(Supplement Fig. 2-4) Proxy data from IODP Site 306-U1314

Subsurface water column dynamics in the subpo-lar North Atlantic were reconstructed in order to improve theunderstanding of the cause of abrupt ice-rafted detritus (IRD)events during cold periods of the early Pleistocene. We usedpaired Mg/Ca andδ18O measurements ofNeogloboquad-rina pachyderma(sinistral - sin.), deep-dwelling planktonicforaminifera, to estimate the subsurface temperatures andseawaterδ18O from a sediment core from Gardar Drift, inthe subpolar North Atlantic. Carbon isotopes of benthic andplanktonic foraminifera from the same site provide informa-tion about the ventilation and water column nutrient gradient . Mg/Ca-based temperatures and seawaterδ18O suggestincreased subsurface temperatures and salinities during ice-rafting, likely due to northward subsurface transport of sub-tropical waters during periods of weaker Atlantic MeridionalOverturning Circulation (AMOC). Planktonic carbon iso-topes support this suggestion, showing coincident increasedsubsurface ventilation during deposition of IRD. Subsurfaceaccumulation of warm waters would have resulted in basalwarming and break-up of ice-shelves, leading to massive ice-berg discharges in the North Atlantic. The release of heatstored at the subsurface to the atmosphere would have helpedto restart the AMOC. This mechanism is in agreement withmodelling and proxy studies that observe a subsurface warm-ing in the North Atlantic in response to AMOC slowdownduring Marine Isotope Stage (MIS) 3.