David Strivay

CARAIB: A global model of terrestrial biological productivity

CARAIB, a mechanistic model of carbon assimilation in the biosphere estimates the net primary productivity (NPP) of the continental vegetation on a grid of 1° × 1° in latitude and longitude. The model considers the annual and diurnal cycles. It is based on the coupling of the three following submodels; a leaf assimilation model including estimates of stomatal conductance and leaf respiration, a canopy model describing principally the radiative transfer through the foliage, and a wood respiration model. Present-day climate and vegetation characteristics allow the discrimination between ecotypes. In particular, specific information on vegetation distribution and properties is successfully used at four levels; the leaf physiological level, the plant level, the ecosystem level, and the global level. The productivity determined by the CARAIB model is compared with local measurements and empirical estimates showing a good agreement with a global value of 65 Gt C yr−1. The sensitivity of the model to the diurnal cycle and to the abundance of C4 species is also tested. The productivity slightly decreases (10%) when the diurnal cycle of the temperature is neglected. By contrast, neglecting the diurnal cycle of solar irradiance produces unrealistically high values of NPP. Even if the importance of this increase would presumably be reduced by the coupling of CARAIB with a nutrient cycle model, this test emphasizes the key role of the diurnal cycle in a mechanistic model of the NPP. Uncertainties on the abundance and spatial distribution of C4 plants may cause errors in the NPP estimates, however, as demonstrated by two sensitivity tests, these errors are certainly lower than 10% at the global scale as shown by two tests.

Stochastic generation of meteorological variables and effects on global models of water and carbon cycles in vegetation and soils

Abstract Global models of water and carbon cycles in continental vegetation and soils are usually forced with monthly mean climatic data-sets and thus neglect day to day variations of the weather. This treatment may be justified for empirical models based on parametrizations validated at a monthly timescale. Mechanistic models handling hydrological and biological processes at much shorter timescales might, however, be largely affected by such an approximation, since the various processes described are highly nonlinear. A random generator of daily precipitations and temperatures applicable at the global scale has thus been developed from worldwide meteorological data covering 6 years of observations. The probability of a wet day is correlated to the weather encountered the previous day. The amount of precipitation, the daily mean temperature and the diurnal range of temperature are described from the statistical point of view by the cumulative distribution functions (CDF) of three random variables. The CDFs relative to temperatures are different for rainy and dry days. This stochastically generated weather field is used as input to IBM (Improved Bucket Model) and CARAIB (CARbon Assimilation In the Biosphere), two global models of respectively soil hydrology and vegetation productivity. Large differences in both the geographical distribution and the global value of soil water, vegetation productivity and carbon stocks are obtained between the model runs using monthly uniform weather on one side and randomly generated weather on the other. The main contribution to this difference at the global scale arises from the precipitation generation occurring as a result of high degree of nonlinearity of the interception scheme used in IBM.

Multi-disciplinary investigation of the tomb of Menna (TT69), Theban Necropolis, Egypt

The archaeometrical survey of the tomb of Menna (TT69), which took place in November-December 2007, is part of the extended research program that aims to study and preserve this tomb in all its aspects. Menna was a high official who served as an overseer of Cadastral surveys during the reigns of pharaohs Tuthmosis IV and Amenhotep III (ca. 1419-1370 BC). The research team aimed to gather information, in a totally non-destructive way, on the materials used and the painting techniques. The technical examinations included photography with normal and raking light, macrophotography, ultra-violet (UV) fluorescence photography, and microscopy. On selected points X-ray fluorescence (XRF) was combined with diffuse reflectance UV-spectrometry, near infrared (NIR) diffuse reflectance spectroscopy and Raman spectroscopy. The technical aspects as well as problems that are inherently associated with an interdisciplinary survey of this extent, are discussed. The project worked with a large team of people with different backgrounds and sensitive technical equipment. Working conditions were quite hostile, including elevated temperatures and dust hampering the examinations.

Use of a variable incidence angle PIXE arrangement for studying pigment multilayers

Abstract Particle induced X-ray emission (PIXE) method is used in the field of archeometry and specially to investigate pigment colored multilayers. The tilting of the sample with respect to the incident proton beam direction allows to modify the relative contribution of each layer to the fluorescence signal. The experimental results coupled to computer simulations lead to semi-quantitative information about the thickness, the position and the composition of the successive layers.

