Reversing buoyancy in turbidity currents: developing a hypothesis for flow transformation and for deposit facies and architecture     

David Pritchard  Charlotte Gladstone   《Marine and Petroleum Geology》2009,26(10):1997

A turbidity current that contains fresher or otherwise less dense water than its surroundings may initially be denser than the ambient and propagate as a bottom-hugging flow, but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient seawater. It is proposed that this reversal in buoyancy may be a significant mechanism controlling the structure and facies of turbiditic deposits. Buoyancy reversal followed by lofting may directly affect the relative distribution of fine and coarse material in the deposit, while buoyancy reversal itself may mediate the transformation between dilute and highly-concentrated suspension flows, particularly in distal regions, and thus lead to the formation of complex turbiditic beds: in particular, the generation of distal co-genetic debrites may be expected. Similar transformations occur within dilute pyroclastic density currents, where a mobile, basal concentrated flow, termed a surge-derived pyroclastic flow, develops through rapid sedimentation from the suspended load of the overlying surge. The physical mechanisms involved in these processes are discussed, leading to the proposal of some associated facies models; these are compared with field data from the Northern Apennines, with some striking similarities being noted as well as some differences. On the basis of this discussion, some directions are suggested for future experimental and modelling work on the topic.  相似文献   

In vitro study of the effects of cadmium on the activation of the estrogen response element using the YES screen     

Denier X  Couteau J  Baudrimont M  Hill EM  Rotchell J  Minier C 《Marine environmental research》2008,66(1):108-110

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Helium on Mars and Venus: EUVE observations and modeling     

Vladimir A. Krasnopolsky  G. Randall Gladstone 《Icarus》2005,176(2):395-407

Long-exposure spectroscopy of Mars and Venus with the Extreme Ultraviolet Explorer (EUVE) has revealed emissions of He 584 Å on both planets and He 537 Å/O⁺ 539 Å and He⁺ 304 Å on Venus. Our knowledge of the solar emission at 584 Å, eddy diffusion in Mars' upper atmosphere, electron energy distributions above Mars' ionopause, and hot oxygen densities in Mars' exosphere has been significantly improved since our analysis of the first EUVE observation of Mars [Krasnopolsky, Gladstone, 1996, Helium on Mars: EUVE and Phobos data and implications for Mars' evolution, J. Geophys. Res. 101, 15,765-15,772]. These new results and a more recent EUVE observation of Mars are the motivation for us to revisit the problem in this paper. We find that the abundance of helium in the upper atmosphere, where the main loss processes occur, is similar to that in the previous paper, though the mixing ratio in the lower and middle atmosphere is now better estimated at 10±6 ppm. Our estimate of the total loss of helium is almost unchanged at 8×¹⁰²³ s⁻¹, because a significant decrease in the loss by electron impact ionization above the ionopause is compensated by a higher loss in collisions with hot oxygen. We neglect the outgassing of helium produced by radioactive decay of U and Th because of the absence of current volcanism and a very low upper limit to the seepage of volcanic gases. The capture of solar wind α-particles is currently the only substantial source of helium on Mars, and its efficiency remains at 0.3. A similar analysis of EUV emissions from Venus results in a helium abundance in the upper atmosphere which is equal to the mean of the abundances measured previously with two optical and two mass spectrometers, and a derived helium mixing ratio in the middle and lower atmosphere of 9±6 ppm. Helium escape by ionization and sweeping out of helium ions by the solar wind above the ionopause is smaller than that calculated by Prather and McElroy [1983, Helium on Venus: implications for uranium and thorium, Science 220, 410-411] by a factor of 3. However, charge exchange of He⁺ ions with CO₂ and N₂ between the exobase and ionopause and collisions with hot oxygen ignored previously add to the total loss which appears to be at the level of 10⁶ cm⁻² s⁻¹ predicted by Prather and McElroy [1983, Science 220, 410-411]. The loss of helium is compensated by outgassing of helium produced by radioactive decay of U and Th and by the capture of the solar wind α-particles with an efficiency of 0.1. We also compare our derived α-particle capture efficiencies for Mars and Venus with observed X-ray emissions resulting from the charge exchange of solar wind heavy ions with the extended atmospheres on both planets [Dennerl et al., 2002, Discovery of X-rays from Venus with Chandra, Astron. Astrophys. 386, 319-330; Dennerl, 2002, Discovery of X-rays from Mars with Chandra, Astron. Astrophys. 394, 1119-1128]. The emissions from both disk and halo on Mars agree with our calculated values; however, we do not see a reasonable explanation for the X-ray halo emission on Venus. The ratio of the charge exchange efficiencies derived from the disk X-ray emissions of Mars and Venus is similar to the ratio of the capture efficiencies for these planets. The surprisingly bright emission of He⁺ at 304 Å observed by EUVE and Venera 11 and 12 suggests that charge exchange in the flow of the solar wind α-particles around the ionopause is much stronger than in the flow of α-particles into the ionosphere.  相似文献   

