Technical Note 20/4

LEG 175

Benguela Current

Modified from Proposal 354 Submitted By

G. Wefer, V Spieß, U. Bleil, M. Breitzke, K. Gohl, R. Schneider, G. Uenzelmann-Neben

Staff Scientist: Carl Richter
Co-Chief Scientists: Wolfgang Berger &; Gerold Wefer


Leg 175 proposes to drill nine APC/XCB sites off Angola and Namibia to reconstruct the Cenozoic history of the Benguela Current and the coastal upwelling off Angola and Namibia between 5°S and 32°S. The Angola/Namibia system is one of the great upwelling regions of the world, and extends over a considerable portion of the western margin of South Africa. Like the Peru system, which was studied during ODP Leg 112, it is characterized by organic-rich sediments that contain an excellent record of productivity history, which can be read on a very fine scale. In addition, this environment provides an excellent setting for natural experiments in diagenesis and in the formation of economically important resources such as petroleum and phosphate.

The individual transects reflect examinations of specific"endmember" upwelling environments, which collectively comprise one of the most concentrated centers of ocean productivity. One of the major aims will be to monitor the evolution of the Benguela Current system and its relationship with the onset of Northern Hemisphere and Antarctic glacial cycles. Many of the proposed sites are expected to have high-sedimentation rates, offering an opportunity to develop very detailed paleoceanographic records, and all proposed sites will expand and refine the partial record of the paleoceanographic and paleoclimatic changes since the early Miocene provided by DSDP Sites 362 and 532. Sediments will be largely diatomaceous and carbonate rich clays with variable, and occasionally very high, organic-carbon contents.

The ocean's role in climatic change, through heat transport and control of carbon dioxide, increasingly is being recognized. This new awareness, and the urgency that must be accorded the attempt to understand the mechanisms of climatic change, have led to the initiation of large integrated efforts in physical oceanography and chemical oceanography. Likewise, the potential of the ocean record for understanding climatic change has received increased attention in recent years (CLIMAP, 1976; COSOD II, 1987). The Angola/Namibia upwelling system needs to be studied because of its importance in the global ocean-carbon cycle, and to provide a counterpoint to the Peru system, for comparison. Only such comparison will allow us to determine which elements of the systems are peculiar, and which have general validity through time and on a global scale. Leg 175 will drill nine APC/XCB sites (Fig. 1) off the southeastern coast of Africa to study the paleoenvironment of the Benguela Current and Angola/Namibia upwelling system, with emphasis on the late Neogene, to further these goals.

Eastern boundary upwelling is strongly involved in modulation of the carbon cycle and hence in control of the partial pressure of calcium carbonate (pCO2) ("biological pumping", Berger and Keir, 1984; Sundquist and Broecker, 1985; Boyle and Keigwin, 1987; Sarnthein et al., 1988). It is now generally thought that such pumping is a crucial factor for the explanation of short-term pCO2 fluctuations, of the type seen in ice cores (Barnola et al., 1987). Along these lines of argument, there is a good correlation between productivity indices in the eastern equatorial Pacific and the ice-core record of pCO2 (Fig. 2).

On a longer time scale, Vincent and Berger (1985) have postulated that depositional pumping by coastal upwelling is responsible for changing the general level of atmospheric pCO2. They propose a climatic preconditioning by upwelling-induced carbon extraction from the ocean atmosphere system for the beginning of the Neogene and, hence, the modern ice-cap dominated world. Their argument is based on the observation that carbon isotopes in deep-sea benthics become 13C enriched just when organic-rich phosphatic sediments begin to accumulate around the Pacific margins (Fig. 3). In this view, eastern boundary upwelling, and therefore upwelling off Angola and Namibia, has global implications for the long-term history of the carbon cycle and climate, and for the evolution of life and biogeography on land and in the sea.

To be able to predict the effects of changes in productivity on the CO2 content of the atmosphere, the interrelationships between ocean circulation, nutrient transport, and the sedimentation of organic compounds and carbonate must be established for the important productivity regions. Until now, there is no information on the Neogene upwelling fluctuations off Angola and Namibia, a region that is probably of considerable importance for the global carbon cycle.

The most important period for understanding the workings of the present system is the development since the Miocene. These developments include the evolution of the present orography, the build-up of ice-caps on both poles, the development of modern wind and upwelling regimes, and a stepwise increase of North Atlantic Deep Water (NADW) production, which dominates the style of deep circulation of the ocean. The present system is characterized by a strong 100,000-yr component in the Milankovitch spectrum, throughout Brunhes time. A somewhat similar spectrum, with high amplitudes but with a lesser 100,000 yr component, exists since 2.5 to 2.4 Ma., in the North Atlantic (Shackleton et al., 1984).

