Severe cooling episodes at the onset of deglaciations on the Southwestern Iberian margin from MIS 21 to 13 (IODP site U1385)

Abstract

Here we reconstruct past sea surface water conditions on the SW Iberian Margin by analyzing planktonic foraminifer assemblages from IODP Site U1385 sediments (37°34.285′N, 10°7.562′W; 2585m depth). The data provide a continuous climate record from Marine Isotope Stages (MIS) 21 to 13, extending the existing paleoclimate record of the Iberian Margin back to the ninth climatic cycle (867ka). Millennial-scale variability in Sea Surface Temperature (SST) occurred during interglacial and glacial periods, but with wider amplitude (>5°C) at glacial onsets and terminations. Pronounced stadial events were recorded at all deglaciations, during the middle Pleistocene. These events are recorded by large amplitude peaks in the percentage of Neogloboquadrina pachyderma sinistral coincident with heavy values of planktonic δ18O and low Ca/Ti ratios. This prominent cooling of surface waters along the Portuguese margin is the result of major reorganizations of North Atlantic surface and deep-water circulation in response to freshwater release to the North Atlantic when ice sheets collapse at the onset of deglaciations. In fact, most of these cooling events occurred at times of maximum or increasing northern Hemisphere summer insolation. The slowdown of deep North Atlantic deep-water formation reduced the northward flow of the warm subtropical North Atlantic Drift, which was recorded on the Iberian margin by enhanced advection of northern cold subpolar waters. Following each episode of severe cooling at the onset of deglaciations, surface water experienced abrupt warming that initiated the climatic optimum during the early phase of interglacials. Abrupt warming was recorded by a sudden increase of the subtropical assemblage that indicates enhanced northward transport of heat through the North Atlantic Drift. At the onset of glaciations, SST along the Portuguese margin remained relatively warm while the surface waters of the North Atlantic experienced cooling, generating a large latitudinal SST gradient.