Paleohydrologie isotopique de l'Oued el Akarit (sud tunisie) au Pleistocene superieur et a l'Holocene
Previous data from Gobert and Howe, Coque, and Page on the Upper Quaternary deposits from Wadi el Akarit are expanded through radiocarbon and stable isotope measurements (18C and 13C from carbonates, 18O and 34S from sulphates). These new results report on detailed stratigraphic profiles and indicate fluviatile and palustrine environments during the late Pleistocene. Vegetal dams linked to spring activity caused water stagnation. Fine-grained sediments were deposited with appreciable amounts of organic matter which locally developed into some peaty levels. In the upper part of the typical cross-section (located on the left bank about 100-m upstream of the railway bridge), Pleistocene deposits show a palustrine carbonate crust. This formation is attributed to the drainage induced by the major marine regression at glacial maximum. 
 
During the Holocene stagnation periods still occurred, interrupted by flood episodes marked by bottom transport and coarse deposits. In the typical cross-section, deposits are well drained and oxidized. However, downstream a stagnation episode takes place with a rich malacofauna dominated by Cerastoderma (“Cardium”). The uneven distribution of rainfall is documented by the alternation of fine and coarse deposits, and it is accentuated by the climatic evolution toward aridity which results in the formation of a gypsum crust after 4000 yr. BP. Later, exceptional floods removed sediments from the wadi channel, cutting the Upper Quaternary deposits for about 13 m along the central course of the wadi. 
 
Radiocarbon ages on carbonates are discussed according to various possible origins of total dissolved carbon: atmospheric CO2, organic activity in the basins and aquifer supply. 
 
A simple exchange-mixing model between aquifer bicarbonate and atmospheric CO2 is used for calculations of initial carbonate activities. Chemical, isotopic and hydraulic steady states are assumed in this treatment. Present-day 14C and 13C contents of aquifer bicarbonate are thus adopted for past values. 
 
The model leads to acceptable corrected ages for Pleistocene carbonate samples. Corrected ages are younger than apparent ages by a few thousands of years and become consistent with ages given by organic matter from nearby levels. However, Holocene samples appear overcorrected. Their crystallization appears to be controlled by modern organogenic CO2 Holocene 14C ages are probably correct. 
 
18O contents of carbonate indicate a continental origin for environmental water. Nodules and carbonate concretions crystallized in bottom sediments through upward leakage of confined aquifers. They show 18O contents compatible with present-day stable isotope compositions of artesian springs. This supports the concept of a good time constancy in stable isotope contents. Shell carbonate values indicate a variable evaporation rate from the same initial waters. 
 
18O and 34S contents of gypsums are also compatible with aqueous sulphates from springs. The data set falls within the range of Permo-Triassic sulphate and suggests deep hydraulic connections within aquifer systems. 
 
The entire formation appears to be of continental origin. The occurence of “Cardium”-rich basins can only be attributed to the initial salinity of springs waters (7–9 g 1−1), sightly increased by evaporation. This signifies that one can not deduce any tectonic uplift from their location with respect to sea level.