Iron-formations and epigenetic ironstones in the Palaeoproterozoic Willyama Supergroup, Olary Domain, South Australia     

P. M. Ashley  B. G. Lottermoser  J. M. Westaway 《Mineralogy and Petrology》1998,64(1-4):187-218

Summary Iron-formations occur as massive to compositionally layered, Fe oxide-rich, concordant bodies in the Palaeoproterozoic Willyama Supergroup of the Olary Domain, South Australia. They have constitutional similarities to those occurring in the neighbouring Broken Hill Domain. The most abundant iron-formations are in the Quartzofeldspathic Suite and comprise magnetite-quartz assemblages (± hematite, barite, actinolite, apatite). Hematite, magnetite, albite, quartz, Ca(Na) amphibole(s), CaNaFe clinopyroxene and andraditic garnet are major constituents of rare calc-silicate iron-formations in the Bimba and Calcsilicate Suites, whereas magnetite, quartz, almandine-spessartine, manganoan fayalite, manganoan grunerite and apatite form manganiferous iron-formations in the Pelite Suite. The pronounced differences in mineralogy of the three iron-formation types are the result of regional metamorphism of diverse hydrothermal precipitates with variable elastic components, together with the local effects of high-temperature metasomatic alteration. Metasomatic fluids were produced as a result of devolatilisation of the evaporite-bearing volcanosedimentary sequence, during and following amphibolite grade metamorphism and deformation, which led to localised and regional-scale hydrothermal alteration. In places, there was extensive metasomatic reconstitution (veining, brecciation, replacement) of iron-formations and associated rocks, caused by high-temperature (350°–650°C), oxidising, saline fluids. The resulting epigenetic ironstones are dominated by magnetite-hematite-quartz with minor sulfides and display enrichment in Fe, Ti, Cu, Au, Sc, U, V, Y, Zn and HREE relative to parental iron-formations.

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Eisenformationen und epigenetische Eisensteine in der Paläoproterozoischen Willyama Supergroup, Olary Domäne, Südaustralien

Zusammenfassung Eisenformationen kommen als massige bis in der Zusammensetzung geschichtete, Eisenoxidreiche, konkordante Körper in der paläoproterozoischen Willyama Supergroup der Olary Domäne, Südaustralien, vor. Sie haben konstitutionelle Ähnlichkeiten mit Vorkommen in der benachbarten Broken Hill Domäne. Die häufigsten Eisenformationen befinden sich in der Quartzofeldspathic Suite and bestehen aus Magnetit und Quarz (± Hämatit, Baryt, Aktinolit, Apatit). Hämatit, Magnetit, Albit, Quarz, Ca(Na) Amphibol(e), CaNaFe Klinopyroxen und andraditischer Granat sind Hauptbestandteile von seltenen Kalksilikat-Eisenformationen in den Bimba und Calcsilicate Suites, während Magnetit, Quartz, Almandin-Spessartin, manganhaltiger Fayalit, manganhaltiger Grunerit und Apatit manganhaltige Eisenformationen in der Pelite Suite bilden. Die ausgeprägten Unterschiede in der Mineralogie der drei Typen von Eisenformationen sind durch Regionalmetamorphose von diversen hydrothermalen Ausfällungen mit variablen klastischen Komponenten verursacht worden, zusammen mit lokalen Wirkungen einer hock-Temperatur metasomatischen Alteration. Metasomatische Fluide wurden während und nach der Amphibolitmetamorphose und Deformation durch Devolatilisation der evaporithaltigen vulkanosedimentären Abfolge produziert, die sowohl zu lokaler, wie auch zu weiträumiger hydrothermaler Alteration führten. Örtlich kam es zu umfangreicher metasomatischer Rekonstitution (Gangbildung, Brekkzierung, Verdrängung) von Eisenformationen und assoziierten Gesteinen verursacht durch hoch-Temperatur (350°–650°C), oxidierte, saline Fluide. Die resultierenden epigenetischen Eisensteine bestehen hauptsächlich aus Magnetit, Hämatit, und Quarz mit Sulfiden und weisen eine Anreicherung in Fe, Ti, Cu, Au, Sc, U, V, Y, Zu und SREE relativ gegenüber den ursprünglichen Eisenformationen auf.

