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121.
Paula S. Fratantoni Michael S. McCartney 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(2):258-283
Historical hydrographic data, spanning the period 1896–2006, are used to examine the annual mean and seasonal variations in the distribution of freshwater along and across the shelf/slope boundary along the Labrador and Newfoundland Shelves and the Grand Banks of Newfoundland. Particular attention is paid to the export of freshwater along the eastern Grand Banks, between Flemish Cap and the Tail of the Grand Banks, as this has long been identified as a preferential region for the loss of mass and freshwater from the boundary. The data are combined into isopycnally averaged long-term annual and monthly mean gridded property fields and the evolving distribution of fresh arctic-origin water is analyzed in fields of salinity anomaly, expressed as departures from the “central water” temperature–salinity relation of the Gulf Stream. The climatology confirms that cold/fresh northern-source waters are advected offshore within the retroflecting Labrador Current along the full length of the boundary between Flemish Cap and the Tail of the Grand Banks. In fact, it is estimated that most of the equatorward baroclinic transport at the boundary must retroflect back toward the north in order to explain the annual mean distribution of salinity in the climatology. While the retroflection of the Labrador Current appears seasonally robust, the freshwater distribution within the retroflection region varies in response to (1) the freshness of the water available for export which is set by the arrival and rapid flushing of the seasonal freshwater pulse at the boundary, (2) seasonal buoyancy forcing at the surface which alters the vertical stratification across the retroflection region, restricting certain isopycnal export pathways, and (3) the density structure along the eastern Grand Banks, which defines the progressive retroflection of the Labrador Current. 相似文献
122.
Paula OGILVIE Roger L. GIBSON W. Uwe REIMOLD Alexander DEUTSCH Ulrich HORNEMANN 《Meteoritics & planetary science》2011,46(10):1565-1586
Shock recovery experiments were performed at 12.5, 25, 34, 40, and 56 GPa at 25 °C, and at 18 and 25 GPa at 400 °C, on a high‐grade, migmatitic, garnet‐cordierite metapelite from the Etivé aureole, Scotland. Objectives for this study were to (1) characterize shock effects in a complex polymineralic rock with a significant proportion of hydrous ferromagnesian minerals, both as a function of variable shock pressure and preshock temperature, and (2) to explore the effects of shock impedance contrast between component minerals on the respective abundances and distribution of these features. At any shock pressure, the order of decreasing intensity of shock metamorphic effects in component phases is: cordierite (Crd)→biotite (Bt)→plagioclase (Pl)→K‐feldspar (Kfs)→quartz (Qtz)→garnet (Grt)→orthopyroxene (Opx). Samples shocked to pressures below 40 GPa (25 °C) were typically characterized by marked heterogeneous distribution of shock effects on both intragranular and intergranular scales. Shock heterogeneity is mainly attributed to shock impedance contrast between contiguous phases, and manifests as shock amplification locally where shock impedance contrast is greatest, and shock suppression where impedance contrast is least. The heterogeneous distribution of shock metamorphic effects in both experiments and natural rocks is a signature of extreme disequilibrium at the submillimeter scale. The heterogeneous distribution of shock metamorphic effects mitigates against the use of shock effects in minerals exclusively as regional shock pressure barometers, and ought to be augmented by additional constraints on shock pressure from numerical models. 相似文献
123.
Paula Lindgren Martin R. Lee Mahmood R. Sofe Michael E. Zolensky 《Meteoritics & planetary science》2013,48(6):1074-1090
Clasts in the CM2 carbonaceous chondrite Lonewolf Nunataks (LON) 94101 have been characterized using scanning and transmission electron microscopy and electron microprobe analysis to determine their degrees of aqueous alteration, and the timing of alteration relative to incorporation of clasts into the host. The provenance of the clasts, and the mechanism by which they were incorporated and mixed with their host material are also considered. Results show that at least five distinct types of clasts occur in LON 94101, of which four have been aqueously altered to various degrees and one is largely anhydrous. The fact that they have had different alteration histories implies that the main part of aqueous activity occurred prior to the mixing and assimilation of the clasts with their host. Further, the presence of such a variety of clasts suggests complex mixing in a dynamic environment involving material from various sources. Two of the clasts, one containing approximately 46 vol% carbonate and the other featuring crystals of pyrrhotite up to approximately 1 mm in size, are examples of unusual lithologies and indicate concentration of chemical elements in discrete areas of the parent body(ies), possibly by flow of aqueous solutions. 相似文献
124.
