Composition and tectonic evolution of the Chinese continental crust constrained by Poisson's ratio     

Shaocheng Ji  Qian Wang  Matthew H. Salisbury 《Tectonophysics》2009,463(1-4):15-30

We have measured both P- and S-wave velocities (V_p and V_s) and Poisson's ratios (υ) of 60 typical ultrahigh pressure (UHP) metamorphic rock samples from the Chinese Continental Scientific Drilling (CCSD) main and pre-pilot holes and surface outcrops in the Sulu–Dabie orogenic belt at hydrostatic confining pressures up to 850 MPa. The experimental results, together with those compiled in Handbook of Seismic Properties of Minerals, Rocks and Ores [ Ji, S.C., Wang, Q., Xia, B., 2002. Handbook of Seismic Properties of Minerals, Rocks and Ores. Polytechnic International Press, Montreal, 630 pp.], reveal that except for monomineralic rocks such as quartzite, serpentinite, anorthosite, limestone, and marble the rest of the rock types have Poisson's ratios falling along an upward convex curve determined from the correlations between elastic moduli and density. Poisson's ratios increase with density as the lithology changes from granite, felsic gneiss and schist, through diorite–syenite, intermediate gneiss and metasediment, to gabbro–diabase, amphibolite, and mafic gneiss, and then decrease as the rocks become ultramafic in composition. Eclogite has a higher density but a lower Poisson's ratio than peridotite. The results were applied to constrain the crustal composition and tectonic evolution of the Chinese continental crust based on crustal thickness (H) and Poisson's ratio (υ) from 248 broadband seismic stations, measured using teleseismic receiver function techniques. The North China, Yangtze, South China and Northeast China blocks and Songpan–Ganzi Terrane are dominated by low (υ < 0.26) and moderate (0.26 ≤ υ < 0.28) υ values (> 70%), suggesting the dominance of felsic composition in the crust. The Lhasa terrane, Qiangtang terrane, and Indochina block are characterized by high proportions (33–42%) of measurements with very high υ values (≥ 0.30 and H is found for the South China block, Northeast block, Lhasa block, Qiangtang terrane and Indochina block, indicating either tectonic thickening of the felsic upper and middle crust by folding and thrusting or the removal of mafic layers from the lower crust into the upper mantle by delamination.  相似文献   

Capital on the move: The changing relation between livestock and labor in Mali, West Africa     

Matthew D. Turner 《Geoforum》2009,40(5):746-81

In dryland areas of the Africa, livestock play important economic roles as commodities, wealth stores, producers of products, and agents of environmental change. Conventional depictions of livestock economies in this region have focused (in support or against) on the need for greater engagement of livestock producers with markets supplying meat to urban areas. This paper argues this singular focus has led analysts to ignore two important aspects of livestock economies: livestock as a preferred store of wealth across a wide range of social groups and the need for specialized labor to manage these livestock across open pastures to maintain their productivity and limit their negative environmental impacts. In the West African Sahel, the capital-like nature of livestock wealth has become more clear with a growing fraction of the region’s livestock owned by investors with little connection to livestock husbandry. Livestock investments are maintained on a day-to-day basis by hired herders who facilitate access to ephemeral pastures and water. A particular concern is the changing geographies of livestock ownership and the herding labor in relationship to regional pastures (to economic and environmental ends). This relationship will be explored using the case study of the Maasina region of central Mali - a historically important livestock region, which is now undergoing significant labor emigration. Building from a long-term ethnographic engagement with local livestock owners and herders, the results of ownership surveys of livestock herds across a 14-year period and interviews of urban-based emigrants from the study area about investment decisions will be used to analyze the changing geographies of livestock investment and herding labor in the Maasina.  相似文献   

The dependence of star formation on initial conditions and molecular cloud structure     

Matthew R. Bate 《Monthly notices of the Royal Astronomical Society》2009,397(1):232-248

We investigate the dependence of stellar properties on the initial kinematic structure of the gas in star-forming molecular clouds. We compare the results from two large-scale hydrodynamical simulations of star cluster formation that resolve the fragmentation process down to the opacity limit, the first of which was reported by Bate, Bonnell & Bromm. The initial conditions of the two calculations are identical, but in the new simulation the power spectrum of the velocity field imposed on the cloud initially and allowed to decay is biased in favour of large-scale motions. Whereas the calculation of Bate et al. began with a power spectrum   P ( k ) ∝ k ⁻⁴  to match the Larson scaling relations for the turbulent motions observed in molecular clouds, the new calculation begins with a power spectrum   P ( k ) ∝ k ⁻⁶  .
Despite this change to the initial motions in the cloud and the resulting density structure of the molecular cloud, the stellar properties resulting from the two calculations are indistinguishable. This demonstrates that the results of such hydrodynamical calculations of star cluster formation are relatively insensitive to the initial conditions. It is also consistent with the fact that the statistical properties of stars and brown dwarfs (e.g. the stellar initial mass function) are observed to be relatively invariant within our Galaxy and do not appear to depend on environment.  相似文献   

