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991.
X-ray diffraction analysis and transmission electron microscopy have been used to study the effects of alpha-decay damage in pyrochlore group minerals, characterized by the general formula A 1?m B 2O6(O,OH,F)1?n ·pH2O. As defined by the XRD intensity ratio I/I 0 , both the saturation dose (for which I/I 0 =0.1?0.0) and the dose which signifies the initial loss of crystallinity (for which I/I 0 =1.0?0.8) increase as a function of geologic age. The increase is attributed to annealing of isolated alpha-recoil tracks back to the original crystalline structure. The tracks have calculated mean lives, τa, on the order of 108 years. In contrast, minerals which remain crystalline (e.g., uraninite, UO2) despite doses of up to 1018 alpha-events/mg have mean alpha-recoil track lives ≈104 years (Eyal and Fleischer 1985). After correcting the calculated dose for annealing of alpha-recoil damage, I/I 0 is observed to decrease exponentially to zero over the dose range 0.02–1.0 × 1016 alpha-events/mg. The relationship between I/I 0 and “corrected” dose was used to calculate an average alpha-recoil track diameter of 4.6 nm, in which < 2600 atoms are displaced. XRD line broadening due to strain dominates the first half of the crystalline-to-metamict transition, reaching a maximum of 0.003, then decreasing to < 0.001. Line broadening due to decreasing crystallite size dominates the latter half of the transition. Estimated crystallite dimensions decrease from 450 nm to < 15 nm prior to reaching the fully metamict state. With increasing dose HRTEM images of microlites from the Harding pegmatite sequentially exhibit: 1) mottled diffraction contrast, 2) isolated 1–5 nm aperiodic areas, 3) coexisting aperiodic and crystalline areas, 4) relict “islands” of crystalline material in an aperiodic matrix, and 5) complete loss of lattice fringe periodicity. With no consideration given to alpha-recoil track fading, the transition covers a dose range of 0.04–1.7 × 1017 alpha-events/mg. Using a value of τa=108 years, this dose range is corrected down to 0.02–1.2 × 1016 alpha-events/mg. The metamict state is characterized by a range of M-M and M-O distances which give rise to bands of diffuse scattering centered at 0.30 nm and 0.18 nm, respectively, in x-ray and electron diffraction patterns. Random image contrast shown by HRTEM is consistent with a random network type structure, an interpretation supported by EXAFS/XANES studies (Greegor et al. 1985a, b, 1987). The structure of metamict pyrochlore consists of an aperiodic framework of corner-sharing B-O polyhedra. Compared to the crystalline precursor, the metamict state displays a reduced M-O coordination number and mean bond length, increased distortion of the B-site, and a slight increase in the average M-M distance. 相似文献
992.
Mechanisms associated with the erosion of sand dune cliffs,Magilligan, Northern Ireland 总被引:1,自引:0,他引:1
A fifteen year history of coastal dune recession at Magilligan, Northern Ireland has revealed both time and space contrasts in processes and morphology. Since 1968 ‘storm’ frequency has increased, while dune retreat reached a peak (5·4 m) in 1978–1979. Three types of dune eroding events are noted, resulting from particular wave/wind/tide combinations. The spatial variability in dune scarping is associated with vegetation and soil development, and slope failures range from granular avalanches to retrogressive rotational slumps. 相似文献
993.
Owen K. Davis 《Climatic change》1994,26(2-3):271-287
Decreased solar activity correlates with positive cosmogenic isotope anomalies, and with cool, wet climate in temperate regions of the world. The relationship of isotope anomalies to climate may be the opposite for areas influenced by monsoonal precipitation, i.e., negative anomalies may be wet and warm. Petersen (1988) has found evidence for increased summer precipitation in the American Southwest that can be shown to be coincident with negative14C anomalies during the Medieval Warm Period. The present study compares palynological indicators of lake level for the Southwest with Petersen's data and with the14C isotope chronology. Percentages of aquatic pollen and algae from three sites within the Arizona Monsoon record greater lake depth or fresher water from A.D. 700–1350, between the Roman IV and Wolf positive isotope anomalies, thereby supporting Petersens's findings. Maximum summer moisture coincides with maximum population density of prehistoric people of the Southwest. However, water depth at a more northern site was low at this time, suggesting a climateisotope relationship similar to that of other temperate regions. Further analysis of latitudinal patterns is hampered by inadequate14C dating. 相似文献
994.
Much research in the physical and human sciences has addressed the concept of sensitivity to adjustment, including the analysis of geomorphological adjustment in fluvial systems. However, geomorphological sensitivity has been defined in numerous ways, partly because current conceptual understanding of the fluvial system exceeds modeling capabilities. Using published literature and analogies from materials behavior as a background, previous definitions are standardized and four hierarchically based interpretations are developed. The four interpretations, which relate to ratios, thresholds, recovery times, and sensitivity analysis, are explained and illustrated using a table that could be modified to apply to other environmental systems. Such standardized definitions of sensitivity should facilitate more precise communication between geomorphologists and guarantee effective use of the concept in analyses of the fluvial system. 相似文献
995.
