Sapphirine+Forsterite and Sapphirine+Humite-Group Minerals in an Ultra-Magnesian Lens from Kuhi-lal, SW Pamirs, Tajikistan: Are these Assemblages Forbidden?   总被引：2，自引：2，他引：0  

GREW  EDWARD S.; PERTSEV  NIKOLAY N.; YATES  MARTIN G.; CHRISTY  ANDREW G.; MARQUEZ  NICHOLAS; CHERNOSKY  JOSEPH V. 《Journal of Petrology》1994,35(5):1275-1293

Sapphirine occurs with humite-group minerals and forsteritein Precambrian amphibole-facies rocks at Kuhi-lal, SW PamirMountains, Tajikistan, a locality also for talc+kyanite magnesiohornblendewhiteschist. Most of these sapphirine-bearing rocks are graphiticand sulfidic (pyrite and pyrrhotite) and contain enstatite,clinohumite or chondrodite, spinel, rutile, gedrite, and phlogopite.A phlogopite schist has the assemblage with X_Fe = Fe/(Fe+Mg)increasing as follows: chlorite (0-003)<phlogopite (0.004–0.005)sapphirine (0.004–0.006) enstatite (0-006)forsterite (0-006–0-007)<spinel (0-014). This assemblage includes the incompatiblepair sapphirine+forsterite, but there is no textural evidencefor reaction. In one rock with clinohumite, X_Fe increases asfollows: clinohumite (0-002) <sapphirine (0-003) <enstatite(0-004–0-006) <spinel (0-010). Ion microprobe and wet-chemicalanalyses give 0-57–0-73 wt.% F in phlogopite and 0-27wt.% F in chlorite in the phlogopite schist; 0-04, 1.5–1.9,and 4.4 wt.% F in forsterite, clinohumite, and chondrodite,respectively; and 0-0-09 wt.% BeO and 0-05–0-21 wt.% B₂O₃in sapphirine. Stabilization of sapphirine+clinohumite or sapphirine+chondroditeinstead of sapphirine+phlogopite is possible at high F contentsin K-poor rocks, but minor element contents appear to be toolow to stabilize sapphirine as an additional phase with forsterite+enstatite+spinel.Although sapphirine+forsterite is metastable relative to spinel+enstatitein experiments conducted at a_H2O=1 in the MgO-Al₂O₃-SiO₂-H₂Osystem, it might be stabilized at a_H2O0.5, P4 kbar, T650–700C.Textures in the Kuhi-lal whiteschists suggest a polymetamorphicevolution in which the rocks were originally metamorphosed atT650C, P 7 kbar, conditions under which sapphirine+clinohumiteand sapphirine+chondrodite are inferred to have formed, andsubsequently affected by a later event at lower P, similar T,and lower a_H2O. The latter conditions were favorable for sapphirine+forsteriteto form in a rock originally containing chlorite+forsterite+spinel+enstatite.  相似文献   

Sediment trend models fail to reproduce small-scale sediment transport patterns on an intertidal beach   总被引：2，自引：0，他引：2  

GERHARD MASSELINK  DANIEL BUSCOMBE  MARTIN AUSTIN†  TIM O'HARE†  PAUL RUSSELL† 《Sedimentology》2008,55(3):667-687

A rigorous test is presented of the application of sediment trend models to an intertidal beach environment characterized by bar morphology. Sediment samples were collected during low tide from a regular grid and their sediment fall velocity distributions, obtained using a settling tube, were analysed using moment analysis. The net sediment transport direction determined from beach surveys, hydrodynamic measurements, wave ripple observations and sediment transport modelling was compared with predictions by sediment trend models based on the spatial distribution of sediment parameters. It was found that the sediment transport pathways and patterns of sedimentation predicted using sediment trend models were at odds with field observations, and varied significantly depending on whether surface or sub‐surface sediment samples were used. The sediment trend models are thought to fail because, in energetic and morphologically variable beach environments, spatial patterns in sediment characteristics are mainly attributed to the presence of different hydrodynamic regions and associated morphology, rather than sediment pathways. The use of sediment trend models cannot replace the collection of morphological, hydrodynamic and sediment transport data in the field to define relationships between flows, forms and sedimentation patterns on a dynamic intertidal beach.  相似文献   

