Extreme subsidence in the Agulhas-Benguela air mass transition     

Mark Jury  Chris Reason 《Boundary-Layer Meteorology》1989,46(1-2):35-51

Analysis of three-dimensional wind profiles recorded by an acoustic sounder near Cape Town has indicated that extreme subsidence (-35 cm s^-1) is a mean feature throughout the atmospheric boundary layer (50–1000m) during summertime southerly winds. Over the SW Cape coast, the atmospheric subsidence translates into a N-S gradient of the mean summer water deficit (-20 to -32 cm month^-1). The rapid drying out of the air mass along a northward trajectory is linked to a number of factors including synoptic-scale divergence of the surface wind and the effects of the local orography which produce a hydraulic jump of the southerly wind. The along-coast reduction in sea surface temperature provides a major constraint on the height of the moist marine layer. As the depth of the marine air mass shrinks, its potential for inland penetration becomes limited. In addition, dry air is entrained towards the surface as evidenced by aerial survey data. A model is formulated which indicates the importance of the surface heat fluxes in reducing the depth of the Agulhas air mass as it passes northward over the SW tip of Africa during summer.  相似文献   

Market performance of agricultural commodities in semi-arid South Kordofan, Sudan     

Mark Speech 《Geoforum》1989,20(4)

Research conducted by the Western Sudan Agricultural Research Project allows examination of the impact of the recent African drought upon agricultural market performance and structure. Price volatility, as well as high prices, was a major destabilizing factor in agricultural commodity markets. Small farmers and petty traders are largely risk avoiders rather than profit maximizers, and tend not to move product when faced with price uncertainty. At the same time, responsibility for marketing functions shifts to these small operators and producers. These are the channel members least able to finance increased functional responsibility. Again, the result is that products may be held off of the market. Policy makers must recognize these marketing factors when devising strategies appropriate to semi-arid regions such as South Kordofan. Policy recommendations can be offered which can facilitate the movement of product, and thus greatly ease the marketing constraints associated with drought.  相似文献   

Adjustments to calculated Quaternary displacement rates across the Fish Springs fault, Owens Valley Fault Zone, California     

Stephen J. Martel  T. Mark Harrison  Alan Gillespie 《Quaternary Research》1989,32(3)

  相似文献   

Book review     

Brian J. Mitchell  Stanisław Rudowski  Jeanne Sauber  Mark S. T. Bukowinski  Ronald Prinn 《Pure and Applied Geophysics》1985,123(3):487-498

  相似文献   

Characterization of Dye Tracer Plumes: In Situ Field Experiments     

Thomas G. Naymik  Mark E. Sievers 《Ground water》1985,23(6):746-752

  相似文献   

Chemical mass transfer in magmatic processes     

Mark S. Ghiorso  Ian S. E. Carmichael 《Contributions to Mineralogy and Petrology》1985,90(2-3):107-120

  相似文献   

Diffusion of Sm,Sr, and Pb in fluorapatite     

E.Bruce Watson  T.Mark Harrison  F.James Ryerson 《Geochimica et cosmochimica acta》1985,49(8):1813-1823

Diffusion coefficients for Sm, Sr, and Pb in fluorapatite at 900°–1250°C were obtained by measuring experimentally-induced diffusional uptake profiles of these elements in the margins of gem-quality apatite crystals. The crystals were immersed in synthetic melts enriched in the trace elements of interest and presaturated in apatite, and the resulting diffusion gradients were characterized by electron microprobe analysis. Except in the case of Pb, the diffusivities define good Arrhenius lines for the respective elements: D_Sm = 2.3 × 10^?6exp(?52,200/RT) D_Sr = 412 exp(?100,000/RT). (Diffusion perpendicular to and parallel to c is measurably different in the case of Sr; the Arrhenius equation given above is an average for the two directions). Results on Pb were erratic, probably because extremely Pb-rich melts were used for some of the experiments. Data believed to be reliable define the following Arrhenius line: D_Pb = 0.035 exp(?70,000/RT). Constraints based on closure of natural apatites with respect to Pb suggest that the experimental data can be extrapolated, with sizeable uncertainty, to temperatures as low as 550°C.When applied to the question of isotopic and trace-element equilibration of residual or entrained apatites with crustal melts, the measured diffusivities indicate that 0.05-cm crystals will rarely preserve the original Pb-isotope characteristics of the source; the same is not true, however, of Sr (and, under some conditions, the REE), which may be unaffected at crystal cores during typical melting events.  相似文献   

