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761.
Alan J. Wainwright Richard M. Tosdal Joseph L. Wooden Frank K. Mazdab Richard M. Friedman 《Gondwana Research》2011,19(3):764-787
Uranium–Pb (zircon) ages are linked with geochemical data for porphyry intrusions associated with giant porphyry Cu–Au systems at Oyu Tolgoi to place those rocks within the petrochemical framework of Devonian and Carboniferous rocks of southern Mongolia. In this part of the Gurvansayhan terrane within the Central Asian Orogenic Belt, the transition from Devonian tholeiitic marine rocks to unconformably overlying Carboniferous calc-alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation. Radiogenic Nd and Pb isotopic compositions (εNd(t) range from + 3.1 to + 7.5 and 206Pb/204Pb values for feldspars range from 17.97 to 18.72), as well as low high-field strength element (HFSE) contents of most rocks (mafic rocks typically have < 1.5% TiO2) are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc. The Late Devonian and Carboniferous felsic rocks are dominantly medium- to high-K calc-alkaline and characterized by a decrease in Sr/Y ratios through time, with the Carboniferous rocks being more felsic than those of Devonian age. Porphyry Cu–Au related intrusions were emplaced in the Late Devonian during the transition from tholeiitic to calc-alkaline arc magmatism. Uranium–Pb (zircon) geochronology indicates that the Late Devonian pre- to syn-mineral quartz monzodiorite intrusions associated with the porphyry Cu–Au deposits are ~ 372 Ma, whereas granodiorite intrusions that post-date major shortening and are associated with less well-developed porphyry Cu–Au mineralization are ~ 366 Ma. Trace element geochemistry of zircons in the Late Devonian intrusions associated with the porphyry Cu–Au systems contain distinct Th/U and Yb/Gd ratios, as well as Hf and Y concentrations that reflect mixing of magma of distinct compositions. These characteristics are missing in the unmineralized Carboniferous intrusions. High Sr/Y and evidence for magma mixing in syn- to late-mineral intrusions distinguish the Late Devonian rocks associated with giant Cu–Au deposits from younger magmatic suites in the district. 相似文献
762.
Zooplankton are an important trophic link and a key food source for many larval fish species in estuarine ecosystems. The
present study documents temporal and spatial zooplankton dynamics in Suisun Bay and the Sacramento–San Joaquin Delta—the landward
portion of the San Francisco Estuary (California, USA)—over a 37-year period (1972–2008). The zooplankton community experienced
major changes in species composition, largely associated with direct and indirect effects of introductions of non-native bivalve
and zooplankton species. A major clam invasion and many subsequent changes in zooplankton abundance and composition coincided
with an extended drought and accompanying low-flow/high-salinity conditions during 1987–1994. In the downstream mesohaline
region, the historically abundant calanoid copepods and rotifers have declined significantly, but their biomass has been compensated
to some extent by the introduced cyclopoid Limnothoina tetraspina. The more upstream estuary has also experienced long-term declining biomass trends, particularly of cladocerans and rotifers,
although calanoid copepods have increased since the early 1990s due to the introduced Pseudodiaptomus spp. In addition, mysid biomass has dropped significantly throughout the estuary. Shifts in zooplankton species composition
have also been accompanied by an observed decrease in mean zooplankton size and an inferred decrease in zooplankton food quality.
These changes in the biomass, size, and possibly chemical composition of the zooplankton community imply major alterations
in pelagic food web processes, including a drop in prey quantity and quality for foraging fish and an increase in the importance
of the microbial food web for higher trophic levels. 相似文献
763.
Paul D. Feldman S. Alan Stern Joel Wm. Parker Michael F. A'Hearn Michel C. Festou 《Icarus》2007,187(1):104-108
We report on spectroscopic observations of periodic Comet 9P/Tempel 1 by the Alice ultraviolet spectrograph on the Rosetta spacecraft in conjunction with NASA's Deep Impact mission. Our objectives were to measure an increase in atomic and molecular emissions produced by the excavation of volatile sub-surface material. We unambiguously detected atomic oxygen emission from the quiescent coma but no enhancement at the 10% (1-σ) level following the impact. We derive a quiescent H2O production rate of 9×1027 molecules s−1 with an estimated uncertainty of ∼30%. Our upper limits to the volatiles produced by the impact are consistent with other estimates. 相似文献
764.
