Peter Rothe

The Benguela Current and associated upwelling on the southwest African Margin: a synthesis of the Neogene-Quaternary sedimentary record at DSDP sites 362 and 532

Abstract Sediments cored at DSDP Sites 362 and 532 on the Walvis Ridge provide a Neogene-Quaternary history of the development of the upwelling system on the southwest African margin. Upwelling occurs principally on the landward side of the Benguela Current. The upwelling centres have shifted northward since the Middle Miocene as the current has intensified and has flowed farther to the north. Changes in productivity are recorded in the types, proportions and preservation of foraminifera, radiolaria, diatoms, organic matter, and clay minerals in the sedimentary record. Prior to the Late Miocene (10 Ma), the Benguela Current did not reach the Walvis Ridge, and enhanced productivity is not evident in the sediments at this location. Between 10 to 5.2 Ma, upwelling was recorded in the DSDP sites in glacial periods, indicating that the Benguela Current intensified during glacial periods and transported evidence of upwelling to Sites 362/532 from near-coastal areas. During interglacial periods the current was not as strong and did not reach the Walvis Ridge, turning instead to the west within the Cape Basin. Strengthening of the current continued such that by the Pliocene and Quarternary the upwelling signal is contained in interglacial sediments. Sediments deposited in these more recent glacial times contain a weak or absent upwelling signal because glacial intensification shifts the Benguela Current system northward to reach the Angola Basin before it turns westward away from its coast-parallel direction.

LIGHT-DARK CYCLES IN OPAL-RICH SEDIMENTS NEAR THE PLIO-PLEISTOCENE BOUNDARY, DSDP SITE 532, WALVIS RIDGE CONTINENTAL TERRACE

Diester-Haass, L., Meyers, P.A. and Rothe, P., 1986. Light-dark cycles in opal-rich sediments near the Plio-Pleistocene boundary, DSDP Site 532, Walvis Ridge continental terrace. Mar. Geol., 73: 1-23. Samples taken at 10 cm intervals from DSDP Core 532B-17 contain variations in carbonate, opal, organic carbon, and terrigenous components that correlate with light-dark cycles in sediment color. The core site, at 1300 m water depth, is well above the CCD, yet the color variations appear to result largely from cyclical fluctuations in carbonate dissolution, which was greater during glacial periods. Higher concentrations of organic carbon and of terrigenons sediment components correlate with enhanced carbonate dissolution, but opal concentrations inversely correlate and suggest that biological productivity at this site diminished during glacial periods. A complicated glacial-interglacial picture emerges from the data. In interglacial times, upwelling associated with the Benguela Current produced abundant opaline material, organic matter was fairly well preserved, and carbonate was only moderately dissolved. In glacial times, the upwelling core shifted as sea level fell and winds intensified. Productivity in the waters over Site 532 decreased, but lateral supply of oxidizable organic matter enhanced carbonate dissolution, giving rise to light-dark cycles in these sediments.

Plio-Pleistocene sedimentation on the Walvis Ridge, Southeast Atlantic (DSDP Leg 75, Site 532) — Influence of surface currents, carbonate dissolution and climate

An investigation of the quantitative composition of the coarse (> 40 μm) and clay (<2 μm) fraction of HPC 532, DSDP Leg 75, in 1300 m water depth on the eastern Walvis Ridge off Southwest Africa yielded the following results: 1. (1) The sediments reflect a complete Latest Miocene to Recent depositional history. Sedimentation rates vary between 2.3 and 7.8 cm 10−3 years. 2. (2) Preservation of calcium carbonate is subject to strong variations: short-term (<100,000 years) and long-term (about 1 m.y.) cycles in carbonate dissolution have been observed, with strongest dissolution occurring during periods of lowered sea level. 3. (3) Upwelling influence from the near-coastal upwelling centre has been detected by means of the opal content: interglacial periods show high opal contents, because the Benguela Current turned westward at about 20°S and carried opal-laden upwelled water to the west. Sediments from glacial periods, however, show opal minima. Besides these short-term cyclic variations in opal content, long-term cycles have been found, with maximum upwelling influence in the latest Pliocene/early Quaternary. 4. (4) Each CaCO3 dissolution minimum (maximum) is correlated with an opal maximum (minimum) throughout the sediment sequence. 5. (5) The oceanographic system off southwest Africa remained essentially unchanged since the latest Miocene: sea level rose and fell periodically on a small and on a large scale, and the Benguela Current flowed southeast-northwest and turned to the west at the latitude of Site 532 during interglacial periods, when sea level was high. 6. (6) The climate in the near-coastal area of southwest Africa in the latitude of Site 532 has probably been arid throughout the investigated period.

Rezente meteorische Diegenese von Mg-Kalziten des Miozäns (Hydrobien-Schichten, Mainzer Becken)

SummaryMiocene Mg-calcite with a maximum of 16 mole% MgCO3 was reported in Hydrobia Beds from the Mainz Basin in 1971. Samples were from a road cut near Mainz Weisenau just opended for highway construction. This locality was resampled in 1973 and 1983.53 samples from a four meter thick vertical section of subhorizontal dark marls and marly clays revealed Mg-calcites with a MgCO3-content of up to 12.4 mole%.Mg-calcites occur (1) as single grains (“wheat grains”) of 4–10 μm length and 1–3 μm diameter, (2) as “clusters” composed of single crystals of about 0.2 μm size, and (3) as oriented coatings on aragonitic fossils.MgCO3-content is highest within homogeneous marls. It decreases gradually, however, approaching the intercalated oncoidsand-layers, each a few centimeters to some decimeters thick, which allow meteoric water to percolate.The diagenetic influence of meteoric water could likewise be demonstrated by analyzing a horizontal section within a single clay layer. MgCO3-content within Mg-calcites decreases at a distance of 25 cm from the present vertical surface from 15 mole% to less than 10 mole%.Mg-calcites within the section studied can, if adequately exposed to meteoric water, be transformed quantitatively into stable calcite within 10–20 years.In this paper we studied the section in the Mainz Basin in further detail in order to reveal conditions for the stability of the Miocene Mg-calcites reported earlier.Miocene Mg-calcite with a maximum of 16 mole% MgCO3 was reported in Hydrobia Beds from the Mainz Basin in 1971. Samples were from a road cut near Mainz Weisenau just opended for highway construction. This locality was resampled in 1973 and 1983.

Sediments on Volcanic Islands — On the Importance of the Exception

Sediments of the Middle Atlantic Islands are discussed along with their position relative to volcanic events. They can be subdivided into prevolcanic, synvolcanic, and postvolcanic sediments.