Dechlorinating and Iron Reducing Bacteria Distribution in a TCE-Contaminated Aquifer     

Carmen Lourdes Yupanqui Zaa  Joan E. McLean  R. Ryan Dupont  Jeanette M. Norton  Darwin L. Sorensen 《Ground Water Monitoring & Remediation》2010,30(1):46-57

Dehalorespiration bioremediation has been considered for chlorinated compound removal from two trichloroethene contaminated groundwater plumes in the OU 5 area of Hill Air Force Base, Utah. The distributions and population densities of the 16S rRNA genes of Bacteria, Dehalococcoides ethenogenes, Desulfuromonas michiganensis, Geobacter spp. and Rhodoferax ferrireducens -like bacteria, as well as the functional genes trichloroethene reductive dehalogenase ( tce A) and vinyl chloride reductase ( vcr A) were determined in contaminated aquifer material samples. The influence of aquifer physical and chemical properties, including iron availability, on these distributions was evaluated. Twenty aquifer cores were collected. DNA was extracted and analyzed using real-time quantitative polymerase chain reaction (RT-qPCR) to quantify the gene densities. Dehalococcoides population densities were low and unevenly distributed. D. michiganensis was found in 12 cores while Geobacter spp. were found in 8 cores. Rhodoferax ferrireducens -like bacteria were widely distributed. The vcr A gene distribution was relatively uniform and broad but the tce A gene was detectable in only 2 cores. Gene distribution was not related to core clusters derived from physical/chemical characteristics.  相似文献   

Mutation analysis of ras gene in the liver of European eel (Anguilla anguilla L.) exposed to benzo[a]pyrene     

Nogueira PR  Lourenço J  Mendo S  Rotchell JM 《Marine pollution bulletin》2006,52(12):1611-1616

Ras is regarded as one of the most important genes involved in carcinogenesis. Such genes have been characterised in several fish species and the presence of ras mutations have already been described in fish populations from hydrocarbon contaminated areas and following experimental exposure to specific contaminants. The aims of this study were to evaluate the DNA integrity by comet assay, to isolate the normal ras gene of Anguilla anguilla and analyse for the presence of ras gene mutations or changes in gene expression levels induced after one month of benzo[a]pyrene (BaP) experimental exposure. The A. anguilla ras cDNA isolated revealed a 189 amino acid protein and alignment with other vertebrate ras proteins revealed conservation of functionally important regions. Following experimental exposure to BaP, an increase in DNA damage was found by comet assay. However, no point mutations or changes in ras gene expression levels were detected when compared to control samples. In contrast to the majority of fish ras gene sequences, a high degree of polymorphic variation was detected in the A. anguilla ras gene.  相似文献   

The ultraluminous X-ray source population of NGC 4485/4490     

Jeanette C. Gladstone  Timothy P. Roberts 《Monthly notices of the Royal Astronomical Society》2009,397(1):124-134

We report the results of spectral and temporal variability studies of the ultraluminous X-ray sources (ULXs) contained within the interacting pair of galaxies NGC 4485/4490, combining Chandra and XMM–Newton observations. Each of the four separate observations provide at least modest quality spectra and light curves for each of the six previously identified ULXs in this system; we also note the presence of a new transient ULX in the most recent observation. No short-term variability was observed for any ULX within our sample, but three out of five sources show correlated flux/spectral changes over longer time-scales, with two others remaining stable in spectrum and luminosity over a period of at least 5 yr. We model the spectra with simple power-law and multicolour disc blackbody models. Although the data are insufficient to statistically distinguish models in each epoch, those better modelled (in terms of their  χ²  fit) by a multicolour disc blackbody appear to show a disc-like correlation between luminosity and temperature, whereas those modelled by a power-law veer sharply away from such a relationship. The ULXs with possible correlated flux/spectral changes appear to change spectral form at  ∼2 × 10³⁹ erg s⁻¹  , suggestive of a possible change in spectral state at high luminosities. If this transition is occurring between the very high state and a super-Eddington ultraluminous state, it indicates that the mass of the black holes in these ULXs is around  10–15 M_⊙  .  相似文献   