Patterns of basin fill in Triassic turbidites of the Nanpanjiang basin: implications for regional tectonics and impacts on carbonate‐platform evolution 下载免费PDF全文
Daniel J. Lehrmann Daniel H. Chaikin Paul Enos Marcello Minzoni Jonathan L. Payne Meiyi Yu Alexa Goers Tanner Wood Paula Richter Brian M. Kelley Xiaowei Li Yanjiao Qin Lingyun Liu Gang Lu 《Basin Research》2015,27(5):587-612
The Nanpanjiang Basin occurs in a key position for resolving controversies of basin tectonics and patterns of plate assembly at the junction between south China and Southeast Asian plates. Paleocurrent measurements indicate that siliciclastic turbidites in the basin were sourced by the Precambrian Jiangnan uplift to the northeast, the Precambrian Yunkai uplift to the southeast and the Triassic Songma suture to the south. Detrital zircon geochronology reveals Archean (2500 Ma), Paleoproterozoic (1800–1900 Ma), Neoproterozoic (900–1000 Ma) and Paleozoic (420–460 Ma) ages consistent with derivation from the Jiangnan and Yunkai uplifts. A large Permian‐Triassic peak of 250 Ma is present in the southern basin and attenuates northward suggesting derivation from an arc developed along the Songma suture. Sandstone QFL compositions average 65/12/23% and plot in the recycled orogen field except for a few samples in the southern basin that fall in the dissected arc field. The compositions are consistent with derivation from Precambrian basement that includes orogenic complexes. In the southern basin, Middle Triassic turbidites contain greater lithics and feldspars and Lower Triassic turbidites have volcaniclastic composition consistent with derivation from a southerly arc. Our preferred interpretation is evolution from remnant basin to a large peripheral foreland with southward subduction and convergence with Indochina along the Songma suture. The previously proposed Dian‐Qiong zone is not a suture as its map location places it within carbonate platforms bounded by identical stratigraphy. The Nan‐Uttaradit zone is too distant to have provided voluminous siliciclastic flux to the basin. The Nanpanjiang Basin provides an example of the evolution of an exceptionally large foreland with far‐field rejuvenation of Precambrian uplifts and carbonate platforms that were significantly influenced by siliciclastic flux. The timing and pattern of turbidite basin fill impacted platform evolution by enabling margin progradation in areas proximal to siliciclastic sources, whereas platforms distant from sources were driven to aggradation and extreme relief with large‐scale gravitational sector collapse. 相似文献
125.
Paula G. Benavidez Daniel D. Durda Brian L. Enke William F. Bottke David Nesvorný Derek C. Richardson Erik Asphaug William J. Merline 《Icarus》2012,219(1):57-76
Collisions are a fundamental process in the creation of asteroid families and in satellite formation. For this reason, understanding the outcome of impacts is fundamental to the accurate modeling of the formation and evolution of such systems. Smoothed-Particle Hydrodynamics/N-body codes have become the techniques of choice to study large-scale impact outcomes, including both the fragmentation of the parent body and the gravitational interactions between fragments. It is now possible to apply this technique to targets with either monolithic or rubble-pile internal structures. In this paper we apply these numerical techniques to rubble-pile targets, extending previous investigations by Durda et al. (Durda, D.D., Bottke, W.F., Enke, B.L., Merline, W.J., Asphaug, E., Richardson, D.C., Leinhardt, Z.M. [2004]. Icarus 170, 243–257; Durda, D.D., Bottke, W.F., Nesvorný, D., Enke, B.L., Merline, W.J., Asphaug, E., Richardson, D.C. [2007]. Icarus 186, 498–516). The goals are to study asteroid–satellite formation and the morphology of the size–frequency distributions (SFDs) from 175 impact simulations covering a range of collision speeds, impact angles, and impactor sizes. Our results show that low-energy impacts into rubble-pile and monolithic targets produce different features in the resulting SFDs and that these are potentially diagnostic of the initial conditions for the impact and the internal structure of the parent bodies of asteroid families. In contrast, super-catastrophic events (i.e., high-energy impacts with large specific impact energy) result in SFDs that are similar to each other. We also find that rubble-pile targets are less efficient in producing satellites than their monolithic counterparts. However, some features, such as the secondary-to-primary diameter ratio and the relative separation of components in binary systems, are similar for these two different internal structures of parent bodies. 相似文献
126.