Use of PIXE-PIGE under variable incident angle for ancient glass corrosion measurements

Abstract Although glass is usually considered as a very stable archaeological material, it can undergo severe degradation. Soda-lime glass, the most common glass throughout ancient times, is particularly sensitive to this problem. The glass surface absorbs moisture from its environment and the contact with CO 2 causes Na 2 O and NaOH to convert to Na 2 CO 3 , which is extremely hygroscopic. The subsequent unstable glass layer can be leached out and causes decomposition of the glass. The non-destructive PIGE–PIXE method of investigation allows detection of this phenomenon even if no visible effect appears. The variable incident angle method is able to discern the depth of the degradation. One aim of such studies is the possible dating or at least fake detecting of archaeological materials. Furthermore, even objects of large size can be investigated with the atmospheric PIGE–PIXE set-up. Some examples of measurements on ancient glass are given.

Study of color centers induced by PIXE irradiation

The particle induced X-ray emission method is perfectly adapted to the study, by external beam, of art objects (like paintings) and allows non-destructive analysis of the atomic composition of the target. However, a strange phenomenon occurs during irradiation on some pigments: dark brownish stains appear, and this could be due to the formation of color centers. In fact, these darkening spots progressively fade out and disappear after a few weeks. Heat and UV light accelerate the decreasing process. The aim of this study is to understand the physical processes of the stain creation and to find a way to make stains disappear, avoiding any damage for the painting.

On relativistic scalar and vector mesons with harmonic oscillatorlike interactions

Relativistic descriptions for spin 0 and 1 particles of nonzero restmasses are known for a long time as the so-called Kemmer or Sakata-Taketani formulations. Through harmonic oscillatorlike interactions, we study the nonrelativistic limit of the corresponding wave equations in connection with expected spin-orbit terms. Typical Foldy-Wouthuysen developments are included in these approaches with exact results up to required orders for interacting vector mesons only.

An empirical formula for L line X-ray production cross-section of elements from Ag to U for protons below 3.5 MeV

When computing element concentration from proton induced X-ray emission analysis, an important parameter is the X-ray production cross-section. There have been numerous experimental and theoretical works in this field. Nonetheless, although there is a simple analytical formula to compute K X-ray cross-sections, there is no such ones for the L lines. We present here analytical formulas for the cross-section of the three main X-ray lines Lα, Lβ and Lγ based on experimental data. So far, nearly 3000 values of cross-sections for elements from Ag to U and proton energy ranging from 0.5 to 3.5 MeV have been collected from various references. This experimental data set has been fitted for each X-ray line with an exponential function depending on the proton energy and on the element atomic number. These fitted values have then been compared to the experimental data and with theoretical values obtained by the ECPSSR theory and Coster–Kronig fluorescence yields.

Synthesis and fading of eighteenth-century Prussian blue pigments: a combined study by spectroscopic and diffractive techniques using laboratory and synchrotron radiation sources

Prussian blue, a hydrated iron(III) hexacyanoferrate(II) complex, is a synthetic pigment discovered in Berlin in 1704. Because of both its highly intense color and its low cost, Prussian blue was widely used as a pigment in paintings until the 1970s. The early preparative methods were rapidly recognized as a contributory factor in the fading of the pigment, a fading already known by the mid-eighteenth century. Herein two typical eighteenth-century empirical recipes have been reproduced and the resulting pigment analyzed to better understand the reasons for this fading. X-ray absorption and Mössbauer spectroscopy indicated that the early syntheses lead to Prussian blue together with variable amounts of an undesirable iron(III) product. Pair distribution functional analysis confirmed the presence of nanocrystalline ferrihydrite, Fe10O14(OH)2, and also identified the presence of alumina hydrate, Al10O14(OH)2, with a particle size of ∼15 Å. Paint layers prepared from these pigments subjected to accelerated light exposure showed a tendency to turn green, a tendency that was often reported in eighteenth- and nineteenth-century books. The presence of particles of hydrous iron(III) oxides was also observed in a genuine eighteenth-century Prussian blue sample obtained from a polychrome sculpture.

Is the external beam PIXE method suitable for determining ancient silver artifact fineness

Abstract In archeometry, the non-destructive characteristic of an analytical method is always very attractive. For most of techniques, besides the need of sampling, difficulties can originate from the shape and the size of the silver artifacts to be analyzed. The external PIXE method should be an answer to this problem but several questions are arising from the implementation of the technique to such a material. The aim of this paper is to specify the experimental procedure as to take into account several parameters, which could induce dramatic errors. Among them it can be pointed out the superficial enrichment in silver, arising from the dissolution of copper and copper oxide, the surface roughness effects, the great importance of the stability of the geometric conditions. The accuracy and the precision of the method are considered within the scope of the results obtained in ancient times by the cupellation with ashbone cupels. In addition, the process involved to obtain silver alloys has been studied in order to define its influence on the real fineness of the objects.