HR: 0800h
AN: PP51B-1337    [Abstracts]
TI: Emergence of the Namib Desert During the Neogene, and Correlations With Global Palaeoclimate Changes
AU: * Segalen, L
EM: loic@science.uct.ac.za
AF: Quaternary Research Centre, Dept of Archaeology, UCT, Rondebosch, Cape Town, 7701 South Africa
AU: * Segalen, L
EM: loic@science.uct.ac.za
AF: Dept de Geologie Sedimentaire, UPMC, CNRS-FR 32, case 116, 4 place Jussieu , Paris, 75252 France
AU: Renard, M
EM: renard@ccr.jussieu.fr
AF: Dept de Geologie Sedimentaire, UPMC, CNRS-FR 32, case 116, 4 place Jussieu , Paris, 75252 France
AU: Lee-Thorp, J A
EM: jlt@science.uct.ac.za
AF: Quaternary Research Centre, Dept of Archaeology, UCT, Rondebosch, Cape Town, 7701 South Africa
AU: Senut, B
EM: bsenut@mnhn.fr
AF: Dept "Histoire de la Terre", MNHN, USM 203 et CNRS-UMR 5143, case 38, 57 rue Cuvier, Paris, 75231 France
AU: Pickford, M
EM: pickford@mnhn.fr
AF: College de France, CNRS-UMR 5143, 11 place Marcellin Berthelot, Paris, 75005 France
AB: The Namib is one of the oldest known desert systems, hence its formation and maintenance are of considerable regional and global importance. Fluctuations in Atlantic climate and oceanic circulation patterns directly affect the ecology of the Namibian desert, and the entire South African west coast area, on a variety of timescales. The discovery of mammalian fossils and ratite eggshells has provided a biochronology, while the ratite eggshells have allowed stable oxygen and carbon isotope studies of this material to pinpoint the plant dietary base of these birds and to provide clues about changing environments through time. The results of the existing multidisciplinary study provide a sequence showing that during the lower Miocene, prior to 17 Ma, (fluviatile environments, terraces of the Proto-Orange river) a tropical/subtropical climate existed. The onset of aridification is correlated with appearance of the first aeolianites in the basal middle Miocene period. In the Southern Namib, cross-stratification in aeolianites represent the lower slipface of crescent dunes. The predominant northeast-northwest azimuths of the cross-bedded strata indicate deposition under a prevailing southerly wind regime. This regime remained in place throughout the Neogene. The aeolianites of the Northern Namib present different lithologies from those in the south: sand sheet deposits rich in cemented roots (stabilization phase of the arid system), cross-stratification of complex dunes and deflation surfaces related to water flowing from nearby inselbergs. The azimuths of these cross-bedded strata indicate deposition under westerly/southerly winds and also easterly and northeasterly winds during the Late Miocene (at present the bergwinds produce similar conditions). Isotopic data from the ratite eggshells shows advance of C4 grasses in this region at the end of the Miocene, and emergent differences in the plant cover of the southern versus northern sectors. Eggshell carbon isotope composition during the C3- period (prior to the spread of any C4 grasses) tracks isotopic variability of C3 plants relative to recently estimated low global atmospheric CO2 concentrations and pluviometry. Comparisons between continental Namib data with oceanic data (oxygen isotopes in benthic foraminifera, and the sea level curve) provide a broader global framework for the history of this desert. Two aridification periods appear to have occurred: (i) during the Middle Miocene (contemporaneous with establishment of the Antarctic ice-cap), and (ii) intensification of aridity during the Lower Pliocene (related to growth of the Arctic Ice Cap and upwelling of the Benguela cold water cells).
DE: 9305 Africa
DE: 9604 Cenozoic
DE: 3344 Paleoclimatology
DE: 1040 Isotopic composition/chemistry
DE: 1600 GLOBAL CHANGE (New category)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting