Seasonal and inter-annual relationships between vegetation and climate in central New Mexico, USA

Linear correlations between seasonal and inter-annual measures of meteorological variables and normalized difference vegetation index (NDVI) are calculated at six nearby yet distinct vegetation communities in semi-arid New Mexico, USA Monsoon season (June–September) precipitation shows considerable positive correlation with NDVI values from the contemporaneous summer, following spring, and following summer. Non-monsoon precipitation (October–May), temperature, and wind display both positive and negative correlations with NDVI values. These meteorological variables influence NDVI variability at different seasons and time lags. Thus vegetation responds to short-term climate variability in complex ways and serves as a source of memory for the climate system.     

Discrimination between climate and human-induced dryland degradation

In this study we present a technique to discriminate between climate or human-induced dryland degradation, based on evaluations of AVHRR NDVI data and rainfall data. Since dryland areas typically have high inter-annual rainfall variations and rainfall has a dominant role in determining vegetation growth, minor biomass trends imposed by human influences are difficult to verify. By performing many linear regression calculations between different periods of accumulated precipitation and the annual NDVI_max, we identify the rainfall period that is best related to the NDVI_max and by this the proportion of biomass triggered by rainfall. Positive or negative deviations in biomass from this relationship, expressed in the residuals, are interpreted as human-induced. We discuss several approaches that use either a temporally fixed NDVI peaking time or an absolute one, a best mean rainfall period for the entire drylands or the best rainfall period for each individual pixel. Advantages and disadvantages of either approach or one of its combinations for discriminating between climate and human-induced degradation are discussed. Depending on the particular land-use either method has advantages. To locate areas with a high likelihood of human-induced degradation we therefore recommend combining results from each approach.     

Resprout characteristics of creosote bush (Larrea tridentata) when subjected to repeated vehicle damage

Studies were conducted in the central Mojave Desert to quantify how creosote bushes (Larrea tridentata) respond to physical damage during large-scale military training exercises. Creosote bush possesses a resilient growth form that recovers from repeated physical damage via resprouts arising from meristems in stem bark below severed or crushed canopy units. At high levels of disturbance by heavy vehicles, nearly all individuals showed measurable breakage, but without additional damage each plant can regain a full canopy within 5 years under arid field conditions. Resprouts exhibited more vigorous growth and doubled the biomass accumulation stimulated by high rainfall of 1998, an El Niño year, vs. a normal year. New shoots of resprouted individuals were markedly different in morphological traits than canopy old growth and had slightly higher predawn shoot water potentials. The natural ability of this evergreen species to recover from cutting and crushing bodes well for re-establishment of creosote bush desert scrub communities following episodes of severe damage by vehicles.     

The use of heavy metal concentrations and dendrochronology in the reconstruction of sediment accumulation, Ma a Panew River Valley, southern Poland

Heavy metal concentrations were investigated in overbank sediments of the Mala Panew River, southern Poland. Samples were collected from seven vertical profiles located within channel infills of a 20th century floodplain at three sites, each up to 50 m wide. In each profile, 15–24 samples were collected and analysed for Ba, Cd, Cu, Pb, and Zn. Sequential extraction of these elements was carried out in the 0.063-mm fraction of selected samples. Additionally, the age of the oldest trees growing close to the profiles has been used to estimate the initiation of sediment accumulation there. Ba, Cu, and Pb, which occur mostly in less mobile, moderately reducible, and residual fractions, were used for sediment dating. Zn and Cd, which in 50–75% occur in the mobile exchangeable fraction, were not suitable for dating. Correlation of Ba, Cu, and Pb concentrations in vertical profiles with changes in the load of effluents discharged to the river showed abrupt changes in the thickness of the strongly polluted sediments across the floodplains. A comparison of the relative changes between heavy metal peaks in sediments of similar age in the different profiles suggests a variable rate of downward metal migration. In general, none of the heavy metals investigated seems to have been mobilised within the stratigraphic layers above the water table. In layers located at stratigraphically lower levels, the Zn and Cd peaks seem to migrate several centimetres to several decimetres down in the profile. In profiles inundated for several weeks every year, Zn and Cd, as well as the relatively less mobile Ba, Cu, and Pb, have migrated downward by several decimetres. The investigation shows that frequent fluctuations of the water table have blurred the original depositional metal patterns of metal concentrations within a period of less than 40 years.     

