Oxygen isotope and 26Al‐26Mg systematics of aluminum‐rich chondrules from unequilibrated enstatite chondrites     

Yunbin Guan  Gary R. Huss  Laurie A. Leshin  Glenn J. MacPherson  Kevin D. McKeegan 《Meteoritics & planetary science》2006,41(1):33-47

Abstract— Correlated in situ analyses of the oxygen and magnesium isotopic compositions of aluminum‐rich chondrules from unequilibrated enstatite chondrites were obtained using an ion microprobe. Among eleven aluminum‐rich chondrules and two plagioclase fragments measured for ²⁶Al‐²⁶Mg systematics, only one aluminum‐rich chondrule contains excess ²⁶Mg from the in situ decay of ²⁶Al; the inferred initial ratio (²⁶Al/²⁷Al)_o = (6.8 ± 2.4) × 10^?6 is consistent with ratios observed in chondrules from carbonaceous chondrites and unequilibrated ordinary chondrites. The oxygen isotopic compositions of five aluminum‐rich chondrules and one plagioclase fragment define a line of slope ?0.6 ± 0.1 on a three‐oxygen‐isotope diagram, overlapping the field defined by ferromagnesian chondrules in enstatite chondrites but extending to more ¹⁶O‐rich compositions with a range in δ¹⁸O of about ?12‰. Based on their oxygen isotopic compositions, aluminum‐rich chondrules in unequilibrated enstatite chondrites are probably genetically related to ferromagnesian chondrules and are not simple mixtures of materials from ferromagnesian chondrules and calcium‐aluminum‐rich inclusions (CAIs). Relative to their counterparts from unequilibrated ordinary chondrites, aluminum‐rich chondrules from unequilibrated enstatite chondrites show a narrower oxygen isotopic range and much less resolvable excess ²⁶Mg from the in situ decay of ²⁶Al, probably resulting from higher degrees of equilibration and isotopic exchange during post‐crystallization metamorphism. However, the presence of ²⁶Al‐bearing chondrules within the primitive ordinary, carbonaceous, and now enstatite chondrites suggests that ²⁶Al was at least approximately homogeneously distributed across the chondrite‐forming region.  相似文献   

Development of a Multimetric Index for Integrated Assessment of Salt Marsh Ecosystem Condition     

Jessica L. Nagel  Hilary A. Neckles  Glenn R. Guntenspergen  Erika N. Rocks  Donald R. SchoolmasterJr  James B. Grace  Dennis Skidds  Sara Stevens 《Estuaries and Coasts》2018,41(2):334-348

Tools for assessing and communicating salt marsh condition are essential to guide decisions aimed at maintaining or restoring ecosystem integrity and services. Multimetric indices (MMIs) are increasingly used to provide integrated assessments of ecosystem condition. We employed a theory-based approach that considers the multivariate relationship of metrics with human disturbance to construct a salt marsh MMI for five National Parks in the northeastern USA. We quantified the degree of human disturbance for each marsh using the first principal component score from a principal components analysis of physical, chemical, and land use stressors. We then applied a metric selection algorithm to different combinations of about 45 vegetation and nekton metrics (e.g., species abundance, species richness, and ecological and functional classifications) derived from multi-year monitoring data. While MMIs derived from nekton or vegetation metrics alone were strongly correlated with human disturbance (r values from ?0.80 to ?0.93), an MMI derived from both vegetation and nekton metrics yielded an exceptionally strong correlation with disturbance (r = ?0.96). Individual MMIs included from one to five metrics. The metric-assembly algorithm yielded parsimonious MMIs that exhibit the greatest possible correlations with disturbance in a way that is objective, efficient, and reproducible.  相似文献   

Variations of sulphur isotope signatures in sulphides from the metamorphosed Ming Cu(−Au) volcanogenic massive sulphide deposit,Newfoundland Appalachians,Canada     

Stefanie M. Brueckner  Stephen J. Piercey  Graham D. Layne  Glenn Piercey  Paul J. Sylvester 《Mineralium Deposita》2015,50(5):619-640

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The causes of land-use and land-cover change: moving beyond the myths   总被引：39，自引：0，他引：39  

Eric F. Lambin  B. L. Turner  Helmut J. Geist  Samuel B. Agbola  Arild Angelsen  John W. Bruce  Oliver T. Coomes  Rodolfo Dirzo  Günther Fischer  Carl Folke  P. S. George  Katherine Homewood  Jacques Imbernon  Rik Leemans  Xiubin Li  Emilio F. Moran  Michael Mortimore  P. S. Ramakrishnan  John F. Richards  Helle Sknes  Will Steffen  Glenn D. Stone  Uno Svedin  Tom A. Veldkamp  Coleen Vogel  Jianchu Xu 《Global Environmental Change》2001,11(4)

Common understanding of the causes of land-use and land-cover change is dominated by simplifications which, in turn, underlie many environment-development policies. This article tracks some of the major myths on driving forces of land-cover change and proposes alternative pathways of change that are better supported by case study evidence. Cases reviewed support the conclusion that neither population nor poverty alone constitute the sole and major underlying causes of land-cover change worldwide. Rather, peoples’ responses to economic opportunities, as mediated by institutional factors, drive land-cover changes. Opportunities and constraints for new land uses are created by local as well as national markets and policies. Global forces become the main determinants of land-use change, as they amplify or attenuate local factors.  相似文献   

Can the Intermediate Western Boundary Current recirculation trigger the Vitória Eddy formation?     

Napolitano  Dante C.  Rocha  Cesar B.  da Silveira  Ilson C. A.  Simoes-Sousa  Iury T.  Flierl  Glenn R. 《Ocean Dynamics》2021,71(3):281-292

Ocean Dynamics - South of the Vitória-Trindade Ridge, a seamount chain off East Brazil, the Brazil Current (BC) meanders cyclonically within Tubarão Bight, occasionally forming the...  相似文献   

Patterns and Controls of Hurricane-Caused Forest Damage: A Landscape-Scale Analysis of Treefall Direction Following Hurricane Katrina∗     

John A. Kupfer  David A. Glenn  Jovian P. Sackett 《The Professional geographer》2013,65(4):478-494

To clarify broad-scale patterns and controls of treefall directionality from Hurricane Katrina, we examined fall directions across a 4,500 km² landscape mosaic in southern Mississippi using georeferenced, planar-rectified aerial photographs. Analyses using directional statistics, measures of local spatial autocorrelation, and general linear modeling indicated that treefall was significantly directional for nearly all of our locations and constrained primarily by mesoscale surface wind directions and landscape setting. None of our plots exhibited fall angles consistent with damage caused by wind reversals following the passage of the storm or by microbursts or tornadoes spawned by the hurricane. When coupled with results from other studies focused on hurricane-caused damage, these results suggest that it is possible to develop empirical, landscape-scale models of wind impacts or stand vulnerability using basic site information (e.g., topography, soils), biotic conditions (e.g., land cover, forest attributes), and generalized, but readily available, estimates of surface wind flow patterns.  相似文献   

Large‐scale 3D inversion of potential field data     

Martin Čuma  Glenn A. Wilson  Michael S. Zhdanov 《Geophysical Prospecting》2012,60(6):1186-1199

