Effect of different soil textures and saturation levels on the equilibration time of oxygen isotopes using the direct liquid-vapor equilibration method

Stable water isotopes of oxygen (d¹⁸O) are used as tracers to study soil pore water. One method to measure d¹⁸O of soil samples is the direct liquid-vapor equilibration (DLVE) method. In this method, test samples are stored in Ziploc bags and equilibrated for three days. After equilibration, the headspace gas is measured using laser spectrometry. The DLVE method requires minimum sample handling, enables direct isotopic measurements without the need of extracting the water, and is highly reliable and comparatively cheaper than other measurement methods. However, the influence of different soil textures and saturation levels on the δ¹⁸O isotope when using the DLVE method is not well understood yet. In this study, three different soil textures (sand, organic carbon rich silt and kaolinite) were oven-dried for three days at 105^°C and saturated to different saturation levels (100%, 80%, 60% and 40%) in laboratory cylinders for a week. The samples were saturated using tap water of known isotopic value and stored in Ziploc bags for different amounts of time. The samples were analyzed after 1, 2, 3, 4 and 7 days using cavity ring down spectroscopy (CRDS), and the isotopic ratios recorded after storage were compared with the isotopic measurements obtained before the sample equilibration. The resulting isotopic deviations were less than the CRDS measurement precision after one day of sample storage for sandy soil regardless of their saturation levels. Likewise, one day was also adequate for 100%, 80% and 40% saturated kaolinite, with 100% saturation allowing for up to seven days of sample storage with only small isotopic deviations (±0.43‰). Contrary to this, for organic-silty soil the required equilibration time depended on the saturation level. The findings from this laboratory-based analysis enhance the understanding of the impact of soil texture and saturation level on the DLVE method.