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21.
Alfred N.N. Muzuka Alfonse M. DubiChristopher A. Muhando Yohanna W. Shaghude 《Estuarine, Coastal and Shelf Science》2010
Coral reefs which are an important resource to coastal communities and nation at large are adversely affected by rate of sediment flux to the reefs. However, there is little information on seasonal trend in sediment flux and its impact at the reefs off Zanzibar. Two years’ monthly data on sedimentation at Chumbe and Bawe reefs were used to assess seasonal variability in sediment flux and its implication on the coral status. Sediment flux to the Bawe reefs for the duration of the study ranged from 0.2 to 41.5 mg cm−2 d−1, while it ranged from 0.8 to 65.8 mg cm−2 d−1 at the Chumbe reefs. Sediment fluxes at Bawe reefs were highest between November and March, while they were highest between April and September at Chumbe reefs. Generally, sediment fluxes at Bawe reefs were low compared to those at Chumbe. The total sediment input to the reefs ranged from 4615 to 123,403 kg d−1 for Bawe reefs and 2750 to 79,636 kg d−1 for Chumbe reefs. High sediment fluxes at Bawe reefs between November and March; and the Chumbe reefs between April and September can be attributed to water currents and wind pattern in the east African region which are under the influence of the monsoons. The observed trend suggests that the period for coral transplant as a management option for the two sites should be different. Coral transplant can be undertaken in such a way that stress of the corals due to sedimentation can be felt after they have overcome stress from transplant process and temperature. The results from this study contribute to the much needed information for coral transplant, restoration, and management. 相似文献
22.
Piston cores in Tongue of the Ocean, Bahamas, revealed an alternating sequence of periplatform ooze and bankderived turbidites of the past 5 glacial and interglacial periods. By using the point-count method we have analyzed the composition of the turbidite sediment. Variations in the bank-derived fraction are clearly linked to the glacial-interglacial cycles. Nonskeletal components (pellets; ooids; grapestones) are most abundant in interglacial turbidites, while skeletal components (calcareous algae; peneroplids and reef builders) dominate in glacial turbidites. We attribute this pattern to the exposure and flooding of the surrounding carbonate platforms during Late Quaternary sea-level cycles. Because nonskeletal sediment is produced in the interior of the platforms, its growth and subsequent export to the flanks are at a maximum when the banks are flooded, i.e. during interglacial periods. During glacials, skeletal grains dominate because the banks are exposed and carbonate production is limited to a narrow belt of skeletal sands and fringing reefs. The resulting compositional signal appeared to have a good correlation with aragonite variations in the periplatform ooze between the turbidites. Further, compositional variation parallels the change in turbidite frequency described earlier (»highstand bundling«). Frequency and composition of turbidites are both unaffected by diagenesis and therefore provide an excellent monitor of sealevel fluctuations.
Zusammenfassung In Kolbenlotkernen aus der Tongue of the Ocean, Bahamas, bestehen die jüngsten fünf Glaziale und Interglaziale aus einer Wechselfolge von Periplattform-Schlamm und Turbiditen mit Flachwasser-Material. Die Zusammensetzung der neritischen Kornfraktion in den Turbiditen wurde durch Punktzählung ermittelt. Sie schwankt deutlich im Verlauf eines glazialen Zyklus. Abiogene Komponenten (Pellets, Ooide, Traubenklumpen) dominieren in den interglazialen Turbiditen, während Biogene (Kalkalgen, peneroplide Foraminiferen und Riffbildner) in glazialen Turbiditen vorherrschen. Wir erklären diese Schwankungen mit dem Auftauchen und Überfluten der Bahama Bänke während der jungquartären Schwankungen des Meeresspiegels. Die abiogenen Komponenten werden auf der Innenseite der Plattformen gebildet. Ihre Produktion und Export zu den Plattformflanken erreichen daher ein Maximum während der interglazialen Hochstände des Meeres. In den Glazialen dominieren biogene Komponenten, weil die Plattformen trocken liegen und die Karbonat-Produktion auf einen schmalen Streifen von Saumriffen und Biogensanden beschränkt ist.Die Zusammensetzung der Turbidite variiert im gleichen Rhythmus wie der Aragonitgehalt der zwischengeschalteten Periplattform-Schlamme. Überdies korreliert die Zusammensetzung gut mit der Häufigkeitsverteilung der Turbidite. Während der Interglaziale sind Turbidite häufiger als während der Glaziale (»Hochstand-Bündel«). Häufigkeitsverteilung und Zusammensetzung der Turbidite zusammen ergeben ein gutes, durch Diagenese nicht verwischbares Abbild der Schwankungen des Meeresspiegels.
