Strength and geometry of the glacial Atlantic Meridional Overturning Circulation

Abstract

During the Last Glacial Maximum, the pattern of Atlantic Meridional Overturning Circulation was different from today. A combination of sediment chemistry and a scavenging model suggests that the glacial circulation was shallower and at least as vigorous as today. The strength and geometry of the Atlantic meridional overturning circulation is tightly coupled to climate on glacial–interglacial and millennial timescales1, but has proved difficult to reconstruct, particularly for the Last Glacial Maximum2. 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 231Pa/230Th have been used to reconstruct the strength of circulation in the North Atlantic Ocean3,4, but the effects of biogenic silica on 231Pa/230Th-based estimates remain controversial5. Here we use measurements of 231Pa/230Th 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 231Pa/230Th 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 231Pa/230Th values. We estimate that the transport of intermediate water during the Last Glacial Maximum was at least as strong as deep water transport today.