Periodic frontogenesis in a region of freshwater influence   总被引：1，自引：0，他引：1  

Jonathan Sharples  John H. Simpson 《Estuaries and Coasts》1993,16(1):74-82

Observations are presented from a series of three conductivity-temperature-depth (CTD) surveys of the salinity and temperature structure of Liverpool Bay, a region that is strongly influenced by the input of fresh water from the rivers of northwest England. The surveys demonstrate the development, seaward movement, and eventual decay of a haline front. The frontogenesis is driven by the relaxation of a freshwater-induced horizontal density gradient following the decrease in tidal range at neap tides. It results in the area of Liverpool Bay being stratified for a period of 8 d before the increase in tidal mixing as the spring tide approaches returns the region to its initial vertically mixed state. In Liverpool Bay this process usually repeats on the spring-neap cycle, though strong wind-mixing may prevent the frontogenesis and subsequent stratification. Analysis with a 1-dimensional numerical model suggests that relaxation of an initially nonlinear horizontal density field, creating the front, is triggered by the stability produced by tidal straining of the water column during the ebb half-cycle. The reduction in tidal mixing energy approaching neap tide does not lead to frontogenesis without this initial stability. Such a regular stratification signal will have a marked effect on the local environment. The periodic frontogenesis will act as a tidal pump, moving buoyant substances in the water column offshore, while the onshore residual currents lower in the water column will more deeper dissolved substances inshore. The cycling of stability on the springneap time scale is considerably faster than the seasonal cycle of thermal stratification in the shelf seas, but is similar in creating the conditions required for phytoplankton blooms. Conditions favorable for enhanced primary production may therefore occur frequently in such regions of freshwater influence.  相似文献   

Crust-mantle interaction in continental arcs: inferences from the Mesozoic arc in the southwestern United States     

Yemane Asmerom  P. Jonathan Patchett  Paul E. Damon 《Contributions to Mineralogy and Petrology》1991,107(1):124-134

Arc magmas ranging in composition from basaltic andesites to rhyolites and intrusive equivalents were emplaced into the western margin of the North American craton starting in Late Triassic time giving way to rift0related sedimentation in the Late Jurassic. The region of this study cuts across Proterozoic basements of contrasting Nd model ages, 1.7–1.8 Ga (average ɛ_Nd∼−11) in eastern Arizona and 2.0 to 2.3 GA (average ɛ_Nd∼−18) in western Arizona and eastern California (Bennett and DePaolo 1987). The Mesozoic rocks have initial ɛ_Nd of -3.4 to-6.4 in the eastern part of the study area and -7.1 to -9.2 in the western part. All of the rocks have elevated ⁸⁷Sr/⁸⁷Sr initial ratios (>0.706). Trends in initial ɛ_Nd values of Mesozoic arc rocks are directly correlated with the Nd model ages of the basement through which they passed. Simple two-component mixing calculations indicate that recycled continental crust in the arc magmas represents on average about 65%. A minimum of 35% mantle input into continental arc magmas, as recent as the Mesozoic, represents a significant contribution to the growth of the continental crust, in the absence of a return flow of continental material into the mantle of similar magnitude. In a detailed study in the Santa Rita Mountains. Arizona, there is a pattern of increase of ɛ_Nd with time: early basaltic andesites have more negative ɛ_Nd than later felsic rocks. A correlated pattern of depletion with time is also observed with trace element and major element data. We attribute this either to progressive hybridization of the lower crust by repeated injection of mantle magmas, or the progressive thinning of the continental crust during prolonged arc magmatism. The present data do not allow distinction between the two models. Progressive decrease in crustal contribution to arc magmas with time may be an important feature of continental arc evolution. Hybridization of the lower crust due to repeated injection of mantle melts during arc magmatism may help contribute to small-scale heterogeneities in lower crust inferred from seismic and xenolith data. Similarly, whether there is a well defined MOHO or sharp crust-mantle boundary in any given segment of the continental crust may in part depend on the extent of crust modification as a result of continental arc magmatism.  相似文献   

A 29Si MAS NMR study of sub-Tg amorphization of stishovite at ambient pressure     

Xianyu Xue  Jonathan F. Stebbins  Masami Kanzaki 《Physics and Chemistry of Minerals》1993,19(7):480-485

Stishovite, a high-pressure SiO₂ polymorph in which each Si is coordinated by six O atoms, transforms to an amorphous phase when undergoing heat treatment below the glass transition temperature at ambient pressure. We have applied ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) to study this amorphization process. We found that the amorphous phase generated after heating stishovite for up to 3 days at around 600 ° C consisted exclusively of four-coordinate Si, similar to glasses quenched from melts at ambient pressure. Furthermore, our data suggest that there are subtle structural differences between the amorphous phase transformed from stishovite at 600 ° C and glasses quenched from melts at ambient pressure: the amorphous phase from stishovite had a smaller mean Si-O-Si angle initially, and it gradually relaxed toward the latter with increasing heating time. There was no detectable change in the stishovite structure even after about 80% of it had been converted to the amorphous phase. The mechanism of the amorphization of stishovite is discussed in light of these results.  相似文献   

Nonpoint source pollution control effectiveness of riparian forests along a coastal plain river   总被引：5，自引：0，他引：5  

