Clay mineral assemblages,siliciclastic input and paleoproductivity at ODP Site 1085 off Southwest Africa: A late Miocene–early Pliocene history of Orange river discharges and Benguela current activity,and their relation to global sea level change |
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| Institution: | 1. Department of Geosciences, University of Bremen, FB5 Klagenfurter Strasse, 28359 Bremen, Germany;2. Instituto Dom Luiz (IDL), University of Lisbon, 1740-016, Portugal;3. Portuguese Hydrographic Institute (IH), Rua das Trinas 49, 1249-093 Lisboa, Portugal;4. Royal Netherlands Institute for Sea Research (NIOZ), Marine Geology Dep., Texel, The Netherlands;5. Department of Geology, Fac. Sciences, University of Lisbon, 1749-016 Lisboa, Portugal;6. Departament d''Informàtica, Matemàtica Aplicada i Estadística, Universitat de Girona, Campus Montilivi, P4, E-17071 Girona, Spain;1. Department of Arctic Geology, The University Centre in Svalbard (UNIS), Longyearbyen, 9171 Svalbard, Norway;2. School of Geography, Queen Mary University of London, London E1 4NS, UK;3. Department of Arctic Biology, The University Centre in Svalbard (UNIS), Longyearbyen, 9171 Svalbard, Norway;1. Geology Center (CEGUL)/Instituto Dom Luiz (IDL), University of Lisbon, 1749-016 Lisboa, Portugal;2. Department of Geology and Chemical Oceanography, Royal Netherlands Institute for Sea Research (NIOZ), Texel, The Netherlands;3. Portuguese Hydrographic Institute (IH), Rua das Trinas 49, 1249-093 Lisboa, Portugal;4. Department of Geology, Faculty of Sciences, University of Lisbon, 1749-016 Lisboa, Portugal;1. Istituto per l''Ambiente Marino Costiero, Consiglio Nazionale delle Ricerche, Calata Porta di Massa, Interno Porto di Napoli, 80133 Naples, Italy;2. Istituto Nazionale di Geofisica e Vulcanologia, Via della Fagiola32, 56126 Pisa, Italy |
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| Abstract: | A late Miocene to early Pliocene sequence drilled on the continental slope of southwest Africa off the Orange river mouth (ODP Site 1085) has been investigated. Clay mineral assemblages, coarse siliciclastics and benthic foraminifer accumulation rates (BFAR) unravel a step by step evolution of marine and continental environments closely related to sea level variations, ocean circulation and global climate: (1) smectite is a typical tracer of the Orange river load, whereas illite is mostly transported by the Benguela current (like chlorite) and winds, and kaolinite is derived from low latitudes by the poleward undercurrent and the North Atlantic Deep Water (NADW); (2) increased erosion and influence of the Orange river after 9.6 Ma is linked to a sea level drop at a time of Antarctic ice-growth. This has been followed by an increased seasonality of precipitation and high productivity, but low oxygen content and associated dissolution of carbonates; (3) increased productivity and dissolution of carbonates, and coeval increase of continental aridity after 8.9 Ma express a further development of the Benguela current and upwelling; (4) better preservation of carbonates and increased contribution of terrigenous material from northern sources at 6.9 Ma are related to increased circulation of NADW after an early stage of northern hemisphere glaciation; (5) increased erosion and contribution from the Orange river and westward shift of the area of higher productivity from 5.9–5.8 Ma to 5.3–5.2 Ma are related to a significant fall of sea level, and encompass the time of the entire Mediterranean salinity crisis; (6) short-term variations of the smectite/illite ratio (S/I) and BFAR suggest a major control of productivity by wind and current activities (and related upwelling), but may express brief variations of sea level in specific intervals before 8.9 Ma and during the late Messinian especially. |
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