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Fonds National Suisse de la Recherche Scientifique (FNSRS) 2017

Assessing the coupled cycles of C and Ca in tropical environments : the significance of terrestrial carbonate deposits in limestone-free watersheds

Kalahari Carbonate Deposit

Titre : Assessing the coupled cycles of C and Ca in tropical environments : the significance of terrestrial carbonate deposits in limestone-free watersheds

Numéro  : 172944

Début/Fin : 01.10.2017 - 30.09.2021

Requérant  : Verrecchia Eric Institut des sciences de la Terre Université de Lausanne

Partenaires : Sebag David Laboratoire M2C Université de Rouen and IRD UMR 6143 CNRS Deschamps Pierre CEREGE Europôle Méditerranéen de l’Arbois Herman Frédéric Institut des dynamiques de la surface terrestre Université de Lausanne

Présentation
In the northern Kalahari desert in Botswana, enigmatic Quaternary formations enriched in carbonate are observed in the landscape. These deposits are in the geographical area of large (paleo-) rivers having their source in the Equatorial zone. In the past, the watershed was related to an inland lacustrine basin, the Makgadikgadi Lake. One of the most intriguing characteristics of these deposits is their large abundance of carbonate nodules and/or beds, in a place where the closest limestone outcrop is several hundreds to thousands of km away. Indeed, in Botswana, large areas are covered with Kalahari sands, but in some places, thick carbonate layers and paleosols unexpectedly outcrop in this predominantly quartz environment. These formations, incorrectly identified as "calcrete" in the past, seem to be palustrine/shallow lacustrine deposits, probably related to the dynamics of the (paleo-)Zambezi-Chobe and/or Linyanti rivers during the Late Quaternary.The proposed research will be conducted to tackle these objectives :(i) The various carbonate deposits need to be properly studied, as they are usually considered as calcrete instead of palustrine limestones. Consequently, the first task targets the study of the various types of deposits in the Chobe Enclave, including diatomites, fluvial and eolian sands, based on modern and conventional methods used in sedimentology/biogeochemistry. (ii) The next aim concerns the Ca sources. How did Ca accumulate enough to contribute to the precipitation of almost pure Ca-carbonate phases ? (iii) The third task, as a consequence, assesses the origin/conditions of the unexpected succession of silicate- and carbonate-rich phases : climatic, tectonic, tectono-climatic, lacustrine, palustrine, eolian ? (iv) Finally, dating of the various deposits in the Chobe Enclave will be assessed in order to propose a chronological sequence based on various methods, i.e. 14C on carbonate and organic matter, OSL, ESR, U-Th, and compare results to the Southern African chronological framework.In conclusion, the aim of the project is to propose a genetic model that can explain the unexpected presence of calcium carbonate in carbonate-free and acidic endorheic landscapes during the Late Quaternary. This research will improve our knowledge of the relationships between regional scale climatic variability and externally-forced climate change in Africa. In addition, conclusions will contribute to understanding some surprising carbonate paleosols in carbonate-free landscapes from the Earth’s deep time and their potential relationships with atmospheric pCO2 and life.

Mots clés : Calcium cycle ; Calcium carbonate ; Calcium oxalate ; Dating methods ; Stable isotopes ; Terrestrial carbonates ; Africa ; Quaternary

Financement : 281’304.00

Fonds National Suisse de la Recherche Scientifique

Page publiée le 7 août 2018