Data-model comparison of soil–water δ18O at a temperate site in N. Spain with implications for interpreting speleothem δ18O

An understanding of how seasonal and longer-term δ18O signals in meteoric precipitation (δ18Op) are modified by percolation through soils is essential to link temporal changes in speleothem δ18O to surface climatic conditions. This study focuses on modifications that occur in a relatively thick soil above a temperate cave site (La Garma, N. Spain). Monthly soil–water δ18O (δ18Osw) values at a depth of 60 cm through the year are only 14% of the range in δ18Op, implying substantial homogenisation and attenuation of seasonal signals. A striking feature is that δ18Osw values at 60 cm depth are lowest in summer and highest in winter, the opposite (anti-phase) to that observed in rainfall. Soil–water residence times of up to circa 6 months in the upper 60 cm of soil, and a matrix flow, piston-type infiltration behaviour with mixing are inferred. Evaporative effects on recovered soil–water δ18O are minimal at this wet temperate site, in contrast with published results from arid and semi-arid sites. A soil–water model is presented to estimate monthly δ18Osw as a function of air temperature and δ18Op, incorporating effects such as variations in the amount of infiltrated water, changes in the ratio between evaporation and transpiration, mixing with antecedent soil moisture and small enrichments in 18O linked to evaporation and summer moisture deficits. Our model reproduces the observed δ18Osw results, and produces δ18O outputs in excellent agreement with δ18O data for two monitored drip-water sites at La Garma cave that exhibit seasonal δ18O variability. We conclude that simple evapotranspiration models that permit infiltration during months that have a positive hydrological balance only, tend to underestimate summer rainfall contributions. Overall, the study provides an improved framework for predicting δ18Osw trends at temperate sites such as La Garma that have a relatively thick soil cover, as well as for understanding seasonal ranges and trends in δ18O in cave drip-sites.