INFLUENCE OF SAND GRAIN SIZE AND DRYING TEMPERATURE ON OXYGEN ISOTOPIC COMPOSITION OF WATER EVAPORATED FROM SOILS

Yu.K. Vasil’chuk^a,b,#, L.B. Bludushkina^b,##, and N.A. Budantseva^a,###

^aFaculty of Geography, Lomonosov Moscow State University, Leninskie Gory 1, Moscow, 119234 Russia

^bFaculty of Geology, Lomonosov Moscow State University, Leninskie Gory 1, Moscow, 119234 Russia

^# E-mail: vasilch_geo@mail.ru
^## E-mail: bludushkina19@mail.ru
^### E-mail: nadin.budanceva@mail.ru

Isotopic composition of pore water is a sensitive monitor of water dynamics in soils during evaporation. The purpose of this study is to reveal the influence of sand grain size and drying temperature on the rate of water evaporation and oxygen isotopic composition of water evaporated from soils, as well as to determine the iso-topic composition of different moisture categories in soils. Silty and medium-grained sands were chosen as objects of research. The method of research consisted in drying disperse soils in a thermostat at different tem-peratures (180, 200, 220 °C), followed by the condensation of evaporated water and its sampling for the sub-sequent study of the isotopic composition. Isotopic analyses of water samples were performed in the isotope laboratory of the Faculty of Geography, Moscow State University, using the Delta-V mass spectrometer with a standard Gas-Bench option. For measurements, international standards V-SMOW, GISP, SLAP, own lab-oratory standards were used. Each category of water in sand, i.e., free, transitional and bound water, has a cer-tain isotopic composition. The isotopic composition of moisture categories in soils becomes heavier in δ¹⁸O values in the series: free water < transitional water < bound water. It was found that water evaporated from a more disperse soil (from silty sand) has a heavier isotopic composition compared to the same amount of water evaporated from a less disperse soil (from medium-grained sand). At a temperature of 220 °C, the effect of the sand grain size on the isotopic composition of evaporated water is not observed. Isotopic composition of the evaporated water varies depending on the evaporation temperature. For medium-grained sand portions of water evaporated at a higher temperature have a heavier isotopic composition. However, for silty sand this trend is not observed. The enrichment of evaporated water with heavy isotopes in time occurs at a constant rate, which depends on the evaporation temperature (and increases linearly with increasing temperature from 180 to 220°C), but it does not depend on sand grain size.

Keywords: sands, water evaporation, temperature of evaporation, isotopic composition, stable isotopes fractionation

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