Hydrography of the Central Atlantic. 3. The North Atlantic deep-water complex

Results of the GEOSECS (Geochemical Ocean Sections) program show that the compositional trends within the NADW complex can be ascribed mainly to geographic variations in the relative proportions of northern (i.e., Deep waters originating in the Norwegian and Labrador seas) and southern components (i.e., of Antarctic Bottom Water). At any given station the fraction of southern component increases with depth within the mass, and at any given depth it increases from north to south. Oxygen, silica, alkalinity, barium, and radium all show anomalies that relate to the decomposition of particulate debris within the deep eastern basin. For oxygen this leads to a pronounced anomaly in the western basin water column at a depth corresponding to the hydrographic sill for the Mid-Atlantic Ridge. The downward mixing of Mediterranean Water produces compositional anomalies in the upper NADW in the latitude range 17 to 33N. A large thermal anomaly is observed within the NADW complex from 4 to 18S in the western basin. As this anomaly is centered high on the Mid-Atlantic Ridge there is a chance that it may reflect an unusually large recent heat release in this region.