Leaf wax n-alkane δD values of field-grown barley reflect leaf water δD values at the time of leaf formation

Leaf wax n-alkanes from barley (Hordeum vulgare) from a field in Switzerland exhibited changes in δD values on the order of 20‰ over a growing season, while source water (soil water) and leaf water varied by 40‰. Additionally the seasonal variability in δD values of leaf wax n-alkanes of different barley leaves can only be found across different leaf generations (i.e. leaves that were produced at different times during the growing season) while n-alkane δD values did not vary significantly within a leaf generation. Interestingly, δD values of n-alkanes correlated best with the δD values of leaf water at midday of the sampling day but showed no significant correlation with soil water (e.g. precipitation) δD values. These results provide empirical evidence that leaf wax δD values record leaf water enrichment, and therefore integrate the isotopic effects of precipitation and evapotranspiration. Our results show that leaf wax n-alkane δD values from grasses are ‘locked in’ early during leaf development and hence record the environmental drivers of leaf water enrichment, such as vapor pressure deficit (VPD). Our data have important implications for the interpretation of paleorecords of leaf wax δD. We suggest that leaf wax n-alkane δD values from sedimentary records could be used to estimate changes in the degree of leaf water enrichment and hence VPD.     

Different methods for modelling the areal infiltration of a grass field under heavy precipitation

The areal infiltration behaviour of a grass field is studied using a data set of 78 sprinkler infiltration experiments. The analysis of the experimental data shows a distinct event dependency: once runoff begins, the final infiltration rate increases with increasing rainfall intensity. This behaviour is attributed to the effects of small‐scale variability. Increasing rainfall intensity increases the ponded area and therefore the portion of the plot which infiltrates at maximum rate. To describe the areal infiltration behaviour of the grass field the study uses two different model structures and investigates different approaches for consideration of subgrid variability. It is found that the effective parameter approach is not suited for this purpose. A good representation of the observed behaviour is obtained by using a distribution function approach or a parameterization approach. However, it is not clear how the parameters can be derived for these two approaches without a large measurement campaign. The data analysis and the simulations show the great importance of considering the effects of spatial variability for the infiltration process. This may be significant even at a small scale for a comparatively homogeneous area. The consideration of heterogeneity seems to be more important than the choice of the model type. Furthermore, similar results may be obtained with different modelling approaches. Even the relatively detailed data set does not seem to permit a clear model choice. In view of these results it is questionable to use very complex and detailed simulation models given the approximate nature of the problem. Although the principle processes may be well understood there is a lack of models that represent these processes and, more importantly, there is a lack of techniques to measure and parameterize them. Copyright © 2002 John Wiley & Sons, Ltd.     

Density Perturbations in the Upper Atmosphere Caused by the Dissipation of Solar Wind Energy

The upper atmosphere constitutes the outer region of the terrestrial gas envelope above about 100 km altitude. The energy budget of this outer gas layer is partly controlled by the dissipation of solar wind energy. Since this energy input is largely irregular, the resulting density changes are considered as perturbations. The properties and physics of such density perturbations are reviewed here. Besides being an important link in the complex chain of solar-terrestrial relations, such disturbances are also of practical interest because they affect the orbits of satellites and space stations and are responsible for ionospheric disturbance effects.     

New high‐resolution age data from the Ediacaran–Cambrian boundary indicate rapid,ecologically driven onset of the Cambrian explosion

The replacement of the late Precambrian Ediacaran biota by morphologically disparate animals at the beginning of the Phanerozoic was a key event in the history of life on Earth, the mechanisms and the time‐scales of which are not entirely understood. A composite section in Namibia providing biostratigraphic and chemostratigraphic data bracketed by radiometric dating constrains the Ediacaran–Cambrian boundary to 538.6–538.8 Ma, more than 2 Ma younger than previously assumed. The U–Pb‐CA‐ID TIMS zircon ages demonstrate an ultrashort time frame for the LAD of the Ediacaran biota to the FAD of a complex, burrowing Phanerozoic biota represented by trace fossils to a 410 ka time window of 538.99 ± 0.21 Ma to 538.58 ± 0.19 Ma. The extremely short duration of the faunal transition from Ediacaran to Cambrian biota within less than 410 ka supports models of ecological cascades that followed the evolutionary breakthrough of increased mobility at the beginning of the Phanerozoic.     

A computationally efficient approach for estimating high-rate satellite clock corrections in realtime

Realtime satellite clock corrections are usually estimated using undifferenced phase and range observations from a global network. Because a large number of ambiguity parameters must be estimated, the computation is time-consuming. Consequently, only a sparse global network of limited number of stations is processed by most IGS Realtime Analysis Centers with an update rate of 5 s. In addition, it is very desirable to build the capability to simultaneously estimate clock corrections for multi-GNSS constellations. Although the estimation can be sped up by epoch-differenced observations that eliminate ambiguities, the derived clocks can contain a satellite-specific bias that diminishes the contribution of range observations. We introduce a computationally efficient approach for realtime clock estimation. Both the epoch-differenced phase and undifferenced range observations are used together to estimate the epoch-differenced satellite clocks and the initial clock bias for each satellite and receiver. The biased clock corrections accumulated from the estimated epoch-differenced clocks are then aligned with the estimated clock biases and provided as the final clock corrections to users. The algorithm is incorporated into the EPOS-RT software developed at GFZ (GeoForschungsZentrum) and experimentally validated with the IGS global network. The comparison with the GFZ rapid products shows that the accuracy of the clock estimation with the new approach is comparable with that of the undifferenced approach, whereas the computation time is reduced to one-tenth. As a result, estimation of high-rate satellite clocks from a large reference network and tracking satellites of multi-GNSS constellations becomes achievable.     

