Expériences sur la précipitation directe de l'apatite dans l'eau de mer: Implication dans la genèse des phosphorites

The direct precipitation of apatite in seawater is inhibited by Mg ions. The action of various factors on this precipitation is studied: addition of Ca, addition of F, reduction of pH and simultaneous effect of reduction of pH and addition either of F or of Ca. It is established that the reduction of the pH of seawater to 7 allows the precipitation of the apatite with a Ca/Mg ratio in the water of 1.2. The favourable environment for the settlement of natural apatite must then be either neutral or acid. This result is corroborated by the re-study of Gulbrandsen's equation. Such an acid environment with a high phosphorus content may be the result of oxidation of large quantities of organic matter due to previous intense biological activity. The acidity is probably brought about by bubbling of CO₂.     

The maximal runaway temperature of Earth-like planets

In Simpson’s (Simpson, G.C. [1927]. Mem. R. Meteorol. Soc. II (16), 69–95) classical derivation of the temperature of the Earth in the semi-gray model, the surface temperature diverges as the fourth root of the thermal radiation’s optical depth. No resolution to this apparent paradox was yet obtained under the strict semi-gray approximation. Using this approximation and a simplified approach, we study the saturation of the runaway greenhouse effect.First we generalize the problem of the semi-gray model to cases in which a non-negligible fraction of the stellar radiation falls on the long-wavelength range, and/or that the planetary long-wavelength emission penetrates into the transparent short wavelength domain of the absorption.Second, applying the most general assumptions and independently of any particular properties of an absorber, we show that the greenhouse effect saturates and that any Earth-like planet has a maximal temperature which depends on the type of and distance to its main-sequence star, its albedo and the primary atmospheric components which determine the cutoff frequency below which the atmosphere is optically thick. For example, a hypothetical convection-less planet similar to Venus, that is optically thin in the visible, could have at most a surface temperature of 1200–1300 K irrespective of the nature of the greenhouse gas.We show that two primary mechanisms are responsible for the saturation of the runaway greenhouse effect, depending on the value of λ_cut, the wavelength above which the atmosphere becomes optically thick. Unless λ_cut is small and resides in the optical region, saturation is achieved by radiating the thermal flux of the planet through the short wavelength tail of the thermal distribution. This has an interesting observational implication, the radiation from such a planet should be skewed towards the NIR. Otherwise, saturation takes place by radiating through windows in the FIR.     

The theory of steady-state super-Eddington winds and its application to novae

We present a model for steady-state winds of systems with super-Eddington luminosities. These radiatively driven winds are expected to be optically thick and clumpy as they arise from an instability-driven porous atmosphere. The model is then applied to derive the mass loss observed in bright classical novae. The main results are as follows.
(i) A general relation between the mass-loss rate and the total luminosity in super-Eddington systems.
(ii) A natural explanation of the long-duration super-Eddington outflows that are clearly observed in at least two cases (Novae LMC 1988 #1 and FH Serpentis).
(iii) A qualitative agreement between the prediction and observations of the mass loss and temperature evolution.
(iv) An agreement between the predicted average integrated mass loss of novae as a function of white dwarf mass and its observations.
(v) A natural explanation for the 'transition phase' of novae.
(vi) Agreement with η Carinae, which was used to double check the theory: the prediction for the mass shed in the star's great eruption agrees with observations to within the measurement error.     

Polarization evolution in strong magnetic fields

Extremely strong magnetic fields change the vacuum index of refraction. Although this polarization-dependent effect is small for typical neutron stars, it is large enough to decouple the polarization states of photons travelling within the field. The photon states evolve adiabatically and follow the changing magnetic field direction. The combination of a rotating magnetosphere and a frequency-dependent-state decoupling predicts polarization phase lags between different wavebands, if the emission process takes place well within the light cylinder. This QED effect may allow observations to distinguish between different pulsar-emission mechanisms and to reconstruct the structure of the magnetosphere.     

Rate of sedimentation in Lake Kinneret,Israel: spatial and temporal variations

The rate of sedimentation in Lake Kinneret was measured over several years by means of sediment traps, in up to seven different locations in the lake. Gross sedimentation rates measured in the sediment traps vary from about 1·5 kg m⁻² a⁻¹ in the deepest part of the lake up to 10 kg m⁻² a⁻¹ near the mouth of the upper Jordan river. The rate of sedimentation near the Jordan's inflow is highly correlated to flow discharge in the river, while in the centre of the lake the seasonal sedimentation pattern is mainly correlated to the bloom period of Peridinium gatunense. During the bloom period of Peridinium gatunense sedimentation rates all over the lake are very similar, indicating that the Peridinium is evenly distributed in the lake. The average suspended sediment discharge of the upper Jordan river flowing into the lake is 41 000 ton a⁻¹.Copyright © 2000 John Wiley & Sons, Ltd.     

A new approach to the problem of tektite genesis

Variations in the major cation ratios in the (micro-) tektites of each of the different strewn fields were studied with the help of Niggli parameters. There is a high degree of variability in almost each individual strewn field which, however, is not random. Invariably, as SiO₂ increases the proportion of alumina and the alkalies among the cations increases, that ofMgO + ΣFeO (total iron as FeO) and CaO decreases; simultaneouslyK₂O/Na₂O increases whileMgO/FeO decreases. This is entirely analogous to magmatic differentiations and is incompatible with an explanation by mechanical mixing of sedimentary parent rocks. The picture can best be explained by partial melting, in analogy to fulgurites and combustion ultrametamorphic glasses.In some strewn fields (e.g. javaites, georgiaites, bediasites), all tektites formed from the same set of parent rocks. In these fields each cation parameter, when plotted against silica, follow a linear slightly curved path with a very small scatter (simple type). In others (e.g. moldavites, philippinites, australites) some or all cation parameters show a strong scatter which sometimes almost obliterates the differentiation trends. These complex diagrams can be reduced to 2–5 diagrams (lineages) of the simple type, each reflecting a different combination of parent rocks. Most lineages again show the influence of partial melting differentiation.If the chemical variations of tektites are caused by partial melting, the silica-poor end of each variation diagram (case of total melting) and not the average composition corresponds to the parent rock combination involved. Thus, neglecting losses by evaporation (which are minor), the chemical compositions of 25 starting materials were established. All except one of these can very closely be matched by a combination, in appropriate proportions, of only two of the most common sedimentary rocks. The only group which cannot be duplicated by a combination of any two sedimentary rocks are the bottle-green microtektites.