Reconstructing ocean circulation in the North Atlantic since the last ice age

Jerry McManus, Summer K Praetorius, Jeanne Gherardi Scao, Candace O Major, Sharon S Hoffman, Natalie Roberts, Fern T Gibbons and Delia W. Oppo

Abstract

Past ocean circulation changes may be expressed in water mass tracers and dynamical proxies. We have examined a combination of the 231Pa/230Th and sediment grain size indicators with nutrient proxies to reconstruct the evolution of the North Atlantic meridional overturning circulation since the last ice age. Deep-sea core locations in the western basin monitor the export of northern-sourced waters, especially along the deep western boundary. Cores from a range of water depths allow reconstruction of both intermediate and deep waters. During the last glacial maximum, northern source waters shoaled relative to the modern circulation and may not have overflowed from the Nordic seas, but they appear to have been well ventilated and part of a vigorous overturning cell. Intermediate depth sediments south of Iceland display a similar grain size spectrum as the modern, and sedimentary 231Pa/230Th at shallower sites was generally reduced, in contrast to higher ratios at deep sites. The combination of reduced volume and approximately equally vigorous overturning requires a reduced residence time for northern sourced waters at the glacial maximum. During the deglaciation, large-amplitude variability characterized the climate as well as the meridional overturning circulation at all depths. The most prominent excursion to high 231Pa/230Th indicating reduced export occurred at deep sites along the western boundary. The circulation rebounded at all depths at the onset of the Bolling Allerod, and declined again during the Younger Dryas. Only smaller oscillations have occurred once the modern configuration of overflows from the Greenland and Norwegian Seas was established at the start of the Holocene.