Lithium isotopic composition and concentration of the upper continental crust   总被引：4，自引：0，他引：4  

F.-Z. Teng  W.F. McDonough  C. Dalpé  P.B. Tomascak  B.W. Chappell 《Geochimica et cosmochimica acta》2004,68(20):4167-4178

The Li isotopic composition of the upper continental crust is estimated from the analyses of well-characterized shales, loess, granites and upper crustal composites (51 samples in total) from North America, China, Europe, Australia and New Zealand. Correlations between Li, δ⁷Li, and chemical weathering (as measured by the Chemical Index of Alteration (CIA)), and δ⁷Li and the clay content of shales (as measured by Al₂O₃/SiO₂), reflect uptake of heavy Li from the hydrosphere by clays. S-type granites from the Lachlan fold belt (-1.1 to -1.4‰) have δ⁷Li indistinguishable from their associated sedimentary rocks (-0.7 to 1.2‰), and show no variation in δ⁷Li throughout the differentiation sequence, suggesting that isotopic fractionation during crustal anatexis and subsequent differentiation is less than analytical uncertainty (±1‰, 2σ). The isotopically light compositions for both I- and S-type granites from the Lachlan fold belt (-2.5 to + 2.7 ‰) and loess from around the world (-3.1 to + 4.5‰) reflect the influence of weathering in their source regions. Collectively, these lithologies possess a limited range of Li isotopic compositions (δ⁷Li of −5‰ to + 5‰), with an average (δ⁷Li of 0 ± 2‰ at 1σ) that is representative of the average upper continental crust. Thus, the Li isotopic composition of the upper continental crust is lighter than the average upper mantle (δ⁷Li of + 4 ± 2‰), reflecting the influence of weathering on the upper crustal composition. The concentration of Li in the upper continental crust is estimated to be 35 ± 11 ppm (2σ), based on the average loess composition and correlations between insoluble elements (Ti, Nb, Ta, Ga and Al₂O₃, Th and HREE) and Li in shales. This value is somewhat higher than previous estimates (∼20 ppm), but is probably indistinguishable when uncertainties in the latter are accounted for.  相似文献   

Petrology and geochemistry of spinel peridotite xenoliths from Hannuoba and Qixia, North China craton   总被引：58，自引：0，他引：58  

Roberta L. Rudnick  Shan Gao  Wen-li Ling  Yong-shen Liu  William F. McDonough 《Lithos》2004,77(1-4):609-637

We report mineralogical and chemical compositions of spinel peridotite xenoliths from two Tertiary alkali basalt localities on the Archean North China craton (Hannuoba, located in the central orogenic block, and Qixia, in the eastern block). The two peridotite suites have major element compositions that are indistinguishable from each other and reflect variable degrees (0–25%) of melt extraction from a primitive mantle source. Their compositions are markedly different from typical cratonic lithosphere, consistent with previous suggestions for removal of the Archean mantle lithosphere beneath this craton. Our previously published Os isotopic results for these samples [Earth Planet. Sci. Lett. 198 (2002) 307] show that lithosphere replacement occurred in the Paleoproterozoic beneath Hannuoba, but in the Phanerozoic beneath Qixia. Thus, we see no evidence for a compositional distinction between Proterozoic and Phanerozoic continental lithospheric mantle. The Hannuoba xenoliths equilibrated over a more extensive temperature (hence depth) interval than the Qixia xenoliths. Neither suite shows a correlation between equilibration temperature and major element composition, indicating that the lithosphere is not chemically stratified in either area. Trace element and Sr and Nd isotopic compositions of the Hannuoba xenoliths reflect recent metasomatic overprinting that is not related to the Tertiary magmatism in this area.  相似文献   

Streetvending in Peruvian Cities: The Spatio-Temporal Behavior of Ambulantes*     

Maureen Hays-Mitchell 《The Professional geographer》1994,46(4):425-438

Informal trade is a historically rooted and empirically important livelihood in Andean America. This paper examines the spatial dimensions of informal commerce by documenting and analyzing the locational patterns and preferences of streetvendors and the locational constraints imposed on their activities in six Peruvian cities. The interplay of streetvendor needs and municipal exigencies is found to determine the spatio-temporal behavior of streetvendors. A fundamental shift in municipal policy to a more pragmatic attitude toward streetvending is noted.  相似文献   

Geomagnetic excursion in the late Cretaceous     

Maureen B. Steiner 《Geophysical Journal International》1983,73(1):17-25

Summary. Two sedimentary cores from the western Pacific display a palaeo-magnetic record of the late Cretaceous long normal interval and the boundary reversed interval corresponding to seafloor spreading anomalies 33–34. Near the young end of this reversed interval, a systematic excursion of inclinations is observed in both cores. Samples are very stable to both alternating field and thermal demagnetization. Blocking temperatures and Curie points suggest that the remanence is carried primarily by magnetite, but with an additional contribution from hematite. Approximate sedimentation rates derived from biostratigraphy suggest that the excursion had a duration of between 46 000 and 54 000yr and occurred about 236000–303000 yr before the succeeding polarity reversal. The excursion, thus, may represent an aborted geomagnetic field reversal.  相似文献   