Alan J. Hewitt Natasha Doss Nikolai F. Zobov Oleg L. Polyansky Jonathan Tennyson † 《Monthly notices of the Royal Astronomical Society》2005,356(3):1123-1126
HDO and D2 O are thought to be present in significant quantities in the coldest regions of the Universe. Partition functions are calculated for these species and used to calculate temperature dependent equilibrium constants for the most important fractionation reactions involving water. HDO may also be a key indicator of the absence of deuterium burning in very low mass stars. The HDO partition function calculation is extended to high ( T ≤ 6000 K) temperatures for this purpose. 相似文献
765.
Jordi LLORCA Josep M. TRIGO‐RODRÍGUEZ Jos L. ORTIZ Jos A. DOCOBO Javier GARCÍA‐GUINEA Alberto J. CASTRO‐TIRADO Alan E. RUBIN Otto EUGSTER Wayne EDWARDS Matthias LAUBENSTEIN Ignasi CASANOVA 《Meteoritics & planetary science》2005,40(6):795-804
Abstract— An impressive daylight fireball was observed from Spain, Portugal, and the south of France at 16h46m45s UTC on January 4, 2004. The meteoroid penetrated into the atmosphere, generating shock waves that reached the ground and produced audible booms. The associated airwave was recorded at a seismic station located 90 km north of the fireball trajectory in Spain, and at an infrasound station in France located 750 km north‐east of the fireball. The absolute magnitude of the bolide has been determined to be ?18 ± 1 from a casual video record. The energy released in the atmosphere determined from photometric, seismic, and infrasound data was about 0.02 kilotons (kt). A massive fragmentation occurred at a height of 28 ± 0.2 km, resulting in a meteorite strewn field of 20 × 6 km. The first meteorite specimen was found on January 11, 2004, near the village of Villalbeto de la Peña, in northern Palencia (Spain). To date, about 4.6 kg of meteorite mass have been recovered during several recovery campaigns. The meteorite is a moderately shocked (S4) L6 ordinary chondrite with a cosmic‐ray‐exposure age of 48 ± 5 Ma. Radioisotope analysis shows that the original body had a mass of 760 ± 150 kg, which is in agreement with the estimated mass obtained from photometric and seismic measurements. 相似文献
766.
Mass selection bias in galaxy cluster peculiar velocities from the kinetic Sunyaev–Zel'dovich effect
Alan C. Peel 《Monthly notices of the Royal Astronomical Society》2006,365(4):1191-1202
Upcoming surveys for galaxy clusters using the Sunyaev–Zel'dovich effect are potentially sensitive enough to create a peculiar velocity catalogue. The statistics of these peculiar velocities are sensitive to cosmological parameters. We develop a method to explore parameter space using N -body simulations in order to quantify dark matter halo velocity statistics which will be useful for cluster peculiar velocity observations. We show that mass selection bias from a kinetic Sunyaev–Zel'dovich velocity catalogue forecasts rms peculiar velocities with a much more complicated Ωm dependency than suggested by linear perturbation theory. In addition, we show that both two-point functions for velocities disagree with linear theory predictions out to ∼40 h −1 Mpc separations. A pedagogical appendix is included developing linear theory notation with respect to the two-point peculiar velocities functions. 相似文献
767.
Gert Finger James W. Beletic Reinhold Dorn Manfred Meyer Leander Mehrgan Alan F. M. Moorwood Joerg Stegmeier 《Experimental Astronomy》2005,19(1-3):135-147
The conversion gain of optical and infrared focal plane CMOS hybrid arrays is a fundamental parameter, whose value computes into the derivation of other parameters characterizing the performance of a detector. The widespread “noise squared versus signal” method used to obtain the conversion gain can overestimate the nodal capacitance of the detector pixel by more than 20% for infrared arrays and by more than 100% for Si-PIN diode arrays. This is because this method does not take account of the capacitive coupling between neighboring pixels. A simple technique has been developed to measure the nodal capacitance directly by comparing the voltage change of an external calibrated capacitor with the voltage change on the nodal capacitor of the detector pixel. The method is elaborated in detail and has been verified with a Si-PIN diode array hybridized to a Hawaii-2RG multiplexer using an Fe
55 X-ray source. It is also in good agreement with a stochastic method based on 2D autocorrelation. 相似文献
768.
