Miocene history of the Benguela Current and Antarctic ice volumes: Evidence from rhythmic sedimentation and current growth across the Walvis Ridge (Deep Sea Drilling Project Sites 362 and 532)

Abstract

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 Copyright 1990 by the American Geophysical Union. Paper number 90PA00559. 0883-8305/90/90PA-00559 $10.00 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 welldeveloped 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 highice-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. for the first time at these locations on the Walvis Ridge Terrace at 10 Ma [Siesser, 1980]. The appearance of this system could be detected by means of abundant opal skeletons, which point to nearcoastal upwelling, and the westward turning Benguela Current transported the opal skeletons from shelf areas to the deeper Walvis Ridge depositional sites. Both long-term and short-term changes in the Benguela Current system have been observed [Diester-Haass and Rothe, 1987; Diester-Haass, 1988]. The most important features are the northward migration of the Benguela Current during the late Miocene and a shifting of the current axis with a glacial-interglacial cyclicity. The overall driving force of these changes is the Antarctic ice mass with its growth and fluctuations during the Miocene. Changes in the volume of Miocene Antarctic ice must have been sufficiently large as to have produced changes in sea level comparable t...

Controls on carbonate and terrigenous deposition in the incipient Benguela upwelling system during the middle to the late Miocene (ODP Sites 1085 and 1087)

“…According to the scheme of Robinson and McCave (1994), the pattern of the terrigenous sortable silt is related to an increase in dust input. This timing may generally be linked to the intensification of upwelling in the studied region, which is paralleled with the results based on nannofossil studied from Krammer et al (2006), and the aridification as a result from strengthened southeastern trade winds (Siesser, 1980;Diester-Haass et al, 1990.…”

“…This coincided with the first establishment of the trade-wind-driven Benguela upwelling system around 10 Ma (Hay and Brock, 1992). Furthermore, Diester-Haass et al (1990 have postulated a northward shift of the Benguela Current since the middle Miocene that might have strengthened upwelling at about 10.5 Ma.…”

The middle to late Miocene climatic development of Southwest Africa derived from the sedimentological record of ODP Site 1085A

“…At about 10 Myr opal signals from the BC are detected at DSDP Sites 362 and 532 (Diester-Haass et al 1990. This is also the time at which climatic changes initiated a desertification of SW Africa, especially in the Namib region, where a Late Miocene sand sea was re-established (Partridge 1997).…”

“…North Atlantic Deep Water (NADW) did not intrude into the Cape Basin before 10 Myr (Diester-Haass et al 2004). During the Middle Miocene the Benguela Current (BC) progressed northwards and reached the Walvis Ridge at about 10 Myr (Diester-Haass et al 1990Krammer et al 2006;Kastanja et al 2006).…”

“…During this period prominent shifts towards a modern setting were achieved; ocean circulation, the onset of the Benguela Upwelling System and the desertification of the hinterland (Diester-Haass et al 1990. For the development of the upwelling cells, seaward blowing winds were required, which prevented coeval landward intrusion of humid air masses from the South Atlantic.…”