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排序方式: 共有98条查询结果,搜索用时 15 毫秒
1.
Jonathan F. Stebbins Emily V. Dubinsky Koji Kanehashi Kimberly E. Kelsey 《Geochimica et cosmochimica acta》2008,72(3):910-925
Configurational changes with temperature are important for the thermodynamic and transport properties of most aluminosilicate melts, but in general are not well understood. Here, we present high-resolution 27Al and 17O NMR data on several calcium aluminosilicate glasses prepared with varying quench rates and thus with fictive temperatures that span ranges up to about 200 K. In all compositions the content of five-coordinated aluminum increases with fictive temperature, in agreement with recent high temperature NMR data on melts. In a glass of CaAl2Si2O8 (“anorthite”) composition, the content of non-bridging oxygens also increases with temperature; however this effect was not observed in a sample with a much higher CaO/Al2O3 ratio. We present a consistent notation for reactions among structural species in these systems that clarify why in some cases, high-coordinated network cations may appear on the same side of the reaction, while in others they occur on the opposite sides: the key difference is in accounting for all coordination changes for oxygens. Mixing of non-bridging oxygens and of high-coordinated aluminum make significant contributions to the overall configurational entropy and heat capacity of the melts, as does the mixing of various bridging oxygens and of tetrahedral network cations. Other, less well known, types of increase in disorder with temperature may be important as well. 相似文献
2.
Diurnal variation in rates of calcification and carbonate sediment dissolution in Florida Bay 总被引:2,自引:0,他引:2
Water quality and criculation in Florida Bay (a shallow, subtropical estuary in south Florida) are highly dependent upon the
development and evolution of carbonate mud banks distributed throughout the Bay. Predicting the effect of natural and anthropogenic
perturbations on carbonate sedimentation requires an understanding of annual, seasonal, and daily variations in the biogenic
and inorganic processes affecting carbonate sediment precipitation and dissolution. In this study, net calcification rates
were measured over diurnal cycles on 27 d during summer and winter from 1999 to 2003 on mud banks and four representative
substrate types located within basins between mud banks. Substrate types that were measured in basins include seagrass beds
of sparse and intermediate densityThalassia sp., mud bottom, and hard bottom communities. Changes in total alkalinity were used as a proxy for calcification and dissolution.
On 22 d (81%), diurnal variation in rates of net calcification was observed. The highest rates of net carbonate sediment production
(or lowest rates of net dissolution) generally occurred during daylight hours and ranged from 2.900 to −0.410 g CaCO3 m−2d−1. The lowest rates of carbonate sediment production (or net sediment dissolution) occurred at night and ranged from 0.210
to −1.900 g CaCO3 m−2 night−1. During typical diurnal cycles, dissolution during the night consumed an average of 29% of sediment produced during the day
on banks and 68% of sediment produced during the day in basins. Net sediment dissolution also occurred during daylight, but
only when there was total cloud cover, high turbidity, or hypersalinity. Diurnal variation in calcification and dissolution
in surface waters and surface sediments of Florida Bay is linked to cycling of carbon dioxide through photosynthesis and respiration.
Estimation of long-term sediment accumulation rates from diurnal rates of carbonate sediment production measured in this study
indicates an overall average accumulation rate for Florida Bay of 8.7 cm 1000 yr−1 and suggests that sediment dissolution plays a more important role than sediment transport in loss of sediment from Florida
Bay. 相似文献
3.
