Morphological Chronoclines among Late Pleistocene Muskrats (Ondatra zibethicus: Muridae, Rodentia) from Northern Florida     

Matthew C. Mihlbachler  C. Andrew Hemmings  S. David Webb 《Quaternary Research》2002,58(3):289

The muskrat (Ondatra zibethicus) is presumed to have undergone a rapid phyletic size decrease near the end of the Pleistocene. Evolutionary changes in the size of middle to late Wisconsinan (ca. 32,000–12,300 ¹⁴C yr B.P.) muskrats from the Aucilla River, Jefferson County, Florida, were reconstructed by examining length and width of the lower first molar (m1). Body mass, estimated from m1 length, was relatively stable from 32,000 to 16,000 ¹⁴C yr B.P. and decreased only slightly by 12,300 ¹⁴C yr B.P. If the size trend found in the Aucilla River material is characteristic of the southeastern United States, a body size decrease after 12,300 ¹⁴C yr B.P. is needed to explain the smaller size of modern populations. It was previously thought that the length/width (l/w) ratio of the muskrat m1 was a paleoenvironmental indicator based on its presumed correlation with latitude in modern populations. We examined the length and width of modern muskrats from several geographic regions and found only a very weak trend in the size of the m1 between northern and southern populations; however, highly significant differences were found between regions of similar latitude. Our data indicate that chronoclines in the m1 of the Aucilla muskrat material and other such documented trends among fossil muskrats have paleoenvironmental significance, but it is not yet clear which environmental variables can best be predicted from them.  相似文献   

Climatic influences on the oxygen isotopic composition of biogenic silica in prairie grass     

Elizabeth A. Webb  Fred J. Longstaffe 《Geochimica et cosmochimica acta》2002,66(11):1891-1904

Samples of Calamovilfa longifolia were collected from across the North American prairies to investigate the relationship between the oxygen-isotope composition of biogenic silica (phytoliths) deposited in this grass and relative humidity, temperature, and the oxygen-18 enrichment of soil water relative to local precipitation. The δ¹⁸O values of silica in nontranspiring tissues were controlled by soil-water composition and temperature, whereas the oxygen-18 content of silica formed in leaf and inflorescence tissues was enriched further by transpiration. Accurate calculation of growing temperature was possible only when the oxygen-isotope compositions of both stem silica and soil water were known. However, the oxygen-isotope values of stem phytoliths can be used to calculate the variation in the isotopic composition of soil water across a North American temperature gradient.As plant organic matter decays and phytoliths are transferred to the soil, the temperature and soil-water signals carried by the oxygen-isotope composition of silica from nontranspiring tissues can be masked by the oxygen-18 enrichment of phytoliths from transpiring tissues. However, the overall oxygen-isotope composition of a soil-phytolith assemblage can be related to temperature using an empirical relationship based on temperature and the difference between soil-phytolith and estimated soil-water oxygen-isotope compositions.  相似文献   

Determination of trace elements in silicate rocks by X-ray fluorescence spectrometry on 1:5 glass discs: comparison of accuracy and precision with pressed powder pellet analysis     

Jacinta Enzweiler  Peter C. Webb 《Chemical Geology》1996,130(3-4):195-202

A fusion procedure commonly used for major element analysis of silicate rocks (1:5 sample to flux ratio, using 0.7 g of sample) was used to determine a small group of trace elements (Cu, Zn, Rb, Sr, Y, Zr, Nb) by X-ray fluorescence. Compton scatter peak ratioing was successfully used for matrix corrections, despite the thickness of the glass discs being only 1.7 mm. Precision and detection limits were compared with pressed pellet values. For most elements, routine analytical precision is better with powder pellets and detection limits are worse in glass discs, by an average factor of 2, considering all elements analyzed. Accuracy was evaluated with international reference materials and for most of the determinations, good or excellent agreement with recommended values was obtained. The main advantages of using glass discs for the trace element determination are the possibility of analyzing small amounts of sample and the suppression of any mineralogical effects in the glass discs, which can also be used to determine major elements.  相似文献   

First measurements from a deep-tow transient electromagnetic sounding system     

Spahr Webb  R. Nigel Edwards  Liming Yu 《Marine Geophysical Researches》1993,15(1):13-26

