Coastal morphology; Southwest Great Abaco Island, Bahamas     

C.N. Raphael 《Geoforum》1975,6(3-4)

An examination of a portion of Great Abaco Island. Bahamas, reveals that the coastal zone may be subdivided into two morphological units: barrier and lagoon complexes, and rock benches with boulder ramparts. The sand barriers, colonized by multistoried vegetation, are stratigraphically thin and are characterized by narrow coralline beaches and beach rock. The imbricate boulder ramparts adjacent to deeper water, reveal that, on occasion, high wave energy conditions occur. It is apparent that hurricanes and tropical storms are significant in modifying coastal southwest Great Abaco.  相似文献   

VSP traveltime inversion for anisotropy in a buried layer     

Michael A. Slawinski  Michael P. Lamoureux  Raphael A. Slawinski  R. James Brown 《Geophysical Prospecting》2003,51(2):131-140

We present a method for calculating the anisotropy parameter of a buried layer by inverting the total traveltimes of direct arrivals travelling from a surface source to a well‐bore receiver in a vertical seismic profiling (VSP) geometry. The method assumes two‐dimensional media. The medium above the layer of interest (and separated from it by a horizontal interface) can exhibit both anisotropy and inhomogeneity. Both the depth of the interface as well as the velocity field of the overburden are assumed to be known. We assume the layer of interest to be homogeneous and elliptically anisotropic, with the anisotropy described by a single parameter χ. We solve the function describing the traveltime between source and receiver explicitly for χ. The solution is expressed in terms of known quantities, such as the source and receiver locations, and in terms of quantities expressed as functions of the single argument x_r, which is the horizontal coordinate of the refraction point on the interface. In view of Fermat's principle, the measured traveltime T possesses a stationary value or, considering direct arrivals, a minimum value, . This gives rise to a key result ‐‐ the condition that the actual anisotropy parameter . Owing to the explicit expression , this result allows a direct calculation of in the layer of interest. We perform an error analysis and show this inverse method to be stable. In particular, for horizontally layered media, a traveltime error of one millisecond results in a typical error of about 20% in the anisotropy parameter. This is almost one order of magnitude less than the error inherent in the slowness method, which uses a similar set of experimental data. We conclude by detailing possible extensions to non‐elliptical anisotropy and a non‐planar interface.  相似文献   

Redox and nonredox reactions of magnetite and hematite in rocks   总被引：1，自引：0，他引：1  

Arno Mücke  Alexandre Raphael Cabral 《Chemie der Erde / Geochemistry》2005,65(3):271-278

Redox and nonredox reactions causing pseudomorphic replacement of hematite by magnetite and magnetite by hematite are compared.Pseudomorphic replacements resulting from redox reactions are known as martitization [replacement of magnetite by hematite due to oxidation; reaction (1)] and mushketovitization [replacement of hematite by magnetite due to reduction; reaction (2)]. These two replacements cause characteristic ore textures and volume changes (reaction (1): increase of 1.66%; reaction (2): decrease of 1.64%). These small volume changes are the reason that martitization and mushketovitization are widespread in many rocks under condition, however, that oxidizing or reducing fluids (solutions) are present.The same initial and end products may also be involved in nonredox reactions. Reaction (3) is the replacement of hematite by magnetite due to simple addition of Fe²⁺ atoms under basic conditions. This reaction causes an increase of the volume of 47.6%. Reaction (4), causing a volume decrease of 32.2%, is the replacement of magnetite by hematite due to leaching of Fe²⁺ atoms under acidic conditions. From these volume changes it is concluded that reaction (4) may occur in many rock types, whereas reaction (3) is restricted to unlithified sediments only. However, ore textures caused by nonredox reactions are unknown and therefore their occurrence in rocks is hypothetical.  相似文献   

Spatial and temporal dynamics of epiphytic microalgae on the cordgrassSpartina alterniflora in North Inlet Estuary, South Carolina     

Gabriella Jackson  Richard Zingmark  Alan J. Lewitus  Raphael G. Tymowski  Joyce Stuckey 《Estuaries and Coasts》2006,29(6):1212-1221

Epiphytic microbial biomass (as chlorophylla) was measured monthly in North Inlet Estuary, South Carolina, for 16 months on spatially distinct stem sections (bottom and middle) of dead and livingSpartina alterniflora growth forms (tall, medium, and short) exposed at low tide. The highest biomass was located on the bottom section of tall plants, presumably due to their relatively longer contact with creek water and associated phytoplankton, and their closer proximity to marsh sediments with associated benthic microalgae, both recruitment sources for epiphytes. Dead plants left standing from the previous year’s growth cycle had higher epiphytic biomass than living plants, which occurred mostly in late spring through fall. Epiphytic biomass was highest in the winter (mean of 1.77 mg chla (m² marsh)⁻¹) and lowest in the summer (mean of 0.34 mg chla (m² marsh)⁻¹). Because phytoplankton andSpartina production are lowest in the winter, the results emphasize the relative importance of epiphytes to growth of herbivores in this season.  相似文献   

A New Multi-Mineral Age Reference Material for ⁴⁰Ar/³⁹Ar, (U-Th)/He and Fission Track Dating Methods: The Limberg t3 Tuff     

Michael Kraml  Raphael Pik  Meinert Rahn  Rune Selbekk  Jean Carignan  Jörg Keller 《Geostandards and Geoanalytical Research》2006,30(2):73-86

The phonolitic Limberg t3 tephra (Kaiserstuhl Volcanic Complex, Germany) was previously dated by the conventional K/Ar method yielding inconsistent results. We have re-dated this tephra layer with three independent methods. Fission Track (FT) external detector analyses on single apatite crystals (16.8 ± 1.3 Ma, 2s) and (U-Th)/He measurements on titanite and apatite (16.5 ± 1.0 Ma, 2s and 16.8 ± 1.0 Ma, 2s, respectively) are in close agreement with laser Ar/Ar dates on incrementally heated single crystals of sanidine (16.3 ± 0.4 Ma, 2s). Due to very rapid cooling, the He, FT and Ar thermochronometers provide one single age representing the eruption event. The different minerals are characterised by favourable properties with respect to their chemical composition, grain size and shape. In particular for the t3 sanidine, homogeneity has been demonstrated by electron microprobe analysis and on a grain-to-grain and grain-internal scale by single crystal incremental laser heating. Based on the agreement between independent methods and the mineral yield of this unit, the Limberg t3 tephra is proposed as multi-method age reference material for single grain laser Ar/Ar, FT and (U-Th)/He dating.  相似文献