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 108Ős 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 < 3 cm. Based upon the amount of diatoms and organic carbon in the sediment, it appears that between 2.5 and 3.0 Ma, coastal upwelling and south equatorial divergence substantially intensified until about 500 ka. The presence of diatoms in the sediments indicates that during the last 3 m.y., stronger eastern-boundary currents in both hemispheres moved cold surface water toward the equator. Higher proportions of clay, silt, and freshwater diatoms from lake basins also suggest a higher quantity of wind-borne detritus, a result of more frequent and severe arid conditions in Africa. Dust fluxes from Africa to the Atlantic, low during the final 3 m.y. of the Miocene, increased markedly in the Plio-Pleistocene. Because prominent changes in long-term dust fluxes preceded Northern Hemisphere glaciation by 1.5 m.y., Northern Hemisphere ice sheets were not the major factor in the evolution of the African climate. Several major long-term changes in climate over Africa and the equatorial Atlantic in synchroneity with changes in the Southern Ocean and South Atlantic suggests long term linkage in the responses of these two regions.

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.