Inversion of gravity and/or magnetic data attempts to recover the density and/or magnetic susceptibility distribution in a 3D earth model for subsequent geological interpretation. This is a challenging problem for a number of reasons. First, airborne gravity and magnetic surveys are characterized by very large data volumes. Second, the 3D modelling of data from large‐scale surveys is a computationally challenging problem. Third, gravity and magnetic data are finite and noisy and their inversion is ill posed so regularization must be introduced for the recovery of the most geologically plausible solutions from an infinite number of mathematically equivalent solutions. These difficulties and how they can be addressed in terms of large‐scale 3D potential field inversion are discussed in this paper. Since potential fields are linear, they lend themselves to full parallelization with near‐linear scaling on modern parallel computers. Moreover, we exploit the fact that an instrument’s sensitivity (or footprint) is considerably smaller than the survey area. As multiple footprints superimpose themselves over the same 3D earth model, the sensitivity matrix for the entire earth model is constructed. We use the re‐weighted regularized conjugate gradient method for minimizing the objective functional and incorporate a wide variety of regularization options. We demonstrate our approach with the 3D inversion of 1743 line km of FALCON gravity gradiometry and magnetic data acquired over the Timmins district in Ontario, Canada. Our results are shown to be in good agreement with independent interpretations of the same data.  相似文献   

A method of accelerating transport simulation when groundwater pumping is simulated     

Zhang J  Randall G  Wei X 《Ground water》2012,50(3):464-471

In solving groundwater transport problems with numerical models, the computation time (CPU processing time) of transport simulation is approximately inversely proportional to the transport time-step size. Therefore, large time-step sizes are favorable for achieving short computation time. However, transport time-step size must be sufficiently small to avoid numerical instability if an explicit scheme is used (and to guarantee enough model accuracy if an implicit scheme is used). For a transport model involving groundwater pumping, a small transport time-step size is often required due to the high groundwater velocities near the pumping well. Small grid spacing often specified near the pumping well also limits the time-step size. This paper presents a method to increase transport time-step size in a transport model when groundwater pumping is simulated. The key to this approach is to numerically decrease the groundwater seepage velocities in grid cells near the pumping well by increasing the effective porosity so that the transport time-step size can be increased without violating stability constraints. Numerical tests reveal that by using the proposed method, the computation time of transport simulation can be reduced significantly, while the transport simulation results change very little.  相似文献   

A bioindicator system for water quality on inshore coral reefs of the Great Barrier Reef     

Fabricius KE  Cooper TF  Humphrey C  Uthicke S  De'ath G  Davidson J  LeGrand H  Thompson A  Schaffelke B 《Marine pollution bulletin》2012,65(4-9):320-332

Responses of bioindicator candidates for water quality were quantified in two studies on inshore coral reefs of the Great Barrier Reef (GBR). In Study 1, 33 of the 38 investigated candidate indicators (including coral physiology, benthos composition, coral recruitment, macrobioeroder densities and FORAM index) showed significant relationships with a composite index of 13 water quality variables. These relationships were confirmed in Study 2 along four other water quality gradients (turbidity and chlorophyll). Changes in water quality led to multi-faceted shifts from phototrophic to heterotrophic benthic communities, and from diverse coral dominated communities to low-diversity communities dominated by macroalgae. Turbidity was the best predictor of biota; hence turbidity measurements remain essential to directly monitor water quality on the GBR, potentially complemented by our final calibrated 12 bioindicators. In combination, this bioindicator system may be used to assess changes in water quality, especially where direct water quality data are unavailable.  相似文献   

The AzTEC mm-wavelength camera     

G. W. Wilson  J. E. Austermann  T. A. Perera  K. S. Scott  P. A. R. Ade  J. J. Bock  J. Glenn  S. R. Golwala  S. Kim  Y. Kang  D. Lydon  P. D. Mauskopf  C. R. Predmore  C. M. Roberts  K. Souccar  M. S. Yun 《Monthly notices of the Royal Astronomical Society》2008,386(2):807-818

AzTEC is a mm-wavelength bolometric camera utilizing 144 silicon nitride micromesh detectors. Here, we describe the AzTEC instrument architecture and its use as an astronomical instrument. We report on several performance metrics measured during a three-month observing campaign at the James Clerk Maxwell Telescope and conclude with our plans for AzTEC as a facility instrument on the Large Millimetre Telescope.  相似文献