A. R?pke  M. von Weber  A. Bachor 《Ocean Dynamics》1998,50(2-3):281

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The emergence of a Galápagos shield volcano, Roca Redonda     

Jeff Standish  Dennis Geist  Karen Harpp  Mark D. Kurz 《Contributions to Mineralogy and Petrology》1998,133(1-2):136-148

Roca Redonda volcano is a mostly submarine shield volcano that rises nearly 3 km from the adjacent seafloor. Over twenty lava flows and palagonite tuff are exposed in a 60 meter high oblong outcrop above sea level, and several other flows are exposed in the shallow water surrounding the islet. Thick, slightly alkaline picritic flows form the base of the section. Thinner picrites interbedded with sparsely porphyritic alkali-olivine basaltic pahoehoe toes characterize the upper section. The subaerial section probably records the filling of a palagonite tuff cone with younger lavas. Numerous fumaroles that may have a magmatic component are present in the shallow (<30 m) submarine zone and indicate that the volcano is probably still active. Three lava types are exposed: the basal picrites with 19% > MgO > 14%, high-Mg basalts with MgO of about 9%, and low-Mg basalts with MgO of about 6%. The Sr and Nd isotopic ratios of the three lava types are within analytical uncertainty. Olivine compositions indicate that the picrites are basaltic liquids that have accumulated olivine whose composition is in equilibrium with the host basaltic liquid. Apparently, basaltic magmas percolated through dunite and troctolite that had crystallized from slightly older Roca Redonda basaltic magma. Lavas from Roca Redonda have enriched trace element contents and isotopic ratios relative to nearby Wolf volcano, but they are quite similar to lavas from Cerro Azul and Ecuador volcanoes. The common characteristic of these volcanoes is that they lie on the periphery of the archipelago and are in a stage of subaerial growth. This suggests that Galápagos volcanoes may go through a juvenile alkaline stage before a mature tholeiitic stage, analogous to the Loihi stage of Hawaiian volcanism. A low ³He/⁴He ratio in olivine from one of the picrites indicates a small contribution by the Galápagos mantle plume. Received: 15 December 1997 / Accepted: 6 May 1998  相似文献   

Electron paramagnetic resonance spectroscopy: A suggested approach to trace metal analysis in marine environments     

Bruce A. Burgess  N. Dennis Chasteen  Henri E. Gaudette 《Environmental Geology》1975,1(3):171-180

Electron paramagnetic resonance (EPR) spectroscopy analysis of marine samples from different environments appears to differentiate between adsorbed and structural Mn (II) and Fe (III) sites in the sediment. This suggests that EPR may provide a means of distinguishing different environmental influences on sediment. Acid extract solutions from sediment samples exhibit clearly defined EPR spectra due to Mn(II), Ti(III), Fe(III), and VO(IV), which are amenable to qualitative and quantitative analysis at concentrations below one part per million. Spectra of several shellfish vary considerably, both between species, and within a species, depending on sampling localities. Resonances from Mn(II), Mo(V), and Fe(III) can be obtained. Mn(II) is substituted for Ca(II) in the calcite structure of some shells. The low detection limits, small sample size, required and identification of oxidation states by EPR complement other analytical techniques and may prove useful in marine systems.  相似文献   

Mercury in fluvial bed sediments subsequent to contamination     

Dennis G. Waslenchuk 《Environmental Geology》1975,1(3):131-136

Once inputs of mercury-bearing effluents to a fluvial system are stopped, levels of contamination will decline slowly to background values. Crystalline mercury phases will dissolve in response to lowered aqueous mercury concentrations, and aqueous Hg(OH)₂ will desorb from ferric hydroxide grain coatings as the coatings age to more ordered phases. In the Ottawa River, mercury concentrations in bed sediments are declining by about 50 percent annually. Editor's note: Waslenchuk is now with the Georgia Institute of Technology School of Geophysical Sciences, Atlanta, Georgia 30332, U.S.A.  相似文献   

