Carbon and oxygen isotope composition of shell carbonate of desert land snails

Variation in¹³C/¹²C and¹⁸O/¹⁶O ratios in the shell carbonate of several species of land snails was studied along a climatic gradient in semi-arid to arid areas in the southern Levant.¹³C was found to be enriched in snails from communities having plants with a C₄ photosynthetic pathway. Depleted δ¹³C values were found in areas with high mean annual rainfall, apparently due to higher input of metabolic CO₂ as a result of greater snail activity. Shell carbonate δ¹⁸O values show a weak relation to the δ¹⁸O values of rainwater. The shell δ¹⁸O values are enriched by 2–8‰ relative to isotopic equilibrium with environmental waters. Enrichment is suggested to result from metabolic effects on body water (with lower activity producing greater enrichment) but evaporation could also be a factor. Consistent differences in both¹³C and¹⁸O were found among species and may relate to the time when shell deposition occurs. As with most paleoenvironmental indicators, the application of shell isotopes is complicated by the multiplicity of controls of isotopic composition.     

Refractory inclusions in the Murchison meteorite

Mineralogical and petrographic studies of a wide variety of refractory objects from the Murchison C2 chondrite have revealed for the first time melilite-rich and feldspathoid-bearing inclusions in this meteorite, but none of these is identical to any inclusion yet found in Allende. Blue spinel-hibonite spherules have textures indicating that they were once molten, and thus their SiO₂-poor bulk composition requires that they were exposed to higher temperatures (>1550°C) than those deduced so far from any Allende inclusion. Melilite-rich inclusions are similar to Allende compact Type A's, but are more Al-, Ti-rich. One inclusion (MUCH-1) consists of a delicate radial aggregate of hibonite crystals surrounded by alteration products, and probably originated by direct condensation of hibonite from the solar nebular vapor. The sinuous, nodular and layered structures of another group of inclusions, spinel-pyroxene aggregates, suggest that these also originated by direct condensation from the solar nebular gas. Each type of inclusion is characterized by a different suite of alteration products and/or rim layers from all the other types, indicating modification of the inclusions in a wide range of different physico-chemical environments after their primary crystallization. All of these inclusions contain some iron-free rim phases. These could not have formed by reaction of the inclusions with fluids in the Murchison parent body because the latter would presumably have been very rich in oxidized iron. Other rim phases and alteration products could have formed at relatively low temperatures in the parent body, but some inclusions were not in the locations in which they were discovered when this took place. Some of these inclusions are too fragile to have been transported from one region to another in the parent body, indicating that low temperature alteration of these may have occurred in the solar nebula.     

Hydrodynamical Modeling Of Oceanic Vortices

Mesoscale coherent vortices are numerous in the ocean.Though they possess various structures in temperature and salinity,they are all long-lived, fairly intense and mostly circular. Thephysical variable which best describes the rotation and the density anomaly associated with coherent vortices is potential vorticity. It is diagnostically related to velocity and pressure, when the vortex is stationary. Stationary vortices can be monopolar (circular or elliptical) or multipolar; their stability analysis shows thattransitions between the various stationary shapes are possible when they become unstable. But stable vortices can also undergo unsteady evolutions when perturbed by environmental effects, likelarge-scale shear or strain fields, -effect or topography. Changes in vortex shapes can also result from vortex interactions. such as the pairing, merger or vertical alignment of two vortices, which depend on their relative polarities and depths. Such interactions transfer energy and enstrophy between scales, and are essential in two-dimensional and in geostrophic turbulence. Finally, in relation with the observations, we describe a few mechanisms of vortex generation.     