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Molecular hydrogen line emission from the reflection nebula Parsamyan 18     

Ryder  Allen  Burton  Ashley  & Storey 《Monthly notices of the Royal Astronomical Society》1998,294(2):338-346

The newly commissioned University of New South Wales Infrared Fabry–Perot (UNSWIRF) has been used to image molecular hydrogen emission at 2.12 and 2.25 μm in the reflection nebula Parsamyan 18. P 18 is known to exhibit low values of the (1–0)/(2–1) S(1) ratio, suggestive of UV-pumped fluorescence rather than thermal excitation by shocks. Our line ratio mapping reveals the full extent of this fluorescent emission from extended arc-like features, as well as a more concentrated thermal component in regions closer to the central exciting star. We show that the emission morphology, line fluxes and gas density are consistent with the predictions of photodissociation region (PDR) theory. Those regions with the highest intrinsic 1–0 S(1) intensities also tend to show the highest (1–0)/(2–1) S(1) line ratios. Furthermore, variations in the line ratio can be attributed to intrinsic fluctuations in the incident radiation field and/or the gas density, through the self-shielding action of H₂. An isolated knot of emission discovered just outside P 18, and having both an unusually high (1–0)/(2–1) S(1) ratio and relative velocity, provides additional evidence for an outflow source associated with P 18.  相似文献   

Alignment and precession of a black hole with a warped accretion disc     

Rebecca G. Martin  J. E. Pringle  Christopher A. Tout 《Monthly notices of the Royal Astronomical Society》2007,381(4):1617-1624

We consider the shape of an accretion disc whose outer regions are misaligned with the spin axis of a central black hole and calculate the steady state form of the warped disc in the case where the viscosity and surface densities are power laws in the distance from the central black hole. We discuss the shape of the resulting disc in both the frame of the black hole and that of the outer disc. We note that some parts of the disc and also any companion star maybe shadowed from the central regions by the warp. We compute the torque on the black hole caused by the Lense–Thirring precession, and hence compute the alignment and precession time-scales. We generalize the case with viscosity and hence surface density independent of radius to more realistic density distributions for which the surface density is a decreasing function of radius. We find that the alignment time-scale does not change greatly but the precession time-scale is more sensitive. We also determine the effect on this time-scale if we truncate the disc. For a given truncation radius, the time-scales are less affected for more sharply falling density distributions.  相似文献   

Sea-level rise research and dialogue in North Carolina: Creating windows for policy change     

Benjamin Poulter  Rebecca L. Feldman  Mark M. Brinson  Benjamin P. Horton  Michael K. Orbach  Samuel H. Pearsall  Enrique Reyes  Stanley R. Riggs  John C. Whitehead 《Ocean & Coastal Management》2009,52(3-4):147-153

Coastal areas are among the world's most vulnerable landscapes to impacts related to climate change, including inundation from sea-level rise (SLR), increased exposure to shoreline erosion, and greater frequency and intensity of storms. The status of research on the physical, ecological, and socio-economic effects of vulnerability to SLR and progress toward planning for its consequences varies from region to region worldwide. Here, we synthesize the results of three decades of SLR research and the development of coastal management policies in North Carolina, USA. We identify the major factors responsible for opening new policy ‘windows’ that address SLR, including how stakeholders have developed an increased understanding of the risks, the extent of public dialogue about potential response strategies, and advances in political receptivity to policy change. Research and policy progress in North Carolina continue to provide a model for other regions to help guide and evaluate the development of coastal policies.  相似文献   

Hyperspectral mapping of alteration assemblages within a hydrothermal vug at the Haughton impact structure,Canada          下载免费PDF全文

Rebecca N. Greenberger  John F. Mustard  Gordon R. Osinski  Livio L. Tornabene  Alexandra J. Pontefract  Cassandra L. Marion  Roberta L. Flemming  Janette H. Wilson  Edward A. Cloutis 《Meteoritics & planetary science》2016,51(12):2274-2292

Meteorite impacts on Earth and Mars can generate hydrothermal systems that alter the primary mineralogies of rocks and provide suitable environments for microbial colonization. We investigate a calcite–marcasite‐bearing vug at the ~23 km diameter Haughton impact structure, Devon Island, Nunavut, Canada, using imaging spectroscopy of the outcrop in the field (0.65–1.1 μm) and samples in the laboratory (0.4–2.5 μm), point spectroscopy (0.35–2.5 μm), major element chemistry, and X‐ray diffraction analyses. The mineral assemblages mapped at the outcrop include marcasite; marcasite with minor gypsum and jarosite; fibroferrite and copiapite with minor gypsum and melanterite; gypsum, Fe³⁺ oxides, and jarosite; and calcite, gypsum, clay, microcline, and quartz. Hyperspectral mapping of alteration phases shows spatial patterns that illuminate changes in alteration conditions and formation of specific mineral phases. Marcasite formed from the postimpact hydrothermal system under reducing conditions, while subsequent weathering oxidized the marcasite at low temperatures and water/rock ratios. The acidic fluids resulting from the oxidation collected on flat‐lying portions of the outcrop, precipitating fibroferrite + copiapite. That assemblage then likely dissolved, and the changing chemistry and pH resulting from interaction with the calcite‐rich host rock formed gypsum‐bearing red coatings. These results have implications for understanding water–rock interactions and habitabilities at this site and on Mars.  相似文献   

Scientific advocacy,environmental interest groups,and climate change: are climate skeptic portrayals of climate scientists as biased accurate?     

