The spreading of Antarctic Bottom Water--the densest global-scale wate
r mass--is highly constrained by ocean-floor topography. In the Atlant
ic Ocean, the Mid-Atlantic Ridge confines this water mass mainly to th
e western basins, the bottom waters in the eastern basins being renewe
d by flows through gaps in the ridge(1). One such gap is the Romanche
fracture zone, a large offset of the ridge which straddles the Equator
. It has been observed(2) that sills within this fracture zone block t
he passage of waters colder than similar to 0.9 degrees C; warmer, les
s dense waters passing over the sills appear to cascade downslope wher
e they are modified by mixing. Here we present direct measurements whi
ch quantify these processes. The flow is vertically sheared and exhibi
ts remarkably intense turbulence, comparable to that seen at the ocean
surface in the presence of winds of similar to 10 m s(-1). This turbu
lence mixes the densest waters passing through the fracture zone with
the warmer, overlying waters, so that the coldest waters exiting this
region have been warmed by similar to 0.6 degrees C during transit, To
pographic obstructions and turbulent mixing together thus determine th
e properties of the flows renewing the deepest waters of the Atlantic
Ocean's eastern basins.