THE BENGUELA ECOSYSTEM .7. MARINE-GEOLOGICAL ASPECTS

The Benguela Ecosystem has made an imprint on the underlying sediments on the continental margin since the Late Tertiary and today the Holocene sediments are the product of the modem Benguela Ecosystem and its interaction with the adjacent hinterland. The Orange River still builds a wave-dominated submarine delta, but wave-driven littoral drift sweeps sand equatorwards to be deflated from log-spiral beaches towards the Namib Sand Sea. A poleward undercurrent carries silt and clay polewards towards Cape Columbine along an extensive belt of terrigenous mud. Across the margin, depth-zoned assemblages of benthic foraminiferans and of ostracods are useful markers of oceanic water-masses, while planktonic foraminiferans are found seaward of the major upwell cells. On the equatorward side of the Luderitz upwell cell, diatoms and organic matter dominate the diatom ooze deposited on the inner shelf. The diatoms are accompanied by a dysaerobic benthic-foraminiferan fauna and fish debris from the mass mortalities that are common in summer as upwelling slackens. This belt of diatom ooze, flanked by quartzose and then micaceous facies along the inner shelf, is one of the only places in the world where concretionary phosphorite is forming today. The landward flank of the ooze is opal-rich, whereas the seaward flank is organic-rich, because more of the highly soluble diatoms dissolve before reaching the greater depths farther out to sea. It is in these organic-rich diatom oozes that varved laminae have been cored. A start has been made in extracting a palaeoclimatic record from the cores by the study of diatoms. foraminiferans (both planktonic and benthic), fish debris, faecal pellets (both planktonic and benthic) and aeolian dust (mica and quartz). The Benguela Ecosystem thus overlies a fascinating assemblage of marine sediments. which, for such an inherently variable system, has very distinct patterns summarising 'average' conditions, while preserving evidence of rare, but significant events. El Ninos, for example.