Recycling in the Puerto Rican mantle wedge, Greater Antilles Island Arc     

Wayne T. Jolly  Edward G. Lidiak  Alan P. Dickin  Tsai-Way Wu 《Island Arc》2002,11(1):10-24

Abstract The initial volcanic phase of Cretaceous island arc strata in central Puerto Rico, at the eastern end of the extinct Greater Antilles Arc, comprises a 6‐km thick pile of lava and volcanic breccia (Río Majada Group). Preserved within the sequence is a conspicuous shift in absolute abundances of the more incompatible elements, including Th, Nb, and the light rare earth elements (LREE: La, Ce, Pr and Nd). The compositional shift is marked by a decrease in La/Sm from averages of 2.11 in the lowest third of the pile (Formation A) to 1.48 at the top (Formation C), and by a distinctive flattening of LREE segments of chondrite‐normalized REE patterns. i⁸⁷Sr/⁸⁶Sr and ?_Nd average about 0.7035 and 8.2, respectively, in early Formation A basalts. These ranges normally overlap samples from later Formations B and C. Isotope compositions of the latter group are more variable, however, and several samples are considerably more radiogenic than Formation A basalts, such that i⁸⁷Sr/⁸⁶Sr averages almost 0.7042 while ?_Nd‐values decrease to 7.5 in Formation B and C basalts. Theoretical models of non‐modal melting processes in both amphibole peridotite and spinel lherzolite sources provide insight into the origin of depleted Th, Nb, and LREE abundances in Puerto Rican basalts. Low Nb concentrations less than normal mid‐oceanic ridge basalts in Formation A basalts indicate the wedge was slightly depleted by low‐volume decompression fusion due to induced convection in the back‐arc region prior to entry of the source into the arc melting zone. However, depleted patterns in Formation C basalts cannot be generated by relatively greater degrees of decompression fusion in the back‐arc, because addition of the La‐enriched slab‐derived component to more depleted source material invariably produces elevated rather than decreased La/Sm. Refluxing of Formation A harzburgitic residua is similarly precluded. In contrast, the observed patterns are readily reproduced by multistage melting models involving hybridized sources containing normal Formation A lherzolite source material blended with recycled, unrefluxed harzburgite residua. Successful models require hybrid sources containing large volumes of recycled harzburgite (up to 50%) during generation of Formation C basalts. Slightly elevated radiometric Sr and Nd isotopes in a few flows from Formation C are attributed to partial refluxing of the hybrid sources within the wedge.  相似文献   

Alpine medicinal plant trade and Himalayan mountain livelihood strategies   总被引：5，自引：0，他引：5  

Carsten Smith Olsen  Helle Overgaard Larsen 《The Geographical journal》2003,169(3):243-254

Commercial alpine medicinal plants are collected from the wild by local rural households throughout the Himalaya and sold in order to increase household incomes. Recent studies indicate that this annual trade amounts to thousands of tonnes of roots, rhizomes, tubers, leaves, etc., worth millions of US dollars. The main market is in India. Based on a national survey, including the most commonly traded species, and a village study this paper investigates the importance of the alpine medicinal plant trade at national and local levels in Nepal. The national survey included standardized open-ended interviews with 232 harvesters, 64 local traders, 66 central wholesalers, 47 regional wholesalers, and 16 production companies. The village survey is based on the daily records of household activities in 15 households in a one-year period. The annual Nepalese alpine and sub-alpine medicinal plant trade is conservatively estimated to vary from 480 to 2500 t with a total harvester value of US$0.8–3.3 million; the average harvester value is estimated at US$66.0 ± 99.0. The trade in 1997/98 amounted to 1600 t with a harvester value of US$2.3 million and an export value equivalent to 2.5% of total export from Nepal. Medicinal plant harvesting was found to constitute an integrated part of local livelihood strategies, contributing from 3 to 44% (average of 12%) of the annual household income. Importance at household level depended on land and animal holdings, and the availability of adult male labour. The validity and reliability of methods and analyses is evaluated, and issues of conservation and management of alpine medicinal plant species under the community forestry scheme are discussed.  相似文献   

Aspects of the loss-on-ignition (LOI) technique in the context of clay-rich, glaciolacustrine sediments     

Jamie G. Smith 《Geografiska Annaler: Series A, Physical Geography》2003,85(1):91-97

