Variability in the deep and intermediate water circulation of the Atlantic Ocean during the past 25,000 years: Northern Hemisphere modulation of the Southern Ocean

The relative flux of North Atlantic Deep Water (NADW) out of the Atlantic can be monitored in the Southern Ocean, where high 813C NADW mixes rapidly with low 813C recirculated Pacific water before forming Antarctic Bottom Water (AABW). By using 813C measured in benthic foraminifera as a water mass tracer, it is shown that the Southern Ocean was dominated by Pacific water, with little NADW contribution during the last glaciation. These tracer results document the direct physical connection between insolation controlled Northern Hemisphere oscillations and changes in the chemical and physical properties of Southern Ocean water. The 813C records from the deep tropical Atlantic document the transition from an Atlantic dominated by Southern Ocean water during the last glaciation to an Atlantic dominated by NADW during the Holocene. With more Southern Ocean water in the deep Atlantic during the last glaciation, the boundary between NADW and Southern Ocean water was north of its modern position, allowing more Southern Ocean water to enter the eastern Atlantic through the Romanche Fracture Zone. Carbon isotope records from the Caribbean and Mediterranean Seas suggest that the influence of Mediterranean Overflow Water (MOW) extended to the Caribbean during the last glaciation, and therefore was a volumetrically much more important water mass in the intermediate-depth Atlantic than it is today.