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

Abstract

Summary An understanding of how seasonal and longer-term δ 18 O signals in meteoric precipitation (δ 18 O p ) are modified by percolation through soils is essential to link temporal changes in speleothem δ 18 O 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 δ 18 O (δ 18 O sw ) values at a depth of 60 cm through the year are only 14% of the range in δ 18 O p , implying substantial homogenisation and attenuation of seasonal signals. A striking feature is that δ 18 O sw 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 δ 18 O 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 δ 18 O sw as a function of air temperature and δ 18 O p , 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 18 O linked to evaporation and summer moisture deficits. Our model reproduces the observed δ 18 O sw results, and produces δ 18 O outputs in excellent agreement with δ 18 O data for two monitored drip-water sites at La Garma cave that exhibit seasonal δ 18 O 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 δ 18 O sw 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 δ 18 O in cave drip-sites.