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The Geological Record of Upwelling Evolution |
Institut de Géologie du Bassin d’Aquitaine (I.G.B.A.), CNRS (U.R.A. 197), Université de Bordeaux 1, Talence, France
Coccoliths are skeletal remains of organisms living in the uppermost part of the oceans, so they reflect the climatological and hydrological variations of the uppermost water layers. The main goal of the present study is to use their fossil remains (calcareous nannofossils) to capture information about the environment where they lived for the last 700 000 years. The work is based on a study of sediment from Site 658, ODP Leg 108 located in a major upwelling area in the northeastern tropical Atlantic, close to the African continent. The hydrological and atmospherical conditions of the area are complex: seasonally windy and alternatively humid and arid. Downcore the effects of these changes are complicated by the presence of climatic stages including various glacial/interglacial alternations.
The interpretation of Plio-Pleistocene climatic history at Site 658 is based on analysis of abundance matrices (calcareous nannofossils as well as other <25 µm organic and inorganic elements) and of two nannofossil-based transfer functions (giving estimates for summer-winter temperatures, summer-winter salinities and productivity). The nannofossils most useful for paleoecological interpretation proved to be Rhabdosphaera (related to increased temperature and decreased productivity), Coccolithus pelagicus (related to sharp decreases in temperature and inversely to salinity), Syracosphaera (related to increased salinity) and Helicopontosphaera (related to sharp decreases in salinity and increased productivity).
A tentative paleoenvironmental and paleoecological model is given for a typical glacial/interglacial alternation. During glacial periods the African continent was arid, with seasonal winds blowing towards the ocean, and the uppermost oceanic waters showing increased salinity. At first oceanic temperatures were low, and organic matter linked with fecal pellets was abundant due to wind-induced upwelling; temperature then increased slightly, before decreasing at the end of the glacial period under the influence of northern ice sheets. At the glacial/interglacial boundary, rainy monsoons caused strong and sudden increased runoff, which lowered salinity; interglacial time was characterized by low salinity surface waters. As interglacials began, fluvial outflows supplied abundant inorganic and organic matter, while the Canary Current brought cooler waters south from northern latitudes, and there was a short record of upwelling. Then, temperature increased slowly, as the area became dominated by tropical waters.
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  F. O. Amore, G. Ciampo, V. Di Donato, P. Esposito, E. Russo Ermolli, and D. Staiti An integrated micropalaeontological approach applied to Late Pleistocene-Holocene palaeoclimatic and palaeoenvironmental changes (Gaeta Bay, Tyrrhenian Sea) Geological Society, London, Special Publications, 2000; 181: 95 - 111. [Abstract] [PDF]
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