Tilting separation flows: a mechanism for intense vertical mixing in the coastal ocean     

D. Farmer  R. Pawlowicz  R. Jiang   《Dynamics of Atmospheres and Oceans》2002,36(1-3)

Observations of a front associated with boundary layer separation from a headland illustrate a mechanism by which horizontal density gradients create intense turbulence and vertical mixing, thus, contributing to water property modification in the coastal zone. Tidal current past an island separates from the coast, creating a shear zone between the primary flow and the slowly moving water in the lee of the island. The density structure on either side of the front may differ due to different origins or degrees of prior mixing. Consequently, there can be horizontal density gradients across the front. Boundary layer separation from the headland begins as a vertical vortex sheet on which instabilities grow to form a sequence of eddies. The presence of horizontal density gradients causes the shear layer to tilt. Tilting and stretching of the sheared flow generates intense circulation. Whirlpools and boils appear at the surface accompanied by vertical motions in which broad areas of upwelling alternate with narrow areas of downwelling. These mix the water throughout its depth; bubbles entrained at the surface reach depths of over 120 m. Such violent mixing weakens stratification associated with the estuarine circulation and aerates water masses passing through the area.  相似文献   

Provenance and diagenesis of the evaporite-bearing Burns formation, Meridiani Planum, Mars     

S.M. McLennan  J.F. Bell III  P.R. Christensen  P.A. de Souza  J. Farmer  D.A. Fike  A. Ghosh  J.P. Grotzinger  K.E. Herkenhoff  J.R. Johnson  B. Jolliff  A.H. Knoll  M.C. Malin  J. Pocock  L.A. Soderblom  N.J. Tosca  M.B. Wyatt 《Earth and Planetary Science Letters》2005,240(1):95-121

Impure reworked evaporitic sandstones, preserved on Meridiani Planum, Mars, are mixtures of roughly equal amounts of altered siliciclastic debris, of basaltic provenance (40 ± 10% by mass), and chemical constituents, dominated by evaporitic minerals (jarosite, Mg-, Ca-sulfates ± chlorides ± Fe-, Na-sulfates), hematite and possibly secondary silica (60 ± 10%). These chemical constituents and their relative abundances are not an equilibrium evaporite assemblage and to a substantial degree have been reworked by aeolian and subaqueous transport. Ultimately they formed by evaporation of acidic waters derived from interaction with olivine-bearing basalts and subsequent diagenetic alteration. The rocks experienced an extended diagenetic history, with at least two and up to four distinct episodes of cementation, including stratigraphically restricted zones of recrystallization and secondary porosity, non-randomly distributed, highly spherical millimeter-scale hematitic concretions, millimeter-scale crystal molds, interpreted to have resulted from dissolution of a highly soluble evaporite mineral, elongate to sheet-like vugs and evidence for minor synsedimentary deformation (convolute and contorted bedding, possible teepee structures or salt ridge features). Other features that may be diagenetic, but more likely are associated with relatively recent meteorite impact, are meter-scale fracture patterns, veins and polygonal fractures on rock surfaces that cut across bedding. Crystallization of minerals that originally filled the molds, early cement and sediment deformation occurred syndepositionally or during early diagenesis. All other diagenetic features are consistent with formation during later diagenesis in the phreatic (fluid saturated) zone or capillary fringe of a groundwater table under near isotropic hydrological conditions such as those expected during periodic groundwater recharge. Textural evidence suggests that rapidly formed hematitic concretions post-date the primary mineral now represented by crystal molds and early pore-filling cements but pre-date secondary moldic and vug porosity. The second generation of cements followed formation of secondary porosity. This paragenetic sequence is consistent with an extended history of syndepositional through post-depositional diagenesis in the presence of a slowly fluctuating, chemically evolving, but persistently high ionic strength groundwater system.  相似文献   

Influences on the extent and record of heavy metal pollution in sediment cores from Loch Tay in a mineralised area of Scotland     

J.G. Farmer  A.B. MacKenzie  L.J. Eades  A. Kirika  A.E. Bailey-Watts 《Journal of Geochemical Exploration》1997,58(2-3)

