DYNAMICS OF WATER SOIL STORAGE IN THE UNSATURATED ZONE OF A SAND DUNE IN A SEMI-ARID REGION TRACED BY HUMIDITY AND CARBON ISOTOPES: THE CASE OF ASHDOD, ISRAEL

摘要

A 22 m sediment core was collected from the unsaturated zone (USZ) in the dunes south of Ashdod, Israel, in a low rainfall year, following an even lower-rainfall year. The mineralogy of was quartz with some clay and carbonate. The local climate is semi-arid. The roots of the sparse vegetation can reach ~8m. The porosity was ~40-55%. The DIC ranged from 2 to 13mmole C/L. Large variability is noted in the soil moisture, with a maximum at the high clay region at ~8m. Oxygen isotopes enter the USZ with rain falling on the surface. There is ~(18)O enrichment in the top of the section due to evaporation during the summer. Below this depth δ~(18)O values of ~ -5‰ prevail as in the local aquifer. CO_2 and the carbon isotopes enter the USZ laterally by exhalation from roots. The 513C varies from -5‰ t o -23‰, representing stages in carbon isotopic fractionation: CO_2gaS becomes dissolved CCKaqueous which then forms carbonic acid and HCO_3~- . The high δ~(13)C, Δ~(14)C, and DIC are found at the 8 m depth. These values along with high humidity are replicated at ~22m depth. They most probably represent recharge water from the previous rainy season at 8 m depth subsequently pushed down 12.8 ± 0.8 m during the last winter.