干旱区生态环境知识资源中心

The history of the northward growth and of the fluctuations of the Benguela Current from the middle Miocene (ca. 14 Ma) to the uppermost late Miocene (ca. 5 Ma) has been reconstructed from analyses of the coarse and clay fractions and geochemical analyses of the organic matter in sediments from Deep Sea Drilling Project Sites 362 and 532. In the middle Miocene the Benguela Current had not yet reached the Walvis Ridge, and consequently, no local upwelling occurred. Carbonate preservation was good during this period of low productivity. Rhythmic changes related to Antarctic ice volumes were weak, leading to slightly higher organic matter concentrations and higher montmorillonite/illite (M/I) ratios in sediments deposited during times of greater continental ice. Montmorillonite was carried north from the Orange River by the Benguela Current, whereas illite originated from the nearby Namib Desert. In the early late Miocene the Benguela Current reached the Walvis Ridge in colder periods and led to well-developed rhythmic increases in opal, which was transported from near-coastal upwelling areas. Organic matter concentration, carbonate dissolution, and M/I ratios were also enhanced during periods of greater ice volume. During the late Miocene, farther northward migration of the Benguela Current led to a change in the uppermost Miocene sediments: opal contents and M/I ratios were high during times of low ice volume rather than in colder times. During the cold periods, local winds from the Namib desert increased the illite supply to the Walvis Ridge and depressed the M/I ratio. In these periods, the Benguela Current flowed farther to the north, reaching the Angola Basin as it did in Quaternary glacial periods. Throughout these changes, sediment organic carbon content was always enhanced in high-ice-volume periods from near-bottom downslope transport of organic matter from shelf regions during regressions, and carbonate dissolution in the sediments was increased as a result of oxidation of this organic matter.