The Angola/Namibia system is one of the five or six great upwelling regions in the world. It extends over a considerable portion of the western margin of South Africa, with productivity values of 180 gC/m2yr, and greater (black areas in Fig. 4). Like the Peru system, which was studied during Leg 112, it is characterized by organic-rich sediments, in part deposited under anaerobic conditions. Such sediments contain an excellent record of productivity history, which, in part, can be read on a very fine scale (e.g., Schrader and Baumgartner, 1983). In addition, this environment provides an excellent setting for "natural experiments" in diagenesis, and in the genesis of economically important resources (e.g., petroleum and phosphate).

Upwelling off southwest Africa is at present centered on the inner shelf and at the shelf edge. The Benguela Current flows roughly parallel to the coast and within ~180 km of it south of 25°S, and then turns to the west over the Walvis Ridge between 23° and 20°S (Fig. 1). At about 20°S, warm, tropical-water masses from the north meet the cold Benguela Current water generating eddies. Eddies of cold, upwelled water contain radiolarian and diatom skeletons, which are transported from the upwelling area to the northern part of the Walvis Ridge, where they have been sampled at DSDP Sites 532 (Hay et al., 1984) and 362 (Bolli et al., 1978).

During the last glacial maximum the eddy formation took place farther north and the Benguela Current flowed parallel to the coast and over the Walvis Ridge to reach the Angola Basin, finally bearing to the west at about 17°S. Sediments deposited at Site 532 during the last glacial period confirm the absence of upwelling eddies by containing zero to very few opal skeletons (Hay et al., 1984; Diester-Haass, 1985). Upwelling may have continued to occur on the African shelf, but the Benguela Current did not transport that upwelling signal to the Walvis Ridge. In contrast, from the distribution of foraminiferal assemblages at Site 532 it was suggested that the northeastern Walvis Ridge was characterized by intensified upwelling and a westward expansion of coastal upwelling cells at glacial periods during the last 500,000 years (Oberhänsli, 1991).

The results from Sites 362 and 532 can be used to reconstruct, tentatively, the evolution of the Benguela Current during the past 10 m.y. The 5.2 m.y. transition between the two upwelling regimes marks an important event. At this time, the late Quaternary-Holocene circulation/upwelling pattern became established, perhaps in response to the migration of the polar front to its modern position at the same time.

In addition, the evolution of the climate of the Northern Hemisphere, and particularly that of northern Europe, is linked to the exchange of heat between the South Atlantic and the North Atlantic Oceans (Fig. 5). This energy transport, operating over large distances, is involved in the formation and magnitude of polar ice caps. In today's world, a net heat transfer from the South Atlantic to the North Atlantic exists in currents above the thermocline (Woods, 1981; McIntyre et al., 1989). A part of the heat contribution from the South Atlantic is believed to originate from the Indian Ocean via the Agulhas Current. The Benguela Current is a connection between the waters north of the polar front in the South Atlantic and the Equatorial Currents of the Atlantic. Northward and southward shifts of the Southern Ocean polar front constrict or expand, respectively, the interchange of heat from the Indian Ocean to the South Atlantic (McIntyre et al., 1989). This interchange has drastic impacts on the heat budget of the Benguela Current and consequently that of the entire Atlantic Ocean. Such variations in heat transfers should appear as changes in the course of the current track and be recorded in the sedimentary accumulations along the southwest African margin.

Paleoceanographic interpretations are derived from the information from a single location off southwest Africa and must be considered preliminary. Given the strong likelihood that the axis and the intensity of the Benguela Current have changed over the past 15 m.y. and that the current has fluctuated with glacial/interglacial cycles, confirmation and refinement of these ideas is needed. Although DSDP Legs 40, 74, and 75 occupied sites in the Cape and Angola Basins and on the Walvis Ridge, these sites are situated too far offshore to provide the needed information. The Benguela Current and its associated upwelling are not recorded well in the sediments of these sites. Even Sites 362 and 532 on the Walvis Ridge are too far offshore to contain a direct record of upwelling. They receive an indirect record of near-coastal upwelling from material transported to their location by the Benguela Current. Furthermore, modern coring technology (APC, XCB) enables high-resolution studies by avoiding much of the drilling disturbance present in the Leg 40 cores. Such high resolution work is crucial if the dynamics of upwelling are to be captured on a scale of glacial-interglacial cycles back to the Miocene. Information from an array of sites situated in the southern and central Cape Basin, on the Walvis Ridge, and in the southern Angola Basin would allow the construction of a coherent picture.

