Simultaneous X-ray and IR variability in the quasar 3C 273     

Ian M  Hardy  Anthony Lawson  rew Newsam  Alan Marscher  Ian Robson  Jason Stevens 《Monthly notices of the Royal Astronomical Society》1999,310(2):571-576

From a combination of high-quality X-ray observations from the NASA Rossi X-ray Timing Explorer ( RXTE ) and IR observations from the UK Infrared Telescope (UKIRT) we show that the medium-energy X-ray (3–20 keV) and near-IR fluxes in the quasar 3C 273 are highly correlated. It is widely believed that the X-ray emission in quasars like 3C 273 arises from Compton scattering of low-energy seed photons, and our observations provide the first reliable detection of correlated variations in 3C 273 between the X-ray band and any lower energy band. For a realistic electron distribution we demonstrate that it is probable that each decade of the seed-photon distribution from the mm to IR wavebands contributes roughly equally to the medium-energy X-ray flux. However, the expected mm variations are too small to be detected above the noise, probably explaining the lack of success of previous searches for a correlation between X-ray and mm variations. In addition, we show that the IR leads the X-rays by  0.75±0.25 d  . These observations rule out the 'External Compton' emission process for the production of the X-rays.  相似文献   

A possible active galactic nucleus in M82?     

K. A. Wills  A. Pedlar  T. W. B. Muxlow  I. R. Stevens 《New Astronomy Reviews》1999,43(8-10)

MERLIN and VLA observations of the galaxy M82 have detected a jet-like feature emanating from one of the compact sources, 44.01+59.6. The proximity of this source to the dynamical centre of M82 led us to suspect that it could be a weak active nucleus rather than an SNR. We imaged this source using the EVN at 15 mas and although it shows a compact shell-like structure which could be consistent with an SNR, we note that the EVN image shows a bright region within the source which is elongated along the jet direction. There are distinct similarities between this source and the Sgr A complex at the centre of our own Galaxy which contains the compact AGN, Sgr A^*.  相似文献   

The metamorphic and alteration history of West Rand Group shales from distal portions of the Witwatersrand Basin   总被引：1，自引：0，他引：1  

G. Stevens  R. F. Preston 《Mineralogy and Petrology》1999,66(1-3):123-147

Summary The maximum metamorphic grade in West Rand Group shales from three boreholes to the east and south of the Vredefort Dome is lower greenschist facies. In most samples the peak metamorphic assemblage consists of quartz + muscovite + chlorite, with the phyllosilicates defining a tectonic cleavage. In many of the samples more coarsely crystalline alteration zones that clearly crosscut both bedding and the tectonic cleavage overprint this assemblage. The alteration zones are localized around minor faults and shears where fluid ingress has occurred. Muscovite was not observed in these zones and the cleaved matrix assemblages has been replaced by assemblages consisting of pyrite + quartz ± chlorite ± stilpnomelane ± calcite ± carbonaceous matter ± Mn-garnet ± epidote ± chalcopyrite. In some of the alteration zones stilpnomelane is a very common product and appears to arise through the breakdown of chlorite + muscovite. K and Fe metasomatism are indicated during this reaction, as well as the obvious mobility of S, Cu and C. Assays on small samples indicate gold introduction into the shales during the alteration event. Chlorite thermometry was used to estimate the equilibration temperature of chlorite from the cleaved matrix and the alteration zones. Temperatures close to 300°C are indicated for both chlorite varieties, and for all three boreholes. These results, and the petrographic data, imply that fluid influx post-dated the formation of the tectonic cleavage, but occurred close to the peak of metamorphism. Comparison between the chlorite thermometry data from this study with those from similar studies in the goldfields indicates very similar peak metamorphic grades in both areas, suggesting that these metamorphic conditions may be typical of almost the entire basin. The one exception is obviously the amphibolite facies metamorphism exposed in the northwestern sector of the collar of the Vredefort Dome. Several recent studies have suggested that this metamorphism is representative of a widespread zone of higher grade metamorphism within the central portions of the basin. The results of this study indicate that this is unlikely to be the case and suggest that these amphibolite facies rocks are probably unique to the northern portions of the dome.

