FRACTIONATION AND RECYCLING OF U AND TH ISOTOPES IN A SEMIARID ENDOREIC DEPRESSION OF CENTRAL SYRIA

Mesozoic and Cenozoic carbonate-rich formations including Late Cretaceous phosphate deposits are exposed in a structural depression which is known as the Palmyra aulacogen. The bottom of this depression has been occasionally filled by water bodies during Quaternary humid episodes. The bottom of the depression now forms a sabkha as a consequence of modern semi-arid climatic conditions. Isotopic activity ratios and concentrations of U and Th were measured in bedrock formations, ground-waters, unsaturated soils, and in Quaternary lacustrine sediments cored in the sabkha in order to investigate the processes governing the fractionation and recycling of U and Th within the endoreic depression. The bedrock samples are close to secular equilibrium with the exception of phosphorites which show a 234U deficit ( 234U 238U = 0.83 ± 0.04). Upper soil horizons have a strong excess of 230Th ( 230Th 234U = 1.7 ± 0.2) as a consequence of U removal. The U loss occurs without fractionation ( 234U 238U = 1.03 ± 0.01) as it is co-dissolved with the CaCO3 matrix; this is also shown by an inverse relationship between CaCO3 vs 232Th in soils. Vertical profiles indicate transitional retention of U in the lower unsaturated soil horizons, where deficiency in 230Th ( 230Th 234U as low as 0.37 ± 0.02) and high U concentrations (up to ca. 100 μ · g t̄) characterize "secondary" co-precipitation of U and CaCO3 which forms in response to downward decreasing pCO2 during water percolation events. A rate of U removal of ca. 10-4 to 10-5 a-1 from the upper soil horizons and a residence time of approximately 40 to 50 ka of soil particles in the U-ablation zone were calculated using a simplified mathematical model based on the assumption that 230Th remains immobile in the soils. The residence time of U corresponds to a rate of erosion of ca. 1 cm for 103 a. The activity ratios of the most recent lacustrine sediments of the sabkha (upper 2 m in cores) define a well correlated (r > 0.97) "dilution curve" of the form: [ 230Th 234U = (1.91 ± 0.03) ( 234U 232Th-1 + 0.15 ±0.01]. This indicates that the radioisotopes inherited with the detrital fraction were diluted by an authigenic/early diagenetic phase of U fixation. A residence time for the diluting U (ca. 18,000 a) can be evaluated from the asymptote value (0.15 ± 0.01). This age corresponds to the last humid episode recorded in central Syria (the equivalent of the last glacial maximum at higher latitudes) which was associated with lacustrine sedimentation in the basin. Detrital particles eroded from the surrounding soils were then characterized by 238U/232Th and 230Th/238U ratios lower than those observed in modern upper soil horizons: i.e., by a longer residence time and a higher U-leaching rate during their pedologic cycle. © 1990.