Résumé Dans le »Tongue of the Ocean« (Bahamas), des sondages à piston effectués dans les sédiments des cinq dernières périodes glaciaires et interglaciaires montrent une alternance de turbidites et de boues déposées en périphérie de plate-forme. La composition de la fraction néritique des turbidites, dé terminée par la méthode du compteur de points, montre une relation claire avec les cycles glaciaires. Les composants abiogènes (pellets, ooïdes, grapestones) dominent dans les turbidites interglaciaires, tandis que les composants biogènes (algues calcaires, foraminifères de type pénéroplide et organismes constructeurs) sont prépondérants dans les turbidites glaciaires. Nous attribuons cette répartition aux émersions et submersions successives du banc des Bahamas, consécutives aux variations du niveau de la mer au Quaternaire récent. Comme les composants abiogènes ont été formés sur la partie interne de la plate-forme, leur développement et leur transport vers les flancs étaient maximaux lorsque le banc était submergé, au cours des périodes interglaci aires. Par contre, les composants biogènes dominaient pendant les épisodes glaciaires, où la plate-forme était exondée et où la production de carbonate était restreinte à une zone étroite de récifs frangeants et de sables biogènes. Ces variations dans la composition des turbidites présentent une bonne corrélation avec la teneur en anhydrite des boues de périphérie de plate-forme intercalées entre les turbidites. Il existe également une bonne corrélation avec la fréquence des turbidites, plus élevée pendant les interglaciaires. La fréquence et la composition des turbidites ne sont pas affectées par la diagenèse et constituent donc un excellent témoin des fluctuations du niveau de la mer.
, «» , , . . . / , , / , , .. , . - . . . , . , . , . , / «Hochstand-Bndel» /. , .相似文献
23.
Fraser Goff Sue J. Goff Sharad Kelkar Lisa Shevenell Alfred H. Truesdell John Musgrave Heinz Rüfenacht Wilmer Flores 《Journal of Volcanology and Geothermal Research》1991,45(1-2)
Results of drilling, logging, and testing of three exploration core holes, combined with results of geologic and hydrogeochemical investigations, have been used to present a reservoir model of the Platanares geothermal system, Honduras. Geothermal fluids circulate at depths ≥ 1.5 km in a region of active tectonism devoid of Quaternary volcanism. Large, artesian water entries of 160 to 165°C geothermal fluid in two core holes at 625 to 644 m and 460 to 635 m depth have maximum flow rates of roughly 355 and 560 l/min, respectively, which are equivalent to power outputs of about 3.1 and 5.1 MW(thermal). Dilute, alkali-chloride reservoir fluids (TDS ≤ 1200 mg/kg) are produced from fractured Miocene andesite and Cretaceous to Eocene redbeds that are hydrothermally altered. Fracture permeabillity in producing horizons is locally greater than 1500 and bulk porosity is ≤ 6%. A simple, fracture-dominated, volume-impedance model assuming turbulent flow indicates that the calculated reservoir storage capacity of each flowing hole is approximately 9.7 × 106 l/(kg cm−2), Tritium data indicate a mean residence time of 450 yr for water in the reservoir. Multiplying the natural fluid discharge rate by the mean residence time gives an estimated water volume of the Platanares system of ≥ 0.78 km3. Downward continuation of a 139°C/km “conductive” gradient at a depth of 400 m in a third core hole implies that the depth to a 225°C source reservoir (predicted from chemical geothermometers) is at least 1.5 km. Uranium-thorium disequilibrium ages on calcite veins at the surface and in the core holes indicate that the present Platanares hydrothermal system has been active for the last 0.25 m.y. 相似文献
24.
Summary With secondary ion mass spectrometry as a new analytical method, which grants trace element determination in the microscale at the ppma-level and below, the abundance, distribution, and correlation of Mn, Fe, Cu, Ga, Ge, As, Ag, Cd, In, Sb, Hg, Tl, and Pb in sphalerites from different occurrences of the Eastern Alps were studied. On the whole the trace element characteristics known from average analyses is confirmed also in the microscale, but in different crystallization stages of one and the same deposit considerable variations can be found. Also in the correlation of elements great differences exist. The eminent inhomogeneity in trace element distribution is an indication for the presence of microcrystals. Thus the assumed Tl-content of already mentioned Pb-As-S-microphases has been confirmed in these investigations.