Luminescence-dating zeroing tests in Lake Hoare, Taylor Valley, Antarctica

In two of the perennially ice-covered lakes in the McMurdo Dry Valleys, lakes Hoare and Bonney in the Taylor Valley, bottom water has ¹⁴C ages of 2.7 ka and 10 ka (respectively), rendering ¹⁴C ages of bottom sediments highly problematic. Consequently, we tested the effectiveness of thermoluminescence (TL) zeroing in polymineral fine silt material from several depositional environments around and on the lake (stream suspensions, ice-surface sand dune, and silty sand from near the top of the more-than-3m-thick ice). We also conducted TL and infrared-stimulated-luminescence (IRSL) dating tests on material from three box cores recovered from the bottom of Lake Hoare, in a transect away from the abutting Canada Glacier. We observed effective zeroing of light-sensitive TL in suspended silt from one input stream and less effective zeroing from another stream. We observed effective zeroing of light-sensitive TL also in silt from a glacier-proximal eolian ice-surface dune and from sand from within the upper 5 cm of ice. In contrast, in box-core 1, the bottom sediment yielded minimum TL apparent ages of 1500-2600 yrs, with no discernable stratigraphic depth trend. IRSL dating applied to the same box-core samples produced significantly lower age estimates, ranging from ~600 ± 200 yrs to 1440 ± 270 yrs top-to-bottom, an improvement over the depth-constant ~2200 yrs TL ages. In two other cores closer to the Canada Glacier, IRSL ages from ~600 ± 200 yrs (top) to ~ 2900± 300 yrs (at depth) were measured. Not only are the IRSL ages a significant improvement over the TL results, but the near-core-top IRSL ages are also a dramatic improvement over the ¹⁴C results (~2.7 ka). IRSL dating has a demonstrated potential to supplant ¹⁴C dating for such antarctic lacustrine deposits.     

Zoning in garnets—Diffusion models

When reference frames and diffusing components are properly selected, the flux equations for diffusion of the major components in natural garnets may often be approximated by: J DC. In such cases it is shown that, for reasonable diffusion coefficients and boundary conditions, observed zoning profiles in natural garnets may be explained with pure diffusion models. These models allow for original inhomogeneities in the host rock and may be used to explain why single hand specimens may show such a variety of zoning profiles within a single mineral species.     

Bay of Biscay and Pyrenees

The different hypotheses proposed for the creation of the Bay of Biscay are reviewed. New geological and geophysical data collected in the last two years in the Bay and in the Pyrenean domain give new insight into the tectogenesis of the Pyrenees. Geological data of the Pyrenean area provide tight constraints on the hypothesis of formation of the Bay. The most probable hypothesis is an opening by rotation of the Iberian Peninsula around a pole of rotation situated near Paris, which resulted in strike-slip motion along the North Pyrenean fault during the Upper Mesozoic. A progressive westward migration of the pole initiated in the late Cretaceous blocked the motion along the fault and led to the main Eocene tectogenetic Pyrenean phase.     

Infrared photometry of comets Bowell and P/Stephan-Oterma

We report broadband infrared photometry of comets P/Stephan-Oterma and Bowell between 1 and 20 μm. Their JHK colors are similar to P/Meier and P/Tuttle and are compatible with scattering of sunlight by micron-sized grains. The thermal emission from P/Stephan-Oterma showed an effective temperature significantly higher than that expected from a blackbody in equilibrium. The thermal emission can be models be fit by models of the dust coma consisting of micron-sized grains. Most of the flux at all observed wavelengths comes from the dust grains rather than form the nucleus.     

Numerical simulation of the plasma double layer

A one-dimensional particle-in-cell computer simulation is used to model the formation of an electrostatic double layer. The conditions for the onset of the layer formation are explored and a relation between the length of the layer and the electrostatic potential difference across is found.     

A theoretical investigation of bed-form shapes

The deformation of movable boundaries under the action of an applied turbulent shear stress is well known. The resulting bed forms often are highly organized and nearly two-dimensional, which makes them an intriguing focus of study considering that they are generated in both steady and oscillatory turbulent flows. Many past studies share a common approach in which an infinitesimal perturbation is prescribed and the resulting growth or decay patterns are examined. In this approach, the bed forms are usually sinusoidal and the perturbation analysis does not provide a theoretical prediction of equilibrium bed-form geometry. An alternative approach is suggested here in which the forcing terms (pressure and stress) are prescribed parametrically and the governing equations are solved for the flow velocity and the associated boundary deformation. Using a multilayered approach, in which the bottom boundary layer is divided into a discrete, yet, arbitrary number of finite layers, analytical solutions for the horizontal current and bed profile are derived. The derivations identify two nondimensional parameters, p₀/u₀² and ₀/kh₀u₀², which modulate the amplitude of the velocity fluctuations and boundary deformation. For the case of combined pressure and stress divergence anomalies, the magnitude of the front face and lee slopes exhibit an asymmetry that is consistent with observed bed forms in steady two-dimensional flows.Responsible Editor: Jens Kappenberg