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31.
The stable isotopic composition (13C and 18O) and elemental (Sr and Mg) of marine molluscs are presented for Carditacea and Solenacea shells collected off the Mediterranean coast of Egypt. Based on shell microstructures and mineralogy, the bivalve shells are preserved in their original mineralogy and chemistry.The Sr and Mg concentrations of the bivalve shells have mean values of 1960 ppm and 226 ppm respectively. The stable isotopic composition generally show high values of 18O and 13C. The 18O values range from +0.1 to –1.8 PDB and most shells are highly enriched in13C; averaging +2.5 PDB. These elemental and isotopic signatures are analogous to modern marine bivalves from other localities.The oxygen and carbon isotopes, together with the calculated temperatures, suggest that the aragonitic bivalve shells were precipitated in isotopic equilibrium from warm marine waters. 相似文献
32.
Detailed studies of the daytime E-region critical frequency at Aberystwyth (geomagnetic latitude +56°) show clear evidence for changes associated with both the axially-symmetric (Dst) and asymmetric (DS) components of the disturbance magnetic field. Comparison of the sensitivity of the E-region peak density to these two influences shows that the changes cannot entirely (if at all) be ascribed to the influence of electric currents in the region. It is suggested that a major role is played by dynamical influences associated with the neutral air “storm circulation” which distributes the energy fed into the auroral region to lower latitudes. 相似文献
33.
A comparison is made between the flux-versus-time profile in the EUV band and the thick target electron flux profile as inferred from hard X-rays for a number of moderately large solar flares. This complements Kane and Donnelly's (1971) study of small flares. The hard X-ray data are from ESRO TD-1A and the EUV inferred from SFD observations.Use of a 2 minimising method shows that the best overall fit between the profile fine structures obtains for synchronism to 5 s which is within the timing accuracy. This suggests that neither conduction nor convection is fast enough as the primary mechanism of energy transport into the EUV flare and rather favours heating by the electrons themselves or by some MHD wave process much faster than acoustic waves.The electron power deposited, for a thick target model, is however far greater than the EUV luminosity for any reasonable assumptions about the area and depth over which EUV is emitted. This means that either most of the power deposited is conducted away to the optical flare or that only a fraction 1–10% of the X-ray emitting electrons are injected downwards. Recent work on H flare heating strongly favours the latter alternative - i.e. that electrons are mostly confined in the corona. 相似文献
34.
The work of Brown and Hoyng (1975) on the betatron acceleration of hard X-ray source electrons trapped in a vibrating flux tube is generalised to include Fermi acceleration by the varying transverse field. This development can explain the trajectory of bursts in a plot of equivalent thick-target electron flux
versus spectral index as inferred from observations obtained by ESRO TD1A. Specifically the loops observed in this
, diagram, unexplained in Brown and Hoyng's anslysis are accounted for by a changing phase relationship between the varying field strength f(t) and magnetic scale length g(t)- Application of the formalism to the detailed TD1A observations of the large events of 1972, August 4 and 7, allows inference of the evolution of f(t), g(t) in these events.Now at: Space Sciences Laboratory, University of California, Berkeley, U.S.A. 相似文献
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36.
Karen L. Hunter Dewayne A. Fox Lori M. Brown Kenneth W. Able 《Estuaries and Coasts》2006,29(3):487-498
Modification of brackish marshes by nonindigenousPhragmites australis has occurred across a broad geographical area in eastern North America. Among its effects on marsh processes,Phragmites may be increasingly unfavorable to marsh surface fishes as its invasion progresses within an estuary. We assessed the effect
of thePhragmites invasion on resident marsh surface fishes by examining the population response ofFundulus heteroclitus (mummichog, 5–48 mm TL) andF. luciae (spotfin killifish, 5–41 mm TL) to four distinct invasion stages in three estuaries of the U.S. mid Atlantic region (New
Jersey, Delaware, and Maryland). We documented precipitous declines in mean catch per unit effort ofF. heteroclitus in pit traps from natural marsh (51.6), through initial (33.8), early (12.3), and late invasion stages (2.4) across all sites.
A similar pattern was documented forF. luciae, with mean catch per unit effort in pit traps declining from natural marsh (48.9), through initial (39.1), early (9.3), and
late invasion stages (2.7). Population structure of both species also changed somewhat across invasion stages such that we
collected a narrower size range of individuals of both species from late invasion stages. Patterns suggest that as thePhragmites invasion progresses, there is a decline in habitat function for larval and juvenileF. heteroclitus and an increased risk of extirpation ofF. luciae from brackish marshes along the east coast of the U.S. 相似文献
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Daniel G. Brown Rick Riolo Derek T. Robinson Michael North William Rand 《Journal of Geographical Systems》2005,7(1):25-47
The use of object-orientation for both spatial data and spatial process models facilitates their integration, which can allow exploration and explanation of spatial-temporal phenomena. In order to better understand how tight coupling might proceed and to evaluate the possible functional and efficiency gains from such a tight coupling, we identify four key relationships affecting how geographic data (fields and objects) and agent-based process models can interact: identity, causal, temporal and topological. We discuss approaches to implementing tight integration, focusing on a middleware approach that links existing GIS and ABM development platforms, and illustrate the need and approaches with example agent-based models. 相似文献
39.
Matthias J. Raab Roderick W. Brown Kerry Gallagher Andrew Carter Klaus Weber 《Tectonophysics》2002,349(1-4)
Namibia's passive continental margin records a long history of tectonic activity since the Proterozoic. The orogenic belt produced during the collision of the Congo and Kalahari Cratons in the Early Proterozoic led to a zone of crustal weakness, which became the preferred location for tectonism during the Phanerozoic. The Pan-African Damara mobile belt forms this intraplate boundary in Namibia and its tectonostratigraphic zones are defined by ductile shear zones, where the most prominent is described as the Omaruru Lineament–Waterberg Thrust (OML–WT). The prominance of the continental margin escarpment is diminished in the area of the Central and Northern Zone of the Damara belt where the shear zones are located. This area has been targeted with a set of 66 outcrop samples over a 550-km-long, 60-km-broad coast-parallel transect from the top of the escarpment in the south across the Damara sector to the Kamanjab Inlier in the north. Apatite fission track age and length data from all samples reveal a regionally consistent cooling event. Thermal histories derived by forward modelling bracket this phase of accelerated cooling in the Late Cretaceous. Maximum palaeotemperatures immediately prior to the onset of cooling range from ca. 120 to ca. 60 °C with the maximum occurring directly south of the Omaruru Lineament. Because different palaeotemperatures indicate different burial depth at a given time, the amount of denudation can be estimated and used to constrain vertical displacements of the continental crust. We interpret this cooling pattern as the geomorphic response to reactivation of basement structures caused by a change in spreading geometry in the South Atlantic and South West Indian Oceans. 相似文献
40.