Assessing the effects of catchment‐scale urban green infrastructure retrofits on hydrograph characteristics 下载免费PDF全文
Run‐off from impervious surfaces has pervasive and serious consequences for urban streams, but the detrimental effects of urban stormwater can be lessened by disconnecting impervious surfaces and redirecting run‐off to decentralized green infrastructure. This study used a before–after‐control‐impact design, in which streets served as subcatchments, to quantify hydrologic effectiveness of street‐scale investments in green infrastructure, such as street‐connected bioretention cells, rain gardens and rain barrels. On the two residential treatment streets, voluntary participation resulted in 32.2% and 13.5% of parcels having green infrastructure installed over a 2‐year period. Storm sewer discharge was measured before and after green infrastructure implementation, and peak discharge, total run‐off volume and hydrograph lags were analysed. On the street with smaller lots and lower participation, green infrastructure installation succeeded in reducing peak discharge by up to 33% and total storm run‐off by up to 40%. On the street with larger lots and higher participation, there was no significant reduction in peak or total stormflows, but on this street, contemporaneous street repairs may have offset improvements. On the street with smaller lots, lag times increased following the first phase of green infrastructure construction, in which streetside bioretention cells were built with underdrains. In the second phase, lag times did not change further, because bioretention cells were built without underdrains and water was removed from the system, rather than just delayed. We conclude that voluntary green infrastructure retrofits that include treatment of street run‐off can be effective for substantially reducing stormwater but that small differences in design and construction can be important for determining the level of the benefit. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
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Kimberly K. Takagi Phaothep Cherdsukjai Izumi Mimura Yuka Yano Kanjana Adulyanukosol Makoto Tsuchiya 《Estuarine, Coastal and Shelf Science》2010
This study aimed to elucidate Dotilla myctiroides (H. Milne-Edwards) food resources and how the soldier crab influences the organic matter composition on the intertidal flat. A preliminary assessment of D. myctiroides size, sex, and population distribution was conducted to clarify the characteristics of the inhabiting population. Overall, D. myctiroides size and abundance was normal across the tidal flat area. Comparison of different sediment types from the soldier crab habitat (surface sediments, igloos, burrow lining, burrow pellets, feeding pellets, and feces) using the fatty acid (FA) analysis technique revealed D. myctiroides removes organic matter via the significantly lower contributions of polyunsaturated FAs, essential FAs, and dinoflagellates in the feeding pellets. Dotilla myctiroides tissue analysis of the hepatopancreas and hindgut in individuals that had been starved for 24 h confirmed not only the consumption of dinoflagellates, but diatoms and macroalgae as well. In addition, the FA metabolism within the digestive system implied the ability of the soldier crab to synthesize highly unsaturated FAs (HUFAs) such as 20:4n–6, 20:5n–3 and 22:6n–3. 相似文献
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8.
For the 1980–2003 period, we analyzed the relationship between crop yield and three climatic variables (minimum temperature,
maximum temperature, and precipitation) for 12 major Californian crops: wine grapes, lettuce, almonds, strawberries, table
grapes, hay, oranges, cotton, tomatoes, walnuts, avocados, and pistachios. The months and climatic variables of greatest importance
to each crop were used to develop regressions relating yield to climatic conditions. For most crops, fairly simple equations
using only 2–3 variables explained more than two-thirds of observed yield variance. The types of variables and months identified
suggest that relatively poorly understood processes such as crop infection, pollination, and dormancy may be important mechanisms
by which climate influences crop yield. Recent climatic trends have had mixed effects on crop yields, with orange and walnut
yields aided, avocado yields hurt, and most crops little affected by recent climatic trends. Yield-climate relationships can
provide a foundation for forecasting crop production within a year and for projecting the impact of future climate changes. 相似文献
9.
Katrina D. van Drongelen Douglas Rumble III Kimberly T. Tait 《Meteoritics & planetary science》2016,51(6):1184-1200
Northwest Africa (NWA) 5232, an 18.5 kg polymict eucrite, comprises eucritic and exogenic CM carbonaceous chondrite clasts within a clastic matrix. Basaltic clasts are the most abundant eucritic clast type and show a range of textures and grain size, from subophitic to granoblastic. Other eucritic clast types present include cumulate (high‐En pyroxene), pyroxene‐lath, olivine rich with symplectite intergrowths as a break‐down product of a quickly cooled Fe‐rich metastable pyroxferroite, and breccia (fragments of a previously consolidated breccia) clasts. A variable cooling rate and degree of thermal metamorphism, followed by a complex brecciation history, can be inferred for the clasts based on clast rounding, crystallization (and recrystallization) textures, pyroxene major and minor element compositions, and pyroxene exsolution. The range in δ18O of clasts and matrix of NWA 5232 reflects its origin as a breccia of mixed clasts dominated by eucritic lithologies. The oxygen isotopic compositions of the carbonaceous chondrite clasts identify them as belonging to CM group and indicate that these clasts experienced a low degree of aqueous alteration while part of their parent body. The complex evolutionary history of NWA 5232 implies that large‐scale impact excavation and mixing was an active process on the surface of the HED parent body, likely 4 Vesta. 相似文献
10.
Pradeep Adhikari Yang Hong Kimberly R. Douglas Dalia Bach Kirschbaum Jonathan Gourley Robert Adler G. Robert Brakenridge 《Natural Hazards》2010,55(2):405-422
Floods have profound impacts on populations worldwide in terms of both loss of life and property. A global inventory of floods
is an important tool for quantifying the spatial and temporal distribution of floods and for evaluating global flood prediction
models. Several global hazard inventories currently exist; however, their utility for spatiotemporal analysis of global floods
is limited. The existing flood catalogs either fail to record the geospatial area over which the flood impacted or restrict
the types of flood events included in the database according to a set of criteria, limiting the scope of the inventory. To
improve upon existing databases, and make it more comprehensive, we have compiled a digitized Global Flood Inventory (GFI)
for the period 1998–2008 which also geo-references each flood event by latitude and longitude. This technical report presents
the methodology used to compile the GFI and preliminary findings on the spatial and temporal distributions of the flooding
events that are contained in the inventory. 相似文献