C.M. Denardini M.A. Abdu E.R. de Paula J.H.A. Sobral C.M. Wrasse 《Journal of Atmospheric and Solar》2005,67(17-18):1665
In this paper, we present the seasonal characteristics of the post-noon rise of the equatorial electrojet 3-m irregularities scattering region observed over São Luís, Brazil (2.3S, 44.2W, dip:-0.5). The study is based on a 1 year data set collected by the 50 MHz coherent back-scatter radar (also known by the acronym RESCO), that started operation in 1998. Using a method to estimate the moments from each individual back-scattered power profiles that constitute the standard range–time–intensity (RTI) maps we were able to determine the following representative parameters of the electrojet: the total power back-scattered by the electrojet irregularities (EJP), the thickness of the electrojet back-scattered power profile (EJT), the height of the center of the back-scatter region, that is, the power profile, (EJC), and the noise level corresponding to each power profile (EJN). The parameterization was applied to all selected daily RTI maps from 2002. The analysis was carried out by grouping the data according to the radar beam angle (tilted 30 westward or eastward), the magnetic disturbance indices Kp and the season, which enables us to quantify the differences in the parameters on these bases. The results are presented and discussed here focusing on the post-noon ascent of the EJC. We will also present results on the east–west asymmetry in the radar back-scattered echo power confirming our previous results, and on the appearance of a scattering region after sunset during magnetically quiet conditions around the southern summer solstice. 相似文献
127.
Griselda Chaparro María Soledad Fontanarrosa María Romina Schiaffino Paula de Tezanos Pinto Inés O’Farrell 《Aquatic Sciences - Research Across Boundaries》2014,76(4):579-594
In floodplains located in temperate regions, seasonal variations in temperature affect biological communities and these effects may overlap with those of the flood regime. In this study we explored if and how timing (with regard to temperature seasonality) influences the responses of planktonic and free-floating plants communities to floods in a warm temperate floodplain lake and assessed its relevance for determining state shifts. We took samples of zooplankton, phytoplankton, picoplankton, heterotrophic nanoflagellates and free-floating macrophytes at four sites of the lake characterized by the presence-absence of emergent or free-floating macrophytes along a 2-year period with marked hydrological fluctuations associated to river flood dynamics. We performed ANOVA tests to compare the responses of these communities to floods in cold and warm seasons and among sites. Planktonic communities developed high abundances in response to floods that occurred in the cold season, while the growth of free-floating macrophytes was impaired by low winter temperatures. Spring and summer floods favored profuse colonization by free-floating plants and limited the development of planktonic communities. The prolonged absence of floods during warm periods caused environmental conditions that favored Cyanobacteria growth, leading to a “low turbid waters” regime. The occurrence of floods early in the warm season caused phytoplankton dilution and promoted free-floating plant colonization and a shift towards a “high clear waters” state. Zooplankton:phytoplankton biomass ratio was very low during floods in warm seasons, thus zooplankton grazing on phytoplankton seemed to play a minor role in the maintenance of the clear regime. 相似文献
128.
Paula Chainho Gilda Silva Michael F. Lane José L. Costa Tadeu Pereira Carla Azeda Pedro R. Almeida Inês Metelo Maria J. Costa 《Estuaries and Coasts》2010,33(6):1314-1326
Long-term monitoring studies are needed to understand changes in ecosystem status when restoration measures are implemented.
A long-term data series (1996–2007) of the Tagus estuary (Portugal) intertidal and subtidal benthic communities was collected
in a degraded area where mitigation measures were implemented. Multivariate analysis was used to analyze spatial and temporal
patterns in benthic community composition and trends in five benthic community metrics (i.e., taxonomic richness, density,
biomass, Shannon–Wiener diversity and the AMBI index) were also examined. The results revealed a clear separation between
intertidal and subtidal assemblages, although they had 50% of taxa in common, including the most abundant. Significant positive
trends were found for all metrics showing that both intertidal and subtidal communities responded to the restoration measures
implemented. Nevertheless, biotic indices need some adaptation before being universally applied to intertidal and subtidal
habitats. 相似文献
129.
130.
Tapani O. Rämö James P. Calzia Paula J. Kosunen 《Contributions to Mineralogy and Petrology》2002,143(4):416-437
Mesozoic granitoid plutons in the southern Death Valley region of southeastern California reveal substantial compositional and isotopic diversity for Mesozoic magmatism in the southwestern US Cordillera. Jurassic plutons of the region are mainly calc-alkaline mafic granodiorites with )Ndi of -5 to -16, 87Sr/86Sri of 0.707-0.726, and 206Pb/204Pbi of 17.5-20.0. Cretaceous granitoids of the region are mainly monzogranites with )Ndi of -6 to -19, 87Sr/86Sri of 0.707-0.723, and 206Pb/204Pbi of 17.4-18.6. The granitoids were generated by mixing of mantle-derived mafic melts and pre-existing crust - some of the Cretaceous plutons represent melting of Paleoproterozoic crust that, in the southern Death Valley region, is exceptionally heterogeneous. A Cretaceous gabbro on the southern flank of the region has an unusually juvenile composition ()Ndi -3.2, 87Sr/86Sri 0.7060). Geographic position of the Mesozoic plutons and comparison with Cordilleran plutonism in the Mojave Desert show that the Precambrian lithosphere (craton margin) in the eastern Mojave Desert region may consists of two crustal blocks separated by a more juvenile terrane. 相似文献