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 9th IAMAS
Scientific
Assembly
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Stable isotope records of Holocene environmental changes from Moroccan Lakes: an emerging synthesis
Abdelfattah Benkaddour, Henry Lamb, Melanie Leng, Françoise Gasse
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Lacustrine sediment is one of the most interesting and reliable non-marine paleo-archives that Morocco can provide for studying climate variability and change in North Africa. The Moroccan mountains (High and Middle-Atlas) contain several small basins with natural lakes. Their geographic position makes these water bodies generally safe from anthropogenic disturbance. Fluctuations in the balance between precipitation and evaporation are translated directly into changes in lake level and salinity. The stable isotope composition and hydrochemistry of such closed-basin lakes respond directly to changes in the water budget through evaporation and concentration of dissolved salts. The sediments of these lakes contain several excellent fossils and geochemical proxies; some of which have the potential for quantitative reconstruction of past water chemistry. However, because of the small size of these lacustrine systems (few hectares to 3 km2), the local effects are amplified. These effects should thus be distinguished from climatic changes.
Five lacustrine sites with eight lakes were investigated. Isotopic contents of meteoric and groundwater show that the rain-producing air masses in the Atlas Mountains have two origins Atlantic and Mediterranean. The Local Meteoric Water Line in the Atlas is approximately ?2H = 8.07 ?18O + 13. Isotopic contents and chemistry of the lakes water are related to the local groundwater. Most of the lakes are fed by the local aquifers and seep by small downstream springs.
Chloride and 18O in lacustrine water reveal a clear evaporation effect. 18O enrichment varies from about 1‰ (Ifni lake) to about 14‰ (Tislit lake). This allowed us to classify the Moroccan lakes in 3 residence time categories: Short (Ifni), intermediate (Tigalmamine and Azigza) and long (Tislit, Isli and Sidi-Ali).
13C and 18O contents of TDIC and biogenic carbonates are in close relation with the water residence time and the water isotopic composition. Isotopic contents of presumed authigenic carbonate are to be taken with care. Moroccan lakes will have been determined by different controls, and they will have responded differently to the same climate forcing.
Keywords: MOROCCO, LAKES, ISOTOPES, HOLOCENE, PALEOCLIMATE
Abdelfattah Benkaddour, Université Cadi Ayyad, Faculté des Sciences et Techniques, 40 000 Marrakech, Morocco, benkaddour@fstg-marrakech.ac.ma
Henry Lamb, University of Wales, Inst. of Earth Studies, SY23 3DB Aberystwyth, UK
Melanie Leng, NERC Isotope Geosciences Laboratory, Nottingham NG12 5GG, UK
Françoise Gasse, CEREGE, 13545 Aix-En-Provence, France
Session: Future Change: Historical Understanding
Sub-Theme: Africa
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