An electromagnetic sounding system has been developed to map the shallow electrical conductivity structure of the deep sea floor. The instrument consists of a magnetic source and several colinear magnetic receivers forming an array which is towed along the seafloor. The source generates a time varying magnetic field; the shape of the resulting magnetic field waveform at the receivers depends on the electrical conductivity below the seafloor between the receivers and the source. The instrument can be towed systematically over a study area under acoustic transponder or GPS navigation to construct a map of the electrical conductivity. Towing speeds of greater than 1 m s^–1 (2 knots) can be achieved without adversely effecting data quality. The instrument is sufficiently robust to survive continual contact with thinly sedimented, abrasive basalt. We present the first results from a deployment in August, 1990 near the Cleft Segment of the Juan de Fuca Ridge along an 8 km track to the west of the spreading center. Unforeseen problems with the instrument restricted the utility of the measurements for constructing detailed vertical conductivity profiles, but the measurements were adequate to determine an average conductivity in the upper 25 m, at more than 70 stations. The conductivity was found to vary from 0.1 to 0.4 S/m along the track.  相似文献   

Soft X-ray observations of large-scale coronal active region brightenings     

David M. Rust  David F. Webb 《Solar physics》1977,54(2):403-417

One-hundred fifty-six large-scale enhancements of X-ray emission from solar active regions were studied on full-disk filterheliograms to determine characteristic morphology and expansion rates for heated coronal plasma. The X-ray photographs were compared with H observations of flares, sudden filament disappearances, sprays and loop prominence systems (LPS). Eighty-one percent of the X-ray events were correlated with H filament activity, but only forty-four percent were correlated with reported H flares. The X-ray enhancements took the form of loops or arcades of loops ranging in length from 60 000 km to 520 000 km and averaging 15 000 km in width. Lifetimes ranged from 3 hr to >24 hr. Event frequency was 1.4 per day. X-ray loop arcades evolved from sharp-edged clouds in cavities vacated by rising H filaments. Expansion velocities of the loops were 50 km s^-1 immediately after excitation and 1–10 km s^-1 several hours later. These long-lived loop arcades are identified with LPS, and it is suggested that the loops outlined magnetic fields which were reconnecting after filament eruptions. Another class of X-ray enhanced loops stretched outside active regions and accompanied sprays or lateral filament ejections. H brightenings occurred where these loops intersected the chromosphere. Inferred excitation velocities along the loops ranged between 300 and 1200 km s^-1. It is suggested that these loops outlined closed magnetic fields guiding slow mode shocks from flares and filament eruptions.  相似文献   

X-ray and Hα observations of a filament-disappearance flare: An empirical analysis of the magnetic field configuration     

S. W. Kahler  D. F. Webb  R. L. Moore 《Solar physics》1981,70(2):335-350

A flare event occurred which involved the disappearance of a filament near central meridian on 29 August 1973. The event was well observed in X-rays with the AS & E telescope on Skylab and in H at BBSO. It was a four-ribbon flare involving both new and old magnetic inversion lines which were roughly parallel. The H, X-ray, and magnetic field data are used to deduce the magnetic polarities of the H brightenings at the footpoints of the brightest X-ray loops. These magnetic structures and the preflare history of the region are then used to argue that the event involved a reconnection of magnetic field lines rather than a brightening in place of pre-existing loops. The simultaneity of the H brightening onsets in the four ribbons and the apparent lack of an eruption of the filament are consistent with this interpretation. These observations are compared to other studies of filament disappearances. The preflare structures and the alignment of the early X-ray flare loops with the H filament are consistent with the schematic picture of a filament presented first by Canfield et al. (1974).  相似文献   

Coronal loops and active region structure     

D. F. Webb  H. Zirin 《Solar physics》1981,69(1):99-118

We intercompared synoptic H, Ca K, magnetograph and Skylab soft X-ray and EUV data for the purpose of identifying the basic coronal magnetic structure of loops in a typical active region and studying its evolution. We focussed on a complex of activity in July 1973, especially McMath 12417. Our principal results are: (1) Most of the brightest loops connected the bright f plage to either the sunspot penumbra or to p satellite spots; no non-flaring X-ray loops end in umbrae; (2) short, bright loops had one or both ends in regions of emergent flux, strong fields or high field gradients; (3) stable, strongly sheared loop arcades formed over filaments; (4) EFRs were always associated with compact X-ray arcades; and (5) loops connecting to other active regions had their bases in outlying plage of weak field strength in McM 417 where H fibrils marked the direction of the loops. We conclude that a typical loop brightens in response to magnetic field activity at its feet, which heats the plasma. This suggests that the loop acts as a trap for gas convected from its base.  相似文献   