Holocene-emerged notches and tectonic uplift along the Jalisco coast, Southwest Mexico

This paper presents the preliminary results from a study of Holocene-emerged shorelines, marine notches, and their tectonic implications along the Jalisco coast. The Pacific coast of Jalisco, SW Mexico, is an active tectonic margin. This coast has been the site of two of the largest earthquakes to occur in Mexico this century: the 1932 (Mw 8.2) Jalisco earthquake and the 1995 (Mw 8.0) Colima earthquake. Measurement and preliminary radiocarbon dating of emergent paleoshorelines along the Jalisco coast provide the first constraints upon the timing for tectonic uplift. Along this coastline, uplifted Holocene marine notches and wave-cut platforms occur at elevations ranging from ca. 1 to 4.5 m amsl. In situ intertidal organisms dated with radiocarbon, the first ever reported for the Jalisco area, provide preliminary results that record tectonic uplift during at least the past 1300 years BP at an average rate of about 3 mm/year. We propose a model in which coseismic subsidence produced by offshore earthquakes is rapidly recovered during the postseismic and interseismic periods. The long-term period is characterized by slow tectonic uplift of the Jalisco coast. We found no evidence of coastal interseismic and long-term subsidence along the Jalisco coast.     

Splash-saltation trajectories of soil particles under wind-driven rain

It is usually recognized that relatively large amounts of soil particles cannot be transported by raindrop splashes under windless rain. However, the splash-saltation process can cause net transportation in the prevailing wind direction since variations in splash-saltation trajectory due to the wind are expected in wind-driven rain. Therefore, determining the combined effect of rain and wind on the process should enable improvement of the estimation of erosion for any given prediction technique. This paper presents experimental data on the effects of slope aspect, slope gradient, and horizontal wind velocity on the splash-saltation trajectories of soil particles under wind-driven rain. In a wind tunnel facility equipped with a rainfall simulator, the rains driven by horizontal wind velocities of 6, 10, and 14 m s⁻¹ were allowed to impact three agricultural soils packed into 20×55 cm soil pans placed at both windward and leeward slopes of 7%, 15%, and 20%. Splash-saltation trajectories were measured by trapping the splashed particles at distances downwind on a 7-m uniform slope segment in the upslope and downslope directions, respectively, for windward and leeward slopes. Exponential decay curves were fitted for the mass distribution of splash-saltation sediment as a function of travel distance, and the average splash-saltation trajectory was derived from the average value of the fitted functions. The results demonstrated that the average trajectory of a raindrop-induced and wind-driven soil particle was substantially affected by the wind shear velocity, and it had the greatest correlation (r=0.96 for all data) with the shear velocity; however, neither slope aspect nor slope gradient significantly predicted the splash-saltation trajectory. More significantly, a statistical analysis conducted with nonlinear regression model of C₁(u_*²/g) showed that average trajectory of splash saltation was approximately three times greater than that of typical saltating sand grain.     

A parameterisation for the threshold shear velocity to initiate deflation of dry and wet sediment

A new parameterisation for the threshold shear velocity to initiate deflation of dry and wet particles is presented. It is based on the balance of moments acting on particles at the instant of particle motion. The model hence includes a term for the aerodynamic forces, including the drag force, the lift force and the aerodynamic-moment force, and a term for the interparticle forces. The effect of gravitation is incorporated in both terms. Rather than using an implicit function for the effect of the aerodynamic forces as reported earlier in literature, a constant aerodynamic coefficient was introduced. From consideration of the van der Waals force between two particles, it was further shown that the effect of the interparticle cohesion force between two dry particles on the deflation threshold should be inversely proportional to the particle diameter squared. The interparticle force was further extended to include wet bonding forces. The latter were considered as the sum of capillary forces and adhesive forces. A model that expresses the capillary force as a function of particle diameter squared and the inverse of capillary potential was deduced from consideration of the well-known model of Fisher and the Young–Laplace equation. The adhesive force was assumed to be equal to tensile strength, and a function which is proportional to particle diameter squared and the inverse of the potential due to adhesive forces was derived. By combining the capillary-force model and the adhesive force model, the interparticle force due to wet bonding was simplified and written as a function of particle diameter squared and the inverse of matric potential. The latter was loglinearly related to the gravimetric moisture content, a relationship that is valid in the low-moisture content range that is important in the light of deflation of sediment by wind. By introducing a correction to force the relationship to converge to zero moisture content at oven dryness, the matric potential–moisture content relationship contained only one unknown model parameter, viz. moisture content at −1.5 MPa. Working out the model led to a rather simple parameterisation containing only three coefficients. Two parameters were incorporated in the term that applies to dry sediment and were determined by using experimental data as reported by Iversen and White [Sedimentology 29 (1982) 111]. The third parameter for the wet-sediment part of the model was determined from wind-tunnel experiments on prewetted sand and sandy loam aggregates. The model was validated using data from wind-tunnel experiments on the same but dry sediment, and on data obtained from simulations with the model of Chepil [Soil Sci. Soc. Am. Proc. 20 (1956) 288]. The experiments showed that soil aggregates should be treated as individual particles with a density equal to their bulk density. Furthermore, it was shown that the surface had to dry to a moisture content of about 75% of the moisture content at −1.5 MPa before deflation became sustained. The threshold shear velocities simulated with our model were found to be in good agreement with own observations and with simulations using Chepil's model.     

