Fluctuations in isothermal spheres     

Joseph Katz  Isao Okamoto 《Monthly notices of the Royal Astronomical Society》2000,317(1):163-169

Isolated isothermal spheres of N gravitationally interacting points with equal mass are believed to be stable when density contrasts do not exceed 709. That stability limit does not, however, take into consideration fluctuations of temperature near the onset of instability. These are important when N is finite.
Here we correlate global mean quadratic temperature fluctuations with the onset of instability. We show that such fluctuations trigger instability when the density contrast reaches a value near 709×exp(−3.3 N ^−1/3). These lower values of limiting density contrasts are significantly smaller than 709 when N is not very large, and this suggests (i) that numerical calculations with small N may not reflect correctly the onset of core collapse in clusters with large N , and (ii) that a greater number of globular clusters than is normally believed may already be in an advanced stage of core collapse, because most of the observed globular clusters with parameters that fit quasi-isothermal configurations are close to marginal stability.  相似文献   

The superdense plasma sheet in the magnetosphere during high-speed-stream-driven storms: Plasma transport timescales     

Michael H. Denton  Joseph E. Borovsky 《Journal of Atmospheric and Solar》2009,71(10-11):1045-1058

The superdense plasma sheet in the Earth's magnetosphere is studied via a superposition of multispacecraft data collected during 124 high-speed-stream-driven storms. The storm onsets tend to occur after the passage of the IMF sector reversal and before the passage of the stream interface, and the storms continue on for days during the passage of the high-speed stream. The superdense phase of the plasma sheet is found to be a common feature of high-speed-stream-driven storms, commencing before the onset of the storm and persisting for about 1 day into the storm. A separate phenomenon, the extra-hot phase of the plasma sheet, commences at storm onset and persists for several days during the storm. The superdense plasma sheet originates from the high-density compressed slow and fast solar wind of the corotating interaction region on the leading edge of the high-speed stream. Tracking the motion of this dense plasma into and through the magnetosphere, plasma transport times are estimated. Transport from the nightside of the dipole to the dayside requires about 10 h. The occurrences of both the superdense plasma sheet and the extra-hot plasma sheet have broad implications for the physics of geomagnetic storms.  相似文献   

Lacustrine turbidites as indicators of Holocene storminess and climate: Lake Tahoe,California and Nevada     

David A. Osleger  Alan C. Heyvaert  Joseph S. Stoner  Kenneth L. Verosub 《Journal of Paleolimnology》2009,42(1):103-122

Sediment cores from Lake Tahoe permit the discrimination of turbidites initiated by seismic-induced debris flows from those generated by severe storms and associated hyperpycnal currents over the last 7000 years using integrated textural, magnetic, and geochemical signatures. Relative to fine-grained ‘background’ sediments, the majority of Tahoe turbidites exhibit coincident trends of increased mean grain size, increased magnetic susceptibility, decreased TOC, higher δ¹³C_org and variable C/N. We interpret these characteristics to record the rapid influx of terrigenous sediments within runoff from the watershed triggered by high-intensity storms. Correlation of multiple, individual turbidites between cores suggests a synchronicity of occurrence, supporting the model of extreme hydrologic events as the trigger for most turbidity currents into Lake Tahoe. In contrast, turbidites generated by seismic collapse of steep lake margins would have textural, magnetic and geochemical signatures that would reflect a homogenized mix of autochthonous biogenic debris and multiple older turbidites. Only one of the turbidites in the cores appears to be seismically generated. A second component of this study tested the hypothesis that turbidite clustering reflects phases of increased storminess, paleoprecipitation and lake level. We correlated broad patterns of turbidite frequency in the Tahoe cores with climate proxies from (1) elsewhere in the Tahoe watershed, (2) the western Great Basin (primarily Pyramid Lake) and (3) the San Francisco bay estuary. The reasonable degree of temporal overlap suggests that apparent trends in severe storm frequency recorded by clusters of turbidites provides a measure of long-term regional paleoprecipitation and lake level. A key finding is an extended phase of dryness and a near absence of major storms between ~3000 and ~900 cal yr B.P. in the Tahoe watershed.  相似文献   

