ACOUSTIC MODELLING ON A GRID OF VERTICALLY VARYING SPACING1

The Fourier spectral method and high-order differencing have both been shown to be very accurate in computing spatial derivatives of the acoustic wave equation, requiring only two and three gridpoints per shortest wavelength respectively. In some cases, however, there is a lack of flexibility as both methods use a uniform grid. If these methods are applied to structures with high vertical velocity contrasts, very often most of the model is oversampled. If a complicated interface has to be covered by a fine grid for exact representation, both methods become less attractive as the homogeneous regions are sampled more finely than necessary. In order avoid this limitation we present a differencing scheme in which the grid spacings can be extended or reduced by any integer factor at a given depth. This scheme adds more flexibility and efficiency to the acoustic modelling as the grid spacings can be changed according to the material properties and the model geometry. The time integration is carried out by the rapid expansion method. The spatial derivatives are computed using either the Fourier method or a high-order finite-difference operator in the x-direction and a modified high-order finite-difference operator in the z-direction. This combination leads to a very accurate and efficient modelling scheme. The only additional computation required is the interpolation of the pressure in a strip of the computational mesh where the grid spacing changes.     

Diatom changes in sediments of basins possessing marine/lacustrine transitions in Frosta, Nord-Trøndelag, Norway

Diatoms in lower sediments from eight basins of the Frosta peninsula, Nord-Trødelag, Norway, have been analysed. All the basins have been isolated from the sea, and a succession from a flora influenced by marine and brackish water to a freshwater flora is always found. An attempt is made to establish a relative salinity range for the fjord basin around the investigated area, from the Younger Dryas chronozone to the end of the Atlantic chronozone. These changes seem to be controlled by the accumulation and ablation of the ice shield and also by major climatic changes in the early and middle Holocene. It is not possible to make a general list of lagoon taxa, because of great variation from basin to basin. The isolation contact (marine/brackish boundary) must be established at the end of the brackish water flora in the absence of lagoon types. The first period after isolation is characterized by a maximum in productivity with dominance of alkaliphilous types among the diatoms. A pioneer flora with dominance of Fragilaria taxa can be traced in the uppermost basins which were isolated just after the deglaciation of the area     

Noble gases and chemical composition of Shergotty mineral fractions,Chassigny, and Yamato 793605: The trapped argon-40/argon-36 ratio and ejection times of Martian meteorites

Abstract— We report the elemental and isotopic composition of the noble gases as well as the chemical abundances in pyroxene, maskelynite/mesostasis glass, and bulk material of Shergotty and of bulk samples from Chassigny and Yamato 793605. The ⁴⁰K-⁴⁰Ar isochron for the Shergotty minerals yields a gas retention age of 196 Ma, which is, within errors, in agreement with previously determined Rb-Sr internal isochron ages. Argon that was trapped at this time has a ⁴⁰Ar/³⁶Ar ratio of 1100. For Chassigny and Y-793605, we obtain trapped ⁴⁰Ar/³⁶Ar ratios of 1380 and 950, respectively. Using these results and literature data, we show that the three shergottites, Shergotty, Zagami, and QUE 94001; the lherzolites ALH 77005, LEW 88516, and Y-793605; as well as Chassigny and ALH 84001 contain a mixture of Martian mantle and atmospheric Ar; whereas, the trapped ⁴⁰Ar/³⁶Ar ratio of the nakhlites, Nakhla, Lafayette, and Governador Valadares cannot be determined with the present data. We show that Martian atmospheric trapped Ar in Martian meteorites is correlated with the shock pressure that they experienced. Hence, we conclude that the Martian atmospheric gases were introduced by shock into the meteoritic material. For the Shergotty minerals, we obtain ³He-, ²¹Ne-, and ³⁸Ar-based cosmic-ray exposure ages of 3.0 Ma, and for the lherzolite Y-793605, 4.0 Ma, which confirms our earlier conclusion that the lherzolites were ejected from Mars ～1 Ma before the shergottites. Chassigny yields the previously known ejection age of 11.6 Ma.     

Evidence for a Picritic, Volatile-rich Magma beneath Mt. Shasta, California

Large, magnesium-rich olivines are plentiful in several Holocenecinder cones within 20 km of Mt. Shasta Summit. Glasses (formerlysilicate melts) included in the olivines are high alumina basalts(tholeiites and olivine tholeiites). In the most magnesian olivinesthe glass inclusions have large vapor bubbles. Surrounding someof the glass inclusions are broad Fe-rich zones and ghost outlines.These facts indicate crystallization of major proportions ofolivine from the initial trapped melts. The initial melts containedan inferred 24 per cent of MgO and were rich in volatiles. Theinferred entrapment temperature of the initial melt is 1410°C. The initial liquid is a possible mantle derived parentof Mt. Shasta basalts and andesites and of some hidden alpineperidotite.