科研之友
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摘要
An understanding of how seasonal and longer-term delta O-18 signals in meteoric precipitation (delta O-18(p)) are modified by percolation through soils is essential to link temporal changes in speleothem delta O-18 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 delta O-18 (delta O-18(sw)) values at a depth of 60 cm through the year are only 14% of the range in delta O-18(p), implying substantial homogenisation and attenuation of seasonal signals. A striking feature is that delta O-18(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 delta O-18 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 delta O-18(sw) as a function of air temperature and 6180, 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 180 linked to evaporation and summer moisture deficits. Our model reproduces the observed delta O-18(sw) results, and produces delta O-18 outputs in excellent agreement with delta O-18 data for two monitored drip-water sites at La Garma cave that exhibit seasonal delta O-18 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 delta O-18(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 delta O-18 in cave drip-sites.
- 出版日期2015-11
