The lipid biomarker
content in selected sediments from several sites along the southwestern African
margin reflects different marine environments. The majority of the extracted
lipids in all samples is of marine origin, with varying contributions of
terrigenous lipids. It should be noted that the limited sample number restricts
detailed paleoceanographic interpretations and that this study is solely
intended to be an inventory. Conclusions are summarized as follows:
- In general, two main areas can be distinguished, separated by
the ABF. Elevated concentrations of terrigenous lipids and lipids from
eustigmatophyte algae were only observed north of the ABF, whereas lipids
from archaea and zooplankton are only present in significantly elevated
amounts south of it.
- The sediments below the Congo River freshwater plume received a
large contribution of lipids from terrestrial higher plants and mangroves.
The contribution of terrigenous lipids is suggested to decrease with
increasing distance from the river mouth. At the shallower site, marine
lipids were mainly derived from haptophyte algae, diatoms, dinoflagellates,
and eustigmatophyte algae. Lipids of these algae were less abundant in
samples from the deeper site, whereas the contribution from pelagic archaea
and zooplankton were slightly higher. The changes in lipid composition with
distance from the river mouth suggest variation of plankton communities with
reduction of river influence.
- The investigated sediments on the Angola margin received a small
amount of terrigenous lipids. A highly productive plankton community of
haptophyte algae, diatoms, dinoflagellates, and unidentified,
keto-ol-producing microalgae probably caused the elevated sedimentary
organic carbon contents. Supply of nutrients from the nearby Angola Dome may
sustain high productivity.
- Walvis Basin sediments received an upwelling signal transported
by the Benguela Current. Major contributors of marine lipids were haptophyte
algae and pelagic archaea. Diatom lipids were detectable in low
concentrations as sulfurized compounds.
- Persistent coastal upwelling off Lüderitz Bay is well reflected
in the sedimentary lipids. The investigated samples contain the highest
lipid contributions of pelagic archaea, haptophyte algae, dinoflagellates,
diatoms, and zooplankton. Anoxic sedimentary conditions were most probably
caused by a large supply of labile organic matter.
- In the southern Cape Basin, sediments probably received plankton
material from seasonal upwelling, indicated by their content of sterols and
sterol ethers. Their marine lipid content is comparable to the lipids of the
sediments in the Walvis Basin. The nearby Orange River did not cause
elevated concentrations of terrigenous lipids in the investigated samples.