Jonathan D. Phillips 《Journal of Hydrology》1989,110(3-4):221-237

A detention-time model of water quality buffer zones is used to evaluate the nonpoint source pollution control effectiveness of riparian forests in a two-county area of the lower Tar River basin, North Carolina. Soil map units, which represent specific combinations of soil, topography, and vegetation characteristics, are compared in terms of their relative ability to filter nitrate in agricultural runoff. All typical riparian forests provide significant water quality protection, but there is a wide variation in buffer effectiveness. This suggests a need for flexibility in determining buffer widths. A range of 15–80 m is appropriate for the soil-landform-vegetation complexes found in riparian zones within the study area. Buffer widths of 60 m — and often much less — are generally adequate on the soils likely to be used for agricultural production.  相似文献   

Magic angle spinning NMR observation of sodium site exchange in nepheline at 500° C     

Jonathan F. Stebbins  Ian Farnan  Evan H. Williams  Jacques Roux 《Physics and Chemistry of Minerals》1989,16(8):763-766

Dynamics in minerals at time scales from seconds to microseconds are important in understanding mechanisms of displacive phase transitions, diffusion, and conductivity. High resolution, magic-angle-spinning (MAS) NMR spectroscopy can directly show the rates of exchange among sites, potentially providing less model-dependent information than more traditional NMR relaxation time measurements. Here we use a newly developed high temperature MAS probe (Doty Scientific, Inc.) to observe the exchange of Na⁺ among the alkali sites in a high-Na nepheline at temperatures as high as 500° C. Observed exchange rates are consistent with correlation times derived from cation diffusivity.  相似文献   

Trends in UK extreme sea-levels: a spatial approach     

Mark J. Dixon  Jonathan A. Tawn 《Geophysical Journal International》1992,111(3):607-616

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Book review     

David Gubbins  Emile A. Okal  Roberto Scarpa  R. E. Anderson  Leszek Czechowski  Rodney W. Page  Bernhard Spörli  Magdalena Kądziałko-Hofmokl  J. A. Majorowicz  Gregory Houseman  Donald L. Turcotte  David F. McTigue  Teng-fong Wong  David Lockner  David V. Wiltschko  David M. Fountain  G. P. Glasby  R. J. Angel  S. J. Gibowicz  Jonathan Franzen  Kacper R. Rybicki  Brian Farrell  Anil Suri  Zbigniew W. Kundzewicz  Witold G. Strupczewski 《Pure and Applied Geophysics》1988,127(4):695-752

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THREE-DIMENSIONAL, CROSS-SEMIVARIOGRAM CALCULATIONS FOR HYDROGEOLOGICAL DATA     

Jonathan D. Lstok  Richard M. Cooper  Alan L. Flint 《Ground water》1988,26(5):638-646

Abstract. Geostatistics is a powerful tool for the analysis of hydrogeological data, but few well-documented computer programs for performing the necessary calculations have been presented in the technical literature. This is especially true for applications that require either three-dimensional or multivariate analyses. This paper describes a FORTRAN subroutine, VARIO, that can be used to compute experimental direct- and cross-semivariograms from a set of sample data, for any specified direction in one-, two-, or three-dimensional space. The subroutine combines into groups those sample pairs that fall within predetermined angular tolerances of the specified direction. The number of sample pairs used to compute the value of the experimental semivariogram at each value of separation can be specified in four different ways, depending on the nature of the available data. Written in FORTRAN 77, VARIO can be used on any computer that supports a FORTRAN 77 compiler. Source code listing, user instructions, and example input and output data for VARIO are presented.  相似文献   

A predictive model for rare earth element partitioning between clinopyroxene and anhydrous silicate melt   总被引：35，自引：17，他引：18  

Bernard J. Wood  Jonathan D. Blundy 《Contributions to Mineralogy and Petrology》1997,129(2-3):166-181

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Hydrocarbon source potential of the Proterozoic Sirban Limestone Formation,NW Himalaya,Jammu     

Naveen Hakhoo  Bindra Thusu  Devleena Mani  Ghulam M. Bhat  Jonathan Craig  Juergen Thurow  Mateen Hafiz  Sudeep Kanungo  Sumita Koul  Waquar Ahmed 《Journal of the Geological Society of India》2016,88(6):685-692

The Proterozoic Sirban Limestone Formation (SLFm) crops out as detached allochthons in the northwest Himalaya (Jammu region, India) and has its coeval equivalents laterally disposed in the west in Salt Range, in the northwest in Abbotabad (Pakistan) and in southeast in Himachal Pradesh (India). The oil and gas occurrences have been reported from the Proterozoic successions globally and the hydrocarbon potential of the SLFm cannot be ruled out.The interbedded shales and algal laminated dolostones within the SLFm have yielded microflora comparable to those reported in the North African Neoproterozoic sandstones and the Late Proterozoic carbonates of the giant oil and gas fields of the Siberian Platform. The SLFm contains a rich and diverse biota comprising ~ 10% of the rock volume in thin section. The rich organic assemblage justified a hydrocarbon source potential analysis of the SLFm, tested in this study by Rock Eval (RE) pyrolysis.RE pyrolysis yielded a total organic carbon (TOC) content of 0.02 to 1 wt. % with very low Hydrogen Index (HI) values for the shales and TOC content averaging 0.02 wt. % for the dolostones. The organically lean shales and dolostones exhibit T_max values indicative of immature to post mature stage. But, since these values are for the samples with complex thermal and tectonic history the results may be unreliable. The highly altered organic matter and kerogen present in the SLFm had the potential to generate hydrocarbons and presently indicates no significant source potential. This study is important for understanding the hydrocarbon occurrences in the SLFm particularly in light of the recent oil and gas discoveries from the coeval Proterozoic successions.  相似文献