H-2 and O-18 depletion of water close to organic surfaces

Hydrophilic surfaces influence the structure of water close to them and may thus affect the isotope composition of water. Such an effect should be relevant and detectable for materials with large surface areas and low water contents. The relationship between the volumetric solidaEuro-:aEuro-water ratio and the isotopic fractionation between adsorbed water and unconfined water was investigated for the materials silage, hay, organic soil (litter), filter paper, cotton, casein and flour. Each of these materials was equilibrated via the gas phase with unconfined water of known isotopic composition to quantify the isotopic difference between adsorbed water and unconfined water. Across all materials, isotopic fractionation was significant (p < 0.05) and negative (on average -0.91aEuro-+/- aEuro-0.22aEuro-aEuro degrees for O-18a center dot 16 and -20.6aEuro-+/- aEuro-2.4aEuro-aEuro degrees for H-2a center dot 1 at an average solidaEuro-:aEuro-water ratio of 0.9). The observed isotopic fractionation was not caused by solutes, volatiles or old water because the fractionation did not disappear for washed or oven-dried silage, the isotopic fractionation was also found in filter paper and cotton, and the fractionation was independent of the isotopic composition of the unconfined water. Isotopic fractionation became linearly more negative with increasing volumetric solidaEuro-:aEuro-water ratio and even exceeded -4aEuro-aEuro degrees for O-18a center dot 16 and -44aEuro-aEuro degrees for H-2a center dot 1. This fractionation behaviour could be modelled by assuming two water layers: a thin layer that is in direct contact and influenced by the surface of the solid and a second layer of varying thickness depending on the total moisture content that is in equilibrium with the surrounding vapour. When we applied the model to soil water under grassland, the soil water extracted from 7 and 20aEuro-cm depth was significantly closer to local meteoric water than without correction for the surface effect. This study has major implications for the interpretation of the isotopic composition of water extracted from organic matter, especially when the volumetric solidaEuro-:aEuro-water ratio is larger than 0.5 or for processes occurring at the solid-water interface.