|
|
Ammar, F. H., Deschamps, P., Chkir, N., Zouari, K., Agoune, A., & Hamelin, B. (2020). Uranium isotopes as tracers of groundwater evolution in the Complexe Terminal aquifer of southern Tunisia. Quaternary International, 547, 33–49.
Abstract: The Complexe Terminal (CT) multi-layer aquifer is formed by Neogene/Paleogene sand deposits, Upper Senonian (Campanian-Maastrichtian limestones) and Turonian carbonates. The chemical composition and isotopes of carbon and uranium were investigated in groundwater sampled from the main hydrogeological units of the (CT) aquifer in southern Tunisia. We paid special attention to the variability of uranium contents and isotopes ratio (234U/238U) to provide a better understanding of the evolution of the groundwater system. Uranium concentrations range from 1.5 to 19.5 ppb, typical of oxic or mildly reducing conditions in groundwaters. The lowest concentrations are found southeast of the study area, where active recharge is supposed to take place. When looking at the isotope composition, it appears that all the samples, including those from carbonate levels, are in radioactive disequilibrium with significant 234U excess. A clear-cut distinction is observed between Turonian and Senonian carbonate aquifers on the one hand, with 234U/238U activity ratios between 1.1 and 1.8, and the sandy aquifer on the other hand, showing higher ratios from 1.8 to 3.2. The distribution of uranium in this complex aquifer system seems to be in agreement with the lithological variability and are ultimately a function of a number of physical and chemical factors including the uranium content of the hosting geological formation, water-rock interaction and mixing between waters having different isotopic signatures. Significant relationships also appear when comparing the uranium distribution with the major ions composition. It is noticeable that uranium is better correlated with sulfate, calcium and magnesium than with other major ions as chloride or bicarbonate. The 14C activities and δ13C values of DIC cover a wide range of values, from 1.1 pmc to 30.2 pmc and from −3.6‰ to −10.7‰, respectively. 14C model ages estimated by the Fontes and Garnier model are all younger than 22 Ka and indicate that the recharge of CT groundwater occurred mainly during the end of the last Glacial and throughout the Holocene.
|
|
|
|
Brook, G. A., Railsback, L. B., & Marais, E. (2011). Reassessment of carbonate ages by dating both carbonate and organic material from an Etosha Pan (Namibia) stromatolite: Evidence of humid phases during the last 20ka. Quaternary International, 229(1), 24–37.
Abstract: Previous research on lacustrine stromatolites from Etosha Pan in Namibia obtained ages on carbonate close to or beyond the limits of radiocarbon dating. These ages suggested that the basin was likely not subject to extensive flooding during the last ca. 40ka. This study shows that AMS radiocarbon ages for the carbonate of a stromatolite from Poacher’s Point are 15–21ka older than ages for organic material in the stromatolite structure. Calibrated ages range from 30 to 40ka for carbonate and 3–19ka for the organic residue. The new ages, together with petrographic and isotopic data for the stromatolite, have provided important new information on past flooding of Etosha Pan including evidence of prolonged lacustrine conditions during the Holocene Climatic Optimum.
|
|
|
|
Stone, A. (2012). Recharge investigations above the Stampriet Aquifer in semi-arid Namibia using geochemical methods and environmental tracers; sand, salt and water. Quaternary International, 279-280, 470–471.
|
|
|
|
Brook, G. A., Marais, E., Srivastava, P., & Jordan, T. (2007). Timing of lake-level changes in Etosha Pan, Namibia, since the middle Holocene from OSL ages of relict shorelines in the Okondeka region. Quaternary International, 175(1), 29–40.
Abstract: In 2003 examination of aerial photographs revealed a series of previously unknown relict shorelines on the arcuate ridge, possibly a clay lunette dune, that marks the western boundary of Etosha Pan in Namibia. The shorelines are 120–600m wide and the most prominent extend for tens of km around the lunette dune. The shorelines were examined on the ground in 2004 and an attempt was made to date the three lowest levels at ca. 5, 2.5 and 1m above the present pan surface. The OSL ages obtained indicate higher and more prolonged lake conditions than today at ca. 6.4, 4.0 and 2.1ka with the youngest shoreline sediments resting on an ancient pan surface dating to ca. 13ka. The evidence indicates dry conditions in the pan at ca. 13ka, wetter conditions and higher lake levels in the middle Holocene followed by a decline in lake levels to the present. Periods of inundation were of sufficient duration to produce shorelines at the southwestern end of the pan due to the prevailing northeasterly winds that would have maximized wave action along this section of the pan margin. The Etosha findings, together with other regional paleoclimate data, suggest four periods of increased wetness in SW Africa during the Holocene at 7–5, 4.5–3.5, 2.5–1.7 and ca. 1.0ka. There is widespread evidence for the oldest of these periods suggesting that it was a prominent and widespread interval of wetness. Prior to ca. 8.0ka the climate may have been drier than today.
|
|