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Die Metamorphose- und Alterationsgeschichte von Peliten der West Rand Group im distalen Bereich des Witwatersrand Beckens

Zusammenfassung Dieser Artikel dokumentiert die Metamorphose und Alteration einiger Tonschiefer der West Rand Group, welche in drei Bohrkernen östlich und südlich des Vredefort Domes aufgeschlossen sind. In allen untersuchten Proben entspricht der maximale Metamorphosegrad der unteren Grünschiefer Fazies. Die maximalen metamorphen Bedingungen sind häufig durch die Mineralparagenese Quarz + Muscovit + Chlorit charakterisiert, wobei die Schichtsilikate eine ausgeprägte Schieferung definieren. Viele der Proben zeichnen sich durch gröber kristalline Bereiche intensiver Alteration aus, die sowohl die Schieferung als auch die Schichtung deutlich schneiden und in denen die metamorphe Mineralparagenese überprägt ist. Diese Alterationszonen befinden sich im Bereich von kleineren Störungen und Scherzonen, entlang derer Fluide eingedrungen sind. Muscovit fehlt in diesen Bereichen und die Matrixminerale sind durch die Paragenese Pyrit ± Quarz ± Chlorit ± Stilpnomelan ± Kalzit ± Kohlenstoff-haltiges Material ± Mn-Granat ± Epidot ± Kupferkies verdrängt worden. In einigen Alterationszonen findet man häufig Stilpnomelan, welcher möglicherweise auf Kosten von Muscovit und Chlorit gewachsen ist. Versuche, diese Reaktion mit Hilfe von gemessenen Mineralzusammensetzungen unter Annahme eines isochemischen Systems auszugleichen, scheiterten. Dies ist wahrscheinlich auf die Mobilität von K und Fe, sowie S, Cu and C zurückzuführen. Untersuchungen an kleinen Proben deuten darauf hin, daß die Alteration mit einer Goldanreicherung verbunden war. Die Abschätzung der Gleichgewichtstemperaturen von Chlorit der geschieferten Matrix sowie der Alterationszone mit Hilfe von Chlorit-Thermometrie ergab Temperaturen um 300°C für beide Chlorit-Varietäten aus allen drei Bohrkernen, Diese thermometrischen und petrographischen Daten sprechen dafür, daß die Fluidinfiltration nach Anlegung der Schieferung nahe dem Metamorphosehöhepunkt erfolgte. Ein Vergleich der Daten aus der Chlorit-Thermometrie dieser Studie mit den Ergebnissen ähnlicher Studien in den Goldfeldern ergibt sehr ähnliche metamporphe Bedingungen, was möglicherweise dafür spricht, daß diese Bedingungen typisch für das ganze Becken sind. Die einzige Ausnahme bildet offensichtlich die amphibolitfazielle Metamorphose in den nordwestlichen Randbereichen des Vredefort Domes. Einige neuere Studien spekulieren, daß diese Metamorphose eventuell für eine breite Zone höhergradiger Metamorphose im zentralen Teil des Beckens repräsentativ ist. Unsere Resultate deuten jedoch eher darauf hin, daß diese amphibolitfaziellen Gesteine auf den nördlichen Teil des Domes beschränkt sind.

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Kīlauea summit overflows: their ages and distribution in the Puna District, Hawai'i     

David A. Clague  Jonathan T. Hagstrum  Duane E. Champion  Melvin H. Beeson 《Bulletin of Volcanology》1999,61(6):363-381

 The tube-fed pāhoehoe lava flows covering much of the northeast flank of Kīlauea Volcano are named the 'Ailā'au flows. Their eruption age, based on published and six new radiocarbon dates, is approximately AD 1445. The flows have distinctive paleomagnetic directions with steep inclinations (40°–50°) and easterly declinations (0°–10°E). The lava was transported ∼40 km from the vent to the coast in long, large-diameter lava tubes; the longest tube (Kazumura Cave) reaches from near the summit to within several kilometers of the coast near Kaloli Point. The estimated volume of the 'Ailā'au flow field is 5.2±0.8 km³, and the eruption that formed it probably lasted for approximately 50 years. Summit overflows from Kīlauea may have been nearly continuous between approximately AD 1290 and 1470, during which time a series of shields formed at and around the summit. The 'Ailā'au shield was either the youngest or the next to youngest in this series of shields. Site-mean paleomagnetic directions for lava flows underlying the 'Ailā'au flows form only six groups. These older pāhoehoe flows range in age from 2750 to <18,000 BP, and the region was inundated by lava flows only three times in the past 5000 years. The known intervals between eruptive events average ∼1600 years and range from ∼1250 years to >2200 years. Lava flows from most of these summit eruptions also reached the coast, but none appears as extensive as the 'Ailā'au flow field. The chemistry of the melts erupted during each of these summit overflow events is remarkably similar, averaging approximately 6.3 wt.% MgO near the coast and 6.8 wt.% MgO near the summit. The present-day caldera probably formed more recently than the eruption that formed the 'Ailā'au flows (estimated termination ca. AD 1470). The earliest explosive eruptions that formed the Keanakāko'i Ash, which is stratigraphically above the 'Ailā'au flows, cannot be older than this age. Received: 10 October 1998 / Accepted: 12 May 1999  相似文献   

