedbacks in that region, such as basin-wide subsurface warming, increased basal melt rates under an ice shelves fronting the Laurentide Ice Sheet, subsequent collapse allowing ice flow surges and eventually iceberg and freshwater discharge into the Labrador Sea that further amplified weakening of the AMOC.