A reconstruction of sea surface warming in the northern North Atlantic during MIS 3 ice-rafting events
Dahl, C. A.
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Marine isotope stage 3(29-59 kyr BP) is characterised by rapid shifts from cold stadial to warm interstadial periods, which may be linked to changes in the vigour of the Atlantic Meridional Overturning Circulation due to variable freshwater input by melting ice. Here we present two northern North Atlantic multi-proxy records of sea surface conditions that indicate warm (near) sea surface conditions during such ice-rafting events. We infer near surface temperature from planktonic foraminiferal counts, Mg/Ca and oxygen isotopes of left-coiling Neogloboquadrina pachyderma and from calcite content. Temperatures increased during ice-rafting and rose rapidly to interstadial values after ice-rafting ceased. This pattern is clearest during Heinrich Event 4, but also present throughout the other millennial scale ice-rafting events. It indicates that stadials in the Greenland ice-cores are concurrent with a (near) surface warming in the North Atlantic, which was probably restricted to the summer, as winter temperatures must have remained low for sediment-laden ice to reach the site. As similar warming during ice-rafting events is seen regionally in the northern North Atlantic it cannot be explained by a rerouting of the North Atlantic Current. Rather, we attribute it to a shoaling of a warm subsurface water mass that was formed as a result of decreased ventilation of the upper waters and a continued northward subsurface flow of warm water. Planktonic carbon isotopes support this suggestion showing coincident decreased ventilation during deposition of ice-rafted detritus (IRD). The absence of a clear meltwater spike in the delta O-18 records during IRD input suggests that besides glacial freshwater, sea ice may have been responsible for the ventilation decrease and associated near surface heat built up. The proposed scenario is in agreement with modelling studies that require the release of heat trapped below the surface to restart the overturning circulation. (C) 2010 Elsevier Ltd. All rights reserved.