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991.
Orpin AR Ridd PV Thomas S Anthony KR Marshall P Oliver J 《Marine pollution bulletin》2004,49(7-8):602-612
Coastal development activities can cause local increases in turbidity and sedimentation. This study characterises the spatial and temporal variability of turbidity near an inshore fringing coral reef in the central Great Barrier Reef, under a wide range of natural conditions. Based on the observed natural variability, we outline a risk management scheme to minimise the impact of construction-related turbidity increases. Comparison of control and impact sites proved unusable for real-time management of turbidity risks. Instead, we suggest using one standard deviation from ambient conditions as a possible conservative upper limit of an acceptable projected increase in turbidity. In addition, the use of regional weather forecast as a proxy for natural turbidity is assessed. This approach is simple and cheap but also has limitations in very rough conditions, when an anthropogenic turbidity increase could prove fatal to corals that are already stressed under natural conditions. 相似文献
992.
Anthony Finizola Jean-Franois Lnat Orlando Macedo Domingo Ramos Jean-Claude Thouret Francesco Sortino 《Journal of Volcanology and Geothermal Research》2004,135(4):216
One of the seven potentially active andesite stratovolcanoes in southern Peru, Misti (5822 m), located 17 km northeast and 3.5 km above Arequipa, represents a major threat to the population (900,000 inhabitants). Our recent geophysical and geochemical research comprises an extensive self-potential (SP) data set, an audio–magnetotelluric (AMT) profile across the volcano and CO2 concentrations in the soil along a radial profile. The SP survey is the first of its kind in providing a complete mapping of a large andesitic stratovolcano 20 km in diameter. The SP mapping enables us to analyze the SP signature associated with a subduction-related active volcano.The general SP pattern of Misti is similar to that of most volcanoes with a hydrogeologic zone in the lower flanks and a hydrothermal zone in the upper central area. A quasi-systematic relationship exists between SP and elevation. Zones with constant SP/altitude gradients (Ce) are observed in both hydrogeologic (negative Ce) and hydrothermal (positive Ce) zones. Transition zones between the different Ce zones, which form a concentric pattern around the summit, have been interpreted in terms of lateral heterogeneities in the lithology. The highest amplitudes of SP anomalies seem to coincide with highly resistive zones. The hydrothermal system 6 km in diameter, which extends over an area much larger than the summit caldera, may be constrained by an older, concealed collapse caldera. A sealed zone has apparently developed through alteration in the hydrothermal system, blocking the migration of CO2 upward. Significant CO2 emanations are thus observed on the lower flanks but are absent above the hydrothermal zone. 相似文献
993.
Hennebelle Patrick Belloche Arnaud Whitworth Anthony André Philippe 《Astrophysics and Space Science》2004,292(1-4):355-360
We present numerical simulations of the evolution of low-mass, isothermal, molecular cores which are subjected to an increase in external pressure. If the external pressure increases very slowly, the core approaches instability quite quasi-statically. However, for faster compressions, a compression wave is driven into the core (Hennebelle, P., Whitworth, A., Gladwin, P. and André, P.: 2003a MNRAS 340, 870). Quantitative comparisons with observational velocity and density profiles are presented. The consequences of this compression for the fragmentation of the cloud is investigated and discussed. 相似文献
994.
995.
The network of interrelationships among a set of 22 geographic journals is investigated, using data on the frequency with which articles published in one journal cite papers published In another. The analysis considers journals published in 1970–1972 and 1980–1982. Journals with a focus on regional science weaken their connection to others in the set. The Annals retains its position at the core of the journal space that is identified using multidimensional scaling; Environment and Planning A invades the niche vacated by the regional science journals. 相似文献
996.
Anthony R. Dobrovolskis 《Icarus》1983,56(1):165-175
Diurnal solar heating of Venus' surface produces variable temperatures, winds, and pressure gradients within a shallow layer at the botton of the atmosphere. The corresponding asymmetric mass distribution experiences a tidal torque tending to maintain Venus' slow retrograde rotation. It is shown that including viscosity in the boundary layer does not materially affect the balance of torques. On the other hand, friction between the air and ground can reduce the predicted wind speeds from ∽5 to ∽1 m/sec in the lower atmosphere, more consistent with the observations from Venus landers and descent probes. Implications for aeolian activity on Venus' surface and for future missions are discussed. 相似文献
997.
Anthony R. Dobrovolskis 《Icarus》1983,54(3):539-542
The periodic absorption and desorption of CO2 by the soil on Venus may buffer daily temperature, pressure, and wind variations in the lower atmosphere. This would effectively eliminate the net tidal torque on the atmosphere. Nevertheless, the redistribution of mass in the crust would generate a sizable torque, which might balance that due to the gravitationally induced tide. 相似文献
998.
We apply an oxygen barometer based on the Fe content of CaTiO3perovskite to estimate the oxygen fugacity (fO2) during thecrystallization and emplacement of kimberlites in differenteruptive phases of a single pipe, or between different pipes,clusters or provinces. Mineral chemical data for perovskitewere compiled from the literature and obtained in our detailedstudy of perovskites from 11 kimberlites at Somerset Islandand Lac de Gras, Canada. Perovskite compositions in kimberlitesrecord a range in fO2 of many orders of magnitude from NNO5to NNO+6 [where log fO2 is given relative to the nickelnickeloxide (NNO) buffer]. The range of fO2 recorded by differentparageneses of perovskite within a single pipe can vary up tothree orders of magnitude with trends toward both oxidationand reduction during crystallization. Kimberlites record someof the greatest ranges, and the highest known fO2 conditionsfor any terrestrial magma. This is attributed to the presenceof deep and oxidized source regions and the variable interplayof ferricferrous vs carbonfluid equilibria duringascent of kimberlite magmas. Three kimberlite pipes from theLac de Gras field show that higher fO2 values correlate withhigher proportions of more resorbed diamonds, suggesting thatthis variable has a measurable effect on the physical propertiesof diamonds in a pipe. KEY WORDS: kimberlites; oxygen fugacity; perovskite; diamond; redox; mantle 相似文献
999.
Sulfur isotopic evidence for chemocline upward excursions during the end-Permian mass extinction 总被引:1,自引:0,他引:1
Anthony L. Riccardi Michael A. Arthur Lee R. Kump 《Geochimica et cosmochimica acta》2006,70(23):5740-5752
The latest Permian was a time of major change in ocean chemistry, accompanying the greatest mass extinction of the Phanerozoic. To examine the nature of these changes, samples from two well-studied marine sections that span the Permian-Triassic boundary have been analyzed: the Meishan and Shangsi sections located in Southern China. Isotopic analysis of the carbonate-associated sulfate in these samples provides a detailed record of several isotopic shifts in δ34SCAS approaching and across the PTB, ranging from +30 to −15‰ (VCDT), with repeated asynchronous fluctuations at the two locations. We interpret the patterns of isotopic shifts, in conjunction with other data, to indicate a shallow unstable chemocline overlying euxinic deep-water which periodically upwelled into the photic zone. These chemocline upward excursion events introduced sulfide to the photic zone stimulating a bloom of phototrophic sulfur oxidizing bacteria. We hypothesize that elemental sulfur globules produced by these organisms and 34S-depleted pyrite produced in the euxinic water column were deposited in the sediment; later oxidation led to incorporation as CAS. This created the large changes to the δ34SCAS observed in the latest Permian at these locations. 相似文献
1000.