Copper dissociation from estuarine humic materials     

Dean L. Olson  Mark S. Shuman 《Geochimica et cosmochimica acta》1985,49(6):1371-1375

Dissolved humic material from three locations on the Ogeechee River Estuary near Savannah, GA, was ultrafiltered into three size fractions and used for kinetic experiments with Cu(II). A Cu(II)-humic mixture was reacted with a colorimetric reagent for Cu(II) and absorbance observed from 50 msec to at least 1835 sec corresponding to rate constants from 0.001–40 sec^?1. The apparent dissociation rate constants were distributed over a wide range, with most bound Cu(II) having k > 1 sec^?1 ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{< 0.7 sec}$). Nearly all the variation seen in the kinetic distribution was among size fractions; as size fraction decreased, the distribution of bound Cu(II) shifted to larger rate constants. Location of sampling stations on the estuary had little effect on results.  相似文献   

Use-wear analysis of flaked stone tools: by P.C. Vaughan, University of Arizona Press, Arizona, 1985, 0-8165-0861-5, £49.50, 104 pp.     

Mark Newcomer 《Quaternary Science Reviews》1985,4(4):xviii-xx

  相似文献   

Helium isotopic variability within single diamonds from the Orapa kimberlite pipe     

Mark D. Kurz  John J. Gurney  William J. Jenkins  Dempsey E. Lott III 《Earth and Planetary Science Letters》1987,86(1)

The distribution and isotopic composition of helium has been measured in a suite of well-characterized one-carat diamonds from the Orapa kimberlite, Botswana. Crushing of the diamonds in vacuo indicates that most of the helium is contained by the matrix (generally greater than 90%), rather than by the inclusions. Step-heating experiments, performed on inclusion-free fragments remaining after crushing, indicate that the³He/⁴He ratio is variablewithin individual diamonds. The fragments, as small as 10 mg, were heated in two timed steps, both at 2000°C. In every case, lower³He/⁴He ratios are observed in the first graphitization step (0.05–3 × atmospheric), while the last heating step releases helium with systematically higher³He/⁴He ratio (30–80 × atmospheric). We suggest that this internal isotopic variability is the result of stepwise graphitization: the first heating step initiates graphitization, which nucleates around defects, and the second heating step graphitizes the relatively defect-free regions of the diamond. The³He/⁴He ratio measured, using the partial graphitization technique, differs by up to a factor of 100 within a single specimen. The inclusion-free fragments release small quantities of helium below 2000°C, which suggests that helium release is obtained only by graphitization. The³He contents of the monocrystalline diamonds are relatively constant (at 3 × 10⁻¹³ cm³ STP/gram) and indicate that most of the isotopic variability is due to radiogenic⁴He. The variations in⁴He content are either related to zoning of Th and U in the diamonds (i.e., in-situ decay), to zoning of inherited⁴He, or to implantation of α-particles from a Th and U rich environment (i.e., kimberlite). Because the Orapa diamonds were mined from roughly 40 m depth in the kimberlite, spallation reactions from cosmic ray interactions are not a significant source of³He. However, calculations based on the age of the kimberlite (90 m.y.) and reasonable Th and U abundances suggest that most of the³He in the Orapa diamonds could be produced by⁶Li(n, α)T in the diamond. Although this may not be true of all diamonds, nuclear reactions in the crust and mantle (including spallation reactions at the surface) can explain many of the high³He/⁴He ratios previously reported for diamonds.  相似文献