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361.
Microgram quantities of lead were volatilized from the chondrites Richardton, Holbrook, Beardsley and Plainview and purified for mass spectrometric analysis. Aliquots of approximately 1 μg were surface-ionized by use of the boric acid technique. The isotopic compositions found range from highly radiogenic lead in the case of Richardton to leads in the other three chondrites which are substantially less radiogenic than average lead in the crust of the earth. p]From our recent concentration determinations by the isotopic dilution method, it seems that the amount of lead in Beardsley is about 0·13 p.p.m. and that in Richardton about 0·06 p.p.m. The leads in these meteorites are consistent within a factor of approximately two with the lead-lead ages and the independently measured uranium and thorium concentrations. The lead—lead age for Richardton is the same as that found by
(1955) for Nuevo Laredo, being 4·6 Æ. The lead concentration measured for Forest City is 0·09 p.p.m., while that for Holbrook is 0·28 p.p.m. Holbrook is quite anomalous in containing 2·5 times the amount of radiogenic lead that can be explained by the accumulation of decay products in situ with a primeval lead which had the isotopic composition of the lead from the troilite in the Canyon Diablo and Henbury iron meteorites. p]The total amount of lead on the surface of a meteorite appears to be comparable to the total amount inside. The isotopic composition of this surface lead indicates that it is principally terrestrial lead. 相似文献
362.
The profiles of the Fe xiv, 5303, and Fe x, 6374, emission lines of the solar corona have been observed at different positions using a photoelectric scanning Fabry-Perot interferometer. These profiles were obtained during the eclipse of 7th March 1970, in Mexico and at the Pic-du-Midi coronagraph in October, 1970. The half-widths of these profiles were determined for both the coronal lines and temperatures were derived from these widths. No systematic temperature variation was discovered, however there was some suggestion of the existence of a fluctuation with time in the width of the emission lines. 相似文献
363.
364.
To determine maximum-likelihood magnitudes for a given seismological network requires estimates of the detection thresholds
of short-period P for the individual stations. Ideally the detection threshold of a station with fixed instrumentation and
operating procedures would be constant. However, some stations reporting to the International Seismological Centre (ISC) show
systematic variations with time. Other stations are unreliable or report amplitudes (A) and periods (T) only intermittently. Ways are therefore needed for choosing stations that have a consistent performance over many years.
Two rough measures are described here that can be used to look at variation in the apparent detection thresholds with time:
one is the number of A/T observations (at distances greater than 20∘) and the other
, for the P detections associated each month by the ISC;
being the average
. Some of the factors that control detection thresholds and clearly revealed by these rough measures are: array stations have
lower thresholds than conventional observatories; installation of automatic detectors can lower the threshold; and annual
variations in noise level are reflected in annual variations in detection thresholds. For some stations an apparent lowering
of the threshold appears not to be due to changes at the station, but is a consequence of the lowering of the ISC reporting
thresholds in the region around the station. For stations of the Swiss seismological network there is a slow rise in the threshold
from the mid-1970s to 1992 which is difficult to explain but seems to be due to a gradual changes in the detail with which
the seismograms are analysed. 相似文献
365.
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
366.
367.
Summary. Short period P -wave amplitudes at Soviet seismic stations are analysed to determine an amplitude–distance curve in the range 0°–180° from the USSR. Station amplitude terms, corrected for crustal amplification effects, are determined from seismic sources located between 30° and 90°. A relationship between these station terms and regional Pn wave speed and regional heat flow is determined and is found to be in close agreement with similar observations in North America. Station amplitude terms were found to be reasonably uniform across the continental portion of the USSR but the Baikal Rift valley is characterized by very low P -wave amplitudes and is the most significant feature discemible from the P -wave amplitude terms. 相似文献
368.