Skillful Seasonal Forecasts of Summer Surface Air Temperature in Western China by Global Seasonal Forecast System Version 5     

Chaofan LI  Riyu LU  Philip E.BETT  Adam A.SCAIFE  Nicola MARTIN 《大气科学进展》2018,35(8):955-964

Variations of surface air temperature(SAT) are key in affecting the hydrological cycle,ecosystems and agriculture in western China in summer.This study assesses the seasonal forecast skill and reliability of SAT in western China,using the Glo Sea5 operational forecast system from the UK Met Office.Useful predictions are demonstrated,with considerable skill over most regions of western China.The temporal correlation coefficients of SAT between model predictions and observations are larger than 0.6,in both northwestern China and the Tibetan Plateau.There are two important sources of skill for these predictions in western China:interannual variation of SST in the western Pacific and the SST trend in the tropical Pacific.The tropical SST change in the recent two decades,with a warming in the western Pacific and cooling in the eastern Pacific,which is reproduced well by the forecast system,provides a large contribution to the skill of SAT in northwestern China.Additionally,the interannual variation of SST in the western Pacific gives rise to the reliable prediction of SAT around the Tibetan Plateau.It modulates convection around the Maritime Continent and further modulates the variation of SAT on the Tibetan Plateau via the surrounding circulation.This process is evident irrespective of detrending both in observations and the model predictions,and acts as a source of skill in predictions for the Tibetan Plateau.The predictability and reliability demonstrated in this study is potentially useful for climate services providing early warning of extreme climate events and could imply useful economic benefits.  相似文献   

THE ALBERTA OIL SANDS FROM BOTH SIDES OF THE BORDER*     

MARTIN J. PASQUALETTI 《Geographical review》2009,99(2):248-267

ABSTRACT. The United States produces less than half of the oil it consumes, a dependence unlikely to subside without drastic improvements in domestic production, development of renewable resources, and greater energy efficiency. All three of these actions, even if ultimately meaningful, will take time to produce results, so the United States is likely to continue to depend on outside suppliers. The most tempting of these suppliers is Canada, especially its massive oil sands in northeastern Alberta Province. In this article I ask why that is true and, more important whether the arrangement is mutually beneficial. The answers are strongly related to location; that is, the location of supply and the location of demand. The view from the south favors Canada above all other countries as a likely source for meeting the growing U.S. oil needs, yet not without accompanying unintended consequences. When viewed from the north, the monetary attraction of the oil sands is weakened by the environmental costs that are likely if their development expands as expected. Weighing these perspectives, the question is whether the combination of demand and environmental concerns leads to, accelerates, or discourages development. Viewing such a prospect from both sides of the border challenges the view that development of natural resources is always inevitable or wise, regardless of apparent profitability and need. Much depends on location.  相似文献   

