Modern ocean thermohaline-driven circulation influences global climate by transporting heat to high latitudes(1-2) and by affecting the exchange of CO2 between ocean and atmosphere(3). North Atlantic Deep Water (NADW) plays a key role in this circulation, and Quaternary climate cycles have been linked to changes in NADW flow(4). General circulation model simulations indicate that before closure, some 3-4 million years ago, of the Central American Isthmus-the narrow strip of land Linking North and South America-the direct now of low-salinity water from the Pacific to the Atlantic Ocean would have led to a smaller NADW flow(5,6). Sedimentation patterns(7) and nutrient proxies(8-11) support these model results by indicating an increase in NADW now around the time of isthmus closure, but these records do not allow changes in different NADW sources to he distinguished, and the overall effect of closure on global ocean circulation is poorly known. Here we present Nd, Pb and Sr isotope records preserved by a hydrogenous ferromanganese crust from the NADW flow-path in the western North Atlantic Ocean. These records indicate that the isotopic signal associated with NADW strengthened around 3-4 million years ago showing that deep water that formed in the Labrador Sea made a gradually increasing contribution to NADW now. These data, taken together with those from the central Pacific Ocean(12), indicate an increased NADW flow since isthmus closure, and suggest that the closure established today’s general pattern of ocean circulation.