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41.
Giant ground figures are widespread in the lower Colorado River area of southwestern North America, yet their chronology has remained unconstrained by numerical ages. Thirteen AMS 14C measurements reported here indicate that geoglyphs were made from before ˜A. D. 1200 to before ˜900 B. C. We account for potential contamination from prior organics in weathering rinds. All other potential errors point to 14C dates being minimum-limiting ages for the manufacturing of geoglyphs. Although these ages indicate considerable chronological complexity among geoglyphs, our data are consistent with the linguistic hypothesis that the Yuman people in the desert of southeastern California migrated from Baja California—rather than from the north. These results must, however, be placed under the cloud of uncertainty that hangs over the entire field of AMS dating of rock art: the untested assumption surrounding contemporeneity of organics in a surface context. © 1995 John Wiley & Sons, Inc. 相似文献
42.
43.
Sarah E. Rothenberg Matthew E. Kirby Broxton W. Bird Margie B. DeRose Chu-Ching Lin Xinbin Feng Richard F. Ambrose Jennifer A. Jay 《Environmental Earth Sciences》2010,60(5):993-1005
In southern California, USA, wildfires may be an important source of mercury (Hg) to local watersheds. Hg levels and Hg accumulation
rates were investigated in dated sediment cores from two southern California lakes, Big Bear Lake and Crystal Lake, located
approximately 40-km apart. Between 1895 and 2006, fires were routinely minimized or suppressed around Big Bear Lake, while
fires regularly subsumed the forest surrounding Crystal Lake. Mean Hg concentrations and mean Hg accumulation rates were significantly
higher in Crystal Lake sediments compared to Big Bear Lake sediments (Hg levels: Crystal Lake 220 ± 93 ng g−1, Big Bear Lake 92 ± 26 ng g−1; Hg accumulation: Crystal Lake 790 ± 1,200 μg m−2 year−1, Big Bear 240 ± 54 μg m−2 year−1). In Crystal Lake, the ratio between post-1965 and pre-1865 Hg concentrations was 1.1, and several spikes in Hg levels occurred
between 1910 and 1985. Given the remote location of the lake, the proximity of fires, and the lack of point sources within
the region, these results suggested wildfires (rather than industrial sources) were a continuous source of Hg to Crystal Lake
over the last 150 years. 相似文献
44.
TitaniQ under pressure: the effect of pressure and temperature on the solubility of Ti in quartz 总被引:5,自引:2,他引:3
Jay B. Thomas E. Bruce Watson Frank S. Spear Philip T. Shemella Saroj K. Nayak Antonio Lanzirotti 《Contributions to Mineralogy and Petrology》2010,160(5):743-759
Quartz and rutile were synthesized from silica-saturated aqueous fluids between 5 and 20 kbar and from 700 to 940°C in a piston-cylinder
apparatus to explore the potential pressure effect on Ti solubility in quartz. A systematic decrease in Ti-in-quartz solubility
occurs between 5 and 20 kbar. Titanium K-edge X-ray absorption near-edge structure (XANES) measurements demonstrate that Ti4+ substitutes for Si4+ on fourfold tetrahedral sites in quartz at all conditions studied. Molecular dynamic simulations support XANES measurements
and demonstrate that Ti incorporation onto fourfold sites is favored over interstitial solubility mechanisms. To account for
the P–T dependence of Ti-in-quartz solubility, a least-squares method was used to fit Ti concentrations in quartz from all experiments
to the simple expression
RTlnX\textTiO 2 \textquartz = - 60952 + 1.520 ·T(K) - 1741 ·P(kbar) + RTlna\textTiO 2 RT\ln X_{{{\text{TiO}}_{ 2} }}^{\text{quartz}} = - 60952 + 1.520 \cdot T(K) - 1741 \cdot P(kbar) + RT\ln a_{{{\text{TiO}}_{ 2} }} 相似文献
45.
Stanislav S. Shabala Jonathan G. Rogers Jamie N. McCallum Oleg A. Titov Jay Blanchard James E. J. Lovell Christopher S. Watson 《Journal of Geodesy》2014,88(6):575-586
We examine the relationship between source position stability and astrophysical properties of radio-loud quasars making up the International Celestial Reference Frame (ICRF2). Understanding this relationship is important for improving quasar selection and analysis strategies, and therefore reference frame stability. We construct flux density time series, known as light curves, for 95 of the most frequently observed ICRF2 quasars at both the 2.3 and 8.4 GHz geodetic very long baseline interferometry (VLBI) observing bands. Because the appearance of new quasar components corresponds to an increase in quasar flux density, these light curves alert us about potential changes in source structure before they appear in VLBI images. We test how source position stability depends on three astrophysical parameters: (1) flux density variability at X band; (2) time lag between flares in S and X bands; (3) spectral index root-mean-square (rms), defined as the variability in the ratio between S and X band flux densities. We find that the time lag between S and X band light curves provides a good indicator of position stability: sources with time lags $<$ 0.06 years are significantly more stable ( $>$ 20 % improvement in weighted rms) than sources with larger time lags. A similar improvement is obtained by observing sources with low $(<$ 0.12) spectral index variability. On the other hand, there is no strong dependence of source position stability on flux density variability in a single frequency band. These findings can be understood by interpreting the time lag between S and X band light curves as a measure of the size of the source structure. Monitoring of source flux density at multiple frequencies therefore appears to provide a useful probe of quasar structure on scales important to geodesy. The observed astrometric position of the brightest quasar component (the core) is known to depend on observing frequency. We show how multi-frequency flux density monitoring may allow the dependence on frequency of the relative core positions along the jet to be elucidated. Knowledge of the position–frequency relation has important implications for current and future geodetic VLBI programs, as well as the alignment between the radio and optical celestial reference frames. 相似文献
46.
