Wyville Thomson Ridge Overflow Water: Spatial and temporal distribution in the Rockall Trough     

Clare Johnson  Toby Sherwin  Denise Smythe-Wright  Tracy Shimmield  William Turrell 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(10):1153-1162

Wyville Thomson Ridge Overflow Water (WTOW), which is the only part of the outflow from the Norwegian Sea not to directly enter the Iceland Basin, is shown to be a significant water mass in the northern Rockall Trough. It is found primarily at intermediate depths (600–1200 m) beneath the northward flowing warm Atlantic waters, and above recirculating Mediterranean influenced waters and Labrador Sea Water (LSW). The bottom of the WTOW layer can be identified by a mid-depth inflexion point in potential temperature–salinity plots. An analysis of historical data reveals that WTOW has been present in all but eight of the last 31 years at 57.5°N in the Rockall Trough. A denser component of WTOW below 1500 m has also been present, although it appears to be less persistent (12 out of the 31 years) and limited to the west of the section. The signature of intermediate WTOW was absent in two periods, the mid-1980s and early 1990s, both of which coincided with a freshening, and probable increase in volume, of LSW in the trough. Potential temperature–salinity diagrams from historical observations indicate that WTOW persists at least as far south as 55°N (and as far west as 20°W in the Iceland Basin) although its signature is quickly lost on leaving the Rockall Trough. We suggest that a transport of WTOW down the western side of the trough exists, with WTOW at intermediate depths entering the eastern trough either via a cyclonic recirculation, or as a result of eddy activity. Further, WTOW is seen on the Rockall–Hatton Plateau and in the deep channels connecting with the Iceland Basin, suggesting additional possible WTOW transport pathways. These suggested transport routes remain to be confirmed by further observational or modelling studies.  相似文献   

The rheology of olivine and spinel magnesium germanate (Mg2GeO4): TEM study of the defect microstructures     

Catherine Dupas-Bruzek  Tracy N. Tingle  Harry W. Green II.  Nicole Doukhan  Jean-Claude Doukhan 《Physics and Chemistry of Minerals》1998,25(7):501-514

Synthetic polycrystals of α-Mg₂GeO₄ (with the olivine structure) and γ-Mg₂GeO₄ (with the spinel structure) deformed at high temperature and pressure in their respective stability fields were investigated by analytical transmission electron microscopy. Specimens with a mean grain size of 20–30 µm deform by dislocation glide and/or climb. The predominance of glide versus climb depends on stress and grain orientation. The defect microstructures of both polymorphs are very similar to those observed in their respective silicate analogues, α- and γ-(Mg,Fe)₂SiO₄, and, in the case of the spinel phase, very similar to those observed in magnesium aluminate spinels. These observations suggest that Mg₂GeO₄ is a good rheological analogue for the Earth’s upper mantle. A spinel specimen deformed under the same conditions of temperature and strain rate as an olivine specimen was approximately three times stronger than olivine. In specimens of both phases deformed at or above 1400 K, a thin amorphous film composed of Mg, Ge, and O was detected along some grain boundaries. Grains ≤10 µm diameter surrounded by a film of amorphous phase (>10 nm thick) exhibited low dislocation densities, and deformation appeared to have occurred by grain boundary sliding.  相似文献   

Theoretical plasma distributions consistent with Ulysses magnetic field observations in a solar wind tangential discontinuity     

J. De Keyser  M. Roth  J. Lemaire  B. T. Tsurutani  C. M. Ho  C. M. Hammond 《Solar physics》1996,166(2):415-422

The overall multi-layer structure of the magnetic field profile observed by Ulysses across a broad solar wind tangential discontinuity can be reproduced fairly well by means of a kinetic model. Such a simulation provides complementary information about the velocity distribution functions, which are not always available from the plasma experiment due to the low time resolution inherent in plasma measurements. The success of such a simulation proves that the kinetic model can be used as a realistic basis for further studies of the structure and stability of solar wind tangential discontinuities.  相似文献   

Pervasive aqueous paleoflow features in the Aeolis/Zephyria Plana region, Mars   总被引：1，自引：0，他引：1  

