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1. Cryptic indicators of provenance from the geochemistry of the Okavango Delta sediments, Botswana

Author

P. Huntsman-Mapila, Bernard Vink, Susan Ringrose, Ali B Kampunzu, Domaines Océaniques (LDO), and Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Observatoire des Sciences de l'Univers-Institut d'écologie et environnement-Centre National de la Recherche Scientifique (CNRS)

Subjects
Provenance, Felsic, Botswana, Stratigraphy, Arenite, Geochemistry, Geology, Weathering, 15. Life on land, 010501 environmental sciences, 010502 geochemistry & geophysics, 01 natural sciences, Diagenesis, Source rock, 13. Climate action, [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry, Okavango Delta, Sedimentary rock, Siliciclastic, 0105 earth and related environmental sciences
Abstract

The siliciclastic sediments of the Okavango inland Delta of northwest Botswana have a modal composition of quartz arenites and result from a complex history, including transport by river and deposition in a nascent rift basin located in a desert environment with input of aeolian sands. The geochemical composition of sediments from the Okavango Delta was determined in order to constrain the role of weathering at the source and the composition of the source rocks. The chemical analyses and the interelement ratios show a broad compositional range usually encompassing the PAAS composition. The chemical index of alteration (CIA) values and the A–CN–K diagram define an evolution trend which can be interpreted using a mixing model involving a strongly weathered component which corresponds to the sedimentary fraction transported by the Okavango River and a relatively immature component which corresponds to the aeolian sand component of the Okavango sediments. Field geological data supported by geochemical ratios involving elements with affinity for mafic–ultramafic and felsic rocks such as Th/Cr, Th/Sc, La/Sc, La/Co and Eu/Eu* support a source area including mafic–ultramafic and felsic rocks, with or without intermediate rocks. The relationships between certain elements (Cr–Ni, Na2O–Al2O3, K2O–Al2O3) refine the interpretation by pointing to the existence of at least three source rock end-members, including a felsic rock source and pyroxene-rich and olivine-rich mafic–ultramafic source rocks. Proterozoic granitoid–gabbro and related volcanic and ortho-metamorphic rock complexes exposed in NW Botswana and adjacent Angola and Namibia are the source rocks of the sediment component which was mixed with aeolian sand and interacted with a variable proportion of diagenetic carbonates to produce the Okavango sediments.