Circumplanetary disc properties obtained from radiation hydrodynamical simulations of gas accretion by protoplanets     

Ben A. Ayliffe  Matthew R. Bate 《Monthly notices of the Royal Astronomical Society》2009,397(2):657-665

We investigate the properties of circumplanetary discs formed in three-dimensional, self-gravitating radiation hydrodynamical models of gas accretion by protoplanets. We determine disc sizes, scaleheights, and density and temperature profiles for different protoplanet masses, in solar nebulae of differing grain opacities.
We find that the analytical prediction of circumplanetary disc radii in an evacuated gap  ( R _Hill/3)  from Quillen & Trilling yields a good estimate for discs formed by high-mass protoplanets. The radial density profiles of the circumplanetary discs may be described by power laws between   r ⁻²  and   r ^−3/2  . We find no evidence for the ring-like density enhancements that have been found in some previous models of circumplanetary discs. Temperature profiles follow a  ∼ r ^−7/10  power law regardless of protoplanet mass or nebula grain opacity. The discs invariably have large scaleheights  ( H / r > 0.2)  , making them thick in comparison with their encompassing circumstellar discs, and they show no flaring.  相似文献   

The H i gas content of galaxies around Abell 370, a galaxy cluster at z= 0.37     

Philip Lah  Michael B. Pracy  Jayaram N. Chengalur  Frank H. Briggs  Matthew Colless  Roberto De Propris  Shaun Ferris  Brian P. Schmidt  Bradley E. Tucker 《Monthly notices of the Royal Astronomical Society》2009,399(3):1447-1470

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On the equivalence between the effective cosmology and excursion set treatments of environment     

Matthew C. Martino  Ravi K. Sheth 《Monthly notices of the Royal Astronomical Society》2009,394(4):2109-2112

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A close look at Saturn's rings with Cassini VIMS     

Philip D. Nicholson  Matthew M. Hedman  Mark R. Showalter  Jeffrey N. Cuzzi  Fabrizio Capaccioni  Gary B. Hansen  Pierre Drossart  Bonnie J. Buratti  Angioletta Coradini 《Icarus》2008,193(1):182-212

Soon after the Cassini-Huygens spacecraft entered orbit about Saturn on 1 July 2004, its Visual and Infrared Mapping Spectrometer obtained two continuous spectral scans across the rings, covering the wavelength range 0.35-5.1 μm, at a spatial resolution of 15-25 km. The first scan covers the outer C and inner B rings, while the second covers the Cassini Division and the entire A ring. Comparisons of the VIMS radial reflectance profile at 1.08 μm with similar profiles at a wavelength of 0.45 μm assembled from Voyager images show very little change in ring structure over the intervening 24 years, with the exception of a few features already known to be noncircular. A model for single-scattering by a classical, many-particle-thick slab of material with normal optical depths derived from the Voyager photopolarimeter stellar occultation is found to provide an excellent fit to the observed VIMS reflectance profiles for the C ring and Cassini Division, and an acceptable fit for the inner B ring. The A ring deviates significantly from such a model, consistent with previous suggestions that this region may be closer to a monolayer. An additional complication here is the azimuthally-variable average optical depth associated with “self-gravity wakes” in this region and the fact that much of the A ring may be a mixture of almost opaque wakes and relatively transparent interwake zones. Consistently with previous studies, we find that the near-infrared spectra of all main ring regions are dominated by water ice, with a typical regolith grain radius of 5-20 μm, while the steep decrease in visual reflectance shortward of 0.6 μm is suggestive of an organic contaminant, perhaps tholin-like. Although no materials other than H₂O ice have been identified with any certainty in the VIMS spectra of the rings, significant radial variations are seen in the strength of the water-ice absorption bands. Across the boundary between the C and B rings, over a radial range of ∼7000 km, the near-IR band depths strengthen considerably. A very similar pattern is seen across the outer half of the Cassini Division and into the inner A ring, accompanied by a steepening of the red slope in the visible spectrum shortward of 0.55 μm. We attribute these trends—as well as smaller-scale variations associated with strong density waves in the A ring—to differing grain sizes in the tholin-contaminated icy regolith that covers the surfaces of the decimeter-to-meter sized ring particles. On the largest scale, the spectral variations seen by VIMS suggest that the rings may be divided into two larger ‘ring complexes,’ with similar internal variations in structure, optical depth, particle size, regolith texture and composition. The inner complex comprises the C and B rings, while the outer comprises the Cassini Division and A ring.  相似文献   