Seafloor spreading history of the Agulhas Basin     

John L. Labrecque  Dennis E. Hayes 《Earth and Planetary Science Letters》1979,45(2):411-428

Data gathered by recent “Islas Orcadas” cruises reveal the seafloor spreading pattern for a region south of the Agulhas/Falkland fracture zone system. The presence of a magnetic anomaly bight about the Agulhas Plateau indicates that the Agulhas Plateau may have developed at the site of a tectonic plate triple junction during the Late Cretaceous. A westward jump in the seafloor spreading center during the Late Maestrichtian (anomaly 34?31) reduced the offset across the Falkland/Agulhas fracture zone system and resulted in the formation of two conjugate aseismic ridges here described as the Meteor and Islas Orcadas Rises. The magnetic lineation pattern in the Agulhas Basin suggests that a tectonic plate (Malvinas Plate) existed during Campanian to Maestrichtian times. Relative rates of motion are calculated for Antarctica, South America, and Africa for the Late Cretaceous.  相似文献   

Antithethic and homothetic faults     

J. G. Dennis  V. C. Kelley 《International Journal of Earth Sciences》1980,69(1):186-193

Antithetic faults are dip slip faults that displace rocks so as to prevent or reduce structural relief, typically producing tilted fault blocks. They may or may not occur in conjugate systems with their opposites, homothetic faults, which serve to increase structural relief. Several examples serve to illustrate the concept and to correct current misunderstandings.

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Zusammenfassung Die Arbeit soll die Begriffe antithetisch und homothetisch klären helfen. Antithetische Verschiebungen wirken dem tektonischen Relief entgegen, homothetische Verwerfungen unterstützen es. Die Begriffe werden an einigen Beispielen erläutert.

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Résumé Ce travail constitue une tentative d'élucider les termes »antithétique« et »homothétique«. Le rejet des failles antithétiques s'oppose à la création d'un relief tectonique. Le rejet des failles homothétiques augmente ce relief. Quelques exemples sont discutés.

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Zur Schwefelisotopenzusammensetzung der Pb-Zn-Vererzung des Grazer Paläozoikums (Ostalpen)     

E. Pak  E. Schroll  L. Weber 《Mineralium Deposita》1980,15(3):315-325

Zusammenfassung Die Schwefelisotopenzusammensetzung von sulfidischen Erzmineralen und Baryten der unterdevonischen schichtgebundenen Bleiglanz-Zinkblende-Baryt-Erzvorkommen des Grazer Paläozoikums (Ostalpen, Österreich) wurde untersucht. Zur Analyse gelangten 64 Proben von etwa 15 Lokalitäten. Die Schwefelisotopenverteilung zeigt Ähnlichkeiten mit anderen an das Devon gebundenen schwerspatführenden Lagerstätten. Die Baryte ergaben ³⁴S-Werte von +23, 1 bis +27, 8, die dem Meerwassersulfat devonischen Alters entsprechen. Die untersuchten Galenite variieren von +2, 4 bis +6, 9 und die Sphalerite von +3, 3 bis +9, 1. Die Werte der Pyrite zeigen einen großen Streubereich (–27, 4 bis +37, 6). Eine massive buntmetallarme Pyritvererzung fällt durch sehr schweren Schwefel auf. Späte Mineralisationsphasen der Sulfide tendieren gleichfalls zur Anreicherung des schweren Schwefels.

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The sulphur isotope compositions of 64 samples of sulphide minerals and barites from the strata-bound Lower Devonian ore deposits of the Paleozoic Complex of Graz in the Eastern Alps (Austria) have been investigated. The ³⁴S-values of galenites and sphalerites show only small variations in the range of +2, 4 up to +6, 9 for PbS and +3, 3 up to +9, 1 for ZnS indicating abiogenic origin. Later remobilized sulphide minerals show a tendency to heavier sulphur. The deposition of pyrites is influenced partly by bacteriogenic processes (–27, 4 up to +37, 6). The enrichment of heavier sulphur characterizes massive pyrite mineralisations being poor on base metals. The sulphur isotope compositions of barites (+23, 1 up to 27, 8) correspond to Devonian sea water sulphate. An ore free barite characterized by increased strontium concentration shows heavier sulphur (+29, 6) than barites from ore beds.