Low-level temperature inversions and their effect on aerosol condensation nuclei concentrations under different large-scale synoptic circulations     

Jun Li  Hongbin Chen  Zhanqing Li  Pucai Wang  Maureen Cribb  Xuehua Fan 《大气科学进展》2015,32(7):898-908

Knowledge of the statistical characteristics of inversions and their effects on aerosols under different large-scale synoptic circulations is important for studying and modeling the diffusion of pollutants in the boundary layer. Based on results generated using the self-organizing map(SOM) weather classification method, this study compares the statistical characteristics of surface-based inversions(SBIs) and elevated inversions(EIs), and quantitatively evaluates the effect of SBIs on aerosol condensation nuclei(CN) concentrations and the relationship between temperature gradients and aerosols for six prevailing synoptic patterns over the the Southern Great Plains(SGP) site during 2001–10. Large-scale synoptic patterns strongly influence the statistical characteristics of inversions and the accumulation of aerosols in the low-level atmosphere. The activity,frequency, intensity, and vertical distribution of inversions are significantly different among these synoptic patterns. The vertical distribution of inversions varies diurnally and is significantly different among the different synoptic patterns. Anticyclonic patterns affect the accumulation of aerosols near the ground more strongly than cyclonic patterns. Mean aerosol CN concentrations increase during SBIs compared to no inversion cases by 16.1%, 22.6%, 24.5%, 58.7%, 29.8% and 23.7% for the six synoptic patterns. This study confirms that there is a positive correlation between temperature gradients and aerosol CN concentrations near the ground at night under similar large-scale synoptic patterns. The relationship is different for different synoptic patterns and can be described by linear functions. These findings suggest that large-scale synoptic patterns change the static stability of the atmosphere and inversions in the lower atmosphere, thereby influencing the diffusion of aerosols near the ground.  相似文献   

Cloud-base distribution and cirrus properties based on micropulse lidar measurements at a site in southeastern China     

Jianjun Liu  Zhanqing Li  Youfei Zheng  Maureen Cribb 《大气科学进展》2015,32(7):991-1004

The cloud fraction(CF) and cloud-base heights(CBHs), and cirrus properties, over a site in southeastern China from June 2008 to May 2009, are examined by a ground-based lidar. Results show that clouds occupied the sky 41% of the time.Significant seasonal variations in CF were found with a maximum/minimum during winter/summer and similar magnitudes of CF in spring and autumn. A distinct diurnal cycle in the overall mean CF was seen. Total, daytime, and nighttime annual mean CBHs were 3.05 ± 2.73 km, 2.46 ± 2.08 km, and 3.51 ± 3.07 km, respectively. The lowest/highest CBH occurred around noon/midnight. Cirrus clouds were present ～36.2% of the time at night with the percentage increased in summer and decreased in spring. Annual mean values for cirrus geometrical properties were 8.89 ± 1.65 km, 9.80 ± 1.70 km, 10.73 ± 1.86 km and 1.83 ± 0.91 km for the base, mid-cloud, top height, and the thickness, respectively. Seasonal variations in cirrus geometrical properties show a maximum/minimum in summer/winter for all cirrus geometrical parameters. The mean cirrus lidar ratio for all cirrus cases in our study was ～ 25 ± 17 sr, with a smooth seasonal trend. The cirrus optical depth ranged from 0.001 to 2.475, with a mean of 0.34 ± 0.33. Sub-visual, thin, and dense cirrus were observed in ～12%, 43%, and 45%of the cases, respectively. More frequent, thicker cirrus clouds occurred in summer than in any other season. The properties of cirrus cloud over the site are compared with other lidar-based retrievals of midlatitude cirrus cloud properties.  相似文献   

Diaspora strategies in the sending states of Southeast Asia: Rights,skills and questions of value          下载免费PDF全文

Maureen Hickey 《Singapore journal of tropical geography》2015,36(2):147-163

Over the past decade there has been a surge of academic and policy interest in the contributions of transnational migrants in furthering national development objectives in their ‘home’ countries. These approaches tend to be bifurcated into two distinct categories: (1) ‘diaspora strategies’ that target the participation in national development objectives of sought‐after elite, high‐skilled migrants and (2) migration‐as‐development strategies, which focus on facilitating and channeling the economic and social remittances of nonelite, low‐skilled migrants. Although these broad categorizations have never been entirely adequate in capturing the complexity of international migration flows, a conceptual division between them persists, with very real consequences for state policies in migrant‐sending countries in Southeast Asia. This paper explores the separation between diaspora strategies and migration‐as‐development frameworks through a focus on rights and skills, and questions of relative labour value. I argue that while diaspora strategies and migration‐as‐development frameworks cannot simply be merged, the academic separation between them should be challenged and more in‐depth theoretical engagement should be encouraged. I further suggest that migration policy makers should strive to evaluate their migration policies to address the increasing complexities of contemporary migration.  相似文献   

