CLOSURE OF THE CENTRAL-AMERICAN ISTHMUS AND ITS EFFECT ON DEEP-WATER FORMATION IN THE NORTH-ATLANTIC

Modern ocean thermohaline-driven circulation influences global climate by transporting heat to high latitudes(1-2) and by affecting the exch ange of CO2 between ocean and atmosphere(3). North Atlantic Deep Water (NADW) plays a key role in this circulation, and Quaternary climate c ycles have been linked to changes in NADW flow(4). General circulation model simulations indicate that before closure, some 3-4 million year s ago, of the Central American Isthmus-the narrow strip of land Linkin g North and South America-the direct now of low-salinity water from th e Pacific to the Atlantic Ocean would have led to a smaller NADW flow( 5,6). Sedimentation patterns(7) and nutrient proxies(8-11) support the se model results by indicating an increase in NADW now around the time of isthmus closure, but these records do not allow changes in differe nt NADW sources to he distinguished, and the overall effect of closure on global ocean circulation is poorly known. Here we present Nd, Pb a nd Sr isotope records preserved by a hydrogenous ferromanganese crust from the NADW flow-path in the western North Atlantic Ocean. These rec ords indicate that the isotopic signal associated with NADW strengthen ed 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 sugge st that the closure established today's general pattern of ocean circu lation.