On the equation of transport for cosmic-ray particles in the interplanetary region     

G. M. Webb  L. J. Gleeson 《Astrophysics and Space Science》1979,60(2):335-351

In this paper we provide two new alternative derivations of the equation of transport for cosmic-ray particles in the interplanetary region. Both derivations are carried out by using particle positionr and timet in a frame of reference fixed in the solar system, and the particle momentump is specified relative to a local frame of reference moving with the solar wind. The first derivation is carried out by writing down a continuity equation for the cosmic rays, taking into account particle streaming and energy changes, and subsequently deriving the streaming and energy change terms in this equation. The momentum change term in the continuity equation, previously considered to be due to the adiabatic deceleration of particles in the expanding magnetic fields carried by the solar wind, appears in the present analysis as a dynamic effect in which the Lorentz force on the particle does not appear explicitly. An alternative derivation based on the ensemble averaged Liouville equation for charged particles in the stochastic interplanetary magnetic field using (r, p,t) as independent coordinates is also given. The latter derivation confirms the momentum change interpretation of the first derivation. We also provide a new derivation of the adiabatic rate as a combination of inverse-Fermi and betatron deceleration processes.  相似文献   

Towards quantifying uncertainty in transient climate change   总被引：2，自引：3，他引：2  

Matthew Collins  Ben B. B. Booth  Glen R. Harris  James M. Murphy  David M. H. Sexton  Mark J. Webb 《Climate Dynamics》2006,27(2-3):127-147

Ensembles of coupled atmosphere–ocean global circulation model simulations are required to make probabilistic predictions of future climate change. “Perturbed physics” ensembles provide a new approach in which modelling uncertainties are sampled systematically by perturbing uncertain parameters. The aim is to provide a basis for probabilistic predictions in which the impact of prior assumptions and observational constraints can be clearly distinguished. Here we report on the first perturbed physics coupled atmosphere–ocean model ensemble in which poorly constrained atmosphere, land and sea-ice component parameters are varied in the third version of the Hadley Centre model (the variation of ocean parameters will be the subject of future study). Flux adjustments are employed, both to reduce regional sea surface temperature (SST) and salinity biases and also to admit the use of combinations of model parameter values which give non-zero values for the global radiation balance. This improves the extent to which the ensemble provides a credible basis for the quantification of uncertainties in climate change, especially at a regional level. However, this particular implementation of flux-adjustments leads to a weakening of the Atlantic overturning circulation, resulting in the development of biases in SST and sea ice in the North Atlantic and Arctic Oceans. Nevertheless, model versions are produced which are of similar quality to the unperturbed and un-flux-adjusted version. The ensemble is used to simulate pre-industrial conditions and a simple scenario of a 1% per year compounded increase in CO₂. The range of transient climate response (the 20 year averaged global warming at the time of CO₂ doubling) is 1.5–2.6°C, similar to that found in multi-model studies. Measures of global and large scale climate change from the coupled models show simple relationships with associated measures computed from atmosphere-mixed-layer-ocean climate change experiments, suggesting that recent advances in computing the probability density function of climate change under equilibrium conditions using the perturbed physics approach may be extended to the transient case.  相似文献   

Carolina Thunder Revisited: Toward a Transcultural View of Winston Cup Racing*     

Derek H. Alderman  Preston W. Mitchell  Jeffrey T. Webb  Derek Hanak 《The Professional geographer》2003,55(2):238-249

Almost three decades ago, cultural geographer Richard Pillsbury documented the national expansion of NASCAR and what he considered the erosion of major‐league stock car racing as a unique southern tradition. This claim is reassessed in light of recent research, leading us to suggest that the sport is actually “transcultural” in nature. It is influenced simultaneously by tradition and transition, as well as regional and national forces. In revisiting Pillsbury's seminal work, we document major changes and continuities in Winston Cup racing and briefly examine two North Carolina cases that provide contradictory views on the current relationship between the sport and the American South. North Wilkesboro illustrates how the changing geography of track locations can devalue and demoralize places associated with the tradition of southern stock car racing. The greater Charlotte area demonstrates that the South remains an important part of NASCAR, serving as a gathering place and “knowledge community” for drivers, racing teams, and fans from across the country. In addition to advancing research in the geography of sport and popular culture, the article encourages readers to think critically about regional cultures and their relationship to the forces of nationalization.  相似文献