Carboxylic monoacids in the air of mayombe forest (Congo): Role of the forest as a source or sink

In the tropical rain forests of the Congo during the dry season, from June to September 1987, carboxylic acid partial pressures (P _gas) in the air above the canopy, at ground level, and at the boundary layer, were estimated from water samples such as fog and rainwater. The concentrations of these acids were also measured in the sap of tree leaves. Tree leaves act as a sink or as a source if the acid P _gas is greater of lower than the acid concentrations in molecular form in sap. For each of these soluble gases, there is a value of P _gas where the exchange is nul. This is called the compensation point. Values of the compensation point for some tree leaves were evaluated according to Henry's law. Henry's law coefficients at ppm levels were redetermined for formic (HCOOH), acetic (CH₃COOH), propionic (CH₃CH₂COOH), and isobutyric (CH₃CH(CH₃)COOH) acids.By comparison of P _gas and compensation points, it is concluded that the forest was a potential source for these acids. The soil-or the litter-acts as a significant source of a carboxylic acid of C₃ or C₄ atoms in the aliphatic chain. This carboxylic acid, not yet fully characterized, could play an important role in the rain acidity in forested zones of the equatorial areas.The direct emission of these carboxylic acids by vegetation was the main source in the boundary layer above the forest. The average emissions were 3.1×10⁹, 7.8×10⁹, and 8.4×10⁹ molecules cm^-2 s^-1 for HCOOH, CH₃COOH, and CH₃CH₂COOH, respectively. The savanna is an exogenous source of HCOOH and CH₃CH₂COOH during moderately rainy days for 30% of the time. The ozonolysis of isoprene seems to be a small source of HCOOH.     

Applicability of the theory of hollow inclusions for overcoring stress measurements in rock

Summary Whenever solid or hollow inclusions are used as instrumented probes in overcoring techniques, residual stresses remain in the overcored rock sample and in the probes. When using such devices for computing the in-situ stress field components from measured strains or displacements, it is common practice to assume that the overcoring diameter is infinite and that there is a perfect bonding between the rock and the probes. The validity of these assumptions depends on the magnitude of the residual stresses at the rock-probe contact as compared to the tensile and shear strengths of the rock-probe bond material. It also depends on the distribution of residual stresses in the overcored sample.In comparison to previous work, new expressions are proposed in this paper for the residual stresses associated with solid or hollow inclusion type stress probes in anisotropic ground. These expressions are presented in dimensionless form and are used to show that the distribution and magnitude of residual stresses depend on the isotropic-anisotropic rock character, the degree and type of rock anisotropy, the orientation of the rock anisotropy with respect to the hole in which the probes are located and the relative deformability of the rock with respect to the deformability of the material comprising the probes. The conditions that are required for neglecting the overcored sample diameter are also discussed. This is shown for rocks that can be described as isotropic, transversely isotropic and orthotropic materials.     

Isotope and trace element geochemistry of sediments from the Barbados Ridge-Demerara Plain region,Atlantic Ocean

Twenty-four piston core sediment samples and 13 sediments and 3 basalts from DSDP Leg 78 Site 543 were analyzed for Sr, Nd and Pb isotopic compositions. The results show sediment with highly radiogenic Pb${}^{206}\text{Pb}{}^{204}\text{Pb}$ up to 19.8) and rather radiogenic Sr and unradiogenic Nd has been deposited in the region since the Cretaceous. The source of this sediment is probably the Archean Guiana Highland, which is drained by the Orinoco River. Pb and Sr isotopic compositions and sediment thickness decrease and${}^{143}\text{Nd}{}^{144}\text{Nd}$ increases northward due to a decrease in turbiditic component. This decrease is partly due to the damming action of basement ridges. Rare earth concentrations in the sediments are somewhat low, due to the abundance of detrital and biogenic components in the sediment and rapid sedimentation rates. Both positive and negative Ce anomalies occur in the surface sediments, but only positive Ce anomalies occur in the Site 543 sediments. It is unlikely that sediment subducted to the source region of Lesser Antilles arc magmas could be the cause of negative Ce anomalies in those magmas.Isotopic compositions of Site 543 basalts show some effect of contamination by seawater-basalt reaction products and sediments. Beyond this, however, they are typical of “normal” depleted MORB.     

The geography of tourism in France: definition,scope and themes

Conclusions The substantial body of literature produced in the last two decades bears witness to the marked development of the geography of tourism in France over this period. This research has been strongly influenced by traditional geographic thought and is notable for the logic which underlies it. At the same time much remains to be done, with various avenues for future research being suggested in this paper. French geographers might also look increasingly outwards and to the work of others. By considering new and different methodologies, theories and fields of interest, the conceptual base of the subject might be enlarged and a more systematic approach developed. This need is keenly felt and this opening up is starting to occur, particularly through the exchange of ideas with foreign geographers at meetings of the national or international (IGU) commissions on the geography of tourism.     

Statistical analysis of isotopic ratios in MORB: the mantle blob cluster model and the convective regime of the mantle

A statistical examination of isotopic distributions for MORB from various ocean ridges leads to the “blob cluster model”, in which the oceanic crust accreting at ridges results from the mixing of two components within the ascending mantle. These are (1) upper mantle material and (2) discrete rising blobs of more radiogenic material. The blobs are fractionated to a variable degree and are distributed in the upper mantle circulation in a manner that is related to the spreading rate.(1) Themean values of the isotopic distributions allow us to calculate the probabilities of the two types of material within the mantle. The results show that theproportion of asthenospheric material in the mixtureincreases with the spreading rate, in agreement with the hypothesis of blob dilution within the upper mantle convection.Mass fluxes can be estimated for the rising blobs from these probabilities, which depend on the respective concentrations in the sources of the two types of material. If the blobs originate in the lower mantle, this flux estimation would suggest that a significant part of the lower mantle has been injected into the upper mantle during earth history.(2) Thestandard deviations of the distributions depend on the “efficiency” of the mixing process:the more imbricated are the asthenospheric and blob materials in the mixture,the smaller is theisotopic spread. This efficiency parameter is shown to increase with the spreading rate, as already suggested by previous comparisons between the East Pacific Rise and the Mid-Atlantic Ridge. Moreover, this feature may also be correlated with other data such as ridge bathymetric variations.     

Viscosity and configurational entropy of silicate melts

With the configurational entropy theory of relaxation processes of Adam and Gibbs (1965), one predicts that the viscosity depends on temperature according to log $\text{η = A}{}_{\mathrm{e}}\text{+}\text{B}{}_{\mathrm{e}}\text{TS}{}_{\mathrm{<mtext>conf</mtext>}}$, where S_conf is the configurational entropy of the liquid. Thermochemical calculations of S_conf performed for some mineral compositions show the importance of non-configurational contributions to the entropy differences between amorphous and crystalline phases. Except for the case of SiO₂, the available thermodynamic data indicate that the above equation for viscosity accounts quantitatively for the experimentally determined temperature dependence of the viscosity of silicate melts. The Adam and Gibbs theory also provides a simple rationale for the non linear variation of the logarithmic viscosity with composition in mixed alkali silicate liquids at low temperatures, the minimum of viscosity resulting from the contribution of the entropy of mixing to S_conf.