Páramo is a term used to describe tropical alpine vegetation between the continuous timberline and the snow line in the Northern Andes. Páramo environments provide important species habitat and ecosystem services. Changes in spatial extent of the páramo ecosystem at Pambamarca in the Central Cordillera of the northern Ecuadorian Andes were analysed using multi-temporal Landsat TM/ETM+ satellite data. The region suffered a loss of 1826.6 ha or 20% of the total area at a rate of 100 ha/annum during 1988-2007 period. It is found that permanent páramo cover decreased from 8350 ha in 1988 to 5864 ha in 2007 at a fairly constant rate(R2=0.94). This loss is attributed to expansion of commercial agriculture and floriculture in the valleys coupled with increased population pressure. Land at higher elevations has been cleared for small scale agriculture. Loss of the páramo ecosystem will exert a number of negative impacts on ecosystem services and livelihoods of the local population at Pambamarca. 相似文献
47.
Donald G. Isaak Earl K. Graham Jay D. Bass Hong Wang 《Pure and Applied Geophysics》1993,141(2-4):393-414
We present new elasticity measurements on single-crystal fayalite and combine our results with other data from resonance, pulse superposition interferometry, and Brillouin scattering to provide a set of recommended values for the adiabatic elastic moduliC
ij and their temperature variations. We use a resonance method (RPR) with specimens that were previously investigated by pulse superposition experiments. The nineC
ij of fayalite are determined from three new sets of measurements. One set of our newC
ij data is over the range 300–500 K. We believe that the relatively large discrep ncies found in someC
ij are due in large part to specimen inhomogeneities (chemical and microstructural) coupled with differences in the way various techniques sample, rather than only systematic errors associated with experimental procedures or in the preparations of the specimens.Our recommendeaC
ij's (GPa) and (C
ij/T)
p
(GPa/K) are:
The resulting values for the isotropic bulk and shear moduli,K
s and , and their temperature derivatives are:K
s=134(4) GPa; =50.7(0.3) GPa; (K
s/T)
p
=–0.024(0.005) GPa/K; and (/T)
p
=–0.013(0.001) GPa/K. An important conclusion is thatK
s increases as the Fe/(Fe+Mg) ratio in olivine is increased. 相似文献
48.
Many regions around the world require improved gravimetric data bases to support very accurate geoid modeling for the modernization
of height systems using GPS. We present a simple yet effective method to assess gravity data requirements, particularly the
necessary resolution, for a desired precision in geoid computation. The approach is based on simulating high-resolution gravimetry
using a topography-correlated model that is adjusted to be consistent with an existing network of gravity data. Analysis of
these adjusted, simulated data through Stokes’s integral indicates where existing gravity data must be supplemented by new
surveys in order to achieve an acceptable level of omission error in the geoid undulation. The simulated model can equally
be used to analyze commission error, as well as model error and data inconsistencies to a limited extent. The proposed method
is applied to South Korea and shows clearly where existing gravity data are too scarce for precise geoid computation. 相似文献
49.
Two approaches of generating pore networks of porous media are presented to capture the pore fabric. The first methodology extracted pore structure from a computer simulated packing of spheres. The modified Delaunay tessellation was used to describe the porous media, and modified Nelder–Mead method in conjunction with three pore‐merging algorithms was used to generate the pore size and coordination number distributions of the randomly packed spheres. The Biconical Abscissa Asymmetric CONcentric bond was used to describe the connection between two adjacent voids. This algorithm was validated by predicting pore structure of a cubic array of spheres of equal radius with known pore sizes, throat sizes and coordination number distributions. The predicted distributions of pore structure agreed well with the measured. Then, the algorithm was used to predict pore structure and permeability of randomly packed spherical particles, and predicted permeability values were compared with published experimental data. The results showed that the predicted permeability values were in good agreement with those measured, confirming the proposed algorithm can capture the main flow paths of packed beds. The second methodology generated an equivalent pore network of porous media, of which the centers of voids were located in a regular lattice with constant pore center distance. However, this network allowed for matching both main geometrical and topological characteristics of the porous media. A comparison of the two approaches suggested that the second approach can also be used as a predictive tool to quantitatively study the microscopic properties of flow through porous media. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
50.
Jay S. Fein 《地球物理与天体物理流体动力学》2013,107(1):213-248
AbstractThe effects of the upper boundary condition on the regime diagram and certain characteristics of the convection within a rotating, differentially heated cylindrical anulus of water were studied in the laboratory. It was found that the regime diagram was not substantially affected by the upper boundary condition. However, the thermal amplitude of the baroclinic waves, as a function of parameter space, and, as expected from previous work, the angular drift velocity of those waves were found to be strongly affected by the upper boundary condition. When the upper surface was free, the amplitude changes were explosive and highly non-linear (as discovered earlier by Kaiser, 1970). When the upper surface was rigid, they were smooth and quite linear. The baroclinic wave patterns drifted round the annulus at rates which were in direct response to the imposed “thermal wind”. However, (as previously observed by Koschmieder, 1972), when the upper surface was rigid they drifted approximately ten times more slowly than when it was free. 相似文献
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