The results from DSDP Sites 362 and 532 suggest that there has been a
general northward migration of the Benguela Current upwelling system
during 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. Leg
175 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 study of the formation of dolomite (Baker and Kastner,
1981; Kulm et al., 1984), phosphorite (Calvert and Price, 1983), and chert
(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 are being addressed during Leg 175 include the
Determine the history of the Benguela Current for the late Neogene. Of
special interest is the changing response to orbital forcing, as seen in
spectral amplitudes and phase relationships (e.g., McIntyre et al., 1989;
Schneider, 1991; Berger and Wefer, 1996; Jansen et al., 1996; Schneider et
al., 1996; Wefer et al., 1996).
Study the history of productivity of the upwelling off Angola and
Namibia and the influence of the Zaire River, extending available
information about the late Quaternary (Bremner, 1983; Jansen et al.,
1996) to earlier periods. The history of opal deposition off the Zaire River
is of interest (Schneider, 1991), as well as the origin of cycles of
carbonate, organic matter-deposition, and diatoms, in each region.
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 Ocean Drilling Program (ODP) Legs 108 and 114 can help
define the past equatorial and polar boundaries of the Benguela Current.
The final aim is to reconstruct the late Neogene paleocirculation pattern
of the South Atlantic Ocean to evaluate implications for the
glacial/interglacial heat balance through time between the South and
North Atlantic. Of special interest is the identification of changes in
modes of circulation, as seen in changes in correlations between proxy
variables, as a function of time.
Determine if changes in the surface-current and upwelling patterns of
the Benguela Current cause, or are related to, changes in climates of
western South Africa. For example, is the origin of the Namib desert
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, and 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).
Examine the effect of sea-level changes, if any, on sedimentation below
the Benguela Current. Published eustatic sea-level curves (Haq et al.,
1987) will be useful for this purpose.
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, off Peru (Suess, von Huene, et al.,
1990). The upwelling sediments off the Peruvian active margin are
deposited in forearc basins in a disturbed tectonic setting, whereas 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.
To 175 Drilling Strategy & Proposed Sites
175 Table of Contents