Narrow associated quasi-stellar object absorbers: clustering, outflows and the line-of-sight proximity effect     

Vivienne Wild  Guinevere Kauffmann  Simon White  Donald York  Matthew Lehnert  Timothy Heckman  Patrick B. Hall  Pushpa Khare  Britt Lundgren  Donald P. Schneider  Daniel Vanden Berk 《Monthly notices of the Royal Astronomical Society》2008,388(1):227-241

Using data from the Sloan Digital Sky Survey data release 3 (SDSS DR3), we investigate how narrow (<700 km s⁻¹) C  iv and Mg  ii quasar absorption-line systems are distributed around quasars. The C  iv absorbers lie in the redshift range 1.6 < z < 4 and the Mg  ii absorbers in the range 0.4 < z < 2.2. By correlating absorbers with quasars on different but neighbouring lines of sight, we measure the clustering of absorbers around quasars on comoving scales between 4 and 30 Mpc. The observed comoving correlation lengths are   r _o∼ 5 h ⁻¹Mpc  , similar to those observed for bright galaxies at these redshifts. Comparing correlations between absorbers and the quasars, in whose spectra they are identified, then implies: (i) that quasars destroy absorbers to comoving distances of ∼300 kpc (C  iv ) and ∼800 kpc (Mg  ii ) along their lines of sight; (ii) that ≳40 per cent of C  iv absorbers within 3000 km s⁻¹ of the quasi-stellar object are not a result of large-scale clustering but rather are directly associated with the quasar itself; (iii) that this intrinsic absorber population extends to outflow velocities of the order of 12 000 km s⁻¹; (iv) that this outflow component is present in both radio-loud and radio-quiet quasars and (v) that a small high-velocity outflow component is also observed in the Mg  ii population. We also find an indication that absorption systems within 3000 km s⁻¹ are more abundant for radio-loud quasars than for radio-quiet quasars. This suggests either that radio-loud objects live in more massive haloes, or that their radio activity generates an additional low-velocity outflow, or both.  相似文献   

The 2dF-SDSS LRG and QSO Survey: evolution of the clustering of luminous red galaxies since z= 0.6     

David A. Wake  Ravi K. Sheth  Robert C. Nichol  Carlton M. Baugh  Joss Bland-Hawthorn  Matthew Colless  Warrick J. Couch  Scott M. Croom  Roberto De Propris  Michael J. Drinkwater  Alastair C. Edge  Jon Loveday  Tsz Yan Lam  Kevin A. Pimbblet  Isaac G. Roseboom  Nicholas P. Ross  Donald P. Schneider  Tom Shanks  Robert G. Sharp 《Monthly notices of the Royal Astronomical Society》2008,387(3):1045-1062

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Linking the Abundance of Estuarine Fish and Crustaceans in Nearshore Waters to Shoreline Hardening and Land Cover     

Matthew S. Kornis  Denise Breitburg  Richard Balouskus  Donna M. Bilkovic  Lori A. Davias  Steve Giordano  Keira Heggie  Anson H. Hines  John M. Jacobs  Thomas E. Jordan  Ryan S. King  Christopher J. Patrick  Rochelle D. Seitz  Heather Soulen  Timothy E. Targett  Donald E. Weller  Dennis F. Whigham  Jim UphoffJr 《Estuaries and Coasts》2017,40(5):1464-1486

Human alteration of land cover (e.g., urban and agricultural land use) and shoreline hardening (e.g., bulkheading and rip rap revetment) are intensifying due to increasing human populations and sea level rise. Fishes and crustaceans that are ecologically and economically valuable to coastal systems may be affected by these changes, but direct links between these stressors and faunal populations have been elusive at large spatial scales. We examined nearshore abundance patterns of 15 common taxa across gradients of urban and agricultural land cover as well as wetland and hardened shoreline in tributary subestuaries of the Chesapeake Bay and Delaware Coastal Bays. We used a comprehensive landscape-scale study design that included 587 sites in 39 subestuaries. Our analyses indicate shoreline hardening has predominantly negative effects on estuarine fauna in water directly adjacent to the hardened shoreline and at the larger system-scale as cumulative hardened shoreline increased in the subestuary. In contrast, abundances of 12 of 15 species increased with the proportion of shoreline comprised of wetlands. Abundances of several species were also significantly related to watershed cropland cover, submerged aquatic vegetation, and total nitrogen, suggesting land-use-mediated effects on prey and refuge habitat. Specifically, abundances of four bottom-oriented species were negatively related to cropland cover, which is correlated with elevated nitrogen and reduced submerged and wetland vegetation in the receiving subestuary. These empirical relationships raise important considerations for conservation and management strategies in coastal environments.  相似文献