The influence of surface layer salinity on wintertime convection in Wilkinson Basin,Gulf of Maine     

Maureen H. Taylor  David G. Mountain 《Continental Shelf Research》2009

Observations suggest that the interannual variability of wintertime convection in Wilkinson Basin (WB) during 1977–2005 is related to the variation in the surface layer salinity in the western Gulf of Maine (wGOM). When the winter convection is stronger (weaker), the resulting deep-layer temperatures in the wGOM are colder (warmer), and the likelihood of deep winter mixing is greater when the wGOM salinities are high. This hypothesis was tested using a one-dimensional mixed layer model to simulate the water column structure over the cooling period. Comparisons were made between the convection potential for the range of observed late-fall salinity values. Model results indicated that the mixed layer depth could deepen by as much as 50% when the surface layer salinities in the wGOM were high. Sensitivity tests revealed that the surface layer salinity variability, and its influence on the water column density structure, was a significant factor, along with local climate variability, in determining the observed time series trend in springtime bottom layer temperatures.  相似文献   

Are trends in SeaWiFS chlorophyll time-series unusual relative to historic variability   总被引：1，自引：0，他引：1  

James A Yoder  Maureen A Kennelly  Scott C Doney  Ivan D Lima 《海洋学报(英文版)》2010,29(2):1-4

For selected locations in the Atlantic and Pacific Ocean, we compared surface ocean chlorophyll time series extracted from SeaWiFS imagery from 1997–2004 with the results of an ocean coupled circulation and biogeochemical model covering the period 1958–2004. During the 1997–2004 time period, linear trends in model and satellite time series were significantly correlated at most of the 44 sites we studied. Eleven sites were selected for further study, and we used the longer time series of the model to assess whether trends observed during the SeaWiFS period at these 11 sites were unusual in relation to those observed over the longer historical period covered by the model. The results show that the trends observed during the SeaWiFS period were not unusual and fell well within the range in magnitude of linear trends observed in other 8-year periods of model output. This result implies that the SeaWiFS satellite ocean color time series is not yet sufficiently long, on its own, to directly observe any long term changes in phytoplankton chlorophyll that may be occurring in the surface waters of the open ocean as a result of increased ocean stratification linked to global climate changed.  相似文献   

Ecosystem responses to recent oceanographic variability in high-latitude Northern Hemisphere ecosystems   总被引：2，自引：0，他引：2  

Franz J. Mueter  Cecilie Broms  Kenneth F. Drinkwater  Kevin D. Friedland  Jonathan A. Hare  George L. Hunt Jr.  Webjrn Melle  Maureen Taylor 《Progress in Oceanography》2009,81(1-4):93

As part of the international MENU collaboration, we compared and contrasted ecosystem responses to climate-forced oceanographic variability across several high latitude regions of the North Pacific (Eastern Bering Sea (EBS) and Gulf of Alaska (GOA)) and North Atlantic Oceans (Gulf of Maine/Georges Bank (GOM/GB) and the Norwegian/Barents Seas (NOR/BAR)). Differences in the nitrate content of deep source waters and incoming solar radiation largely explain differences in average primary productivity among these ecosystems. We compared trends in productivity and abundance at various trophic levels and their relationships with sea-surface temperature. Annual net primary production generally increases with annual mean sea-surface temperature between systems and within the EBS, BAR, and GOM/GB. Zooplankton biomass appears to be controlled by both top-down (predation by fish) and bottom-up forcing (advection, SST) in the BAR and NOR regions. In contrast, zooplankton in the GOM/GB region showed no evidence of top-down forcing but appeared to control production of major fish populations through bottom-up processes that are independent of temperature variability. Recruitment of several fish stocks is significantly and positively correlated with temperature in the EBS and BAR, but cod and pollock recruitment in the EBS has been negatively correlated with temperature since the 1977 shift to generally warmer conditions. In each of the ecosystems, fish species showed a general poleward movement in response to warming. In addition, the distribution of groundfish in the EBS has shown a more complex, non-linear response to warming resulting from internal community dynamics. Responses to recent warming differ across systems and appear to be more direct and more pronounced in the higher latitude systems where food webs and trophic interactions are simpler and where both zooplankton and fish species are often limited by cold temperatures.  相似文献