The loss‐on‐ignition (LOI) technique for organic matter determination is evaluated with reference to clay‐rich, glaciolacustrine sediments. Examples of the use of LOI in glaciolacustrine literature are inconsistent in their methodology, and standardisation is called for. Problems with application of the technique to glaciolacustrine sediments stem mainly from the effects of additional weight loss due to release of structurally bound water and exposure time to ignition temperatures. Exposure time should be increased from 1 hour to 2.5 hours in the light of significant additional weight loss occurring between 2 and 2.5 hours in clay‐rich samples. The reason for this sudden increase in LOI after 2 hours (up to 86% of total LOI in some cases) appears to be the build‐up of a distinct surface crust during heating that insulates the core of the sample from ignition temperatures. Some time between 2 and 2.5 hours, the crust breaks down and exposes the sample core to ignition temperatures, increasing weight loss significantly. Overestimation of LOI due to loss of structural water is thought to be insignificant. Replicates of LOI at 550 and 1000°C in non‐calcareous, clay‐rich (>40%) sediments, show differences of <0.25%, which, for the purpose of inferring broad changes in glacier extent in lake catchments, is an acceptable error.  相似文献   

(Re)surveying the uses of Geographical Information Systems in Health Authorities 1991–2001     

Darren P Smith  Myles I Gould  Gary Higgs† 《Area》2003,35(1):74-83

This paper provides a comparative analysis of two separate national surveys of the uptake and use of Geographical Information Systems (GIS) by Health Authorities (HAs) in England and Wales. The surveys were undertaken at two different time points (1991 and 2001), and the intervening period has been characterized by major (and continuing) health sector restructuring. The substantial number of HA mergers between 1991 and 2001 provide a methodological challenge in attempting to make comparisons. A pragmatic approach is adopted and the presentation of results has to be restricted solely to English and Welsh HA data. Our findings show that there has been a considerable increase in the use of GIS since the early 1990s, and there is some evidence of more sophisticated uses for a wider range of policy-related and application tasks. However, we conclude that there is still considerable scope for the wider uptake and use of GIS within the NHS. In addition, our findings provide a baseline with which to monitor the impacts of future organizational restructuring on the uses of GIS within the NHS. We suggest that this is important given the replacement of 95 English HAs with 28 new Strategic HAs (StHAs) from April 2002, and the repercussions of the anticipated shift in GIS responsibility to Primary Care Trusts.  相似文献   

Recent temperature and precipitation increases in West Siberia and their association with the Arctic Oscillation     

Karen E. Frey  Laurence C. Smith 《Polar research》2003,22(2):287-300

Surface air temperature and precipitation records for the years 1958-1999 from ten meteorological stations located throughout West Siberia are used to identify climatic trends and determine to what extent these trends are potentially attributable to the Arctic Oscillation (AO). Although recent changes in atmospheric variability are associated with broad Arctic climate change, West Siberia appears particularly susceptible to warming. Furthermore, unlike most of the Arctic, moisture transport in the region is highly variable. The records show that West Siberia is experiencing significant warming and notable increases in precipitation, likely driven, in part, by large-scale Arctic atmospheric variability. Because this region contains a large percentage of the world's peatlands and contributes a significant portion of the total terrestrial freshwater flux to the Arctic Ocean, these recent climatic trends may have globally significant repercussions. The most robust patterns found are strong and prevalent springtime warming, winter precipitation increases, and strong association of non-summer air temperatures with the AO. Warming rates for both spring (0.5-0.8 °C/decade) and annual (0.3-0.5°C/decade) records are statistically significant for nine often stations. On average, the AO is linearly congruent with 96% (winter), 19% (spring), 0% (summer), 67% (autumn) and 53% (annual) of the warming found in this study. Significant trends in precipitation occur most commonly during winter, when four of ten stations exhibit significant increases (4-13 %/decade). The AO may play a lesser role in precipitation variability and is linearly congruent with only 17% (winter), 13% (spring), 12% (summer), 1% (autumn) and 26% (annual) of precipitation trends.  相似文献   

Wind erosion from military training lands in the Mojave Desert, California, U.S.A.   总被引：1，自引：0，他引：1  

Simon J. van Donk  Xuewen Huang  Edward L. Skidmore  Alan B. Anderson  Dick L. Gebhart  Valerie E. Prehoda  Elizabeth M. Kellogg 《Journal of Arid Environments》2003,54(4):687-703