Submillimeter-wave and observations of the event on 28 November, 2001

We present a detailed study of a 1B/M6.9 impulsive flare combining high time resolution (1 ms) and instantaneous emission source localization observations at submillimeter frequencies (212 GHz), obtained with the solar submillimeter telescope (SST), and Hα data from the Hα solar telescope for argentina (HASTA). The flare, starting at 16:34 UT, occurred in active region (AR) 9715 (NOAA number) on November 28, 2001, and was followed by an Hα surge. We complement our data with magnetograms from the Michelson Doppler Imager (SOHO/MDI). SST observed a short impulsive burst at 212 GHz, presenting a weak bulk emission (of about 90 sfu) composed of a few shorter duration structures. The integrated Hα and the 212 GHz light curves present a remarkable agreement during the impulsive phase of the event. The delay between both curves stays below 12 s (the time resolution of the Hα telescope). The flare as well as the surge are linked to new flux emergence very close to the main AR bipole. Taking into account the AR magnetic field evolution, we infer that magnetic field reconnection, occurring at low coronal levels, could have been at the origin of the flare; while in the case of surge this would happen at the chromospheric level.     

Direct Current Plasma Emission Spectrometric Determination of Major, Minor and Trace Elements in Microwave Oven Acid Leachates of Powdered Whole Coal Samples

Major concentrations of Al₂O₃, Fe₂O₃, MgO, CaO, Na₂O and K₂O, minor levels of TiO₂, P₂O₅ and thirty petrologically, geochemically and environmentally significant trace elements have been determined in microwave oven acid leachates of whole powdered coal samples by direct current plasma‐atomic emission spectrometry (DCP‐AES). A single sample preparation procedure was suitable for all the determinations with no additional dilution step for major elements solution. Dried samples (0.5 g) were treated in low‐pressure PFA digestion vessels with HF/HCl/HNO₃/HClO₄ acids to quantitatively extract the analytes from the bulk material, while leaving the major part of organic matrix as a residue. The major constituents of geological samples, in particular the easily ionised elements (EIEs) such as alkali and alkaline earths, may complicate the instrumental determinations in DCP‐AES because of differential enhancements of elemental emission intensities and stray light interferences. Taking account of these factors, the coal matrix is considered to have very low major oxide totals as compared to many other common geo‐environmental and related materials (rocks, sediments, soil, ashes etc.). The sample size employed here, while yielding a relatively concentrated solution to cover a wide range of elemental determinations, provided a sample matrix that significantly diminished interferences for DCP measurements. The need for closely matching the unknowns and calibrators was eliminated except for overall acidity and an excess quantity of caesium for EIE buffering. Calibration of the spectrometer was accomplished by simple aqueous single element solutions as high concentration calibrators in addition to a reagent blank as a low concentration calibrator. Two point working curves were established to allow for the maximum concentrations of each element expected in the unknowns. The precision of determinations under routine conditions as well as the reproducibility of the leaching and precision of instrumental measurements have been evaluated. Relative standard deviations (RSD) were of 1–2% for those elements whose concentrations in solid samples were well above the limits of quantification. Method detection limits in the buffered solutions were also evaluated. To evaluate the accuracy of the microwave oven‐DCP method a suite of eight certified coal reference materials of differing rank, were analysed with good agreement with the certified and/or available published data. Results are presented for the uncertified major oxides in the AR series reference materials.     

Variations of U and Sr isotope ratios in Alsace and Luxembourg rain waters: origin and hydrogeochemical implications

Major and trace element concentrations, as well as Sr and U isotope ratios, were measured in rainwater samples collected in three different locations in Alsace (East of France) and Luxembourg: a mid-altitude mountain site (Aubure Environment HydroGeochemical Observatory), an urban site (Strasbourg) and a peri-urban site located in an area of well developed industrial activity (Esch-sur-Alzette in Luxembourg). Results highlight the quite high spatial and temporal variability of the chemical and isotopic characteristics of rainwater at the regional scale. They also suggest a quite systematic contribution of a local component in the chemical composition of rainwater. In urban and peri-urban sites, the local component is certainly linked to human activities, as it is well illustrated in this study with the Esch-sur-Alzette samples. On the other hand, for the Aubure site, i.e. a small forested watershed of mid-altitude mountain, data presented in this study demonstrate the influence of the vegetation on the chemical composition of rainwater for alkali and calc-alkali elements, as well as for the Sr isotope ratios. Such a result questions the reliability of the method classically used to estimate the rainwater contribution on the river chemical budget. In addition, data of the present study confirm the very low content of uranium in rainwater and demonstrate, especially through the U isotope analysis of Aubure rainwater, the negligible effect of rainwater on U budget of river waters. This work thus outlines the property of U to be a geochemical tracer specific of weathering fluxes carried by rivers. To cite this article: F. Chabaux et al., C. R. Geoscience 337 (2005).