LINKS BETWEEN PROGLACIAL STREAM SUSPENDED SEDIMENT DYNAMICS,GLACIER HYDROLOGY AND GLACIER MOTION AT MIDTDALSBREEN,NORWAY     

IAN C. WILLIS  KEITH S. RICHARDS  MARTIN J. SHARP 《水文研究》1996,10(4):629-648

Two-hourly suspended sediment concentration variations observed during the summer of 1987 in the proglacial stream draining Midtdalsbreen, Norway are modelled using multiple regression and time series techniques. Suspended sediment fluctuations are influenced by stream discharge variations, diurnal hysteresis effects, medium-term sediment supply and transport variations and the recent suspended sediment concentration history of the stream. They do not appear to be influenced by seasonal exhaustion or rainfall variations. Possible reasons for this are discussed. Large positive residuals from the fitted models are major pulses of suspended sediment unrelated to discharge variations; these sediment flushes correlate with periods of enhanced glacier motion. They cannot be explained by enhanced sediment production by subglacial erosion, but are probably due to the tapping of subglacially stored sediment during sudden changes in the hydraulics and/or configuration of the subglacial hydrological system. Seasonal changes in the lag between glacier motion peaks and suspended sediment flushes suggest that the subglacial hydrological system evolves over the summer from a distributed to a more channelized configuration.  相似文献   

Estimating the size and travel distance of Klapperhorn Mountain debris flows for risk analysis along railway, Canada   总被引：6，自引：0，他引：6  

Hengxing LAN  C. Derek MARTIN  C. H. ZHOU 《国际泥沙研究》2008,23(3):275-282

Debris flows occurring on Klapperhorn Mountain in the Yellowhead Pass in the Canadian Rocky Mountains pose a significant hazard to railway operations at the base of the mountain. The size （volume） and travel distance of these debris flows play an important role in assessing the risk to the railway. GIS analysis, airphoto interpretation together with field work were undertaken on two debris flows located at track mileage 54.0 and 54.3. Characteristics of these two debris flow events were analyzed, including debris flow path morphology and event behavior. Their sizes and travel distances were estimated using an empirical-statistical model （UBCDFLOW） under different initiation conditions. Their potential impact on the railway bridge was evaluated using a bridge blockage ratio.  相似文献   

DIVIDING THE OCEAN SEA*     

MARTIN W. LEWIS 《Geographical review》1999,89(2):188-214

ABSTRACT. The conventional view of global hydrography, which maps three or four oceans (Pacific, Atlantic, Indian, and, sometimes, Arctic) did not emerge until the late nineteenth and early twentieth centuries. Previously, markedly different conceptions of sea space prevailed, conceptions that changed not only to reflect new discoveries but also in accordance with changing intellectual fashions. By examining the history of global hydrography and by entertaining novel schemes of oceanic division, one can see the world afresh and perhaps discover connections that are obscured by conventional geographical divisions.  相似文献   

Assessment and control of organic and other contaminants associated with the Stardust sample return from comet 81P/Wild 2     

Scott A. SANDFORD  Saša BAJT  Simon J. CLEMETT  George D. CODY  George COOPER  Bradley T. DEGREGORIO  Vanessa De VERA  Jason P. DWORKIN  Jamie E. ELSILA  George J. FLYNN  Daniel P. GLAVIN  Antonio LANZIROTTI  Thomas LIMERO  Mildred P. MARTIN  Christopher J. SNEAD  Maegan K. SPENCER  Thomas STEPHAN  Andrew WESTPHAL  Sue WIRICK  Richard N. ZARE  Michael E. ZOLENSKY 《Meteoritics & planetary science》2010,45(3):406-433

Abstract– Numerous potential sources of organic contaminants could have greatly complicated the interpretation of the organic portions of the samples returned from comet 81P/Wild 2 by the Stardust spacecraft. Measures were taken to control and assess potential organic (and other) contaminants during the design, construction, and flight of the spacecraft, and during and after recovery of the sample return capsule. Studies of controls and the returned samples suggest that many of these potential sources did not contribute any significant material to the collectors. In particular, contamination from soils at the recovery site and materials associated with the ablation of the heatshield do not appear to be significant problems. The largest source of concern is associated with the C present in the original aerogel. The relative abundance of this carbon can vary between aerogel tiles and even within individual tiles. This C was fortunately not distributed among a complex mixture of organics, but was instead largely present in a few simple forms (mostly as Si‐CH₃ groups). In most cases, the signature of returned cometary organics can be readily distinguished from contaminants through their different compositions, nonterrestrial isotopic ratios, and/or association with other cometary materials. However, some conversion of the carbon indigenous to the flight aerogel appears to have happened during particle impact, and some open issues remain regarding how this C may be processed into new forms during the hypervelocity impact collection of the comet dust.  相似文献   

