Geochronology and geochemistry of Grenvillian igneous suites in the northern Oaxacan Complex, southern Mexico: tectonic implications

Chemical and U–Pb isotopic analyses of metaigneous rocks in the northern Oaxacan Complex in southern Mexico indicate that they form part of two granitic–gabbroic suites intruded at 1157–1130 and 1012 Ma, which were metamorphosed under granulite facies conditions between 1004 and 980 Ma. Although the older suite has both within-plate and arc geochemical signatures, the arc characteristics (enrichment of La and Ce relative to Nb, Ta, and Th) are inferred to result from crustal contamination, a conclusion consistent with their negative _Nd signatures. The younger suite is spatially associated with anorthosites (from which we were unable to acquire a protolith age), suggesting that collectively it forms part of anorthosite–mangerite–charnockite–granite (AMCG) suites. The tholeiitic nature of the mafic rocks along with the within-plate character of the felsic rocks suggests that they were intruded during extension related to either farfield backarc rifting, rifting above a slab window, or anorogenic intercontinental rifting. Potentially correlative AMCG suites are widespread in Mexico, the Grenville Province of eastern Canada and northeastern USA, and the Andean massifs of Colombia, however, Pb isotopic data most closely resemble those in South America. These data are consistent with published hypotheses that suggest Oaxaquia represents an exotic terrane derived from Amazonia.     

Verification of the RAMS-based operational weather forecast system in the Valencia Region: a seasonal comparison

Natural Hazards - The meteorological model Regional Atmospheric Modeling System (RAMS) in its version 4.4 has been applied operationally within the Valencia Region. The model output is being used...     

基于综合指标法的中国生态系统服务保护有效性评价研究

将生态系统服务保护纳入到传统的以保护区为基础的自然保护之中是生态保护研究的新领域。利用综合指标方法,分析了中国从2000-2010年的4种生态系统服务时空变化特征;并对中国的七大生态保护和恢复区域的服务空间保护成效进行评价,提出了国家尺度上的生态系统服务保护优先性分级。主要结论为：① 中国的生态系统服务自西北向东南逐渐增加,显著增加区主要分布在黄土高原地区,显著减少区主要分布在东北及华北地区。黄土高原的生态系统服务提高主要是农田向草地和林地、草地向林地的转换引的;华北地区的降低主要是耕地的减少以及人工用地的增加引起的;东北地区是由于林地向耕地草地、耕地向湿地与人工用地的转换引起的。② 七大生态保护和恢复区的生态系统服务均为增加趋势而且显著增加面积大于减少面积;显著增加面积最大的为三北防护林工程区、最小的为重点生态功能区;显著减少最大为重要生态功能区、最小的为三北防护林工程区。而且,对服务的保护成效在重要生态功能区最高,其次为天然林保护工程区,其余区域均小于全国平均水平。③ 提出的生态系统服务保护优先性等级中,极高级别占到了国土面积的26.18%,保护的生态系统服务量占到全国总量的64.19%,保护成效为全国平均水平的2.47倍。     

Structure of the thermal lithosphere and asthenosphere beneath oceans and continents

The lithosphere and asthenosphere make up a common geodynamic system but are characterized by different physical parameters. The former has a temperature of 1200–1300°C, a density of 3.3 g/cm³, and a viscosity of 10²² poise, while the latter has a density of 3.23 g/cm³, a viscosity in the range 10²¹-10^18–19 poise, and a temperature from 1200–1300°C to 1600–1700°C. The asthenosphere is distinguished by a great variability of its physical state in the lateral and vertical directions. This circumstance necessitates the recognition of the different types of the asthenosphere: seismic (LVZ zone), electrical, thermal, and seismological. The structure and the physical state of the thermal asthenosphere is considered in this paper on the basis of P-T parameters. Its state normally fits viscous Newtonian liquid beneath the continents and provides partial (5–20%) melting in spreading zones and along continental margins. No partial melting is detected beneath the main portion of the continents. The interaction between the asthenosphere and lithosphere is characterized by spatiotemporal migration of partial melting zones and asthenosphere upwelling, and such interaction determines the entire range of geodynamic processes from spreading and rifting to collision and vertical motions of different senses.     