Devon M. Burr  Marie-Therese Enga  James R. Zimbelman  Tracy A. Brennand 《Icarus》2009,200(1):52-76

A survey of THEMIS visible wavelength images in the Aeolis/Zephyria Plana region over the two western lobes of the equatorial Medusae Fossae Formation (MFF) shows ∼150 sinuous ridges having a variety of morphologies and contexts. To systematize investigation, we use a classification scheme including both individual ridge and ridge network types, as well as associations with impact craters and fan-shaped features. The morphology of the ridges, their location downslope from higher topography (e.g., crater rims and scarps), and their association with fan-shaped forms indicate that most sinuous ridges formed through overland aqueous flow. Analysis of observations by individual ridge type leads to interpretation of most of these sinuous ridges as inverted fluvial channels or floodplains and a few as possible eskers, with the origin of the remaining ridges under continuing investigation. About 15% of the sinuous ridges are associated with impact craters, but data analysis does not support a genetic relationship between the craters and the sinuous ridges. Instead, analysis of one sinuous ridge network associated with a crater indicates that the water source for the network was atmospheric in origin, namely, precipitation runoff. The broad areal distribution of these ∼150 ridges and the network morphologies, in particular the branched and subparallel types, suggest that an atmospheric water source is generally applicable to the population of sinuous ridges as a whole. This concentration of sinuous ridges is the largest single population of such landforms on Mars and among the youngest. These ridges are situated at a paleoscarp between Cerberus Palus and the Aeolis highlands, suggesting that the precipitation that formed them was orographic in origin. The ages of the equatorial MFF units in which this population of sinuous ridges is found imply that this orographic rain and/or snow fell during some period from the late Hesperian through the middle Amazonian.  相似文献   

Partial melting of H6 ordinary chondrite Kernouvé: Constraints on the effects of reducing conditions on oxidized compositions     

Rena L. FORD  Gretchen K. BENEDIX  Timothy J. McCOY  Tracy RUSHMER 《Meteoritics & planetary science》2008,43(8):1399-1414

Abstract— Partial melting experiments at temperatures of 950–1300 °C were conducted on the H6 chondrite Kernouvé under reducing conditions using CO‐CO₂ gas mixing and graphite‐buffered sealed silica tubes to examine the effect of reducing conditions during melting of starting materials that are more oxidized relative to the oxygen fugacity conditions of the experiments. The experiments produced a range of mineralogical and compositional changes. Olivine exhibits significant reduction to compositions of Fa_2–5 at temperatures of 1300 °C. In contrast, orthopyroxene exhibits only slight reduction until the highest temperatures. Chromite is sometimes consumed by intruding sulfides, and displays increasingly magnesian compositions ranging as low as Fe/Fe + Mg of 0.1 at a constant Cr/Cr + Al ratio. The compositional changes with increasing temperature reflect a complex set of reactions, including oxidation‐reduction. One application of these experiments address whether primitive achondrites could have formed from ordinary chondrite‐like precursors by partial melting under reducing conditions. While changes observed in olivine and troilite compositions might support such an idea, differences in oxygen isotopic composition, Cr/Cr + Al in chromite, orthopyroxene compositions, and thermodynamic evidence against reduction during melting of primitive achondrites (Benedix et al. 2005) firmly refute such an idea.  相似文献   

Carbonate stromatolites from a Messinian hypersaline setting in the Caltanissetta Basin, Sicily: petrographic evidence of microbial activity and related stable isotope and rare earth element signatures     

ELVIRA OLIVERI  RODOLFO NERI  ADRIANA BELLANCA  ROBERT RIDING†  Associate Editor: Tracy Frank 《Sedimentology》2010,57(1):142-161

