Abstract

The circulation of Antarctic Intermediate Water (AIW) is thought to make an important contribution to the global ocean–climate system, but the details of this interaction are not fully understood. Furthermore, the behaviour of AIW under glacial and interglacial conditions is not well constrained. Here we present a 25,000-year-long record of neodymium isotopic variations—a tracer of water-mass mixing—from the middle depths of the tropical Atlantic Ocean. Our data reveal abruptly enhanced northward advection of AIW during periods of reduced North Atlantic overturning circulation during the last deglaciation. These events coincide with an increase in the formation of Antarctic Intermediate Water and warming in the southwest Pacific Ocean, which suggests a tight link with Southern Hemisphere climate. In contrast, the initial incursion of southern source water into the North Atlantic 19,000 years ago coincided with weak AIW formation in the Pacific and reduced overturning in the North Atlantic. We conclude that reduced competition at intermediate water depth at this time allowed expansion of AIW into the North Atlantic. This early incursion of AIW may have contributed to freshening of the North Atlantic, perhaps spurring the subsequent collapse of North Atlantic deep convection.