Geosciences Dept. Seminar: Dynamics of Water Soil storage in the Unsaturated Zone of Ashdod, Israel, traced by humidity and carbon isotopes

Abstract: 

In October 2010 a 22m sediment core was collected from the Unsaturated Zone (USZ) on the Israeli coastal plain, in the dunes, south of Ashdod.  The mineralogy of the sediment was dominantly quartz sands with ancillary clay and carbonate.The local climate is semiarid with winter precipitation only. The vegetation, of C3 type plants, is sparse. It is composed mostly of Retama raetam, whose roots can reach in excess of 8m. Many land snails are found living upon it. The soil moisture (%) and CO2 (from the dissolved inorganic carbon [DIC]) were quantitatively abstracted from the sediment. The porosity was high, ranging from ~40 to 55%.   The DIC ranged from 2 to 13 mmole C /liter.  Nearby to the site of the sediment core, it was found that rainfall recharge traverses the entire sediment profile (22m) in a single season in a piston-type mode (Rimon et.al. 2007). The present core was sampled in a low rainfall year, which followed an even sparser rainfall year. Large variability is noted in the extracted moisture in the field capacity with a maximum at the high clay region. The oxygen isotopes enter the unsaturated zone with rain from the surface. The soil moisture shows δ18O enrichment in the top 6 meters due to evaporation during the 5 months sojourn following the winter rains. Below this depth δ18O values of ~ -5‰(smow) prevail, as in the local aquifer, where it is similar to the rain recharge values. The CO2 and the carbon isotopes (δ13C and Δ14C) enter the USZ laterally from exhalation from plant roots.  The δ13C varies from -5‰ to -23‰(PDB), which represent the various stages in carbon isotopic fractionation that have taken place: CO2gas at 8 m becomes dissolved CO2aqueous which then forms carbonic acid and HCO3- and perhaps suggests CaCO3 precipitation at certain levels. The carbonic acid may dissolve allogenic carbonate within the USZ. These processes occur in distinct isotopic steps, preserved in the field capacity which is "frozen in" during the summer. The high humidity, δ13C and Δ14C noted at 8m depth from root exhalation of the flora. The almost equally high humidity, δ13C, Δ14C at the 22m depth and of the DIC at 20.5most probably represents those of 8m depth from the previous 2008/9 rainy season that was subsequently pushed down 13.4±0.7 meters during the winter of 2009/10. The lowermost Δ14C peak was retained in the field capacity for almost two rainy seasons.

Seminar Organizer: Prof. Eyal Haifetz