LEG 108
The Eastern Tropical Atlantic

The eastern Atlantic Ocean contains a critical boundary zone of surface-water oceanography that includes the Intertropical Convergence Zone and the thermal equator. In the deep water, the Sierra Leone Rise forms an almost continuous, tectonically inactive barrier between the basins of the south and north Atlantic. Long-term changes in, and along, those major equatorial boundaries are linked closely to the history of Neogene global climate change. Leg 108Õs objective was to study whether these changes are controlled by polar components of the climate system, to what degree the low-latitude ocean-atmosphere components evolved independently, and the relative importance of the two polar regions in influencing climate change near the equator.

During Leg 108, new correlation techniques to obtain fine-scale, high-resolution analyses were applied at twelve sites (Site 657 to Site 668). P-wave velocity and magnetic susceptibility signals, which contain orbital-scale rhythms carrying much of the key paleoclimatic responses, were measured at intervals of
Earlier than 2.5 to 3 Ma, sediment cycles rich in calcium carbonate occurred. A concomitant lack of biogenic opal, freshwater diatoms, and land-derived silt and clay suggests that oceanic productivity of both equatorial divergence and coastal upwelling was much lower at that time. Sedimentation rates strongly increased at about 4.5 Ma at sites in water depths of less than 4000 m and at about 4 Ma at deeper sites. The changes in the calcium compensation at different depths reflect a gradual but major displacement of deep-water masses.