Hydrography of the central Atlantic—I. The two-degree discontinuity

A discontinuity in the slope of the property-property relationships for the deep water in the central Atlantic is clearly confirmed by the Geochemical Ocean Sections (GEOSECS) data. The potential temperature and salinity of the discontinuity water lie within the limits $\text{2.08 ± 0.15°}\text{C}\text{and}\text{34.90 ± 0.01‰}$ from 40°N to 30°S. The horizon slopes up from a depth of 4.0 km (σ_θ = 27.92) at the northern to 3.2 km (σ_θ = 27.90) at the southern end of this region. The discontinuity water consists of 89% water of northern and 11% water of southern origin. It is proposed that this water represents the outflow from the topographically confined abyssal Atlantic. If so, the ratio of the input rate to the northern end member to that of the southern end member is about 8. The changes in abundance of the nutrient elements along the discontinuity horizon are best explained if respiration and opal dissolution take place primarily in the eastern basin. An alternate hypothesis for the origin of the discontinuity involves the active erosion of a previously more extensive mixing zone between North Atlantic Deep Water and Antarctic Bottom Water.