The violent August 16–17, 2006 Tungurahua eruption in Ecuador witnessed the emplacement of numerous scoria flows and the deposition
of a widespread tephra layer west of the volcano. We assess the size of the eruption by determining a bulk tephra volume in
the range 42–57 × 106 m3, which supports a Volcanic Explosivity Index 3 event, consistent with calculated column height of 16–18 km above the vent
and making it the strongest eruptive phase since the volcano’s magmatic reactivation in 1999. Isopachs west of the volcano
are sub-bilobate in shape, while sieve and laser diffraction grain-size analyses of tephra samples reveal strongly bimodal
distributions. Based on a new grain-size deconvolution algorithm and extended sampling area, we propose here a mechanism to
account for the bimodal grain-size distribution. The deconvolution procedure allows us to identify two particle subpopulations
in the deposit with distinct characteristics that indicate dissimilar transport-depositional processes. The log-normal coarse-grained
subpopulation is typical of particles transported downwind by the main volcanic plume. The positively skewed, fine-grained
subpopulation in the tephra fall layer shares close similarities with the elutriated co-pyroclastic flow ash cloud layers
preserved on top of the scoria flow deposits. The area with the higher fine particle content in the tephra layer coincides
with the downwind prolongation of the pyroclastic flow deposits. These results indicate that the bimodal distribution of grain
size in the Tungurahua fall deposit results from synchronous deposition of lapilli from the main plume and fine ash elutriated
from scoria flows emplaced on the western flank of the volcano. Our study also reveals that inappropriate grain-size data
processing may produce misleading determination of eruptive type. 相似文献
The 1959 summit eruption of Kīlauea volcano produced the highest recorded Hawaiian fountain in Hawai‘i. Quantitative analysis
of closely spaced samples from the final two high-fountaining episodes of the eruption result in a fine-scale textural study
of pyroclasts and provide a record of postfragmentation processes. As clast vesicularity increases, the vesicle number density
decreases and vesicle morphology shifts from small and round to larger and more irregular. The shift in microtexture corresponds
to greater degrees of postfragmentation expansion of clasts with higher vesicularity. We suggest the range of clast morphologies
in the deposit is related to thermal zonation within a Hawaiian fountain where the highest vesicularity clasts traveled in
the center and lowest traveled along the margins. Vesicle number densities are greatest in the highest fountaining episode
and therefore scale with intensity of activity. Major element chemical analyses and fasciculate crystal textures indicate
microlite-rich zones within individual clasts are portions of recycled lava lake material that were incorporated into newly
vesiculating primary melt. 相似文献
We describe the selection of candidate radio-loud quasars obtained by cross-matching radio source positions from the low-frequency (151-MHz) 7C survey with optical positions from five pairs of EO POSS-I plates scanned with the Cambridge Automatic Plate-measuring Machine (APM). The sky region studied is centred at RA 10h 28m, Dec.+41° and covers ≈0.057 sr. We present VLA observations of the quasar candidates, and tabulate various properties derived from the radio maps. We discuss the selection criteria of the resulting '7CQ' sample of radio-loud quasars. The 70 confirmed quasars, and some fraction of the 36 unconfirmed candidates, constitute a filtered sample with the following selection criteria: 151-MHz flux density S 151>100 mJy; POSS-I E -plate magnitude E ≈ R <20; POSS-I colour ( O E )<1.8; the effective area of the survey drops significantly below S 151≈200 mJy. We argue that the colour criterion excludes few if any quasars, but note, on the basis of recent work by Willott et al., that the E magnitude limit probably excludes more than 50 per cent of the radio-loud quasars. 相似文献
The transition from the Late Glacial to the Early Holocene in the endorheic Salines sequence, which is characterized cyclical sedimentation, occurs between 5.50 and 2.85 m depth. From 5.50-3.50 m depth the cycles are composed of a centimetre alternation of layers of dolomitic marls and gypsarenites and from 3.50-2.85 m depth by the alternation of calcitic marls and calcarenites.Pollen, biotic assemblages and geochemistry provide evidence of a gap with respect to the new hydrological conditions that characterized the beginning of the Holocene. Mesic pollen taxa increased their percentages at the beginning of the Holocene, indicating climate improvement, which coincides with the 14C radiocarbon age of 10,000 years BP. The first biotic remains (gastropods, ostracods and foraminifers) found in this sequence appeared later, at 3.80 m depth, which corresponds to 9,500 years BP, whereas the mineralogical change occurred at 3.50 m depth, which corresponds to 9,000 years BP. The advanced adaptation of the vegetation and biotic aquatic assemblages with respect to the mineralogical response corresponds to a process of a gradual increase in water availability into the lacustrine system. During the Boreal, the calcitic cycles reached their maximum thickness, suggesting a more continuous water input. This assumption has also been corroborated by the expansion of the mesic pollen taxa and the occurrence of biota taxa which depend on a permanent water body for their development.The multiproxy approach in paleoclimate scenarios is an essential tool for understanding the ecosystem adjustment during climate changes. Our results demonstrate an interval of 1000 years between the vegetal and the mineralogical reaction. 相似文献
A total of 51 potential pathogenic vibrios were isolated from moribund silver sea bream (Sparus sarba) collected from fish farms in Hong Kong. Using the API 20E system and the scheme of Alsina and Blanch (1994), 7 species were identified from all isolates. These species were Vibrio alginolyticus (24 strains), Vibrio vulnificus (12 strains), Vibrio parahaemolyticus (7 strains), Vibrio logei (4 strains), Vibrio pelagius II (2 strains), Vibrio fluvialis (1 strain) and Vibrio meditterranei (1 strain). The three dominant species (V. alginolyticus, V. vulnificus and V. parahaemolyticus) were confirmed to be virulent to sea bream by experimental challenge.
