An additional deep-water mass in Drake Passage as revealed by 3He data

Abstract

We present 3He data from a repeat section across Drake Passage, from three sections off the South American continent in the Pacific, at 28°S, 35°S, and 43°S, and from three sections in the Atlantic, eastward of the Malvinas, close to 35°W, and near the Greenwich Meridian. In Drake Passage, a distinct high- 3He signal is observed that is centered just above the boundary of the Lower and the Upper Circumpolar Deep Water (LCDW, UCDW), and is concentrated towards the northern continental slope. 3He concentrations in the Antarctic Circumpolar Current (ACC) upstream of Drake Passage (World Ocean Circulation Experiment section P19 at 88°W) are markedly lower than those found in Drake Passage, and a regional source of primordial helium in the path of the ACC that might cause the high- 3He feature can be ruled out. We explain the feature by addition of high- 3He waters present at the 43°S Pacific section. This supports a previous, similar interpretation of a low-salinity anomaly in Drake Passage (Naveira Garabato et al., Deep-Sea Research I 49 (2002) 681), that is strongly related to the high- 3He feature. Employing multiparameter water mass analysis (including 3He as a parameter), we find that deep waters as met at the 43°S Pacific section, flowing south along the South American continental slope, contribute substantially to the ACC waters in Drake Passage (fractions exceed 50% locally). Lesser, but laterally more extended contributions are found east of the Malvinas, and still smaller ones are present at 35°W and at the Greenwich Meridian. Using velocity measurements from one of the two Drake Passage sections, we estimate the volume transport of these waters to be 7.0±1.2 Sv, but the average transport may be somewhat lower as the other realization had a less pronounced signal. The enhanced 3He signature in Drake Passage is essentially confined north of the Polar Front. Further downstream the signature crosses this front, to the extent that at 35°W the contributions south and north of it are of similar magnitude. At the same time, the 3He levels north of the front are reduced due to a substantial admixture of low- 3He North Atlantic Deep Water, such that 3He becomes highest south of the front. The flow of Southeast Pacific deep slope waters entering the ACC constitutes the predominant exit pathway of the primordial helium released in the deep Pacific, and represents a considerable fraction of the deep water return flow from the Pacific into the ACC. Therefore and also because the density range of the added deep slope waters is intermediate between those of UCDW and LCDW, they must be considered a distinct water mass.