Shallow lake trophic status linked to late Holocene climate and human impacts   总被引：1，自引：1，他引：0  

Matthew N. Waters  Michael F. Piehler  Antonio B. Rodriguez  Joseph M. Smoak  Thomas S. Bianchi 《Journal of Paleolimnology》2009,42(1):51-64

Lake Mattamuskeet, North Carolina, USA is a large (162 km²) and shallow (mean depth = 1 m) coastal lake, which was significantly modified to support agricultural activities following European settlement in 1850. Paleolimnological proxies measured on a 400-cm sediment core collected from Lake Mattamuskeet reveal shifts in organic matter input and primary producer community structure in response to climatic and human impacts on the lake during the late Holocene. Stratigraphic changes in organic matter content, nutrients, metals, lignin phenols and photosynthetic pigments were used to divide the sediment core into three intervals. Interval I includes sediment deposited between A.D. 360–1584 and indicates a clear-water, sand-bottom state with low algal abundance. In addition, the lake catchment area experienced two significant fires during this interval that were recorded as charcoal layers in the core around A.D. 360 and A.D. 1435 (calibrated ¹⁴C AMS dates). Trophic structure changed with the onset of Interval II (A.D. 1584–1860) when total algal abundance increased, and the primary producer community was comprised primarily of diatoms, chrysophytes, cryptophytes and cyanobacteria. During this interval there was also an increase in terrestrial organic material input into the lake as well as a shift in plant type from woody gymnosperms to non-woody angiosperms as determined from lignin data. Sediment deposited in Lake Mattamuskeet following European settlement (Interval III, A.D. 1860-present) suggests a dramatic increase in organic-matter deposition, metals, primary-producer abundance and the onset of cyanobacterial dominance. Sedimentary evidence indicates that shallow-water primary producers can respond rapidly to climate change and human development.  相似文献   

Flow through macropores of different size classes in blanket peat     

Joseph Holden 《Journal of Hydrology》2009,364(3-4):342-348

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The effects of Hurricanes Katrina and Rita on seabed polycyclic aromatic hydrocarbon dynamics in the Gulf of Mexico     

Siddhartha Mitra  Joseph J. Lalicata  Mead A. Allison  Timothy M. Dellapenna 《Marine pollution bulletin》2009,58(6):851-857

To assess the extent to which Hurricanes Katrina and Rita affected polycyclic aromatic hydrocarbons (PAH) in the Gulf of Mexico (GOM), sediment cores were analyzed in late 2005 from: a shallow shelf, a deeper shelf, and a marsh station. Sediment geochronology, fabric, and geochemistry show that the 2005 storms deposited ∼10 cm of sediment to the surface of a core at 5-12A. Bulk carbon geochemistry and PAH isomers in this top layer suggest that the source of sediment to the top portion of core 5-12A was from a relatively more marine area. Particulate PAHs in the marsh core (04 M) appeared unaffected by the storms while sediments in the core from Station 5-1B (deeper shelf) were affected minimally (some possible storm-derived deposition). Substantial amounts of PAH-laden particles may have been displaced from the seabed in shallow areas of the water column in the GOM by these 2005 storms.  相似文献   

Seeing below the basalt – offshore Faroes     

Joseph W. Gallagher  Peter W. Dromgoole 《Geophysical Prospecting》2008,56(1):33-45

Exploration in the basalt covered areas of the Faroes offshore has always suffered from poor seismic imaging below the basalt. Long offset 2D and 3D seismic data were acquired and a significant improvement in the seismic image below top basalt has been achieved. Deep towing of the source and receiver cables helped by extending the seismic bandwidth towards lower frequencies. Bubble‐tuned rather than conventional peak‐tuned source arrays gave little, if any, incremental benefit. The improvement in the imaging comes primarily from the approach to processing the data. High frequencies (dominantly noise) are filtered out of the data early in the processing to concentrate on the low frequency data. Careful multiple removal is important with several passes of demultiple being applied to the data using both Surface‐Related Multiple Elimination (SRME) and Radon techniques. Velocity analysis is performed as an iterative process taking into account the geological model. Reprocessing legacy 2D surveys, acquired with wide‐ranging parameters, using these processing techniques improved these datasets significantly, indicating that sub‐basalt imaging seems to be more sensitive to processing than to the choice of acquisition parameters.  相似文献   