Military training activities reduce vegetation cover, disturb crusts, and degrade soil aggregates, making the land more vulnerable to wind erosion. The objective of this study was to quantify wind erosion rates for typical conditions at the Marine Corps Air Ground Combat Center, Twentynine Palms, CA, U.S.A. Five Big Spring Number Eight (BSNE) sampler stations were installed at each of five sites. Each BSNE station consisted of five BSNE samplers with the lowest sampler at 0·05 m and the highest sampler at 1·0 m above the soil surface. Once a month, sediment was collected from the samplers for analysis. Occurrence of saltating soil aggregates was recorded every hour using Sensits, one at each site. The site with the most erosion had a sediment discharge of 311 kg m⁻¹ over a period of 17 months. Other sites eroded much less because of significant rock cover or the presence of a crust. Hourly sediment discharge was estimated combining hourly Sensit count and monthly sediment discharge measured using BSNE samplers. More simultaneously measured data are needed to better characterize the relationship between these two and reconstruct a detailed time-series of wind erosion. This measured time-series can then be used for comparison with simulation results from process-based wind erosion models such as the Wind Erosion Prediction System (WEPS), once it has been adapted to the unique aspects of military lands.  相似文献   

The Belize margin revisited: 1. Holocene marine facies     

Edward G. Purdy  Eberhard Gischler 《International Journal of Earth Sciences》2003,92(4):532-551

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Understanding Climatic Impacts, Vulnerabilities, and Adaptation in the United States: Building a Capacity for Assessment     

Edward A. Parson  Robert W. Corell  Eric J. Barron  Virginia Burkett  Anthony Janetos  Linda Joyce  Thomas R. Karl  Michael C. MacCracken  Jerry Melillo  M. Granger Morgan  David S. Schimel  Thomas Wilbanks 《Climatic change》2003,57(1-2):9-42

Based on the experience of the U.S. National Assessment, we propose a program of research and analysis to advance capability for assessment of climate impacts, vulnerabilities, and adaptation options. We identify specific priorities for scientific research on the responses of ecological and socioeconomic systems to climate and other stresses; for improvement in the climatic inputs to impact assessments; and for further development of assessment methods to improve their practical utility to decision-makers. Finally, we propose a new institutional model for assessment, based principally on regional efforts that integrate observations, research, data, applications, and assessment on climate and linked environmental-change issues. The proposed program will require effective collaboration between scientists, resource managers, and other stakeholders, all of whose expertise is needed to define and prioritize key regional issues, characterize relevant uncertainties, and assess potential responses. While both scientifically and organizationally challenging, such an integrated program holds the best promise of advancing our capacity to manage resources and the economy adaptively under a changing climate.  相似文献   

I. R. Young and G. J. Holland, Atlas of the Oceans: Wind and Wave Climate     

S.D. Smith  Y. Geshelin 《Boundary-Layer Meteorology》1997,84(1):169-171

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Calculations of horizontal mixing rates using222Rn and the controls on hypoxia in western Long Island Sound, 1991     

Thomas Torgersen  Edward DeAngelo  James O’Donnell 《Estuaries and Coasts》1997,20(2):328-345

Late summer hypoxia (<3 ppm oxygen) in western Long Island Sound (WLIS) is a persistent environmental and management issue whose controlling processes are poorly understood. Measured rates of sediment and water-column oxygen consumption in the bottom water suggest that a condition of no oxygen should be attained on the time scale of 13–30 d. Observations, however, indicate the onset of hypoxia is of the order 150 d. Therefore, horizontal and/or vertical transport of oxygen into the area of hypoxia must play an important role. Hypoxia decreases benthic activity and the sediment flux of²²²Rn. The resulting horizontal gradient in bottom water²²²Rn was measured and used to estimate the effective horizontal transport rate (>5–50 m² s^?1), which is considerably slower than previous estimates. Scale analysis of the hypoxia process indicates that horizontal transport rates alone can explain the slow progression of hypoxia in XLIS but that vertical processes may also be capable of delaying the onset of hypoxia especially under conditions of weak stratification or weak intermediate layer oxygen consumption. This scale analysis indicates a delicately balanced process that is sensitive to both climatologically-driven variability in the rates of horizontal and vertical transport as well as the biologically-driven rates of oxygen consumption. An improved ability to predict and/or control hypoxia must be based on a better understanding of temporal and spacial variations in circulation, mixing, and stratification as well as the biological processes in the water column and the sediments.  相似文献