A replicated field study using rainfall simulation and overland flow application was conducted in central Oahu, Hawaii, on a clay‐dominated Oxisol with a 9% slope. Three main treatment groups were examined: a bare treatment, a group of four rolled erosion control systems (RECSs) with open weave designs, and a group of five randomly oriented fibre RECSs. A total of 1122 measurements of runoff and erosion were made to examine treatment differences and to explore temporal patterns in runoff and sediment flux. All erosion control systems significantly delayed the time required to generate plot runoff under both simulated rainfall (35 mm h?1) and the more intense trickle flow application (114 mm h?1). Once runoff was generated during the rainfall application phase, the bare treatment runoff coefficients were significantly lower than those from the two groups of RECSs, as surface seal disruption by rilling is inferred to have enhanced infiltration in the bare treatments. During the more intense phase of overland flow application, the reverse pattern was observed. Interrill contributing‐area roughness was reduced on the bare treatment, facilitating increased runoff to well‐developed rill networks. Meanwhile, the form roughness associated with the RECSs delayed interrill flow to the poorly organized rills that formed under some of the RECSs. Regardless of runoff variations between treatments, sediment output was significantly lower from all surfaces covered by RECSs. The median cumulative sediment output from the bare surfaces was 6·9 kg, compared with 1·2 kg from the open‐weave RECSs and 0·2 kg from the random‐fibre RECSs. The random‐fibre systems were particularly effective under the more stressful overland flow application phase, with 63 times less sediment eroded than the bare treatments and 12 times less than that from the open‐weave systems. Architectural design differences between the two groups of RECSs are discussed in light of their relation to erosion process dynamics and shear stress partitioning. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
769.
Benjamin Panter Raul Jimenez Alan F. Heavens Stephane Charlot 《Monthly notices of the Royal Astronomical Society》2007,378(4):1550-1564
We present the results of a moped analysis of ∼3 × 105 galaxy spectra from the Sloan Digital Sky Survey Data Release 3 (SDSS DR3), with a number of improvements in data, modelling and analysis compared with our previous analysis of DR1. The improvements include: modelling the galaxies with theoretical models at a higher spectral resolution of 3 Å, better calibrated data, an extended list of excluded emission lines and a wider range of dust models. We present new estimates of the cosmic star formation rate (SFR), the evolution of stellar mass density and the stellar mass function from the fossil record. In contrast to our earlier work the results show no conclusive peak in the SFR out to a redshift around 2 but continue to show conclusive evidence for 'downsizing' in the SDSS fossil record. The star formation history is now in good agreement with more traditional instantaneous measures. The galaxy stellar mass function is determined over five decades of mass, and an updated estimate of the current stellar mass density is presented. We also investigate the systematic effects of changes in the stellar population modelling, the spectral resolution, dust modelling, sky lines, spectral resolution and the change of data set. We find that the main changes in the results are due to the improvements in the calibration of the SDSS data, changes in the initial mass function and the theoretical models used. 相似文献
770.
Alan S. Collins Chris Clark K. Sajeev M. Santosh David E. Kelsey Martin Hand 《地学学报》2007,19(2):141-147
The Mozambique Ocean closed as Gondwana formed. Its suture has been identified in Madagascar (Betsimisaraka suture), but its continuation, into India, is controversial. The Palghat‐Cauvery shear system appears an ideal candidate as it: (i) lies along strike of the Betsimisaraka suture in Gondwana; (ii) forms a high‐pressure granulite belt; and (iii) separates crustal domains with different geological histories. However, existing age constraints have been used to suggest that the structure is Archaean/Palaeoproterozoic. Here we date metamorphic zircons using secondary ion mass spectrometry (535.0 ± 4.9 Ma) and monazites using electron probe micro‐analysis (537 ± 9, 532 ± 8, 525 ± 10 Ma). No evidence for an earlier metamorphic event was found. The identification of Palghat‐Cauvery high‐pressure metamorphism as Cambrian, and recognition that it bounds crustal domains of contrasting origin, points to it being the southern continuation of the Betsimisaraka suture and southern margin of Neoproterozoic India. 相似文献