Vegetation patterns, recent pollen deposition and distribution of non-pollen palynomorphs in a polygon mire near Chokurdakh (NE Yakutia, NE Siberia)     

PIM DE KLERK  NORMAN DONNER  HANS JOOSTEN  NIKOLAY S. KARPOV  MERTEN MINKE  NINA SEIFERT  MARTIN THEUERKAUF 《Boreas: An International Journal of Quaternary Research》2009,38(1):39-58

Fine-scaled distribution patterns of vegetation and palynomorphs in a polygon mire in NE Siberia were studied in 31 × 31 plots of 1 m² (vegetation) and along a transect at intervals of 1 m (palynomorphs). Pollen and non-pollen-palynomorph contents clearly reflect differences between polygon ridges and centres. For most single pollen types, however, no unambiguous relation with actual plant abundances and distribution exists. High pollen values are restricted to the immediate vicinity of the pollen sources, which is attributed to the low height of the vegetation. For several plant taxa, high pollen values were found to be restricted to particular plant specimens in the population. High values of pollen ascribed to taxa currently absent at the relevant plot show that the sampled mosses contain a pollen signal of several years. Betula exilis produces two morphologically clearly distinct pollen types. The distribution of selected non-pollen palynomorphs correlates to microrelief, moisture regime and/or trophic conditions. The study shows that single palaeo-sections from arctic peatlands should not be over-interpreted with respect to their palaeoecological value. Local vegetation types and site conditions, however, can be accurately reconstructed from the total assemblages and from combined occurrences of specific palynomorph types.  相似文献   

Chironomids as indicators of the Holocene climatic and environmental history of two lakes in Northeast Greenland     

STEFFI SCHMIDT  BERND WAGNER  OLIVER HEIRI  MARTIN KLUG  OLE BENNIKE  MARTIN MELLES 《Boreas: An International Journal of Quaternary Research》2011,40(1):116-130

Schmidt, S., Wagner, B., Heiri, O., Klug, M., Bennike, O. & Melles, M. 2010: Chironomids as indicators of the Holocene climatic and environmental history of two lakes in Northeast Greenland. Boreas, 10.1111/j.1502‐3885.2010.00173.x. ISSN 0300‐9483. Two Holocene sediment sequences from arctic lakes on Store Koldewey, an island in Northeast Greenland, were investigated for fossil chironomid assemblages. A total of 18 and 21 chironomid taxa were identified in 290‐ and 252‐cm‐long sediment sequences from Duck Lake and Hjort Lake, respectively. The chironomid assemblages were very similar in the two lakes. Canonical correspondence analysis (CCA) was used to compare fossil chironomid assemblages from Store Koldewey with chironomid assemblages and environmental conditions presently found in Canadian Arctic lakes and, hence, to infer environmental changes for Northeast Greenland. The first chironomids appeared at c. 9500 cal. a BP in Hjort Lake, and 500 years later in Duck Lake. Taxa typical for cold and nutrient‐poor arctic lakes dominated the earliest assemblages. Chironomid assemblages with taxa typical of higher summer air temperatures and lakes with higher nutrient availability occur between 8000 and 5000 cal. a BP. This period probably marks the regional Holocene thermal maximum, which is relatively late compared with some palaeoenvironmental records from East Greenland. One possible reason could be the location of Store Koldewey at the very outer coast, with local climatic conditions strongly influenced by the cold East Greenland Current. From around 5000 cal. a BP, chironomid assemblages in Duck Lake and Hjort Lake again became more typical of those presently found in Northeast Greenland, indicating relatively cold and nutrient‐poor conditions. This shift coincides with an increase of ice‐rafting debris off East Greenland and an intensification of the East Greenland Current.  相似文献