Significant seasonal variation in the hydrogen isotopic composition of leaf-wax lipids for two deciduous tree ecosystems (Fagus sylvativa and Acerpseudoplatanus)

Compound specific hydrogen isotope ratios (δD) of long chain sedimentary n-alkanes, which mostly originate from the leaf waxes of higher terrestrial plants, are increasingly employed as paleoclimate proxies. While soil water is the ultimate hydrogen source for these lipids and the isotopic fractionation during biosynthesis of lipids is thought to remain constant, environmental parameters and plant physiological processes can alter the apparent hydrogen isotopic fractionation between leaf-wax lipids and a plant’s source water. However, the magnitude and timing of these effects and their influence on the isotopic composition of lipids from higher terrestrial plants are still not well understood. Therefore we investigated the seasonal variability of leaf-wax n-alkane δD values for two different temperate deciduous forest ecosystems that are dominated by two different tree species, Beech (Fagus sylvatica) and Maple (Acerpseudoplatanus).We found significant seasonal variations for both tree species in n-alkane δD values of up to 40‰ on timescales as short as one week. Also, the isotopic difference between different n-alkanes from the same plant species did vary significantly and reached up to 50‰ at the same time when overall n-alkane concentrations were lowest.Since δD values of soil water at 5 and 10 cm depth, which we assume represent the δD value of the major water source for the investigated beech trees, were enriched in autumn compared to the spring by 30‰, whereas n-alkane δD values increased only by 10‰, we observed variations in the apparent fractionation between beech leaf derived n-alkanes and soil water of up to 20‰ on a seasonal scale. This observed change in the apparent fractionation was likely caused by differences in leaf water isotopic enrichment. Based on mechanistic leaf water models we conclude that changes in the isotopic difference between water vapor and soil water were the most likely reason for the observed changes in the apparent fractionation between n-alkanes and soil water.The large variability of n-alkane concentrations and δD values over time implies a continuous de novo synthesis of these compounds over the growing season with turnover times possibly as short as weeks. The signal to reach the soil therefore represents an integrated record of the last weeks before leaf senescence. This holds true also for the sedimentary record of small catchment lakes in humid, temperate climates, where wind transport of leaf-wax lipids is negligible compared to transfer through soil and the massive input of leaves directly into the lake in autumn.     

The fractal properties of geochemical landscapes as an indicator of weathering and transport processes within the Eastern Alps

Variations in surface morphology and lithology provide an opportunity to study lithologic and morphologic influences on the spatial pattern of stream-sediment geochemistry within two contrasting environments of the Eastern Alps (Hohe Tauern Range and Gurktaler Alpen Range). The fractal dimension, a measure of surface roughness over a variety of scales, is used to model the dissipation of erosive products due to climatic controlled denudation and fluvial mass transport. Based on a spatial correlation analysis, specific elemental concentrations are used as indicators for a dominant lithotype. Fractal geometry of these elements has been estimated by sequential Gaussian simulation of the area/perimeter relationship (Dal) and by the estimation of multifractal spectra. It is shown that within a 510–780 km² survey area the spatial variations of Al, Ga, Ni and Ca can be approximated by single fractals but for those of Ag and Sn multifractal models must be used. Fractal properties derived from simulated surfaces are explainable by the process controlling the spatial structure of the data. Climatic and tectonic parameters apparently influences Dal at large scales. At smaller scales rock-type variation exert an additional influence on Dal.     

Three-dimensional numerical study of cell broadening during cold-air outbreaks

The boundary-layer development and convection-pattern transition typically occurring in cold-air outbreaks is studied using three-dimensional simulations. The simulations include the secondary-flow transition starting with the relatively small-scale boundary-layer rolls developing during the initial phase and ending with mesoscale cellular convection patterns. The application of a computational grid, whose horizontal mesh size enables the resolution of the small-scale initial patterns and whose domain size is large enough to capture mesoscale convection patterns, overcharges even state-of-the-art supercomputers. In order to bypass the computer storage problem, the horizontal size of the model domain and the horizontal resolution of the computational grid are adjusted to the scale of the dominant convective structures. This enables the simulation of convection cells whose horizontal scales increase up to values exceeding the size of the initial model domain. The model is applied to conditions of a cold-air outbreak observed during the ARKTIS 1991 experiment. The most important characteristics of the observed situation are revealed by the model. Sensitivity studies are performed in order to investigate the relation between cell broadening and various physical processes. The artificial cutoff of liquid-water formation prevents the enlargement of convective scales. Latent heating due to condensation and especially radiative cloud-top cooling are identified as processes leading to cell broadening. We propose a conceptual model that elucidates the mechanism by which cloud-top cooling may generate larger aspect ratios.