Published
2005

2. Determinants of woody cover in African savannas

Author

Alioune Diouf, Steven I. Higgins, Mahesh Sankaran, Feetham Banyikwa, Jonas Ardö, William B. Sea, Niall P. Hanan, Nick Zambatis, Xavier Le Roux, Susan Ringrose, Herbert H. T. Prins, Kelly K. Caylor, Andries Bronn, Jacques Gignoux, Jörg Tews, Gabriela Bucini, Pierre Hiernaux, Brian S. Cade, Halszka Hrabar, Christie J. Feral, Michael B. Coughenour, Jayashree Ratnam, David J. Augustine, Edmund C. February, Robert J. Scholes, Kristine L. Metzger, Jeff Worden, Peter G. H. Frost, Wellington N. Ekaya, Fulco Ludwig, Natural Resource Ecology Laboratory [Fort Collins] (NREL), Colorado State University [Fort Collins] (CSU), Division of Forest Science and Technology, USDA Forest Service, Commanche National Grassland, US Geological Survey [Fort Collins], United States Geological Survey [Reston] (USGS), Fonctionnement et évolution des systèmes écologiques (FESE), École normale supérieure - Paris (ENS-PSL), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Centre National de la Recherche Scientifique (CNRS), Helmholtz Zentrum für Umweltforschung = Helmholtz Centre for Environmental Research (UFZ), Laboratoire d'Ecologie Microbienne - UMR 5557 (LEM), Institut National de la Recherche Agronomique (INRA)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Ecole Nationale Vétérinaire de Lyon (ENVL)-VetAgro Sup - Institut national d'enseignement supérieur et de recherche en alimentation, santé animale, sciences agronomiques et de l'environnement (VAS)-Centre National de la Recherche Scientifique (CNRS), CSIRO Centre for Environment and Life Sciences, Commonwealth Scientific and Industrial Research Organisation [Canberra] (CSIRO), Department of Physical Geography and Ecosystem Science [Lund], Lund University [Lund], Department of Botany, University of Dar es Salaam (UDSM), Department of Agriculture and Game Management, Port Elizabeth Technikon, Department of Civil and Environmental Engineering, Princeton University, Centre de Suivi Ecologique [Dakar] (CSE), Department of Range Management, University of Nairobi (UoN), Environmental Sciences Department, University of Virginia, University of Cape Town, Institute of Environmental Studies, University of Zimbabwe (UZ), Centre d'études spatiales de la biosphère (CESBIO), Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Centre National de la Recherche Scientifique (CNRS)-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE), University of Pretoria [South Africa], Department of Zoology, University of Wisconsin-Madison, Resource Ecology Group, Wageningen University and Research [Wageningen] (WUR), Harry Oppenheimer Okavango Research Center, University of Botswana, Plant Ecology & Nature Conservation, University of Potsdam = Universität Potsdam, Scientific Services, Kruger National Park, Natural Resource Ecology Laboratory, University of Colorado Boulder [Boulder], United States Geological Survey [Reston] ( USGS ), Fonctionnement et évolution des systèmes écologiques ( FESE ), École normale supérieure - Paris ( ENS Paris ) -Université Pierre et Marie Curie - Paris 6 ( UPMC ) -Centre National de la Recherche Scientifique ( CNRS ), Umweltforschungszentrum Leipzig Halle, Sekt Okosyst Anal, Ecologie microbienne ( EM ), Centre National de la Recherche Scientifique ( CNRS ) -Ecole Nationale Vétérinaire de Lyon ( ENVL ) -Université Claude Bernard Lyon 1 ( UCBL ), Université de Lyon-Université de Lyon-Institut National de la Recherche Agronomique ( INRA ) -VetAgro Sup ( VAS ), Commonwealth Scientific and Industrial Research Organisation [Canberra] ( CSIRO ), Department of Physical Geography and Ecosystems Analysis [Lund], University of Dar es Salaam ( UDSM ), Centre de Suivi Ecologique, University of Nairobi ( UoN ), University of Virginia [Charlottesville], University of Zimbabwe, Centre d'études spatiales de la biosphère ( CESBIO ), Université Paul Sabatier - Toulouse 3 ( UPS ) -Institut national des sciences de l'Univers ( INSU - CNRS ) -Observatoire Midi-Pyrénées ( OMP ) -Centre National d'Etudes Spatiales ( CNES ) -Centre National de la Recherche Scientifique ( CNRS ), Mammal Research Institute, University of Wisconsin-Madison [Madison], Wageningen University and Research Centre [Wageningen] ( WUR ), Potsdam University, École normale supérieure - Paris (ENS Paris), Université de Lyon-Université de Lyon-Centre National de la Recherche Scientifique (CNRS)-VetAgro Sup - Institut national d'enseignement supérieur et de recherche en alimentation, santé animale, sciences agronomiques et de l'environnement (VAS)-Ecole Nationale Vétérinaire de Lyon (ENVL), Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE)-Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Université Fédérale Toulouse Midi-Pyrénées-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS), Universität Potsdam, École normale supérieure - Paris (ENS Paris)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Centre National de la Recherche Scientifique (CNRS), Helmholtz Zentrum für Umweltforschung (UFZ), Centre National de la Recherche Scientifique (CNRS)-Ecole Nationale Vétérinaire de Lyon (ENVL)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Institut National de la Recherche Agronomique (INRA)-VetAgro Sup - Institut national d'enseignement supérieur et de recherche en alimentation, santé animale, sciences agronomiques et de l'environnement (VAS), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Observatoire Midi-Pyrénées (OMP), Université Fédérale Toulouse Midi-Pyrénées-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS), and Wageningen University and Research Centre [Wageningen] (WUR)

Subjects
0106 biological sciences, Canopy, 010504 meteorology & atmospheric sciences, quantile regression, Rain, Biome, Plant Ecology and Nature Conservation, [SDV.BID]Life Sciences [q-bio]/Biodiversity, Poaceae, 010603 evolutionary biology, 01 natural sciences, [ SDV.EE ] Life Sciences [q-bio]/Ecology, environment, Trees, Soil, DISPONIBILITE D'ELEMENT NUTRITIF, vegetation, Animals, Ecosystem, Biomass, 0105 earth and related environmental sciences, [ SDV.BID ] Life Sciences [q-bio]/Biodiversity, [SDV.EE]Life Sciences [q-bio]/Ecology, environment, Herbivore, [ SDE.BE ] Environmental Sciences/Biodiversity and Ecology, mechanisms, Multidisciplinary, WIMEK, SUBTANCE NUTRITIVE, Ecology, FEU, Vegetation, 15. Life on land, stability, PE&RC, Arid, Wood, Geography, Disturbance (ecology), Wildlife Ecology and Conservation, ZONE SEMI-ARIDE, Africa, Plant cover, Plantenecologie en Natuurbeheer, systems, Desert Climate, [SDE.BE]Environmental Sciences/Biodiversity and Ecology, fire, tree-grass coexistence
Abstract

Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than approximately 650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered 'stable' systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of approximately 650 mm, savannas are 'unstable' systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation may considerably affect their distribution and dynamics.

Published
2005

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