Reference Minerals for the Microanalysis of Light Elements

The quantitative determination of light element concentrations in geological specimens represents a major analytical challenge as the electron probe is generally not suited to this task. With the development of new in situ analytical techniques, and in particular the increasing use of secondary ion mass spectrometry, the routine determination of Li, Be and B contents has become a realistic goal. However, a major obstacle to the development of this research field is the critical dependence of SIMS on the availability of well characterized, homogeneous reference materials that are closely matched in matrix (composition and structure) to the sample being studied. Here we report the first results from a suite of large, gem crystals which cover a broad spectrum of minerals in which light elements are major constituents. We have characterized these materials using both in situ and wet chemical techniques. The samples described here are intended for distribution to geochemical laboratories active in the study of light elements. Further work is needed before reference values for these materials can be finalized, but the availability of this suite of materials represents a major step toward the routine analysis of the light element contents of geological specimens.     

Self-diffusion of Si and O in diopside-anorthite melt at high pressures

Self-diffusion coefficients for Si and O in Di₅₈An₄₂ liquid were measured from 1 to 4 GPa and temperatures from 1510 to 1764°C. Glass starting powders enriched in ¹⁸O and ²⁸Si were mated to isotopically normal glass powders to form simple diffusion couples, and self-diffusion experiments were conducted in the piston cylinder device (1 and 2 GPa) and in the multianvil apparatus (3.5 and 4 GPa). Profiles of ¹⁸O/¹⁶O and ^29,30Si/²⁸Si were measured using secondary ion mass spectrometry. Self-diffusion coefficients for O (D(O)) are slightly greater than self-diffusion coefficients for Si (D(Si)) and are often the same within error. For example, D(O) = 4.20 ± 0.42 × 10⁻¹¹ m²/s and D(Si) = 3.65 ± 0.37 × 10⁻¹¹ m²/s at 1 GPa and 1662°C. Activation energies for self-diffusion are 215 ± 13 kJ/mol for O and 227 ± 13 kJ/mol for Si. Activation volumes for self-diffusion are −2.1 ± 0.4 cm³/mol and −2.3 ± 0.4 cm³/mol for O and Si, respectively. The similar self-diffusion coefficients for Si and O, similar activation energies, and small, negative activation volumes are consistent with Si and O transport by a cooperative diffusion mechanism, most likely involving the formation and disassociation of a high-coordinated intermediate species. The small absolute magnitudes of the activation volumes imply that Di₅₈An₄₂ liquid is close to a transition from negative to positive activation volume, and Adam-Gibbs theory suggests that this transition is linked to the existence of a critical fraction (∼0.6) of bridging oxygen.     

The new X-ray universe

Phil Charles and Andy Fabian review the changes brought to X-ray astronomy by the two orbiting observatories, XMM-Newton and Chandra.
We describe here the changes to X-ray astronomy that are being brought about by the two new orbiting X-ray observatories, NASA's Chandra and ESA's XMM-Newton. Between them they bring a dramatic gain in sensitivity, spatial resolution (now comparable to ground-based optical and infrared telescopes) and spectral resolution. The superb Chandra mirrors rapidly resolved the diffuse X-ray background and have produced stunning detailed images of a wide variety of cosmic objects. Large gains have been made by both missions in spectroscopic performance, with individual X-ray emission lines now detectable in hot stellar coronae, supernova remnants, X-ray binaries, active galactic nuclei and galaxy clusters.     

Impact of imperfect rainfall knowledge on the efficiency and the parameters of watershed models

It is crucial to analyze the sensitivity of watershed (rainfall-runoff) models to imperfect knowledge of rainfall input, in order to judge whether or not they are reliable and robust, especially if they are to be used for operational purposes. In this paper, a new approach to sensitivity analysis is proposed, based on a comparison between the efficiency ratings and parameter values of the models and the quality of rainfall input estimate (GORE and BALANCE indexes, assessing the quality of rainfall time distribution and the total depth respectively). Data from three watersheds of increasing size (71, 1120, and 10700 km²), are used to test three watershed models of varying complexity (three-parameter GR3J model and six-parameter modified versions of TOPMODEL and IHACRES).