With 7 Figures
The abbreviations, ppma for atomic parts per million, and ppm for parts per million, will be used throughout. 相似文献
Mikrobereichs-Spurenelementcharakterisierung von Zinkblenden aus ostalpinen Pb–Zn-Vorkommen
Zusammenfassung Mit der Sekundärionen-Massenspektrometrie als neuer analytischen Methode, die die Spurenbestimmung im Mikrobereich bis in den ppma-Bereich und darunter gestattet, wurde Anwesenheit, Verteilung und Korrelierbarkeit von Mn, Fe, Cu, Ga, Ge, As, Ag, Cd, In, Sb, Hg, Tl und Pb in Zinkblenden aus verschiedenen ostalpinen Vorkommen untersucht. Im großen und ganzen wird die aus Durchschnittsanalysen bekannte generelle Spurenelementcharakteristik auch im Mikrobereich bestätigt, doch sind in verschiedenen Kristallisationsphasen von Proben ein und derselben Lagerstätte beträchtliche Variationen festzustellen. Weiters können auch in der Korrelierbarkeit der Begleitelemente große Unterschiede gefunden werden. Die ausgeprägten Inhomogenitäten in der Elementverteilung können auch Hinweise für das Vorliegen kleinster Mikrokristallausscheidungen liefern. So konnte der vermutete Tl-Gehalt der bereits bekannten Pb-As-S-Mikrophasen bestätigt werden.
With 7 Figures
The abbreviations, ppma for atomic parts per million, and ppm for parts per million, will be used throughout. 相似文献
25.
David M. Harris Alfred T. Anderson Jr. 《Contributions to Mineralogy and Petrology》1984,87(2):120-128
The products of the 1974 eruption of Fuego, a subduction zone volcano in Guatemala, have been investigated through study of
silicate melt inclusions in olivine. The melt inclusions sampled liquids in regions where olivine, plagioclase, magnetite,
and augite were precipitating. Comparisons of the erupted ash, groundmass, and melt inclusion compositions suggest that the
inclusions represent samples of liquids present in a thermal boundary layer of the magma body. The concentrations of H2O and CO2 in glass inclusions were determined by a vacuum fusion manometric technique using individual olivine crystals (Fo77 to Fo71)
with glass inclusion compositions that ranged from high-alumina basalt to basaltic andesite. Water, Cl, and K2O concentrations increased by a factor of two as the olivine crystals became more iron-rich (Fo77 to Fo71) and as the glass
inclusions increased in SiO2 from 51 to 54 wt.% SiO2. The concentration of H2O in the melt increased from 1.6 wt.% in the least differentiated liquid to about 3.5% in a more differentiated liquid. Carbon
dioxide is about an order of magnitude less abundant than H2O in these inclusions. The gas saturation pressures for pure H2O in equilibrium with the melt inclusions, which were calculated from the glass inclusion compositions using the solubility
model of Burnham (1979), are given approximately by P(H2O)(Pa)=(SiO2−48.5 wt.%) × 1.45 × 107. The concentrations of water in the melt and the gas saturation pressures increased from about 1.5% to 3.5% and from 300
to 850 bars, respectively, during pre-eruption crystallization. 相似文献
26.
Annie Souriau Alfred Piuzzi Micheline Etchegorry Philippe Machetel 《Journal of Geodesy》1984,58(1):53-72
One of the possibilities of the Doppler positioning from satellite is to provide geodetic measurements continuous in time
without the need for reference stations. If measurements of sufficient accuracy can be achieved they may be used to study
local surface displacements in relation to tectonic activity. A Doppler receiver of the MEDOC network is located near Djibouti
in the Ghoubhat-Asal rift region which corresponds to the accreting plate boundary between the Arabian and African plates.
In November 1978, a seismic and volcanic crisis occured in this area. Surface geodetic measurements and levellings performed
in 1973 and 1978–79, just after the crisis, reveal a 60–80 cm sinking of the graben floor and a lateral extension of about
2 meters.
Here we analyse Doppler measurements for the period January 1977 to November 1980. Point positions are computed for 7 to 10
day intervals using a precise ephemeris, and a moving window analysis is applied to the data. An apparent 2 meter uplift preceding
the November 1978 seismic crisis is detected at Djibouti, whereas no similar phenomenon is observed at the two closest stations,
Pretoria and Uccle-Brussels. However, field observations rule out a tectonic origin for this uplift.
In Djibouti, the correlation between the apparent vertical station position and the electron density in the ionospheric F-layer
reveals that a bias may be induced by the third order term of the ionospheric refractive index not previously taken into account,
or more probably by the ray curvature through the ionosphere. This bias is particularly strong for our data set, from a station
located close to the magnetic equator, and related to a period of rapid increase in the solar activity.
Although our analysis fails to detect any tectonic displacement related to the rift seismic activity, it shows that similar
tectonic studies by the Doppler method will be possible once the residual ionospheric errors are removed, for example by use
of higher radio frequencies. 相似文献
27.