Lower Messinian stromatolites of the Calcare di Base Formation at Sutera in Sicily record periods of low sea‐level, strong evaporation and elevated salinity, thought to be associated with the onset of the Messinian Salinity Crisis. Overlying aragonitic limestones were precipitated in normal to slightly evaporative conditions, occasionally influenced by an influx of meteoric water. Evidence of bacterial involvement in carbonate formation is recorded in three dolomite‐rich stromatolite beds in the lower portion of the section that contain low domes with irregular crinkly millimetre‐scale lamination and small fenestrae. The dominant microfabrics are: (i) peloidal and clotted dolomicrite with calcite‐filled fenestrae; (ii) dolomicrite with bacterium‐like filaments and pores partially filled by calcite or black amorphous matter; and (iii) micrite in which fenestrae alternate with dark thin wispy micrite. The filaments resemble Beggiatoa‐like sulphur bacteria. Under scanning electron microscopy, the filaments consist of spherical aggregates of dolomite, interpreted to result from calcification of bacterial microcolonies. The dolomite crystals are commonly arranged as rounded grains that appear to be incorporated or absorbed into developing crystal faces. Biofilm‐like remains occur in voids between the filaments. The dolomite consistently shows negative δ¹³C values (down to ?11·3‰) and very positive δ¹⁸O (mean value 7·9‰) that suggest formation as primary precipitate with a substantial contribution of organic CO₂. Very negative δ¹³C values (down to ?31·6‰) of early diagenetic calcite associated with the dolomite suggest contribution of CO₂ originating by anaerobic methane oxidation. The shale‐normalized rare earth element patterns of Sutera stromatolites show features similar to those in present‐day microbial mats with enrichment in light rare earth elements, and M‐type tetrad effects (enrichment around Pr coupled to a decline around Nd and a peak around Sm and Eu). Taken together, the petrography and geochemistry of the Sutera stromatolites provide diverse and compelling evidence for microbial influence on carbonate precipitation.  相似文献   

The emplacement of pahoehoe toes: field observations and comparison to laboratory simulations     

Tracy?K.?P.?GreggEmail author  Laszlo?P.?Keszthelyi 《Bulletin of Volcanology》2004,66(5):381-391

We observed active pahoehoe lobes erupted on Kilauea during May-June 1996, and found a range of emplacement styles associated with variations in local effusion rate, flow velocity, and strain rate. These emplacement styles were documented and quantified for comparison with earlier laboratory experiments.At the lowest effusion rates, velocities, and strain rates, smooth-surfaced lobes were emplaced via swelling, where new crust formed along an incandescent lip at the front of the lobe and the rest of the lobe was covered with a dark crust. At higher effusion rates, strain rates and velocities, lobes were emplaced through tearing or cracking. Tearing was characterized by ripping of the ductile crust near the initial breakout point, and most of the lobe surface was incandescent during its emplacement. This mechanism was observed to generate both smooth-surfaced lobes, and, when the lava encountered an obstacle, folded lobes. Cracking lobes were similar to those emplaced via tearing, but involved breaking of a thicker, brittle crust at the initial breakout of the lobe and therefore required somewhat higher flow rates than did tearing. Cracking lobes typically formed ropy folds in the center of the lobe, and smooth margins. At the highest effusion rates, strain rates, and flow velocities, the lava formed open channels with distinct levees.The final lobe morphologies were compared to results from laboratory simulations, which were designed to infer effusion rate from final flow morphology, to quantitatively test the laboratory results on the scale of individual natural pahoehoe lobes. There is general agreement between results from laboratory simulations and natural lavas on the scale of individual pahoehoe lobes, but there are disparities between laboratory flows and lava flows on the scale of an entire pahoehoe lava flow field.Editorial responsibility: A. Woods  相似文献   

Archaean lode gold mineralisation in banded iron formation at the Kalahari Goldridge deposit, Kraaipan Greenstone Belt, South Africa     

