Primordial 3He in South Atlantic deep waters from sources on the Mid-Atlantic Ridge

Helium isotope data from three zonal WOCE sections (11°S, 19°S and 30°S) in the South Atlantic are presented. Among other features we find a distinct δ3He-maximum above the Mid-Atlantic Ridge (MAR) at all three latitudes. Using a hydrographic multiparameter analysis, we separate 3He emanating from the MAR from the large-scale 3He background. To our knowledge, this is the first confirmation of input of primordial 3He at the MAR in the South Atlantic. The source appears to be weak compared with the Pacific sources, causing 3He elevations (relative to background values) of only 2–3% directly above the MAR. This exceeds by several times the statistical and systematic data uncertainties, which amount to 0.35% each, so that detailed contouring of the MAR-derived 3He is possible. At 30°S and 11°S, a significant signal extends westward over at least 2000 km, whereas at 19°S the signal is more confined to the ridge area. The westward extensions indicate westward flow at depths near the ridge crest elevation, contradicting flow directions deduced previously by Reid (1989).