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Derzeit Oberste Bergbehörde, Bundesministerium für Handel, Gewerbe und Industrie (Wien)  相似文献   

Atmospheric effects in the remote sensing of phytoplankton pigments   总被引：3，自引：0，他引：3  

Howard R. Gordon  Dennis K. Clark 《Boundary-Layer Meteorology》1980,18(3):299-313

We investigate the accuracy with which relevant atmospheric parameters must be estimated to derive phytoplankton pigment concentrations (chlorophyll a plus phaeophytin a ) of a given accuracy from measurements of the ocean's apparent spectral radiance at satellite altitudes. The analysis is limited to an instrument having the characteristics of the Coastal Zone Color Scanner scheduled to orbit the Earth on NIMBUS-G. A phytoplankton pigment algorithm is developed which relates the pigment concentration (C) to the three ratios of upwelling radiance just beneath the sea surface which can be formed from the wavelengths () 440, 520 and 550 nm. The pigment algorithm explains from 94 to 98% of the variance in log₁₀ C over three orders of magnitude in pigment concentration. This is combined with solutions to the radiative transfer equation to simulate the ocean's apparent spectral radiance at satellite altitudes as a function of C and the optical properties of the aerosol, the optical depth of which is assumed to be proportioned to ^-n . A specific atmospheric correction algorithm, based on the assumption that the ocean is totally absorbing at 670 nm, is then applied to the simulated spectral radiance, from which the pigment concentration is derived. Comparison between the true and derived values of C show that: (1) n is considerably more important than the actual aerosol optical thickness; (2) for C 0299-1 0.2 g l^-1 acceptable concentrations can be determined as long as n is not overestimated; (3) as C increases, the accuracy with which n must be estimated, for a given relative accuracy in C, also increases; and (4) for C greater than about 0.5 g 1^-1, the radiance at 440 nm becomes essentially useless in determining C. The computations also suggest that if separate pigment algorithms are used for C 1gl^-1 and C 1 gl^-1, accuracies considerably better than ±± in log C can be obtained for C 1 g l^-1 with only a coarse estimate of n, while for C 10 gl^-1, this accuracy can be achieved only with very good estimates of n.Contribution No. 387 from the NOAA/ERL Pacific Environmental Laboratory.On leave from Department of Physics, University of Miami, Coral Gables, Florida.  相似文献   

Frequency Changes of Weather Types in the Alpine Region since 1945     

G. Stefanicki  P. Talkner  R. O. Weber 《Theoretical and Applied Climatology》1998,60(1-4):47-61

Summary The annual occurrence of different weather types of Schüepp’s synoptic classification in the Alpine region has significantly changed since the beginning of its recording in 1945. The annual frequency (number of days per year) has shifted towards more convective and less advective weather types. Since 1945 the number of long-lasting convective episodes rose and the number of long-lasting advective episodes lessened. Most of these changes took place in winter. The annual frequencies of weather types and the annual mean of certain local meteorological parameters are significantly correlated. On the large scale there is a strong interdependence between the high pressure weather type and the North Atlantic Oscillation (NAO) index which is based on the sea-level pressure difference between Portugal and Iceland. Received July 28, 1997 Revised November 18, 1997  相似文献   

Geochronology: Radiometric dating of rocks and minerals: Christopher T. Harper (Editor), 1973. Benchmark Papers in Geology. Dowden, Hutchinson and Ross, Stroudsburg, Pa., xv + 469 pp., £ 12.00     

Jon N. Weber 《Earth》1974,10(4):349

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