A regime shift in sediment export from a coastal watershed during a record wet winter,California: Implications for landscape response to hydroclimatic extremes          下载免费PDF全文

Amy E. East  Andrew W. Stevens  Andrew C. Ritchie  Patrick L. Barnard  Pamela Campbell‐Swarzenski  Brian D. Collins  Christopher H. Conaway 《地球表面变化过程与地形》2018,43(12):2562-2577

Small, steep watersheds are prolific sediment sources from which sediment flux is highly sensitive to climatic changes. Storm intensity and frequency are widely expected to increase during the 21st century, and so assessing the response of small, steep watersheds to extreme rainfall is essential to understanding landscape response to climate change. During record winter rainfall in 2016–2017, the San Lorenzo River, coastal California, had nine flow peaks representing 2–10‐year flood magnitudes. By the third flood, fluvial suspended sediment showed a regime shift to greater and coarser sediment supply, coincident with numerous landslides in the watershed. Even with no singular catastrophic flood, these flows exported more than half as much sediment as had a 100‐year flood 35 years earlier, substantially enlarging the nearshore delta. Annual sediment load in 2017 was an order of magnitude greater than during an average‐rainfall year, and 500‐fold greater than in a recent drought. These anomalous sediment inputs are critical to the coastal littoral system, delivering enough sediment, sometimes over only a few days, to maintain beaches for several years. Future projections of megadroughts punctuated by major atmospheric‐river storm activity suggest that interannual sediment‐yield variations will become more extreme than today in the western USA, with potential consequences for coastal management, ecosystems, and water‐storage capacity. The occurrence of two years with major sediment export over the past 35 years that were not associated with extremes of the El Niño Southern Oscillation or Pacific Decadal Oscillation suggests caution in interpreting climatic signals from marine sedimentary deposits derived from small, steep, coastal watersheds, to avoid misinterpreting the frequencies of those cycles. Published 2018. This article is a U.S. Government work and is in the public domain in the USA.  相似文献   

Mineralogical approaches to sourcing pipes and figurines from the eastern woodlands,U.S.A.     

Sarah U. Wisseman  Duane M. Moore  Randall E. Hughes  Mary R. Hynes  Thomas E. Emerson 《Geoarchaeology》2002,17(7):689-715

Provenance studies of stone artifacts often rely heavily upon chemical techniques such as neutron activation analysis. However, stone specimens with very similar chemical composition can have different mineralogies (distinctive crystalline structures as well as variations within the same mineral) that are not revealed by multielemental techniques. Because mineralogical techniques are often cheap and usually nondestructive, beginning with mineralogy allows the researcher to gain valuable information and then to be selective about how many samples are submitted for expensive and somewhat destructive chemical analysis, thus conserving both valuable samples and funds. Our University of Illinois team of archaeologists and geologists employs Portable Infrared Mineral Analyzer (PIMA) spectroscopy, X‐ray diffraction (XRD), and Sequential acid dissolution/XRD/Inductively coupled plasma (SAD‐XRD‐ICP) analyses. Two case studies of Hopewellian pipes and Mississippian figurines illustrate this mineralogical approach. The results for both studies identify sources relatively close to the sites where the artifacts were recovered: Sterling, Illinois (rather than Ohio) for the (Hopewell) pipes and Missouri (rather than Arkansas or Oklahoma) for the Cahokia figurines. © 2002 Wiley Periodicals, Inc.  相似文献   

Sub‐meter acoustic surveying     

Lester Q. Spielvogel  Narendra K. Saxena  Stevens P. Tucker 《Marine Geodesy》2013,36(2-3):137-151

Abstract

During May 1985, a comprehensive GPS and acoustic navigation data set was collected off the Monterey, California coast. Three types of GPS units, a LORAN‐C, and a Miniranger operated concurrently with an OCEANO acoustic system to resolve state‐of‐the‐art accuracies for at‐sea geodetic positioning. This report details the acoustic system which displayed baseline errors of only ±0.25 m over distances to 2600 m. Unfiltered point‐to‐point acoustic navigation errors had a standard deviation of ± 1.25 m, which included ship motion errors in addition to surveying errors. Ninety percent of the stations had navigation standard deviations below ±0.75 m The experiment showed that sub‐meter acoustic surveying is the state‐of‐the‐art.  相似文献   

