Strength and geometry of the glacial Atlantic Meridional Overturning Circulation

The strength and geometry of the Atlantic meridional overturning circulation is tightly coupled to climate on glacial-interglacial and millennial timescales(1), but has proved difficult to reconstruct, particularly for the Last Glacial Maximum(2). Today, the return flow from the northern North Atlantic to lower latitudes associated with the Atlantic meridional overturning circulation reaches down to approximately 4,000 m. In contrast, during the Last Glacial Maximum this return flow is thought to have occurred primarily at shallower depths. Measurements of sedimentary Pa-231/Th-230 have been used to reconstruct the strength of circulation in the North Atlantic Ocean(3,4), but the effects of biogenic silica on Pa-231/Th-230-based estimates remain controversial(5). Here we use measurements of Pa-231/Th-230 ratios and biogenic silica in Holocene-aged Atlantic sediments and simulations with a two-dimensional scavenging model to demonstrate that the geometry and strength of the Atlantic meridional overturning circulation are the primary controls of Pa-231/Th-230 ratios in modern Atlantic sediments. For the glacial maximum, a simulation of Atlantic overturning with a shallow, but vigorous circulation and bulk water transport at around 2,000 m depth best matched observed glacial Atlantic Pa-231/Th-230 values. We estimate that the transport of intermediate water during the Last Glacial Maximum was at least as strong as deep water transport today.