Previous studies confirm that sand can be entrained at the wind velocities recorded on Venus. Present results describe bedforms produced in the Venus Wind Tunnel (VWT) simulating the average Venusian environment. Even at the low wind speeds measured on Venus, dunelike structures form in fine-grained quartz sands (particles 50–200 μm in diameter). The dunelike structures, referred to as microdunes, are considered to be true dunes analogous to those on Earth because they have (1) slip faces, (2) a lack of particle-size sorting, (3) a low ratio of saltation path length to dune length, and (4) internal cross-bedding. The microdunes typically produced in the VWT are 9 cm long and 0.75 cm high. It is proposed that there may be fields of microdunes on Venus that are capable of very fast rates of migration and that they may grow into features much larger than those observed in the VWT. However, neither dunes nor other types of features develop above a wind speed of ~ 1.5 m/sec; at this wind speed, the bed is flat and featureless. Thus, it is predicted that relatively short periods of higher winds may destroy microdunes and other small bedforms which could account for the sparsity of definitive aeolian features observed in Venera images. Some apparent cross-bedding observed in Venera images, however, could represent preserved aeolian structures. 相似文献
369.
The source of metasomatic fluids in iron-oxide–copper–gold districts is contentious with models for magmatic and other fluid sources having been proposed. For this study, δ
18O and δ
13C ratios were measured from carbonate mineral separates in the Proterozoic eastern Mt Isa Block of Northwest Queensland, Australia. Isotopic analyses are supported by petrography, mineral chemistry and cathodoluminescence imagery. Marine meta-carbonate rocks (ca. 20.5‰ δ
18O and 0.5‰ δ
13C calcite) and graphitic meta-sedimentary rocks (ca. 14‰ δ
18O and −18‰ δ
13C calcite) are the main supracrustal reservoirs of carbon and oxygen in the district. The isotopic ratios for calcite from the cores of Na–(Ca) alteration systems strongly cluster around 11‰ δ
18O and −7‰ δ
13C, with shifts towards higher δ
18O values and higher and lower δ
13C values, reflecting interaction with different hostrocks. Na–(Ca)-rich assemblages are out of isotopic equilibrium with their metamorphic hostrocks, and isotopic values are consistent with fluids derived from or equilibrated with igneous rocks. However, igneous rocks in the eastern Mt Isa Block contain negligible carbon and are incapable of buffering the δ
13C signatures of CO2-rich metasomatic fluids associated with Na–(Ca) alteration. In contrast, plutons in the eastern Mt Isa Block have been documented as having exsolved saline CO2-rich fluids and represent the most probable fluid source for Na–(Ca) alteration. Intrusion-proximal, skarn-like Cu–Au orebodies that lack significant K and Fe enrichment (e.g. Mt Elliott) display isotopic ratios that cluster around values of 11‰ δ
18O and −7‰ δ
13C (calcite), indicating an isotopically similar fluid source as for Na–(Ca) alteration and that significant fluid–wallrock interaction was not required in the genesis of these deposits. In contrast, K- and Fe-rich, intrusion-distal deposits (e.g. Ernest Henry) record significant shifts in δ
18O and δ
13C towards values characteristic of the broader hostrocks to the deposits, reflecting fluid–wallrock equilibration before mineralisation. Low temperature, low salinity, low δ
18O (<10‰ calcite) and CO2-poor fluids are documented in retrograde metasomatic assemblages, but these fluids are paragenetically late and have not contributed significantly to the mass budgets of Cu–Au mineralisation. 相似文献
370.
We analyzed microbial diversity and community composition from four salt marsh sites that were impounded for 40?C50?years and subsequently restored and four unimpounded sites in southeastern Connecticut over one growing season. Community composition and diversity were assessed by terminal restriction fragment length polymorphism (TRFLP) and sequence analysis of 16S ribosomal RNA (rRNA) genes. Our results indicated diverse communities, with sequences representing 14 different bacterial divisions. Proteobacteria, Bacteroidetes, and Planctomycetes dominated clone libraries from both restored and unimpounded sites. Multivariate analysis of the TRFLP data suggest significant site, sample date, and restoration status effects, but the exact causes of these effects are not clear. Composition of clone libraries and abundance of bacterial 16S rRNA genes were not significantly different between restored sites and unimpounded sites, but restored sites showed greater temporal and spatial variability of bacterial communities based on TRFLP profiles compared with unimpounded sites, and variability was greatest at sites more recently restored. In summary, our study suggests there may be long-lasting effects on stability of bacterial communities in restored salt marshes and raises questions about the resilience and ultimate recovery of the communities after chronic disturbance. 相似文献