Gregory E. Faiers Barry D. Keim Katherine K. Hirschboeck 《The Professional geographer》1994,46(2):156-163
This research classifies extreme three-hour and 24-hour rainfall events in Louisiana by synoptic weather types (Frontal, Gulf Tropical Disturbance, and Air Mass). These classes were used to analyze differences in frequencies and intensities of extreme rainfall. Significant differences were found in the storm frequencies by weather type. However, no significant differences were detected in rainfall intensities between weather types during either duration. These findings have a bearing on statistical procedures associated with estimating storm probabilities in the region. 相似文献
996.
997.
Land subsidence and sea level rise on the Atlantic Coastal Plain of the United States 总被引:1,自引:0,他引:1
George H. Davis 《Environmental Geology》1987,10(2):67-80
Land subsidence due to decline in head in confined aquifers, related to municipal and industrial water pumpage, is widespread
in the Atlantic Coastal Plain. Although not a major engineering problem, subsidence greatly complicates adjustment of precise
leveling and distorts prediction of future sea-level rise. When preconsolidation stress equivalent to about 20 m of head decline
is exceeded compaction of fine-grained sediments of the aquifer system begins, and continues until a new head equilibrium
is attained between fine and coarse units. The ratio subsidence/head decline is quite consistent, ranging from 0.0064 in southeastern
Virginia to 0.0018 at Dover, Delaware and Atlantic City, New Jersey. Higher values are related to the occurrence of montmorillonite
as the predominant clay mineral present. Review of tide gauge records indicates that gauges not affected by land subsidence
or other local secular effects have been sinking relative to sea level since 1940 at rates averaging about 2.5 mm/yr, of which
0.6 mm/yr is ascribed to glacio-isostatic adjustment to unloading of North America resulting from melting of late Pleistocene
glaciers, and about 0.9 mm/yr is ascribed to steric sea-level rise related to ocean warming. The residual 1 mm/yr of relative
sea-level rise is not well understood, but may be related to regional tectonic subsidence of the Atlantic coast. 相似文献
998.
999.
John I. Hedges Gregory L. Cowie John R. Ertel R. James Barbour Patrick G. Hatcher 《Geochimica et cosmochimica acta》1985,49(3):701-711
Spruce, alder, and oak woods deposited in coastal sediments were characterized versus their modern counterparts by quantification of individual neutral sugars and lignin-derived phenols as well as by scanning electron microscopy, 13C NMR, and elemental analysis. The buried spruce wood from a 2500 yr old deposit was unaltered whereas an alder wood from the same horizon and an oak wood from an open ocean sediment were profoundly degraded. Individual sugar and lignin phenol analyses indicate that at least 90 and 98 wt% of the initial total polysaccharides in the buried alder and oak woods, respectively, have been degraded along with 15–25 wt% of the lignin. At least 75% of the degraded biopolymer has been physically lost from these samples. This evidence is supported by the SEM, 13C NMR and elemental analyses, all of which indicate selective loss of the carbohydrate moiety. The following order of stability was observed for the major biochemical constituents of both buried hardwoods: vanillyl and lignin structural units > syringyl lignin structural units > pectin > α-cellulose > hemicellulose. This sequence can be explained by selective preservation of the compound middle lamella regions of the wood cell walls. The magnitude and selectivity of the indicated diagenetic reactions are sufficient to cause major changes in the chemical compositions of wood-rich sedimentary organic mixtures and to provide a potentially large in situ nutrient source. 相似文献
1000.
Six specimens of unusual chondritic materials were analyzed by neutron activation for 30 elements in order to assess their degree of chondritic compositional pristinity and to search for evidence of genetic links to other chondrites. Five have highly recrystallized textures; the other, the Cumberland Falls chondrite, has suffered minor metamorphic recrystallization. Acapulco and Allan Hills A77081, are closely related and have subpristine compositions; they are more distantly related to Enon which has an altered composition. Udei Station appears to be a IAB meteorite even though its ratio is slightly above the IAB field. The highly weathered meteorite Tierra Bianca is closely related to IAB but has a δ18O value 5 standard deviations higher than the IAB mean and is designated ungrouped. Udei Station and Tierra Bianca have altered compositions; rare earth element patterns indicate loss of a phosphate phase. The elemental composition of the Cumberland Falls chondrite is virtually identical to that of LL chondrites and its O-isotope composition is closely similar to those of some unequilibrated ordinary chondrites including LL Semarkona. The ratios in its olivine are generally much lower than those in pyroxene, a relationship we show to be indicative of in situ reduction resulting from exchange with the aubritic host. The names winonaites and forsterite chondrites have no taxonomic utility. 相似文献