Uranium Removal from Ground Water Using Zero Valent Iron Media   总被引：1，自引：0，他引：1  

James Farrell  William D. Bostick  Robert J. Jarabek  Joseph N. Fiedor 《Ground water》1999,37(4):618-624

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Baseline Sediment Trace Metals Investigation: Steinhatchee River Estuary, Florida, Northeast Gulf of Mexico     

Candace A. Trimble  Ronald W. Hoenstine  A. Brad Highley  Joseph F. Donoghue  Paul C. Ragland 《Marine Georesources & Geotechnology》1999,17(2):187-197

This Florida Geological Survey U.S. Department of the Interior, Minerals Manage ment Service Cooperative Study provides baseline data for major and trace metal concentrations in the sediments of the Steinhatchee River estuary. These data are intended to provide a benchmark for comparison with future metal concentration data measurements. The Steinhatchee River estuary is a relatively pristine bay located within the Big Bend Wildlife Management Area on the North Central Florida Gulf of Mexico coastline. The river flows 55 km through woodlands and planted pines before emptying into the Gulf at Deadman Harbor. Water quality in the estuary is excellent at present. There is minimal development within the watershed. The estuary is part of an extensive system of marshes that formed along the Florida Gulf coast during the Holocene marine transgression. Sediment accretion rate measurements range from 1.4 to 4.1 mm yr on the basis of lead-210 measurements. Seventy-nine short cores were collected from 66 sample locations, representing four lithofacies: clay- and organic-rich sands, organic-rich sands, clean quartz sands, and oyster bioherms. Samples were analyzed for texture, total organic matter, total carbon, total nitrogen, clay mineralogy, and major and trace-metal content. Follow ing these analyses, metal concentrations were normalized against geochemical reference elements (aluminum and iron) and against total weight percent organic matter. Metals were also normalized granulometrically against total weight percent fines (0.062 mm). Concentrations were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) for all metals except mercury. Mercury concentrations were determined by cold-flameless atomic absorption spectrometry (AAS). Granulo metric measurements were made by sieve and pipette analyses. Organic matter was determined by two methods: weight loss upon ignition and elemental analysis (by Carlo-Erba Furnace) of carbon and nitrogen. X-ray diffraction was used to deter mine clay mineralogy. Trace-metal concentrations were best correlated when normalized with respect to sediment aluminum concentrations. Normalizations indicate that most major and trace-metal concentrations fall within 95 % prediction limits of the expected value. This finding suggests that little significant metal contamination occurred within this system prior to 1994 sediment sampling. Exceptions include lead, mercury, copper, zinc, potassium, and phosphorous. Lead and mercury are elements that generally enter this watershed through atmospheric deposition; thus, anomalous levels of these metals are not necessarily associated with activities within the watershed of the Steinhatchee River estuary. Anomalous concentrations of other metals such as zinc, copper, and phosphorous probably do originate within the Steinhatchee watershed. Copper failed to correlate well with any geochemical or granulometric normalizer, and this condition was not limited to a single facies or area within the estuary. This finding may indicate copper contamination in the system. Increased zinc and copper levels may be attributed to marine paints. Phosphorous levels also appeared to be elevated in a few locations in the two marsh facies sampled. This may be due to nutrient loading from two small communities, Jena and Steinhatchee, or from the application of this element in fertilizer to reduce moisture stress to young planted pines on tree farms within the watershed.  相似文献   

Yanshanian and Himalayan Evolution of the Western Tarim Area(Xinjiang China): From the Jurassic Trough up to the Neogere and Quaternary Flexural Basin     

Joseph CANEROT  et al 《地学前缘》2000,(Z1)

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