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Middle and Late Quaternary oceanography and climatology of the Zaire-Congo fan and the adjacent eastern Angola Basin
Jansen, J.H.F.; van Weering, T.C.E.; Gieles, R.; van Iperen, J.M. (1984). Middle and Late Quaternary oceanography and climatology of the Zaire-Congo fan and the adjacent eastern Angola Basin. Neth. J. Sea Res. 17(2-4): 201-249
In: Netherlands Journal of Sea Research. Netherlands Institute for Sea Research (NIOZ): Groningen; Den Burg. ISSN 0077-7579; e-ISSN 1873-1406, meer
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| Auteurs | | Top |
- Jansen, J.H.F.
- van Weering, T.C.E., meer
- Gieles, R.
- van Iperen, J.M.
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| Abstract |
We established Quaternary stratigraphy of the Zaire fan on the basis of downcore carbonate variations, microfossil investigations, and 14C and 230Th datings in 33 piston cores. The rates of carbonate and non-carbonate accumulation in combination with varbonate preservation patterns allow us to distinguish between the 3 principal factors responsible for the carbonate contents: production, dissolution, and non-carbonate dilution. High carbonate production occurred during the interglacial climatic optima, while smaller carbonate peaks are probably also due to increased carbonate accumulation. The carbonate patterns were emphasized by the effects of dissolution, which is most severe in glacials, and fluctuations in non-carbonate sedimentation, with high rates during cold episodes. A lobe of increased carbonate production, consistently located in the central fan area, is not related to high phytoplankton productivity. The lobe is the result of increased activity of zooplankton that live on a near shore river-induced phytoplankton bloom, during the Holocene and interglacial stages. The sediments of the glacial carbonate lobes do contain micropalaeontological evidence of high nutrient supply and phytoplankton productivity. A glacial intensification of the surface current system probably moved a zone of oceanic upwelling towards the latitude of the Zaire fan. Near by the shelf edge high carbonate production is documented between ~27 000 and 15400 yBP, pointing to coastal upwelling of Benguela Current water, and a decrease in the rates of non-carbonate sedimentation and organic carbon burial at ~14 500 yBP. High car- bonate accumulation lasted till ~12 500 yBP which resulted in a regional carbonate peak and preservation spike at 14 000 yBP, not related to any oceanic feature. A short postglacial intensification of the Benguela Current took place 11 400 yBP. The carbonate compensation depth (CCD) and lysocline were about 1000 m shallower during the maximum glacial periods (~4400 and 3800 m) than at present (5600 m and 4800 m). The interglacial CCD and lysocline approximate to the glacial rather than to the Holocene positions, due to post-depositional carbonate dissolution during subse- quent glacial episodes. The hydrography of the Angola Basin provides no evidence that Antartic Bottom Water fluctuations have caused the Quaternary carbonate dissolution cycles in the Angola Basin. A lobe of high sedimentation rates in the central fan area indicates a mainly terrigenous source. In the cold periods terrigenic sedimentation was most intense. It supplied burnt plant fragments, phytoliths, larger proportions of continental organic carbon (based on δ13C measurements), and high inorganic sedimentation rates, and is correlated with arid stages in equatorial Africa and low sea levels. Terrigenous organic carbon reaches the ocean floor with an average delay of 1000 years. A sudden decrease in sedimentation rates suggests that a long-term climatic warming and sea-level rise took place around 350000 to 300000 yBP. Organic carbon burial rates are controlled by the decomposition of organic matter in relation to the sedimentation rates. Both downcore and core to core variations follow these rates closely. |
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