Barbados     

Stephen K. Donovan  & David A.T. Harper 《Geology Today》2009,25(4):151-158

In the words of the noted Antillean geologist C.T. Trechmann (1937 , p.337), 'Barbados, 21 miles long, 14 miles wide, rising to 1101 feet, is probably the most considerable Pleistocene non-volcanic bleb on the face of our planet, at least in the Antillean region.' The island's geology shows a stark contrast between the allochthonous siliciclastic succession of an accretionary prism, deposited in water depths measurable in kilometres, and the autochthonous Pleistocene raised reef cap exposed over 90 per cent of Barbados.  相似文献   

1973 supplement to exploration geochemistry bibliography     

H.E. Hawkes  F.C. Canney  P.R. Donovan  G.H. Friedrich  C.F. Gleeson  M.B. Mehrtens  P. Lowenstein 《Journal of Geochemical Exploration》1974,3(1):89-128

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The relative importance of tropical variability forced from the North Pacific through ocean pathways     

Amy Solomon  Sang-Ik Shin  Michael A. Alexander  Julian P. McCreary 《Climate Dynamics》2008,31(2-3):315-331

To what extent is tropical variability forced from the North Pacific through ocean pathways relative to locally generated variability and variability forced through the atmosphere? To address this question, in this study we use an anomaly-coupled model, consisting of a global, atmospheric general circulation model and a 4½-layer, reduced-gravity, Pacific-Ocean model. Three solutions are obtained; with coupling over the entire basin (CNT), with coupling confined to the tropics and wind stress and heat fluxes in the North and South Pacific specified by climatology (TP), and with coupling confined to the Tropics and wind stress and heat fluxes in the North Pacific specified by output from CNT (NPF). It is found that there are two distinct signals forced in the North Pacific that can impact the tropics through ocean pathways. These two signals are forced by wind stress and surface heat flux anomalies in the subtropical North Pacific. The first signal is relatively fast, impacts tropical variability less than a year after forcing, is triggered from November to March, and propagates as a first-mode baroclinic Rossby wave. The second signal is only triggered during springtime when buoyancy forcing can effectively generate higher-order baroclinic modes through subduction anomalies into the permanent thermocline, and it reaches the equator 4–5 years after forcing. The slow signal is found to initiate tropical variability more efficiently than the fast signal with one standard deviation in subtropical zonal wind stress forcing tropical SST anomalies centered on the equator at 135°W of approximately 0.5°C. Allowing extratropically forced tropical variability is found to shift primarily 2-year ENSO variability in a tropics-alone simulation to a more realistic range of 2–6 years.  相似文献   

Sub-micron scale distributions of trace elements in zircon   总被引：2，自引：1，他引：1  

Amy E. Hofmann  John W. Valley  E. Bruce Watson  Aaron J. Cavosie  John M. Eiler 《Contributions to Mineralogy and Petrology》2009,158(3):317-335

Sub-micron scale zoning of Ti concentrations and correlations between concentrations of Ti and other trace elements (P, Ce, and Y) and cathodoluminescent (CL) banding is observed in natural zircons. Ion images were made using the Caltech Microanalysis Center’s CAMECA NanoSIMS 50L with an O⁻ primary beam focused to ~300 nm on the sample surface. The high spatial resolution of this technique allows for interrogation of chemical variations at or below the scale of CL banding in natural zircons. Images produced in this manner display two types of correlations among Ti, P, Ce, and Y (which appears to be a proxy for CL intensity): strong (correlation coefficients >0.8) and subtle (correlation coefficients ~0.15–0.4). Strongly correlated images, which display Ti variations of ca. a factor of 3 between adjacent CL bands and overall elevated trace element concentrations in CL-dark bands, were found within an oscillatory-zoned, trace element enriched sector of a CL sector-zoned zircon. Three possible causes for such correlations include: temperature-dependent equilibrium partitioning, trace element partitioning limited by diffusion in the host melt and surface-controlled, non-equilibrium growth. Comparison of our data with the expected results of these processes suggests that: (1) Ti partitioning in zircon is dependent upon non-equilibrium effects in addition to temperature and/or (2) the incorporation of elements that co-vary with Ti in zircon (e.g., Y, P and Ce) is also temperature-dependent. Sub-micron scale, high-Ti regions are also found within Proterozoic Adirondack and >4 Ga Jack Hills zircons as well as trace element enrichments (including Ti) along cracks within Jack Hills zircons.  相似文献   

Aerial electromagnetic sounding of the lithosphere of Venus     

Robert E. Grimm  Amy C. Barr  Keith P. Harrison  David E. Stillman  Kerry L. Neal  Michael A. Vincent  Gregory T. Delory 《Icarus》2012,217(2):462-473

