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Comparing Carbonate and Organic Ams-14C Ages in Lake Abiyata Sediments (Ethiopia): Hydrochemistry and Paleoenvironmental Implications

Published online by Cambridge University Press:  18 July 2016

Elisabeth Gibert
Affiliation:
CNRS-UMR 8616, Equipe «Hydrologie, Paléohydrologie et Paléoenvironnement», Université Paris-Sud, Bâtiment 504, 91405 Orsay cedex, France. Email: egibert@geol.u-psud.fr
Yves Travi
Affiliation:
Laboratoire d'Hydrogéologie, Département de Géologie, Faculté des Sciences d'Avignon, 33 rue Louis Pasteur, 84000 Avignon, France
Marc Massault
Affiliation:
CNRS-UMR 8616, Equipe «Hydrologie, Paléohydrologie et Paléoenvironnement», Université Paris-Sud, Bâtiment 504, 91405 Orsay cedex, France. Email: egibert@geol.u-psud.fr
Tesfaye Chernet
Affiliation:
Laboratoire d'Hydrogéologie, Département de Géologie, Faculté des Sciences d'Avignon, 33 rue Louis Pasteur, 84000 Avignon, France
Florent Barbecot
Affiliation:
CNRS-UMR 8616, Equipe «Hydrologie, Paléohydrologie et Paléoenvironnement», Université Paris-Sud, Bâtiment 504, 91405 Orsay cedex, France. Email: egibert@geol.u-psud.fr
Fatima Laggoun-Defarge
Affiliation:
Laboratoire de Géologie de la Matière Organique, Université d'Orléans, 45000 Orleans, France
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Abstract

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We studied a 12.6-m-long sequence from Lake Abiyata (Central Ethiopia) to establish a reliable and accurate chronology for use in global paleoclimatic reconstructions. The 26 accelerator mass spectrometry radiocarbon (AMS 14C) ages, performed on carbonates and organic matter, define 2 parallel chronologies, representing the complete Holocene period. However, these chronologies show a significant discrepancy from 500 to 900 BP in depth; ages obtained on carbonates were always older than those on organic matter. The hydrogeological and geochemical behavior of the Lake Abiyata basin has shed light on this discrepancy. We found that the carbonate crystallization is due mainly to the mixing of lake waters with groundwaters from the multi-layered aquifer contained in the 600-m-thick basement of the lake. The 14C activity of total dissolved inorganic carbon (TDIC) measured by AMS from bottom and surface lake waters (111.4 and 111.8 pMC, respectively) confirms that the mixing occurs at the water-sediment interface. This evidence of groundwater participation in the carbonate crystallization calls into question the current paleoclimatic reconstructions based on inorganic carbonates in lakes. Specific attention should thus be given to the respective proportions of each end-member in the mixing for the quantitative estimation of the groundwater input. This will help to validate the paleoenvironmental reconstructions and to highlight an eventual diagenetical evolution of inorganic carbonates during burial, via the study of pore waters.

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Articles
Copyright
Copyright © 1999 by the Arizona Board of Regents on behalf of the University of Arizona 

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