Observing and Modeling Earth’s Energy Flows   总被引：1，自引：0，他引：1  

Bjorn Stevens  Stephen E. Schwartz 《Surveys in Geophysics》2012,33(3-4):779-816

This article reviews, from the authors’ perspective, progress in observing and modeling energy flows in Earth’s climate system. Emphasis is placed on the state of understanding of Earth’s energy flows and their susceptibility to perturbations, with particular emphasis on the roles of clouds and aerosols. More accurate measurements of the total solar irradiance and the rate of change of ocean enthalpy help constrain individual components of the energy budget at the top of the atmosphere to within ±2 W m^?2. The measurements demonstrate that Earth reflects substantially less solar radiation and emits more terrestrial radiation than was believed even a decade ago. Active remote sensing is helping to constrain the surface energy budget, but new estimates of downwelling surface irradiance that benefit from such methods are proving difficult to reconcile with existing precipitation climatologies. Overall, the energy budget at the surface is much more uncertain than at the top of the atmosphere. A decade of high-precision measurements of the energy budget at the top of the atmosphere is providing new opportunities to track Earth’s energy flows on timescales ranging from days to years, and at very high spatial resolution. The measurements show that the principal limitation in the estimate of secular trends now lies in the natural variability of the Earth system itself. The forcing-feedback-response framework, which has developed to understand how changes in Earth’s energy flows affect surface temperature, is reviewed in light of recent work that shows fast responses (adjustments) of the system are central to the definition of the effective forcing that results from a change in atmospheric composition. In many cases, the adjustment, rather than the characterization of the compositional perturbation (associated, for instance, with changing greenhouse gas concentrations, or aerosol burdens), limits accurate determination of the radiative forcing. Changes in clouds contribute importantly to this adjustment and thus contribute both to uncertainty in estimates of radiative forcing and to uncertainty in the response. Models are indispensable to calculation of the adjustment of the system to a compositional change but are known to be flawed in their representation of clouds. Advances in tracking Earth’s energy flows and compositional changes on daily through decadal timescales are shown to provide both a critical and constructive framework for advancing model development and evaluation.  相似文献   

Low temperature (LT) combustion of sediments does not necessarily provide accurate radiocarbon ages for site chronology     

Fiona Brock  Duane G. Froese  Richard G. Roberts 《Quaternary Geochronology》2010,5(6):625-630

Radiocarbon dating of soils and sediments is notoriously problematic for the purposes of dating a specific event due to their heterogeneous mix of multiple organic fractions, each of which may have a different radiocarbon age. Numerous studies have failed to agree on which sedimentary fraction or radiocarbon pre-treatment method, if any, provides the closest agreement between the age of a sedimentary fraction and that of associated plant macrofossils or charcoal. We tested the stepped-combustion method of McGeehin et al. (2001), as well as standard radiocarbon humin and humic extraction techniques, using samples from a chronologically well-constrained perennially-frozen site at Quartz Creek, Yukon Territory, Canada. The ages in closest agreement with associated radiocarbon-dated plant macrofossils and with the overlying Dawson tephra were given by the humic and humin fractions, but even these were still older than the macrofossil ages by up to 4195 ± 260 radiocarbon years. The low temperature (LT) humin method recommended by McGeehin et al. (2001) yielded ages older than the macrofossils by up to nearly 4425 ± 240 radiocarbon years. These fractions, while still providing information on the mobility and potential residence times of carbon in soils and sediments, should not be relied upon to provide consistently accurate site chronologies.  相似文献   

Spheres of discharge of springs   总被引：3，自引：0，他引：3  

Abraham E. Springer  Lawrence E. Stevens 《Hydrogeology Journal》2009,17(1):83-93

Although springs have been recognized as important, rare, and globally threatened ecosystems, there is as yet no consistent and comprehensive classification system or common lexicon for springs. In this paper, 12 spheres of discharge of springs are defined, sketched, displayed with photographs, and described relative to their hydrogeology of occurrence, and the microhabitats and ecosystems they support. A few of the spheres of discharge have been previously recognized and used by hydrogeologists for over 80 years, but others have only recently been defined geomorphologically. A comparison of these spheres of discharge to classification systems for wetlands, groundwater dependent ecosystems, karst hydrogeology, running waters, and other systems is provided. With a common lexicon for springs, hydrogeologists can provide more consistent guidance for springs ecosystem conservation, management, and restoration. As additional comprehensive inventories of the physical, biological, and cultural characteristics are conducted and analyzed, it will eventually be possible to associate spheres of discharge with discrete vegetation and aquatic invertebrate assemblages, and better understand the habitat requirements of rare or unique springs species. Given the elevated productivity and biodiversity of springs, and their highly threatened status, identification of geomorphic similarities among spring types is essential for conservation of these important ecosystems.  相似文献