Climate-Induced Variability in Mediterranean Outflow to the North Atlantic Ocean During the Late Pleistocene

Mediterranean Outflow Water (MOW) adds salt and density to open ocean intermediate waters and is therefore an important motor of Atlantic meridional overturning circulation (AMOC) and climate variability. However, the variability in strength and depth of MOW on geological timescales is poorly documented. Here we present new detailed records, with excellent age control, of MOW variability from 416 ka to present from rapidly accumulated marine sediments recovered from the West Iberian Margin during Integrated Ocean Drilling Program (IODP) Expedition 339. Our records of X-ray fluorescence (XRF), physical grain size, and paleocurrent information from the anisotropy of magnetic susceptibility (AMS) indicate (i) a close relationship between the orientation of principle AMS axes and glacial-interglacial cycles and (ii) two distinct regimes of MOW behavior over the last similar to 416 kyr in grain-size and AMS variability at orbital (mainly precessional) and suborbital timescales. Between marine isotope stage (MIS) 10 and MIS 4, MOW was focused at a generally shallow depth on the West Iberian Margin, and changes in MOW strength were strongly paced by precession. A transition interval occurred during MISs 5 and 4, when MOW deepened and millennial-scale variability in flow strength was superimposed on orbitally paced change. During MIS 11 and from MIS 3 to present, MOW was deeply focused and millennial-scale variability dominated. We infer that late Pleistocene variability in MOW strength and depth were strongly climate influenced and that changes in circum-Mediterranean rainfall climate were likely a primary control. Plain Language Summary Mediterranean Outflow Water (MOW) is a salty, dense water body flowing from the Mediterranean into the North Atlantic. Today, MOW encourages North Atlantic overturning circulation and influences regional and global climate. However, changes in MOW strength and depth on geological timescales are poorly documented limiting our understanding of the relationships between climate, ocean circulation, and MOW. We analyzed magnetic properties, chemical composition, and grain size of sediment cores recovered off the Portuguese coast. We dated these archives by correlating variability in their chemical composition to a nearby site with well-established chronology. We find two modes of variability in MOW strength and depth. Between 370 and 130 ka, MOW was located above our study site during warm intervals and was present during cold intervals. Changes in MOW strength were paced by the influence of the precession (wobble) of Earth's rotation axis on incoming solar radiation. After 130 ka, variabilities in MOW strength and depth transitioned to a shorter timescale mode of operation, in step with abrupt climate events. The sign of change in MOW strength and the pattern of change in pacing indicate that the primary forcing factor was changes in rainfall climate in and around the Mediterranean.