Deep Ocean Overturning — Then and Now

Knowledge of how fast the deep ocean overturned during the last glacial period is important for understanding how Earth's climate could enter such an extremely cold state. Radiocarbon ages from surface and deep-dwelling foraminifera can indirectly measure the overturning rate. In their Perspective, Adkins and Pasquero discuss new data from the glacial Equatorial Pacific (see report by Broecker et al.) that show that at the time of the Last Glacial Maximum, mid-depth waters were about as "old" (that is, they had spent as much time below the surface) as they are today. Coupled with other records from the deeper ocean, the data might help to determine what drives the strength of the overturning circulation in the first place.