Biogenic and terrigeneous components in ODP Leg 108 holes

Five Ocean Drilling Program sites (657-661), which form a north-south transect off the western periphery of the Sahara, were selected to measure the long-term history of Saharan/Sahelian dust flux and fluvial sediment discharge and the fluxes of marine CaCO3 and opal over the last 8 m.y. Sites 658 and 659 served for high-resolution studies, and Sites 657, 660, and 661 for insights into the spatial patterns of dust flux. The nearshore mean flux of opal off Cap Blanc (21 °N) showed an abrupt increase about 3 Ma that appears to reflect the main onset of coastal upwelling fertility and enhanced trade winds. At the same time, the input of river-borne clay strongly decreased, suggesting a dry up of the central Saharan rivers. Later, marked short-lived spikes of clay and opal may indicate ongoing ephemeral pulses of fluvial runoff linked to peak interglacial stages. Given the zonal dust discharge centered near 18 °N at Site 659, the aridification of the south Sahara and Sahel increased in several steps: at 4.6, 4.3, and especially at 4.0, 3.6, and 2.1 Ma, and again, at 0.8 Ma. The late Miocene and earliest Pliocene were humid. Although the central and north Saharan climate appears to be linked to the glaciation history of the Northern Hemisphere, the long-term aridification further south followed a different schedule. The spatial distribution of quartz accumulation suggests that the dust outbreaks linked to the Intertropical Convergence Zone during summer did not shift in latitude back to 4.0 Ma, at least. The short-term variations of dust output over the last 0.5 m.y. followed orbital scale pulses with a strong precessional signal, showing a link of Sahelian humidity changes to the variation of sea-surface temperature and evaporation in the tropical Atlantic.

Composition and distribution of silica in sediments of the Broken Ridge

Within a dipping sequence of middle Cretaceous to Eocene sediments on Broken Ridge, opal-A, opal-CT, and quartz occur as minor constituents in carbonate and ash-rich sediments. Biogenic opal-A is mainly derived from diatoms and radiolarians. Opal-A and almost all siliceous microfossils disappear within a narrow (<20-m-thick) transition zone below which authigenic opal-CT and quartz are present. These latter silica polymorphs occur together within a 750-m-thick interval, but the ratio of quartz/opal-CT increases with increasing age and depth within the pre-rift sediment sequence. The boundary between opal-A- and opal-CT-bearing sediments is also a physical boundary at which density, P-wave velocity, and acoustic impedance change. This physical transition is probably caused by infilling of pore space by opal-CT lepispheres.

Biogenic and terrigeneous components in Pliocene-Pleistocene sediments of the equatorial Atlantic

High-resolution analyses of sediments at equatorial Atlantic Sites 662, 663, and 664 define the accumulation rates of biogenically produced CaC03 and opal and of eolian dust from North Africa over the last 3.7 m.y. The mean flux of opal increased abruptly by 60%-70% near 2.5 Ma (2.65 to 2.3 Ma), reflecting pulses of increased opal productivity along the equator due mainly to increased upwelling. The mean winter-plume dust influx from Sahelian and Saharan Africa also increased at this time by between 35% and 75%, following smaller increases earlier in the late Pliocene. The increased opal flux implies a stronger zonal component of the southern trade winds in Southern Hemisphere winter. Consistent with this wind configuration, the stronger dust flux suggests a weaker southwesterly monsoonal flow into Africa in Northern Hemisphere summer, thus increasing Sahelian aridity and winter-plume dust fluxes. Dust fluxes to the equator may possibly have also been enhanced by stronger Northern Hemisphere winter trade winds and a more southerly position of the Intertropical Convergence Zone over Africa. These late Pliocene biogenic and terrigenous flux changes coincided with the appearance of Northern Hemisphere ice sheets, implying an ultimate causal link. The immediate control on changes in tropical circulation may, however, have been changes in the Atlantic sector of the Southern Ocean. A steady background trend of increasing winter-plume dust flux occurred from the late Pliocene until the middle Pleistocene. This may reflect a progressive, tectonically induced aridification of northern and eastern Africa because of the gradual uplift of the Tibetan Plateau.