摘要:

Auger electron spectroscopy (AES) is a surface‐sensitive analytical technique that derives from the interaction of an electron beam and atoms in residence at the surface of a sample; inner shell ionizations produce Auger electrons, which have an escape depth of only a few tens of angstroms. This technique has numerous applications to chemical weathering and other geochemical processes that operate on mineral surfaces. The analytical capabilities of AES include chemical analyses of the near surfaces of minerals for all elements except H and He, semiquantitative analysis of the relative atomic concentration of surface components, high lateral spatial resolution (>0.1 µm) of the analyzed area to determine the composition of discrete domains on mineral surfaces and to identify unknown minerals on a submicrometer scale, and elemental depth profiling using an ion sputter gun. These capabilities can be used to address the nature of mineral dissolution processes, providing discrimination among mechanisms such as congruent versus incongruent dissolution, surface‐controlled reactions versus diffusion through a leached or armored layer, uniform versus heterogeneous distribution of reacted layers, and identification of alteration phases. The effectiveness of AES analysis is limited by charging of nonconducting samples (e.g., silicate minerals) and sample degradation under the electron beam; these problems may be mitigated by careful sample preparation and instrument operation procedures. AES can be used in concert with numerous other microbeam techniques to fully characterize the surface composition and structure of reacting mine

ISSN:8755-1209    

DOI:10.1029/RG028i004p00337

年代:1990

数据来源: WILEY

摘要:

Recent developments in side‐scan sonar technology have increased the potential for fundamental changes in our understanding of ocean basins. Developed in the late 1960s, “side looking” sonars have been widely used for the last two decades to obtain qualitative estimates of the acoustic properties of the materials of the seafloor. Modern developments in the ability to obtain spatially correct digital data from side‐scan sonar systems have resulted in images that can be subsequently processed, enhanced, and quantified. With appropriate processing, these acoustic images can be made to resemble easily recognizable optical photographs. Any geological interpretation of these images requires an understanding of the inherent limitations of the data acquisition system. When imagery is collected, these limitations are largely centered on the concept of resolution. In side‐scan sonar images, there are several different types of resolution, including along‐ and across‐track resolution, display resolution, and absolute instrumental resolution. All of these parameters play a critical role in our ability to calibrate and ultimately to interpret the new pictures of the ocean floor. Acoustic image processing is a new application of an old and well‐established technique. Digital optical images have benefited from several decades of development in processing techniques. These relatively sophisticated techniques have been applied to photographic images from satellites and spacecraft, images which are “noisy” and difficult to obtain but extremely valuable. Side‐scan sonar systems, on the other hand, have only recently been able to produce spatially correct, digital images of the seafloor. The application of digital signal‐processing techniques to side‐scan sonar data will now allow us to quantify what had been previously very subjective and qualitative interpretations of images of the seafloor. The goal of all this processing of acoustic images remains clear: the development of an interpretable map of th

ISSN:8755-1209    

DOI:10.1029/RG028i004p00357

年代:1990

数据来源: WILEY

摘要:

Marine biogenic cycles play a key role in controlling atmospheric CO2concentrations. To predict future atmospheric CO2levels and to interpret past changes that may have been associated with global climatic events, it is necessary to determine on a global scale the rates at which carbon is cycled through the ocean and the factors that may alter the rates of transfer. Because of uncertainties and limitations in existing measurement techniques and the extreme spatial and temporal variability of ocean properties and processes, studies have not yet produced an accurate, comprehensive description of the marine carbon cycle. While some advances in the measurement of specific components of the carbon cycle have been made, the fundamental problem of undersampling an extremely heterogeneous ocean has remained. Recently, satellite‐deployed sensors have provided large‐scale, nearly synoptic images of ocean surface water properties, such as temperature and color, which have revealed mesoscale features not observed previously. In addition, moored and free‐drifting observation systems are being developed to determine directly the temporal variability of surface and subsurface water characteristics. With these remotely sensed properties to provide the space and time framework by which individual measurements of ocean processes and material fluxes can be extrapolated throughout ocean basins and over decadal time scales, we are poised to make a fundamental improvement in our understanding of ocean elemental c

ISSN:8755-1209    

DOI:10.1029/RG028i004p00381

年代:1990

数据来源: WILEY

摘要:

The propagation of seismic waves in the Earth's mantle can be significantly affected by relaxation processes, causing attenuation and velocity dispersion (reduction). This paper reviews the solid‐state mechanisms of relaxation processes based on the theory of defect microdynamics in solids together with some experimental observations on defects in minerals (particularly in olivine). For a given mechanism to have a significant effect on seismic wave propagation, both the density and the mobility of the defects must be in an appropriate range. The examination of the densities (and geometry) and mobilities of defects in olivine shows that dislocation and/or grain boundary mechanisms can have a significant effect on seismic wave propagation, although wide distributions of geometrical factors (such as spacing of pinning points) and of mobilities are required to explain all available data. Point defect mechanisms, however, are unlikely to be important because their densities are too small and/or their mobilities are too large. Since the dislocation density and/or grain size are determined in most cases by the long‐term tectonic stress, seismic wave attenuation and velocity dispersion (reduction) involving these defects are likely to depend on the magnitude of the tectonic stress as well as the temperature. Theoretical considerations suggest a wide range of dependence of seismic wave attentuation (and velocity dispersion) on the long‐term tectonic stress. This is particularly the case for dislocation mechanisms and warrants careful experimental investigation. Dislocations and/or grain boundaries cause anelastic behavior (relaxation peaks) when they are pinned or blocked at some points. Pinning or blocking becomes ineffective at high temperatures and/or low frequencies, causing a transition to viscoelastic behavior. Both laboratory and seismological observations of internal friction are dominated by the “high‐temperature background” where internal friction increases monotonically with temperature, which can be interpreted as a gradual transition to viscoelastic behavior or to a wide distribution of relaxation times. However, in most experimental studies to date, the dislocation densities or the grain sizes were not well controlled, making it difficult to identify the attenuation mechanisms and preventing any quantitative applications to Earth. The need for better characterization of defect microstructures in experimental specimens is

ISSN:8755-1209    

DOI:10.1029/RG028i004p00399

年代:1990

数据来源: WILEY