The results from DSDP Sites 362 and 532 suggest that there has been a general northward migration of the Benguela Current upwelling system during at least the last 14 m.y. Because the shape of the South Atlantic has not changed appreciably during this time, the changes in the upwelling system must reflect large-scale, perhaps global, changes in ocean circulation. This leg will focus primarily on the paleoceanographic and paleoclimatic aspects of the area. However there is interest in investigating samples from the upwelling area off Angola and Namibia with regard to early diagenetic processes taking place in this unique environment. Possible work includes the formation of dolomites (Baker and Kastner, 1981), phosphorites (Calvert and Price, 1983), and cherts (see articles in Garrison et al., 1984). We also hope to examine the organic matter type and distribution as a function of time and climatic cycles. Important questions that can be answered with additional drill sites in this region include:

1. Determine the history and early evolution of the Benguela Current.

2. Study the history of productivity of the upwelling off Angola and Namibia and the influence of the Zaire River.

3. Determine what kind of oceanographic changes occur simultaneously in the Atlantic Ocean (Agulhas Current, polar front position, equatorial current, Argentine Current) with the shifting of the Benguela Current. Results from ODP Legs 108 and 114 can help define the equatorial and polar boundaries of the Benguela Current in the past. The final aim is to reconstruct the Oligocene to Quaternary paleocirculation pattern of the South Atlantic Ocean and try to evaluate the glacial-interglacial heat balance between the South and North Atlantic through time.

4. Determine if changes in the surface current/upwelling pattern of the Benguela Current cause, or are related, to changes in climates of western South Africa. For example, is desertification of the Namib related to the initiation of upwelling off Southwest Africa? Sites close to the continent probably contain enough information (clay minerals, grain size of terrigenous material, pollen, phytoliths, fresh water diatoms) to allow reconstruction of continental climatic changes and to determine whether these changes are synchronous with oceanographic changes (i.e., the establishment of upwelling off Southwest Africa).

5. Examine the effect of sea level changes, if any, on the Benguela Current. Published eustatic sea level curves (Haq et al., 1987) can be tested off Southwest Africa not only seismically, but also by sedimentological investigations.

6. Study early diagenetic processes in environments with very high organic carbon and opal contents, which will offer an interesting contrast to the studies undertaken during Leg 112 (Suess, von Huene, et al., 1990) off Peru. The upwelling sediments off the Peruvian active margin are deposited in fore-arc basins in a disturbed tectonic setting, while off Angola/Namibia sedimentation occurs on a steadily sinking passive margin with quite stable conditions. Therefore, we expect a more continuous and longer record in comparison to the sites drilled off Peru, although the sedimentation rate might not be quite as high. Furthermore, sediments should be higher in organic carbon and opal content (Kulm et al. [1984] for Peru sediments, and Bremner [1983] for Namibia sediments).

Leg 175 will drill nine sites as part of a latitudinal transect between 5°S and 32°S. These sites are located in the Northern Angola Basin (NAB), Mid-Angola Basin (MAB), Southern Angola Basin (SAB), Walvis Ridge (WR), Northern Cape Basin (NCB), and Southern Cape Basin (SCB).

1. Northern Angola Basin (NAB)
Two NAB sites will sample a complex environment dominated by riverine input, seasonal coastal upwelling, and incursions from the southern equatorial counter current (Fig. 6). While these two sites represent the same depositional environment, they are located at varying distances from the shelf break, in different water depths, and at different positions with respect to the river plume and Congo Canyon.

2. Mid Angola Basin (MAB)
The MAB sites, off the bight of Angola near 12°S, were chosen to provide a section of "most nearly normal" margin sedimentation, being neither influenced by riverine input, or by sustained year-round upwelling. Upwelling, which is predominately related to the Angola Thermal Dome, is seasonal and productivity is relatively weak, compared with adjacent regions to the north and south (Schneider, 1991). This setting allows a maximum participation of a general, pelagic signal, in the regional high productivity record (Figs. 4 and 6). All proposed drill sites are located on seismic line GeoB 93-015, because the southerly profile GeoB 93-017 is significantly influenced by slumping of shelf sediments and turbidity currents. For each of the proposed sites a crossing seismic profile was shot perpendicular to line GeoB 93-015.

