THE distributions of heat, salt and trace substances in the ocean ther
mocline depend on mixing along and across surfaces of equal density (i
sopycnal and diapycnal mixing, respectively). Measurements of the inva
sion of anthropogenic tracers, such as bomb tritium and He-3 (see, for
example, refs 1 and 2), have indicated that isopycnal processes domin
ate diapycnal mixing, and turbulence measurements have suggested that
diapycnal mixing is small3,4, but it has not been possible to measure
accurately the diapycnal diffusivity. Here we report such a measuremen
t, obtained from the vertical dispersal of a patch of the inert compou
nd SF6 released in the open ocean. The diapycnal diffusivity, averaged
over hundreds of kilometres and five months, was 0.11 +/- 0.02 cm2 S-
1, confirming previous estimates1-4. Such a low diffusivity can suppor
t only a rather small diapycnal flux of nitrate into the euphotic zone
; it justifies the neglect of diapycnal mixing in dynamic models of th
e thermocline25-27, and implies that heat, salt and tracers must penet
rate the thermocline mostly by transport along, rather than across, de
nsity surfaces.