Rebecca Bromley-Trujillo  James W. Stoutenborough  Arnold Vedlitz 《Climatic change》2015,131(4):607-620

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A new chronological framework for Middle and Upper Pleistocene landscape evolution in the Sussex/Hampshire Coastal Corridor, UK     

Martin R. Bates  Rebecca M. Briant  Jean-Luc Schwenninger 《Proceedings of the Geologists' Association. Geologists' Association》2010,121(4):369-392

The unique Middle and Late Pleistocene sedimentary record preserved along the Sussex/Hampshire Coastal Corridor between Romsey and Brighton contains a wealth of deposits including highstand marine sediments associated with a variety of different aged beaches, fluvial sediments associated with rivers crossing the coastal plain and cold stage deposits accumulating above the marine and fluvial sediments. Although quarrying activity has been extensive across much of the area it has been undertaken in flooded workings due to the high level of the watertable. Consequently little is known in detail about the sequences except where they outcrop on the foreshore around the coast. This paper examines recent work from the lower coastal plain using a multi-disciplinary approach these deposits to elucidate the age of the sequences and their associated environments of deposition.OSL dates from two of the beaches, the Aldingbourne and Brighton/Norton Beaches, place both within MIS 7. Although these OSL dates cannot differentiate between sub-stages within MIS 7, coupling these results with inferences from local geography, lithology and contained microfossils it is clear that the beaches belong to two different phases within MIS 7. These two beaches are clearly divided by a major phase of erosion and downcutting associated with a fall in sea-level. Fluvial sediments from Solent Terrace 2 and Arun Terrace 4 also date within MIS 7 and are tentatively ascribed to the downcutting event between the beaches. Together this information allows us to propose, for the first time, a robust independently dated framework for the lower parts of the coastal plain integrating for the first time the marine and terrestrial record.  相似文献   

Phytoliths infer locally dense and heterogeneous paleovegetation at FLK North and surrounding localities during upper Bed I time, Olduvai Gorge, Tanzania     

Doris Barboni  Gail M. Ashley  Manuel Dominguez-Rodrigo  Henry T. Bunn  Audax Z.P. Mabulla  Enrique Baquedano   《Quaternary Research》2010,74(3):344-354

The phytolith content of 10 samples collected immediately under Tuff IF (~ 1.785 Ma) at FLK N and other surrounding localities (~ 2 km²) provides a direct botanical evidence for woody vegetation in the eastern margin of Olduvai Gorge during uppermost Bed I time. Observation and counting of 143 phytolith types (5 to >150 μ) reveal dense but heterogeneous woody cover (~ 40–90%) of unidentified trees and/or shrubs and palms associated to the freshwater springs surrounding FLK N, and more open formation (presumably ~ 25–70% woody cover) in the southeast at localities VEK, HWK W and HWK E. The paleovegetation is best described as groundwater palm forest/woodland or bushland, which current analogue may be found near Lake Manyara in similar hydrogeological context (freshwater springs near saline/alkaline lake). Re-evaluating the published pollen data based on this analogy shows that 70% of the pollen signal at FLK N may be attributed to thicket-woodland, Acacia groundwater woodland, gallery and groundwater forests; while < 30% is attributed to swamp herbage and grasslands. Micro-botanical, isotopic, and taphonomical studies of faunal remains converge on the same conclusion that the area surrounding FLK N, which attracted both carnivores and hominins in the early Pleistocene, was densely wooded during uppermost Bed I time.  相似文献   

Reply to “Rip Current Misunderstandings”     

Victor A. Gensini  Walker S. Ashley 《Natural Hazards》2010,55(2):163-165

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Intraspecific Variation in Growth of Marsh Macrophytes in Response to Salinity and Soil Type: Implications for Wetland Restoration     

Rebecca J. Howard 《Estuaries and Coasts》2010,33(1):127-138

Genetic diversity within plant populations can influence plant community structure along environmental gradients. In wetland habitats, salinity and soil type are factors that can vary along gradients and therefore affect plant growth. To test for intraspecific growth variation in response to these factors, a greenhouse study was conducted using common plants that occur in northern Gulf of Mexico brackish and salt marshes. Individual plants of Distichlis spicata, Phragmites australis, Schoenoplectus californicus, and Schoenoplectus robustus were collected from several locations along the coast in Louisiana, USA. Plant identity, based on collection location, was used as a measure of intraspecific variability. Prepared soil mixtures were organic, silt, or clay, and salinity treatments were 0 or 18 psu. Significant intraspecific variation in stem number, total stem height, or biomass was found in all species. Within species, response to soil type varied, but increased salinity significantly decreased growth in all individuals. Findings indicate that inclusion of multiple genets within species is an important consideration for marsh restoration projects that include vegetation plantings. This strategy will facilitate establishment of plant communities that have the flexibility to adapt to changing environmental conditions and, therefore, are capable of persisting over time.  相似文献