The distribution and Inventories of Pb in the sediments of Loch Tay have been strongly influenced by inputs from past Pb mining activity, as demonstrated by correlation with the characteristic ²⁰⁶Pb/²⁰⁷Ph signature of the Tyndrum ore deposit. A major depositional event, perhaps associated with the cessation of Pb mining in the area, has been dated at 1906–1928 by the ²¹⁰Pb method. Zinc and Cu have been influenced to a lesser extent by mining, as shown by comparison with inventories in Loch Lomond. Profiles of As, Mn and Fe have been affected by post-depositional diagenetic remobilisation processes. The results serve as a benchmark, with Au mining set to commence at Cononish, near Tyndrum, in 1996.  相似文献   

Column abundance and the long-term trend of hydrogen chloride (HCl) above the Jungfraujoch Station     

R. Zander  G. Roland  L. Delbouille  A. Sauval  C. B. Farmer  R. H. Norton 《Journal of Atmospheric Chemistry》1987,5(4):395-404

The integrated column amount of hydrogen chloride has been monitored above the International Scientific Station of the Jungfraujoch (Switzerland) during the last 8 years. The results deduced from solar absorption measurements near 3.42 m indicate a secular trend equivalent to (0.75±0.2) % increase per year since 1978, superimposed on a significant short-term variability which can be partly attributed to the tropospheric component of the total HCl burden. Based on an intensified set of measurements carried out over the last three years, a seasonal component in the total content of HCl has been established for the first time, showing a minimum occuring in early winter and a maximum during the spring.  相似文献   

A physical model of an underground coal mine prototype     

T. N. Singh  I. W. Farmer 《Geotechnical and Geological Engineering》1985,3(4):319-326

Conclusions A physical model simulated qualitative ground movement above a retreating longwall face in a shallow coal mine with considerable accuracy, after modification to allow for scale effects. It is considered that in prototype geometries such as mines where strata deformation involves rock fracture, the use of physical models may have advantages over other types of model.  相似文献   

Design constraints for full-face tunnelling machines in coal mines     

I. W. Farmer  N. H. Glossop 《Geotechnical and Geological Engineering》1983,1(1):57-70

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Late spring ultraviolet levels over the United Kingdom and the link to ozone     

J. Austin  C. M. H. Driscoll  S. F. G. Farmer  M. J. Molyneux 《Annales Geophysicae》1999,17(9):1199-1209

Erythemally-weighted ultraviolet (UV_ery) levels measured over southern England, during anticyclonic weather between 30 April and 2 May, 1997, were almost 50 higher than normally expected for clear skies and were similar to mid-summer values for the first time since measurements began in 1990. Investigation of this episode suggests that a combination of both meteorological and chemical effects were responsible for generating record low ozone amounts for the time of year. Further, comparisons between the A band ultraviolet (315 to 400 nm wavelength) amounts, and radiative calculations confirm that the high UV_ery was primarily due to the reduction in total ozone. These results are contrasted with a similar period for 1998, in which near climatological ozone amounts were measured. The prospects for enhanced UV_ery levels in future years are briefly reviewed in the light of expected increases in stratospheric halogen levels and greenhouse gases.  相似文献   

Spoke formation under moving plasma clouds     

Alison J. Farmer  Peter Goldreich 《Icarus》2005,179(2):535-538

Goertz and Morfill [Goertz, C.K., Morfill, G., 1988. Icarus 53, 219-229] propose that spokes on Saturn's rings form under radially moving plasma clouds produced by meteoroid impacts. We demonstrate that the speed at which a plasma cloud can move relative to the ring material is bounded from above by the difference between the Keplerian and corotation velocities. The radial orientation of new spokes requires radial speeds that are at least an order of magnitude larger than this upper limit, thus the model advanced by Goertz and Morfill fails to make radial spokes.  相似文献   

Self-similar force-free wind from an accretion disc     

Ramesh Narayan  Jonathan C. McKinney  Alison J. Farmer 《Monthly notices of the Royal Astronomical Society》2007,375(2):548-566

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Book reviews     

Ian Farmer  I. W. Farmer 《Geotechnical and Geological Engineering》1988,6(2):177-180

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