Experimental Study on Isotope Fractionation of Soil water in Arid Environments

Abstract:

Soil water dynamics within a thick vadose (unsaturated) zone is a key component in the hydrologic cycle in arid regions. The partitioning of precipitation and soil water into fluxes of percolation to the subsurface, surface runoff and evapotranspiration at the land-atmosphere-vegetation interface is accompanied by characteristic δ²H and δ¹⁸O values of water. The isotopic composition of the transpiration flux is very similar to soil water, since the uptake by plant roots is usually not associated with isotope fractionation. The isotopic composition of evaporation flux from unsaturated soils, which becomes an important flux in arid regions, has extensively been modeled by the Craig-Gordon model with the assumption that equilibrium isotopic fractionation between adsorbed/pored condensed water within soils and water vapor is identical to that between bulk liquid water and vapor. To test this critical assumption, we have conducted laboratory experiments on equilibrium isotope fractionation between adsorbed water in mesoporous silica (15nm pore, as soil analog) and the vapor. Firstly, the adsorption/desorption isotherms of H₂O and N₂ in the silica are determined at 30°C and liq-N₂ temperature, respectively, and large hysteresis loops were observed. Secondly, the isotope fractionation factors between condensed water in the silica pores and the vapor (¹⁸α_{silica water-vapor} and ²α_{silica water-vapor} for oxygen and hydrogen isotopes, respectively) were determined at 30°C along the adsorption curve from near saturation pressure (p/p_o=1). We found that ¹⁸α_{silica water-vapor} values are smaller than that between free liquid-vapor (1.0088) and progressively decreased from1.0083 at p/p_o= 0.9 to 1.0054 at p/p_o=0.5, establishing a trend very similar to the isotherm curve. The corresponding ²α_{silica water-vapor} values are also smaller than that of free liquid-vapor system (1.0740) and decreased from 1.0651 at p/p_o=0.9 to 1.0295 at p/po=0.5. Our experimental results challenge the long-held assumption and the finding needs to be considered in the interpretation and modeling of the isotopic compositions of soil water and paleoclimate proxies in arid regions.