Identification of excess 40Ar by the 40Ar/39Ar age spectrum technique

⁴⁰Ar/³⁹Ar incremental heating experiments on igneous plagioclase, biotite, and pyroxene that contain known amounts of excess⁴⁰Ar indicate that saddle-shaped age spectra are diagnostic of excess⁴⁰Ar in igneous minerals as well as in igneous rocks. The minima in the age spectra approach but do not reach the crystallization age. Neither the age spectrum diagram nor the⁴⁰Ar/³⁶Ar versus³⁹Ar/³⁶Ar isochron diagram reliably reveal the crystallization age in such samples.     

A comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials: Some results from lake sediments

A new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility (χ_ARM, i.e. specific ARM obtained in a 1 Oe steady field), which is particularly sensitive to the single domain (SD) and small pseudo-single domain (PSD) grains of the finer magnetite fraction. A second parameter, low-field susceptibility (χ), is relatively more sensitive to the coarser magnetite fraction (larger PSD and smaller multidomain (MD) grains). We can then obtain a measure of the ratio of coarse- to fine-grain magnetite for large numbers of samples by plotting χ_ARversusχ. A simple idealized model based on sized magnetite samples is proposed to explain the use of the χ_ARMversusχ plot for detecting relative grain-size changes in the magnetic content of natural materials. The sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of χ_ARM and χ are used to test the model.The model is used to interpret the magnetic variations observed, and the interpretations are supported by high-field hysteresis measurements of the same sediments. The combination of the high-field hysteresis method of Day et al. [1] and the χ_ARM vs. χ method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite. The χ_ARMvs.χ method should be used as an intial means of identifying distinct groups of samples.The high-field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation.     

The equity aspects of carbon dioxide-induced climate change

There is a growing scientific consensus that additional carbon dioxide being added to the earth's atmosphere (primarily as a result of fossil fuel combustion and deforestation) will eventually result in an alteration of global climates. While much uncertainty remains regarding the timing, nature and magnitude of such climatic changes, another controversy surrounds the issue of what, if anything, should be done either to avert such changes or to prepare for them. Some studies advocate a reduction of fossil fuel use, and a reversal of present deforestation trends in order to prevent climate changes from occurring. Other studies advocate prediction and adaptation as the preferred course of action. The present paper falls into this latter category, but attempts to examine a number of fundamental obstacles which will prevent fossil fuel use reductions or changes in deforestation activities. An understanding of these moral and ethical issues is crucial for approaching not only the specific question of global climate change, but also the general class of environmental problems that will require international co-operation to resolve.     

The Luna 20 lithic fragments,and the composition and origin of the lunar highlands

Luna 20 soil 22003,1 (250–500 μ) is similar to Apollo 16 soil 61501,47 (250–500 μ) in terms of the percentage of different types of particles. However, among the lithic fragments, the Apollo 16 sample contains a greater percentage of fragments with more than 70 wt. % modal plagioclase and a significantly greater proportion of KREEP-rich particles. Modal analyses of non-mare lithic fragments in Luna 20 and Apollo 11, 14, 15 and 16 indicate that the KREEP-poor highland regions (the bulk of the lunar terrae), though relatively feldspathic, are compositionally inhomogeneous, ranging in plagioclase content from approximately 35 to 100 wt. %. The average plagioclase content lies in the range 45–70 wt.%. Luna 20 pyroxene analyses cluster in two groups, one more magnesian than the other. The groups persist when pyroxene analyses from KREEP-poor noritic, troctolitic and anorthositic lithic fragments from Apollo 11, 14, 15 and 16 and Luna 20 are included. Olivine compositions mimic these pyroxene groups.Within each pyroxene group Cr₂O₃ and TiO₂ decrease as $\text{Fe}\text{(Fe + Mg)}$ increases, suggesting a relationship by fractional crystallization. The two groups suggest that at least two magma compositions were involved. To account for these observations we envisage a Moon-wide magma system in which initial accretionary heterogeneities were imperfectly erased by diffusion and convection. During the cooling of this magma system fractional crystallization was effected by the flotation of plagioclase and sinking of pyroxene, olivine and perhaps ilmenite. The endproduct was an upper layer enriched in plagioclase and a lower layer enriched in mafic silicates. KREEP-rich rocks, which are predominantly noritic in major element composition, may be mechanical mixtures of KREEP-poor norite and material residual after fractional crystallization of the surface magma system.     

Signal-to-noise enhancement in ground-based intensity observations of solarp modes

Intensity observations of solarp modes are needed to form a complete picture of wave propagation in the photosphere. Ground-based intensity observations are severely hampered by terrestrial atmospheric noise. Partial cancellation of the noise power can be achieved if two spectra having disparate signal-to-noise ratios, and based on time series acquired simultaneously at the same site, are combined. A method of combining the spectra is suggested in which one amplitude is scaled and subtracted from the other. The result is squared yielding a positive-definite power density. To test the method, the intensity of light scattered by the Earths atmosphere was recorded at fifteen-second intervals in two narrow bands centered on 0.5 and 1.6. When the two resulting spectra were combined, the noise power was attenuated by a factor of 2.7. The scale factor was varied about its optimum value, revealing that noise peaks have a different signature than signal peaks, and opening up the possibility of a new tool in discrimination against noise peaks. Maxima at symmetry-allowed frequencies and minima at symmetry-forbidden frequencies indicate that the probability that these results are obtained by chance is only 6.1 × 10^–4. The positions of these maxima and minima also support the solar-cycle dependent frequency shifts found by Pallé, Régulo, and Roca Cortés (1990a).     

Measurement of persistent organic pollutants (POPs) in plastic resin pellets from remote islands: toward establishment of background concentrations for International Pellet Watch

Plastic resin pellets collected from remote islands in the Pacific, Atlantic, and Indian Oceans and the Caribbean Sea were analyzed for polychlorinated biphenyls (PCBs), dichloro-diphenyltrichloroethane and its degradation products (DDTs), and hexachlorocyclohexanes (HCHs). Concentrations of PCBs (sum of 13 congeners) in the pellets were 0.1-9.9 ng/g-pellet. These were 1-3 orders of magnitude smaller than those observed in pellets from industrialized coastal shores. Concentrations of DDTs in the pellets were 0.8-4.1 ng/g-pellet. HCH concentrations were 0.6-1.7 ng/g-pellet, except for 19.3 ng/g-pellet on St. Helena, where current use of lindane is likely influence. This study provides background levels of POPs (PCBs<10 ng/g-pellet, DDTs <4 ng/g-pellet, HCHs <2 ng/g-pellet) for International Pellet Watch. Sporadic large concentrations of POPs were found in some pellet samples from remote islands and should be considered in future assessments of pollutants on plastic debris.     

Benefits of Software GPS Receivers for Enhanced Signal Processing

In this article, the architecture of a software Global Positioning System (GPS) receiver is described and an analysis is included of the performance of a software GPS receiver when tracking the GPS signals in challenging environments. Results are included that demonstrate the advantage of the software GPS receiver in tracking the GPS signals in low signal-to-noise or jamming scenarios. Various current and previous applications of the software GPS receiver are also described. ? 2000 John Wiley & Sons, Inc.