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Sven-Ulf Weber Michael Grodzicki Werner Lottermoser Günther J. Redhammer Dan Topa Gerold Tippelt Georg Amthauer 《Physics and Chemistry of Minerals》2009,36(5):259-269
Natural sinhalites, MgAlBO4, from the Ratnapura District, Sri Lanka, and from Bodnar Quarry near Hamburg, Sussex Co., New Jersey, USA, have been characterized by 57Fe Mössbauer spectroscopy, electron microprobe, X-ray single-crystal diffractometry and by electronic structure calculations in order to determine the oxidation state and site occupancy of iron in the sinhalite structure. The samples contain about 3.35 and 1.46 wt% of total iron oxide, respectively. The structure refinement is successful and reproduces the total iron content provided that the substitution of Mg2+ by Fe2+ on the M2 position only is assumed. The 57Fe Mössbauer spectra at 77, 293, 573 and 773 K can be resolved into two doublets with hyperfine parameters common for octahedrally coordinated high-spin Fe2+. There is no evidence for iron in the tetrahedral site. Electronic structure calculations in local spin density approximation yield hyperfine parameters for Fe2+ on the M2-site at 0, 293, 573 and 773 K in quantitative agreement with experiments. Calculated spectroscopic properties for Fe2+ on the M1-site are at variance with the experimental data and, thus, indicate that substitution of Al3+ by Fe2+, if occurring at all, must be accompanied by considerable local expansion and distortion of the M1-octahedron. 相似文献
23.
Detailed 14C AMS data and isotope based stratigraphies from high-resolution paleoceanographic records for the last 22 ka of cores from
the upper continental slope off NE Brazil reveal sedimentation rates of up to 100 cm per 1000 yr. Variations in the sediment
composition relate to changes in the input of terrigenous material. The sedimentation is controlled by sea level and by the
climatic regime of the hinterland. Short-term changes in the tropical wind field may act as a climatic trigger. The zonality
of the SE trades was probably increased and the monsoonal activity over Africa reduced during the Younger Dryas period. 相似文献
24.
Correlated optical, radio and X-ray observations are presented for a pair of consequently homologous flares which occurred on March 17, 1970. A rich complexity of behavior in a bright sub-flare with maximum at 1444 UT is repeated in a flare of importance 1B with maximum at 22:49 UT. The unusual and interesting aspect of these flares is that the second flare developed at approximately half the rate of the first. A difference in the trigger mechanism of the two flares is suggested as a possible explanation. 相似文献
25.
The solar radius at 35 GHz has been determined from solar radio maps made with a pencil beam antenna of half-power beam width 2.8 arcmin at the La Posta Astrogeophysical Observatory during 1973 and 1974. The 35 GHz radius was found to be 2.57% ±0.88% larger than the photospheric radius. The sensitivity of the result to the method of determination is discussed. 相似文献
26.
Saber?Al-RousanEmail author Jürgen?P?tzold Salim?Al-Moghrabi Gerold?Wefer 《International Journal of Earth Sciences》2004,93(6):1066-1076
The stable carbon isotopic composition of the planktonic foraminifera Globigerinoides sacculifer and G. ruber (white) and sedimentary organic matter from the northern Gulf of Aqaba have been investigated to estimate changes in 13CDIC in surface waters during the last 1,000 years. The high sedimentation rates at the core sites (about 54 cm/Kyear) provide high temporal resolution (~10 years). Recent sediments at the top of the cores reflect conditions younger than 1950. The 13C records of the planktonic foraminifera from three multicores display similar trends, showing a uniform and consistent pattern before the 1750s, and a gradual decrease of approximately 0.63 over the last two centuries. This decrease seems to track the decrease of 13CDIC in surface waters, which is mainly caused by the increase of anthropogenic input of 13C-depleted CO2 into the atmosphere. Similarly, a trend towards lighter values of the carbon isotopic composition of sedimentary organic matter (13Corg) during the last 200 years supports the interpretation obtained from the planktonic foraminiferal 13C. Furthermore, direct measurements of seawater show that 13C of the dissolved inorganic carbon (DIC) in the northern Gulf of Aqaba has decreased by about 0.44 during the period 1979–2000. The average annual decrease is 0.021, which is similar to that observed globally. The 13C values of planktonic foraminifera combined with organic matter 13C from marine sediments are good indicators for reconstructing past changes in atmospheric CO2 concentrations from the northern Gulf of Aqaba. 相似文献
27.