All isolates were screened for plasmid DNA by agarose gel electrophoresis and tested for susceptibility to 16 antibiotics by the agar dilution method. Of the 51 isolates examined, all strains were sensitive to ceftriaxone, streptomycin, nalidixic acid and rifampicin, and almost all were sensitive to ceftazidime, netilimicin, chloramphenicol and sulfamethoxazole. Most isolates were resistant to ampicillin (60.8%), cefuroxime (66.7%), amikacin (55%), kanamycin (58.8%) and trimethoprim (76.5%). Fifteen of the 51 isolates harboured 1–4 plasmids, with sizes ranging from 9 to 123 kb. Both the plasmids and the associated antibiotic resistance (ampicillin, cefuroxime and trimethoprim) of 9 isolates could be transferred to a recipient by single-step conjugation. However, the frequencies were very low, ranging from 10−11 to 10−9. The present results indicate that resistance to these antibiotics is chromosomal. 相似文献
Two key challenges regarding the design and operation of aquifer recharge and recovery (ARR) systems are evaluating aquifer heterogeneity and understanding hydrochemical interactions. Uncertainty in this respect can impact the volume of recoverable water and the improvement in water quality. The objective of this research is to leverage the advantages of geophysical measurements and hydrochemical sampling to reveal the properties of an ARR site to inform current ARR system operations and future design decisions. Electrical resistivity tomography was used to image the subsurface below two key infiltration/extraction areas of an ARR site in Colorado, USA. Hydrochemical measurements on transects intersecting the geophysical measurements resolved bulk parameters (i.e., total organic carbon, nitrate, and major cations and anions) and trace organic chemicals (e.g., pharmaceuticals, personal care products). Conservative tracers were also used to estimate degrees of mixing and water travel times and to better assess the performance of the ARR site regarding water quality changes and water recovery. The electrical resistivity measurements suggest that certain areas of the infiltration basins have hydraulic connections to the extraction wells through preferential flow paths, compared with other infiltration basins that are separated by fine‐grained materials from their respective extraction wells. The hydrochemical results indicate that consistent improvements in water quality can be achieved in these preferential flow paths within relatively short travel times (<5 d) at this ARR site. 相似文献
Studies published in the late 1990s and early 2000s identified the presence of exceptionally long methyl tert‐butyl ether (MTBE) plumes (more than 600 m or 2000 feet) in groundwater and have been cited in technical literature as characteristic of MTBE plumes. However, the scientific literature is incomplete in regard to the subsequent behavior and fate of these MTBE plumes over the past decade. To address this gap, this issue paper compiles recent groundwater monitoring records for nine exceptional plumes that were identified in prior studies. These nine sites exhibited maximum historical MTBE groundwater plume lengths ranging from 820 m (2700 feet) to 3200 m (10,500 feet) in length, exceeding the lengths of 99% of MTBE plumes, as characterized in multiple surveys at underground storage tank sites across the United States. Groundwater monitoring data compiled in our review demonstrate that these MTBE plumes have decreased in length over the past decade, with five of the nine plumes exhibiting decreases of 75% or more compared to their historical maximum lengths. MTBE concentrations within these plumes have decreased by 93% to 100%, with two of the nine sites showing significant decreases (98% and 99%) such that the regulatory authority has subsequently designated the site as requiring no further action. 相似文献