Abstract:
An analysis of the coarse (>40 mu m) and clay fractions of Neogene sediments from Sites 135, 139, 140, 141 (DSDP Leg 14) has been carried out. The Early Miocene sediments of Sites 139 and 140 are eolian sand turbidites consisting of coarse quartz sand and shallow water particles. They have been formed during a period with arid climate and lowered sea-level. Upwelling on the upper slope produced a rather high opal content of the allochthonous sediments. Smectite (70-85%) and kaolinite (15-30%) are the dominant clay minerals. The clay assemblage of the Late Miocene of Site 141 indicates a climate with contrasting seasons on the African continent, the arid season being most important. The Early Pliocene (Sites 135, 141) and part of the Late Pliocene (141) reveal a mainly arid climate on the continent, with only small scale changes. Variations in detrital clay mineral contents, especially chlorite, mixed-layers and smectite, suggest some more humid periods on the continent, which cannot be detected from coarse terrigenous components, because the general trend in climate is more arid than humid and because transport agents were too weak to transport major amounts of terrigenous particles >40 mu m. There is a good correlation between coarse and clay sized terrigenous components in the Late Pliocene and the Pleistocene of Site 141. Terrigenous components >40 mu m reveal a constantly arid climate and changing trade wind velocities in the latitude of 20 degrees N. Clay minerals, transported by the Canary current from African regions northeast of Site 141, suggest changes in humidity and in soil formation and soil destruction. Strong trade wind velocities in 20 degrees N correlate with a humid climate north of 20 degrees N ("glacial type"), small trade wind velocities with arid climate ("interglacial type"). The correspondence of results is due to rather strong transport energies, which supply clay and >40 mu m sized terrigenous material. Calcium carbonate dissolution is strongest (100% fragmentation of planktonic foraminifers) in the Late Miocene of Site 141, decreasing in the Pliocene and showing cycles in the Late Pliocene/Pleistocene which might be correlated to periods of advancing cold arctic/antarctic bottom water. Sorting effects in the Late Pliocene/Pleistocene of Site 141 in those sections with strong CaCO (sub 3) dissolution and strong trade winds ("glacial type of climate") are tentatively interpreted by increased velocities of bottom currents, perhaps of arctic or antarctic origin.