Salinity and temperature variability in the Caribbean and the North Atlantic gyre during the last three ice age cycles on millennial and orbital time scales

Changes in the density of polar waters, which are governed strongly by both temperature and salinity, may be the fundamental reason for north Atlantic meridional overturning circulation (MOC) instability. In order to determine the role of density-driven changes in past ocean circulation, it is critical to understand past temperature and salinity variation in north Atlantic surface waters. Mg/Ca-SST estimates are combined with measurements of oxygen isotopes in the planktonic foraminifera Globigerinoides ruber from Colombian Basin cores ODP 999A and VM28-122 to reconstruct surface water delta 18O (delta18OSW, a proxy for surface salinity) during the last three glacial cycles (0-360 ky). Results show that Caribbean sea surface salinity (SSS) oscillated between saltier conditions during the cold marine isotope stages (MIS) 2, 4, 6, 8 and 10 and lower salinities during warm MIS 3, 5, 7 and 9, covarying with the strength of North Atlantic MOC. At the initiation of the Bolling/Allerod warm interval, Caribbean SSS decreased abruptly, suggesting that the advection of salty tropical waters into the North Atlantic amplified MOC and contributed to high-latitude warming. Comparison of Caribbean SSS change with similar estimates of tropical Pacific SSS variability over the past 360 ky reveals shifts in the east-west tropical SSS gradient that are consistent with both a southward migration of the glacial Intertropical Convergence Zone and with a glacial El Nino-like mode of tropical circulation.; Reconstruction of subtropical North Atlantic delta18O SW in the Blake Outer Ridge core ODP 1060 across Dansgaard-Oeschger (D-O) cycles 12-16 (45.9-59.2 ka) demonstrates that salinity increases abruptly at the initiation of each cold stadial event and then rapidly returns to fresher interstadial conditions. These punctuated centennial-scale salinity shifts are followed by plateaus of stable gyre salinity, contradicting the hypothesis that gradual changes in North Atlantic salinity regulate D-O cycles. We theorize that preconditioned stadial salinities played a critical role on interstadial transitions by maintaining elevated North Atlantic surface-water density during periods of increased freshwater flux, thereby enabling an abrupt return to deep overturning circulation and high-latitude warming.