Electromagnetic (EM) investigation depths are larger on Venus than Earth due to the dearth of water in rocks, in spite of higher temperatures. Whistlers detected by Venus Express proved that lightning is present, so the Schumann resonances ～10–40 Hz may provide a global source of electromagnetic energy that penetrates ～10–100 km. Electrical conductivity will be sensitive at these depths to temperature structure and hence thermal lithospheric thickness. Using 1D analytic and 2D numerical models, we demonstrate that the Schumann resonances—transverse EM waves in the ground-ionosphere waveguide—remain sensitive at all altitudes to the properties of the boundaries. This is in marked contrast to other EM methods in which sensitivity to the ground falls off sharply with altitude. We develop a 1D analytical model for aerial EM sounding that treats the electrical properties of the subsurface (thermal gradient, water content, and presence of conductive crust) and ionosphere, and the effects of both random errors and biases that can influence the measurements. We initially consider specified 1D lithospheric thicknesses 100–500 km, but we turn to 2D convection models with Newtonian temperature-dependent viscosity to provide representative vertical and lateral temperature variations. We invert for the conductivity-depth structure and then temperature gradient. For a dry Venus, we find that the error on temperature gradient obtained from any single local measurement is ～100%—perhaps enough to distinguish “thick” vs. “thin” lithospheres. When averaging over thousands of kilometers, however, the standard deviation of the recovered thermal gradient is within the natural variability of the convection models, <25%. A “wet” interior (hundreds of ppm H₂O) limits EM sounding depths using the Schumann resonances to <20 km, and errors are too large to estimate lithospheric properties. A 30-km conductive crust has little influence on the dry-interior models because the Schumann penetration depths are significantly larger. We conclude that EM sounding of the interior of Venus is feasible from a 55-km high balloon. Lithospheric thickness can be measured if the upper-mantle water content is low. If H₂O at hundreds of ppm is present, the deeper, temperature-sensitive structure is screened, but the “wet” nature of the upper mantle, as well as structure of the upper crust, is revealed.  相似文献   

Ice grain size and the rheology of the martian polar deposits     

Amy C. Barr  Sarah M. Milkovich 《Icarus》2008,194(2):513-518

The history and dynamics of the martian polar deposits (MPD), the largest known water reservoirs on Mars, are of great interest, but estimates of ice grain size are required before detailed modeling can be performed. We clarify the microphysical processes that may control grain size in the MPD. If the MPD are ∼2% dust by mass, the maximum ice grain size is ∼1 mm due to grain boundary pinning by silicate microparticles. Relatively dusty layers in the MPD will have smaller grain sizes. If MPD ice has a very low impurity content and has experienced a significant amount of strain, grains may reach a steady state size of ∼1.5 to 3 mm due to dynamic recrystallization, wherein a steady state grain size is maintained due to the balance of grain growth and destruction during flow. If the near-bed ice in the MPD is warmed close to its melting point and has been extensively sheared, grain sizes at its base may be between 10 and 40 mm, by analogy with warm, dirty, near-bed ice in terrestrial ice sheets.  相似文献   

Notes on Mississippian echinoderms from Hurdlow,Derbyshire, central England     

Stephen K. Donovan  A.J. de Winter 《Proceedings of the Geologists' Association. Geologists' Association》2019,130(5):582-589

The fossil echinoderms of the Lower Carboniferous (Mississippian) of Derbyshire remain understudied, principally due to the nature of the preservation rather than any lack of biodiversity. Echinoids and crinoids are described in float blocks of limestone from near Hurdlow, Derbyshire, which have been etched naturally after many years of being washed by weakly acidic rain. Surface detail is variable, commonly poor, but rare specimens retain enough features for tentative identification. Two species of echinoids are identified from rare disarticulated plates, namely Melonechinus? sp. (ambulacral plates) and archaeocidarid sp. (interambulacral plate); more and superior material will be necessary to confirm this division. Crinoids include a cladid brachial ossicle sp. with a distinctive sculpture; a columnal of a monobathrid camerate platycrinitid sp.; Annulocolumnus (col.) sp. cf. A. annulus Donovan, a columnal morphotaxon with an unusually broad axial canal; and Cyclocyclicus (col.) spp. This is the tenth echinoid site to be recognized from the Mississippian of the White Peak. Examination of etched float blocks provides a further method of investigation of the echinoderm fauna of this and other limestone areas.  相似文献   

Breakpoint: Reckoning with America’s Environmental Crisis,Jeremy B.C. Jackson,Steve Chapple,Yale University Press,New Haven,Connecticut (2018). xx+296 pp., Hbk., ISBN: 978-0-300-17939-2. US$26.00     

Stephen K. Donovan 《Proceedings of the Geologists' Association. Geologists' Association》2019,130(6):794

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Human Evolution: A Very Short Introduction (second edition) Bernard Wood,Oxford University Press,Oxford (2019). xviii+140 pp., Pbk., ISBN: 978-0-19-883174-7. UK£8.99.     

Stephen K. Donovan 《Proceedings of the Geologists' Association. Geologists' Association》2019,130(6):795

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Wouter Südkamp,Life in the Devonian: Identification Book Hunsrück Slate Fossils, 2017, Verlag Dr. Friedrich Pfeil; Munich, 176 pp., Hbk., ISBN: 978-3-89937-221-2. €48.00.     

Stephen K. Donovan 《Proceedings of the Geologists' Association. Geologists' Association》2019,130(1):114

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