While shoreface-connected sand ridges may be molded by storm-generated waves and currents, calmweather counterparts may determine their longevity in the German Bight. Fair-weather flow measurements on shoreface ridges off Spiekeroog Island show that: (1) peak velocities (U100 max) mostly range from 30 to 60 cm s–1 and are flood asymmetric, except at neap tide when ebb flows are dominant in ridge troughs; (2) velocity contrast between accelerating and decelerating flow phases is higher for ebb than flood currents, suggesting intense interaction between inlet and shoreface ebb currents; and (3) tidal currents play a primary role in ridge maintenance. 相似文献
28.
Oscar E. Romero Dierk Hebbeln Gerold Wefer 《Deep Sea Research Part I: Oceanographic Research Papers》2001,48(12):46
Flux of siliceous plankton and taxonomic composition of diatom and silicoflagellate assemblages were determined from sediment trap samples collected in coastal upwelling-influenced waters off northern Chile (30°S, CH site) under “normal” or non-El Niño (1993–94) and El Niño conditions (1997–98). In addition, concentration of biogenic opal and siliceous plankton, and diatom and silicoflagellate assemblages preserved in surface sediments are provided for a wide area between 27° and 43°S off Chile. Regardless of the year, winter upwelling determines the maximum production pattern of siliceous microorganisms, with diatoms numerically dominating the biogenic opal flux. During the El Niño year the export is markedly lower: on an annual basis, total mass flux diminished by 60%, and diatom and silicoflagellate export by 75%. Major components of the diatom flora maintain much of their regular seasonal cycle of flux maxima and minima during both sampling periods. Neritic resting spores (RS) of Chaetoceros dominate the diatom flux, mirroring the influence of coastal-upwelled waters at the CH trap site. Occurrence of pelagic diatoms species Fragilariopsis doliolus, members of the Rhizosoleniaceae, Azpeitia spp. and Nitzschia interruptestriata, secondary components of the assemblage, reflects the intermingling of warmer waters of the Subtropical Gyre. Dictyocha messanensis dominates the silicoflagellate association almost year-around, but Distephanus pulchra delivers ca. 60% of its annual production in less than three weeks during the winter peak. The siliceous thanatocoenosis is largely dominated by diatoms, whose assemblage shows significant qualitative and quantitative variations from north to south. Between 27° and 35°S, the dominance of RS Chaetoceros, Thalassionema nitzschioides var. nitzschioides and Skeletonema costatum reflects strong export production associated with occurrence of coastal upwelling. Both highest biogenic opal content and diatom concentration at 35° and 41°–43°S coincide with highest pigment concentrations along the Chilean coast. Predominance of the diatom species Thalassiosira pacifica and T. poro-irregulata, and higher relative contribution of the silicoflagellate Distephanus speculum at 41°–43°S suggest the influence of more nutrient-rich waters and low sea surface temperatures, probably associated with the Antarctic Circumpolar Water. 相似文献
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30.
Harald Gerold Dill 《Arabian Journal of Geosciences》2011,4(1-2):123-146
Jordan, located at the western edge of the Arabian Plate, stands out from the remaining part of the Arabian Peninsula by its abundance in radioactive elements, mainly uranium, in a way so far not found elsewhere on the Arabian Peninsula. Uranium (U) and thorium in Jordan are concentrated in eight different types of ore mineralization: (1) intrusive-related (intramagmatic), (2) vein-type, (3) superficial, (4) sandstone-hosted (5) limestone-hosted, (6) U-Th-REE placer-type, (7) black shales, and (8) phosphorites. The major concentration of radioactive elements are synsedimentary and diagenetic in nature, mainly in near-shore marine depositional environments where uranium contents are abnormally high in the late Cretaceous to Paleogene phosphorites and increasing towards the mobile shelf of the Tethys ocean. These uraniferous phosphorites form the source of uranium that was redeposited within terrigenous chemical residues of lacustrine-fluvial depositional systems in Central Jordan (calcretes). Faultbound radiometric anomalies are caused by hot brines being vented along with the Jordan-Dead-Sea rifting. Presumably, low-grade U accumulation in (hot) black shales and marls of Silurian age are responsible for these radiometric anomalies. In the present paper, the Jordanian uranium concentrations are compared with reference types of uranium deposits elsewhere in the world to get an idea if the geological, chemical, and mineralogical features of analogue uranium mineralization in Jordan are indicative of economic targets. The uranium concentration in Jordanian phosphorites has been tracked beyond the border into Syria, Iraq, Israel, and Saudi Arabia. The uranium potential in neighboring countries is assessed based on the current geological data available for the Mediterranean Phosphorite Belt which is poised to become a another string to the bow with respect to energy supply on the Arabian Peninsula. 相似文献