Napoleon Q. Hammond  John M. Moore 《Mineralium Deposita》2006,41(5):483-503

The Kalahari Goldridge Mine is located within the Archaean Kraaipan Greenstone Belt, about 60 km southwest of Mafikeng in the North West Province, South Africa. The ore body thickness varies from 15 to 45 m along a strike length of about 1.5 km within approximately N–S striking banded iron formation (BIF). The stratabound ore body is hosted primarily by BIF, which consists of alternating chert and magnetite–chlorite–stilpnomelane–sulphide–carbonate bands of millimetre- to centimetre scale. A footwall of sericite–carbonate–chlorite schist underlain by mafic amphibolite occurs to the west and carbonaceous metapelites in the hanging wall to the east. Overlying the hanging wall, carbonaceous metapelites, units of coarse-grained metagreywackes fining upwards, become increasingly conglomeratic up the stratigraphy. Small-scale isoclinal folds, brecciation, extension fractures and boudinage of cherty BIF units reflect brittle-ductile deformation. Fold axial planes have foliation, with subvertical plunges parallel to prominent rodding and mineral lineation in the footwall rocks. Gold mineralisation is associated with two generations of quartz–carbonate veins, dipping approximately 20° to 40° W. The first generation consists of ladder-vein sets (group IIA) preferentially developed in centimetre-scale Fe-rich mesobands, whereas the second generation consists of large quartz–carbonate veins (group IIB), which locally crosscut the entire ore body and extend into the footwall and hanging wall. The ore body is controlled by mesoscale isoclinal folds approximately 67° E, orthogonal to the plane of mineralised, gently dipping veins, defining the principal stretching direction and development of fluid-focussing conduits. The intersections of the mineralised veins and foliation planes of the host rock plunges approximately 08° to the north. Pervasive hydrothermal alteration is characterised by chloritisation, carbonatisation, sulphidation and K-metasomatism. Gold is closely associated with sulphides, mainly pyrite and pyrrhotite, and to a lesser extent, with bismuth tellurides and carbonate minerals. Mass balance transfer calculations indicate that hydrothermal alteration of BIF involved enrichment of Au, Ag, Bi, Te, S and CO₂ (LOI), MgO, Ba, K and Rb, but significant depletion of SiO₂ and, to a lesser extent, Fe₂O₃. Extensive replacement of magnetite and chlorite in BIF and other pelitic sedimentary rocks by sulphide and carbonate minerals, both on mesoscopic and microscopic scales, is evidence of interaction of CO₂- and H₂S-bearing fluids with the Fe-rich host rocks. The fineness of gold grains ranges from 823 to 921, similar to that of other epigenetic Archaean BIF-hosted gold deposits, worldwide.  相似文献   

Cell adhesion of <Emphasis Type="Italic">Shewanella oneidensis</Emphasis>to iron oxide minerals: Effect of different single crystal faces     

Andrew?L?NealEmail author  Tracy?L?Bank  Michael?F?HochellaJr  Kevin?M?Rosso 《Geochemical transactions》2005,6(4):77

The results of experiments designed to test the hypothesis that near-surface molecular structure of iron oxide minerals influences adhesion of dissimilatory iron reducing bacteria are presented. These experiments involved the measurement, using atomic force microscopy, of interaction forces generated between Shewanella oneidensis MR-1 cells and single crystal growth faces of iron oxide minerals. Significantly different adhesive force was measured between cells and the (001) face of hematite, and the (100) and (111) faces of magnetite. A role for electrostatic interactions is apparent. The trend in relative forces of adhesion generated at the mineral surfaces is in agreement with predicted ferric site densities published previously. These results suggest that near-surface structure does indeed influence initial cell attachment to iron oxide surfaces; whether this is mediated via specific cell surface-mineral surface interactions or by more general interfacial phenomena remains untested.  相似文献   

Influence Of Advection Differencing Error Upon Large-Eddy Simulation Accuracy     

John W. Glendening  Tracy Haack 《Boundary-Layer Meteorology》2001,98(1):127-153

Accurate prediction of turbulent features by alarge-eddy simulation (LES) model requires accuratenumerical evaluation of the nonlinear advective terms. In the presence of a mean wind, use of second-order orfourth-order finite differencing produces large lossesof turbulent energy at high wavenumbers, an inaccuracyconsistent with idealized numerical properties ofthese schemes. The discrete differencing inaccuraciesalso affect the small wavenumber ``large eddy'structure and the horizontally-averaged turbulentquantities. With a high accuracy pseudospectraltechnique, this energy loss does not occur andspectral behaviour corresponds to filtered Kolmogorovtheory predictions out to the de-aliasing cutoffwavenumber. The inaccuracy of the finite differenceschemes is greatly ameliorated through a Galileantransformation of the advective term.  相似文献