Using Spatial Statistics to Examine the Pattern of Urban Land Development in Halifax–Dartmouth

This article examines the locational patterns of population and employment and the resultant changes in urban form in the Halifax–Dartmouth region between 1970 and 1996. The article employs the univariate and bivariate K function to measure spatial dependence or clustering within and between the classes of residential and commercial land parcels. The results of the K function estimates suggest that residential land parcels cluster together, commercial land parcels cluster together, and residential and commercial land parcels have become more clustered over time. Evidence of clustering provides insight into the changing urban form of the region and possible multinucleation.     

Seasonal δS variations in two high elevation snow pits measured by S-S double spike thermal ionization mass spectrometry

δ³⁴S and sulfate concentrations were determined in snow pit samples using a thermal ionization mass spectrometric technique capable of 0.2‰ accuracy and requires ≈5 μg (0.16 μmol) natural S. The technique utilizes a ³³S-³⁶S double spike for instrumental mass fractionation correction, and has been applied to snow pit samples collected from the Inilchek Glacier, Kyrgyzstan and from Summit, Greenland. These δ³⁴S determinations provide the first high-resolution seasonal data for these sites, and are used to estimate seasonal sulfate sources. Deuterium (δD) and oxygen (δ¹⁸O) isotope data show that the Inilchek and Summit snow pit samples represent precipitation over ≈20 months.The δ³⁴S values for the Inilchek ranged from +2.6 ± 0.4‰ to +7.6 ± 0.4‰ on sample sizes ranging from 0.3 to 1.8 μmol S. δ³⁴S values for Greenland ranged from +3.6 ± 0.7‰ to +13.3 ± 5‰ for sample sizes ranging from 0.05 to 0.29 μmol S. The concentration ranged from 92.6 ± 0.4 to 1049 ± 4 ng/g for the Inilchek and 18 ± 9 to 93 ± 6 ng/g for the Greenland snow pit. Anthropogenic sulfate dominates throughout the sampled time interval for both sites based on mass balance considerations. Additionally, both sites exhibit a seasonal signature in both δ³⁴S and concentration. The thermal ionization mass spectrometric technique has three advantages compared to gas source isotopic methods: (1) sample size requirements of this technique are 10-fold less permitting access to the higher resolution S isotope record of low concentration snow and ice, (2) the double spike technique permits δ³⁴S and S concentration to be determined simultaneously, and (3) the double spike is an internal standard.     

Characterization of interface roughness of rippled sand off Fort Walton Beach, Florida

As part of the environmental characterization to model acoustic bottom scattering during the high-frequency sediment acoustics experiment (SAX99), fine-scale sediment roughness of a medium sand was successfully measured within a 600 /spl times/ 600-m area by two methods: stereo photography and a technique using a conductivity system. Areal coverage of the two methods, representing approximately 0.16 m/sup 2/ of the sea floor, was comparable, resulting in the depiction and quantification of half-meter wavelength sand ripples. Photogrammetric results were restricted to profiles digitized at 1-mm intervals; sediment conductivity results generated gridded micro-bathymetric measurements with 1- to 2-cm node spacing. Roughness power spectra give similar results in the low-spatial-frequency domains where the spectra estimated from both approaches overlap. However, spectra derived from higher resolution photogrammetric results appear to exhibit a multiple-power-law fit. Roughness measurements also indicate that spectrum changes as a function of time. Application of statistical confidence bounds on the power spectra indicates that roughness measurements separated by only 1-2 m may be spatially nonstationary.     

The concentration of molecular H2 and CH4 in the stratosphere

Stratospheric profiles of CH₄ and H₂ over eastern Texas have been derived up to 31 km altitude from air samples collected aboard a balloon and analyzed by gas chromatography. For H₂, contamination during flight and measurement presented a problem which has been resolved only recently. The earlier profiles require corrections which are rather large for the highest altitudes.The three profiles indicate an increase of the H₂ concentration in the lower stratosphere from about 0.5 p.p.m. per volume at the tropopause to about 0.8 p.p.m. at around 27 km altitude. Above that altitude the H₂ concentration decreases again. An air sample collected between 44 and 62 km by a rocket-borne cryogenic sampler had an H₂ concentration of 0.4 p.p.m.The five CH₄ profiles showed a decrease in CH₄ concentration with altitude generally with a steeper gradient directly above the tropopause and a weaker gradient at higher altitudes reaching 0.9 p.p.m. at 30 km altitude. The CH₄ concentration in the rocket sample was 0.25 p.p.m., in good agreement with the gradient obtained from the balloon samples.The National Center for Atmospheric Research is sponsored by the National Science Foundation.