We constructed high resolution (3 kyr) benthic and planktonic foraminiferal stable isotope stratigraphies for Sites 925 (water depth: 3042 m) and 929 (4361 m) from Ceara Rise in the western equatorial Atlantic. These records provide a unique opportunity to assess the impact of the closure of the central American Seaway on the surface and deep water hydrography. Stable isotope results indicate a marked change in deep and surface water hydrography at 3.7 Ma. Between 3.7 and 4.7 Ma, the region is characterized by a "reversed" deep water oxygen isotope gradient, with benthic values at the shallower Site, 925, slightly higher than those at the deeper Site, 929. Furthermore, the vertical carbon isotope gradient steepens at 3.7 Ma as values of the thermocline dwelling planktonic foraminifera Neogloboquadrina dutertrei increase, while those of Cibicidoides slightly decline. These results reflect a decrease in the vertical deep water thermal/density gradient and an increase in the thermocline nutrient content at 3.7 Ma. We hypothesize that these changes relate to the termination of Atlantic-Pacific surface water communication via the central American Seaway. Cross-spectral analyses of these records against northern hemisphere summer insolation and Pacific time series should identify any changes in the climatic response to orbital forcing at 3.7 Ma.