These models are able to cope with imperfect rainfall input estimates, and react to improvements in rainfall input accuracy by better performance and reduced variability of efficiency. Two different types of model behavior were identified: the models either benefit from improved rainfall data by producing more consistent parameter values, or they are unable to take advantage of the improvements. Although the watershed size seems to be immaterial, the smaller watersheds appear to need more precise areal rainfall estimates (a higher concentration of raingages) to ensure good modeling results.     

First Results for the GPS Survey of South Africa Tide Gauges

Tide gauge measurements are used for a variety of scientific purposes, not least of which are the definition of vertical data and the detection of long-term variations in mean sea level. GPS measurements at tide gauge sites provide a means of separating local verticl motions from sea level rise, and a means of unifying vertical data in a single reference system. This paper describes a GPS survey to determine the positions and heights of reference stations at South African tide gauge sites. The data were processed in baseline mode using a commercial software package. The heights of the tide gauge stations relative to the fixed ITRF reference station HRAO were determined at a precision of around 3 cm – better than 0.1 ppm. Analysis of the error sources showes that use of the precise ephemeris contributed to a substantial improvement in accuracy, as did the use of ionosphere-free fixed integer baseline solutions. Variations in the antenna phase centers also contributed significant changes in height. ? 2001 John Wiley & Sons, Inc.     

Mapping the spatial and temporal distributions of woody debris in streams of the Greater Yellowstone Ecosystem, USA

The objectives of this study were: (1) to document spatial and temporal distributions of large woody debris (LWD) at watershed scales and investigate some of the controlling processes; and (2) to judge the potential for mapping LWD accumulations with airborne multispectral imagery. Field surveys were conducted on the Snake River, Soda Butte Creek, and Cache Creek in the Greater Yellowstone Ecosystem, USA. The amount of woody debris per kilometer is highest in 2nd order streams, widely variable in 3rd and 4th order streams, and relatively low in the 6th order system. Floods led to increases in woody debris in 2nd order streams. Floods redistributed the wood in 3rd and 4th order streams, removing it from the channel and stranding it on bars, but appeared to generate little change in the total amount of wood throughout the channel system. The movement of woody debris suggests a system that is the reverse of most sediment transport systems in mountains. In 1st and 2nd order tributaries, the wood is too large to be moved and the system is transport-limited, with floods introducing new material through undercutting, but not removing wood through downstream transport. In the intermediate 3rd and 4th order channels, the system displays characteristics of dynamic equilibrium, where the channel is able remove the debris at approximately the same rate that it is introduced. The spatial distribution and quantity of wood in 3rd and 4th order reaches varies widely, however, as wood is alternatively stranded on gravel bars or moved downstream during periods of bar mobilization. In the 6th order and larger channels, the system becomes supply-limited, where almost all material in the main stream can be transported out of the central channel by normal stream flows and deposition occurs primarily on banks or in eddy pool environments. Attempts to map woody debris with 1-m resolution digital four-band imagery were generally unsuccessful, primarily because the imagery could not distinguish the narrow logs within a pixel from the surrounding sand and gravel background and due to problems in precisely coregistering imagery and field maps.     

Soil radon,permeability, and indoor radon prediction

Attempts to predict which geographic areas should be associated with a high percentage of homes with unusually high indoor radon levels have been based on estimates of soil radon and soil permeability for geological units. In northern Virginia and southern Maryland, it appears that predictions of indoor radon based on estimates of homesite soil radon and soil permeability are very useful.     

Distributions of horizon-limited direct beam potential radiation in steep pastures

Agricultural production from steep terrain is frequently limited to pasture systems, where forage harvesting is done by grazing animals. Intercepted solar radiation determines the energy available for forage production. A model was developed for estimating potential direct beam radiation in areas with a wide range of aspects, landscape positions, and various shadow sources. The model was applied to a small, steep pasture watershed in central Appalachia to determine the spatial distribution of potential direct beam radiation. Effective horizon was an important parameter affecting the energy budget of the study watershed. A simple means for estimating global radiation from the modelled potential direct beam radiation, given atmospheric transmissivity and cloudiness, was presented. When the proportion of global radiation that was diffuse radiation was increased the effective horizon was less important and the spatial variability of modelled radiation decreased.