Data-model comparison of soil–water δ 18 O at a temperate site in N. Spain with implications for interpreting speleothem δ 18 O

An understanding of how seasonal and longer-term d18O signals in meteoric precipitation (d18Op) are modified by percolation through soils is essential to link temporal changes in speleothem d18O 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 d18O (d18Osw) values at a depth of 60 cm through the year is only 14% of the range in d18Op, implying substantial homogenisation and attenuation of seasonal signals. A striking feature is that d18Osw 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 is inferred. Evaporative effects on recovered soil-water d18O 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 d18Osw as a function of air temperature and d18Op, 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 d18Osw results, and produces d18Osw outputs in excellent agreement with d18O data for two monitored drip-water sites at La Garma cave that exhibit seasonal d18O variability. We conclude that simple evapotranspiration models that permit infiltration during months that have a positive hydrological balance only tend to under-estimate summer rainfall contributions. Overall, the study provides an improved framework for predicting d18Osw 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 d18O in cave drip-sites

Illuminating hydrological processes at the soil-vegetation-atmosphere interface with water stable isotopes

Funded by DFG research project “From Catchments as Organised Systems to Models based on Functional Units” (FOR 1Peer reviewedPublisher PDFPublisher PD

Laser Emission from Electrospun Polymer Nanofibers

A study was conducted to demonstrate lasing action by composite electrospun polymer nanofibers emitting in the visible and near-infrared (NIR) spectral region. It also demonstrated the significant potential of electrospun and flexible gain nanofibers as active waveguides and lasers. Single-mode optically pumped lasing sources on individual electrospun nanofibers emitting at 585 nm with linewidth 0.3 nm and excitation threshold fluence of 60 μJ cm -2 were demonstrated as a prototype system. The single fiber surface was found to exhibit a root-mean-square roughness of about 6 nm using atomic force microscopy (AFM). The fibers were tested for laser emission and it was revealed that each fiber constituted a Fabry-Pérot cavity for spectral selection and light amplification, due to the high confinement of the emitted light

Comparisons of observed and modelled lake δ18O variability

With the substantial number of lake sediment δ18O records published in recent decades, a quantitative, process-based understanding of these systems can increase our understanding of past climate change. We test mass balance models of lake water δ18O variability against five years of monthly monitoring data from lakes with different hydrological characteristics, in the East-Midlands region of the UK, and the local isotope composition of precipitation. These mass balance models can explain up to 74% of the measured lake water isotope variability. We investigate the sensitivity of the model to differing calculations of evaporation amount, the amount of groundwater, and to different climatic variables. We show there is only a small range of values for groundwater exchange flux that can produce suitable lake water isotope compositions and that variations in evaporation and precipitation are both required to produce recorded isotope variability in lakes with substantial evaporative water losses. We then discuss the potential for this model to be used in a long-term, palaeo-scenario. This study demonstrates how long term monitoring of a lake system can lead to the development of robust models of lake water isotope compositions. Such systematics-based explanations allow us to move from conceptual, to more quantified reconstructions of past climates and environments

Water-stable isotopes in the LMDZ4 general circulation model: Model evaluation for present-day and past climates and applications to climatic interpretations of tropical isotopic records

International audienceWe present simulations of water-stable isotopes from the LMDZ general circulation model (the LMDZ-iso GCM) and evaluate them at different time scales (synoptic to interannual). LMDZ-iso reproduces reasonably well the spatial and seasonal variations of both delta O-18 and deuterium excess. When nudged with reanalyses, LMDZ-iso is able to capture the synoptic variability of isotopes in winter at a midlatitude station, and the interannual variability in mid and high latitudes is strongly improved. The degree of equilibration between the vapor and the precipitation is strongly sensitive to kinetic effects during rain reevaporation, calling for more synchronous vapor and precipitation measurements. We then evaluate the simulations of two past climates: Last Glacial Maximum (21 ka) and Mid-Holocene (6 ka). A particularity of LMDZ-iso compared to other isotopic GCMs is that it simulates a lower d excess during the LGM over most high-latitude regions, consistent with observations. Finally, we use LMDZ-iso to explore the relationship between precipitation and delta O-18 in the tropics, and we discuss its paleoclimatic implications. We show that the imprint of uniform temperature changes on tropical delta O-18 is weak. Large regional changes in delta O-18 can, however, be associated with dynamical changes of precipitation. Using LMDZ as a test bed for reconstructing past precipitation changes through local delta O-18 records, we show that past tropical precipitation changes can be well reconstructed qualitatively but not quantitatively. Over continents, nonlocal effects make the local reconstruction even less accurate

Water isotopes in desiccating lichens

The stable isotopic composition of water is routinely used as a tracer to study water exchange processes in vascular plants and ecosystems. To date, no study has focussed on isotope processes in non-vascular, poikilohydric organisms such as lichens and bryophytes. To understand basic isotope exchange processes of non-vascular plants, thallus water isotopic composition was studied in various green-algal lichens exposed to desiccation. The study indicates that lichens equilibrate with the isotopic composition of surrounding water vapour. A model was developed as a proof of concept that accounts for the specific water relations of these poikilohydric organisms. The approach incorporates first their variable thallus water potential and second a compartmentation of the thallus water into two isotopically distinct but connected water pools. Moreover, the results represent first steps towards the development of poikilohydric organisms as a recorder of ambient vapour isotopic composition

Modern precipitation δ18O and trajectory analysis over the Himalaya-Tibet Orogen from ECHAM5-wiso simulations

Variations in oxygen isotope ratios (δ18O) measured from modern precipitation and geologic archives provide a promising tool for understanding modern and past climate dynamics and tracking elevation changes over geologic time. In areas of extreme topography, such as the Tibetan Plateau, the interpretation of δ18O has proven challenging. This study investigates the climate controls on temporal (daily and 6 h intervals) and spatial variations in present-day precipitation δ18O (δ18Op) across the Tibetan Plateau using a 30 year record produced from the European Centre/Hamburg ECHAM5-wiso global atmospheric general circulation model (GCM). Results indicate spatial and temporal agreement between model-predicted δ18Op and observations. Large daily δ18Op variations of 25 to +5‰ occur over the Tibetan Plateau throughout the 30 simulation years, along with interannual δ18Op variations of ~2‰. Analysis of extreme daily δ18Op indicates that extreme low values coincide with extreme highs in precipitation amount. During the summer, monsoon vapor transport from the north and southwest of the plateau generally corresponds with high δ18Op, whereas vapor transport from the Indian Ocean corresponds with average to low δ18Op. Thus, vapor source variations are one important cause of the spatial-temporal differences in δ18Op. Comparison of GCM and Rayleigh Distillation Model (RDM)-predicted δ18Op indicates a modest agreement for the Himalaya region (averaged over 86°–94°E), confirming application of the simpler RDM approach for estimating δ18Op lapse rates across Himalaya