3. Southern Angola Basin (SAB)
The Southern Angola Basin sites will sample the northern end of the Angola/Namibia upwelling region. The transect should nicely complement previous results obtained from Walvis Ridge. This transect is important not only for the history of the Benguela Current and coastal upwelling migration, but also for its contribution to the climatic history of southern Africa. The Kunene River, reaching the coast at ~17°S, is at the climatological barrier between an illite zone in arid areas to the south and a kaolinite zone from tropical weathering areas to the north (Bornhold, 1973). The proposed sites are situated on a climatic boundary, and should sensitively reflect changes in the position of continental climatic zones. Suitable drill sites were identified from seismic lines in water depths between 2200 and 3000 meters. The bathymetric survey confirmed the complex nature of the depositional environment. Although the survey was not sufficient to analyze all structures in detail, it is clear from the combined Hydrosweep and PARASOUND echosounder data set that few potential drill sites may be found in the area. Seismic line GeoB 93-030 lies across the proposed drill sites (Table 1). Stratigraphic data from two gravity cores (GeoB 1023-5, 17°09.4'5, 11°00.7'E, water depth 1918 m; GeoB 1024-2, 17°09.8'E, water depth 2799 m) show high Pleistocene sedimentation rates (10-50 cm/1000 yr) (Schneider et al., 1992; Wefer et al., 1988).

4. Walvis Ridge (WR)
Site WR 1 together with DSDP Sites 532 and 362 (Legs 75 and 40) in 1300 m water depth form a transect that is central to the reconstruction of the history of the Benguela Current. The DSDP sites are seaward of the upwelling center, but contain an upwelling signal that has been transported to this location by the Benguela Current. At the other end of the transect, proposed site WR 1 will give a better record of the upwelling itself. This transect, situated as it is on the only topographic high over which the Benguela Current passes, is central to the reconstruction of the history of the current. Two cores ~11-m long (GeoB 1705-1, 19°30.3'S, 11°23.9'E, water depth 642 m and GeoB 1706-2, 19°33.7'S, 11°10.5'E, water depth 980 m) taken near proposed site WR 1 show sedimentation rates of 4-7 cm/1000 yr (Schulz et al. 1992).

5. Northern Cape Basin (NCB)
The NCB site will help document the northward migration of the Benguela Current system from the Miocene (perhaps Oligocene) to the Quaternary as well as the shoreward/seaward migration of the upwelling center (Figs. 4 and 5). The site will also provide a record of maximum productivity in the system. Previous work in this area (for a summary see Dingle et al., 1987) has documented anaerobic, in part varved, sedimentation in the upper margin regions. Phosphatic deposits also are abundant (Calvert and Price, 1983).

The results of Emery et al. (1975) and Austin and Uchupi (1982) show a thick hemipelagic wedge sitting on "rifted continental crust." Slumps would not seem to pose a problem. Noteworthy is the confirmation of a thick sequence below the shelf region. A close tie-in between pelagic and terrigenous sedimentation is expected to be present within the slope record. During the SONNE cruise SO-86, vertical profiles were shot over MCS line AM-1 collected by the University of Texas to obtain detailed data for the planned site. A first stratigraphy on an 11-m-long core taken in the high-production upwelling area off Namibia (GeoB 1711-4, 23°18.9'S, 12°22.6'E) from a depth of 1967 m indicated a sedimentation rate of 11 cm/1000 yr (Schulz et al., 1992).

6. Southern Cape Basin (SCB)
This site is located in the southernmost area of the Cape Basin (Fig. 1) to explore the early history of the Benguela Current in the southern Cape Basin and to detect possible Agulhas Current influences. The site is located close to the continent to detect upwelling signals and signals from continental climates (pollen, clay minerals, coarser terrigenous matter) and signals related to sea level changes. South of the proposed transect the margin becomes too steeply sloped to support undisturbed sediments. Site SCB 1 is located along MSC line AM-54 collected by the University of Texas. For an 11-m-long core from about the same water depth (GeoB 1719-7, 28°55.6'S, 14°10.7'E, water depth 1010 m) but about 150 miles to the north, a sedimentation rate of about 5 cm/1000 yr was determined (Schulz et al. 1992).

As recommended by the OHP, we could probably reach the Paleogene at Site SCB 1 with a relatively deep hole. However, drilling through 1300 m of sediment would take 9.5 days instead of 3.5 days (Hole SCB 1, to 600 m). It would not be possible to include the additional 6 days in the one-leg proposal. Another possibility for obtaining a Paleogene core might be Site WR 1 (750 m water depth on the Walvis Ridge).

To Leg 175 Proposed Site Information

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