Soil morphology and organic matter dynamics under cheatgrass and sagebrush-steppe plant communities     

Jay B. Norton  Thomas A. Monaco  Jeanette M. Norton  Douglas A. Johnson  Thomas A. Jones 《Journal of Arid Environments》2004,57(4):445

Widespread cheatgrass (Bromus tectorum L.) invasion represents a major shift in species dominance that may alter ecosystem processes across much of the western US. To investigate differences following such conversion, soil morphology and organic matter under cheatgrass-dominated and native shrub-steppe vegetation were compared by standard soil analysis procedures at seven paired sites in Idaho and Utah. Results suggest that, following conversion to cheatgrass dominance, increased porosity and labile organic inputs enhance microbial decomposition in near-surface horizons beneath cheatgrass compared to adjacent soils under native vegetation. Enhanced decomposition could result in depletion of long-term SOM, leading to impoverished sites difficult to restore to native perennial vegetation.  相似文献   

The Crustal Structure From Teleseismic P-Wave Coda-Ii. Application to Data of the Nars Array In Western Europe and Comparison With Deep Seismic Sounding Data     

Jeanette Visser  Hanneke Paulssen 《Geophysical Journal International》1993,112(1):26-38

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Effects of artificial openings of intermittently opening estuaries on macroinvertebrate assemblages of the entrance barrier     

William Gladstone  Nicole Hacking  Vanessa Owen 《Estuarine, Coastal and Shelf Science》2006,67(4):708-720

Intermittently opening estuaries are artificially opened to manage flood risk, water quality, recreational amenity, and fisheries; however, the ecological impacts of this management technique are incompletely understood. During 2001 and 2004, this study assessed the impacts of artificial openings on the macroinvertebrates of entrance barriers of intermittently opening estuaries in New South Wales (Australia). In 2001 macroinvertebrates were sampled once before artificial opening and 9 and 25 d after re-formation of the entrance barrier. A multiple before–after-control-impact analysis found that, although entrance barriers were destroyed by the artificial openings and then re-formed naturally by wave action, significant interactions for taxonomic richness, density of the amphipod Paracalliope australis (Gammaridae) and density of the gastropod mollusc Aschoris victoriae (Hydrobiidae) meant that the effects of this disturbance could not be distinguished from the natural variations that occurred in unopened estuaries. Multivariate analyses found that assemblages at both opened and unopened estuaries changed from before to after the openings, and the magnitude of the dissimilarity between times varied between estuaries. In 2004, macroinvertebrates were sampled on three randomly selected days within each of three periods (before, 3 d and 42 d after) at one opened and three unopened estuaries. Asymmetrical analysis of this modified before–after-control-impact study found that the change in taxonomic richness at the opened estuary from before to after opening did not differ from temporal changes that occurred in unopened estuaries. Short-term variation (i.e. between days) in total density of macroinvertebrates and density of P. australis in the re-formed entrance barrier of the opened estuary also did not differ from the variation in the control estuaries. Additionally, assemblage structure was not significantly changed by the opening and assemblages at two control estuaries were also unchanged over the same time. Individual taxa and assemblages of macroinvertebrates in entrance barriers of these intermittently open estuarine systems appear to be resilient to the habitat disturbance caused by artificial openings.  相似文献   

Genetic heterogeneity of Atlantic bluefin tuna caught in the eastern North Atlantic Ocean south of Iceland   总被引：2，自引：0，他引：2  

Carlsson  Jens; McDowell  Jan R.; Carlsson  Jeanette E.L.; Olafsdottir  Droplaug; Graves  John E. 《ICES Journal of Marine Science》2006,63(6):1111-1117

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