Alfred A. Gübeli Peter A. Hochuli Walter Wildi 《International Journal of Earth Sciences》1984,73(3):1081-1114
The paper describes the palynology, stratigraphy, sedimentology and palaeogeographic setting of Lower Cretaceous turbiditic and terrigenous deep-sea sediments at the Tethyan margins of Africa and Alboran (External Domain of the Rif, and Flysch Nappes). During the Early Cretaceous terrigenous turbiditic deep-water sedimentation characterizes two different palaeogeographic domains in the southern part of the western Tethys: In the northern area of the External Domain (Ketama Unit of the Rif) alternating turbiditic arenites and pelites are interpreted as sediments of a distal part of a fan system on the Tethyan margin of Africa. The main sources for the terrigenous material were situated in Central and Western Algeria; only little sand transited through the Prerif and the Mesorif zones of Morocco. The terrigenous sedimentation began in the Hauterivian, but the main turbiditic cycles are of Aptian to Lower and Middle Albian age. The Flysch Domain probably was situated far to the Northeast with respect to the Rif basin, at the western and southern margins of the Alboran microplate. Relatively proximal turbidites form most of the Tisirène Nappe, whereas more distal turbidites constitute the series of the Melloussa and the Chouamat Nappes. The existence of two different source areas is demonstrated, one to the NE and the other to the NW of the depositional area. The turbidites probably were deposited on a E-W oriented fan system which progradated into a longitudinal trench-like trough. In the Central Rif area, the Tisirène Flysch is of Valanginian to Albian (pre-Vraconian) age. In the Western Rif and in the Melloussa and Chouamat Nappes of the whole area no sediments older than Aptian have been found. The stratigraphy of the investigated series is based on a new tentative palynostratigraphic zonation, using pollen, spores and dinoflagellate cysts. Rich and well preserved assemblages have been found in the Western Rif only, whereas the samples of the Tisirène Nappe and the Chouamat Nappe in the Central and Eastern Rif have been affected by some thermal alteration. Black and mostly opaque palynomorphs from the Ketama Unit reflect the strong thermal influence in the External Domain of the Central and Eastern Rif. 相似文献
28.
Alfred Hirn Nicole Girardin Jean-Pierre Viodé Sylvain Eschenbrenner 《Bulletin of Volcanology》1987,49(6):723-728
Continuous seismic monitoring at Martinique since the 1902 eruption of the Montagne Pelée volcano did not detect local earthquakes for the first 70 years. For the only eruption which occurred in this time span in 1929 the seismograph was 20 km away and of a standard type, not particularly suited for the detection of small-scale local seismicity. Improvement of the monitoring array over the last 15 years with the installation of sensors on the volcano itself allowed the detection of signals of local origin which were interpreted as being due to surface sources, such as rockfalls and landslides. Since December 1985 seismic sources in the volcano itself, i.e. small earthquakes at shallow depth, were identified and located with the aid of a temporary upgrading of the array close to these weak sources. Such an onset of local seismicity could not have been detected with previous seismic equipment; such episodes of seismicity in the volcano might have occurred in the past, apparently quiescent history of the volcano as the reinterpretation of seismograms of some events in 1976 would indicate, without evolving to more important volcanic phenomena. For seismographs on volcanoes the constant upgrading of observation capabilities is certainly perferred to a strict continuity of standard observations. 相似文献
29.
Polymetamorphic relations in iron ores from the Iron Quadrangle,Brazil: The correlation of oxygen isotope variations with deformation history 总被引:2,自引:0,他引:2
Joachim Hoefs Georg Müller Alfred K. Schuster 《Contributions to Mineralogy and Petrology》1982,79(3):241-251
This study deals with the oxygen isotope composition of hematite-rich ore bodies in the Iron Quadrangle, Brazil. The area studied can be divided into two different regions: a western (W) zone of greenschist facies assemblages and an eastern (E) zone of amphibolite facies with transitions into granulite facies assemblages.The
18O values of 136 quartz-iron oxide pairs have been determined and temperatures of formation have been calculated. The
18O values of quartz vary between +6 and 20 except one value near +23, whereas the iron oxides fall between –4 and +10, with nearly 80% of the iron oxide values between –0.5 and 4.0. The regional distribution of the
18O values is as follows: in the W-region 85% of the quartz are >12, whereas in the E-region only 46% fall in this range, In contrast to quartz the iron oxides do not show any regional differences.The variation of oxygen isotope fractionations between quartz and iron oxides is obviously related to the complex deformation history of the iron ores. Samples with a primary schistosity (S1) only represent peak metamorphic conditions. In the E-region the (S1) high temperatures >700° C seem to correspond to orogenic events in the Archaen basement 2,700 m.y. ago. In the W-region S1-temperatures between 460° and 560° C seem to represent peak metamorphic conditions of the Proterozoic Minas metamorphism 2,000 m.y. ago. Iron ores which have been overprinted by later deformation events are selectively reset to lower isotopic temperatures. The more closely spaced the schistosity planes the larger the extent of a temperature lowering.The genetic processes associated with these hematite-rich ore bodies appear to be sedimentary-metamorphic rather than metasomatic processes. Furthermore, there is no evidence for secondary leaching by weathering solutions. 相似文献
30.