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排序方式: 共有67条查询结果,搜索用时 31 毫秒
1.
Ø. Hammer D.K. Dysthe H. Lund P. Meakin B. Jamtveit 《Geochimica et cosmochimica acta》2008,72(20):5009-5021
We present a 2D numerical model for the growth of calcite from supersaturated aqueous solutions under laminar, open-channel flow conditions. The model couples solution chemistry, precipitation at solution/calcite interfaces, hydrodynamics, diffusion and degassing. The model output is compared with experimental results obtained using an oversaturated calcite solution produced by mixing CaCl2 and Na2CO3. The precipitation rate is observed to increase when the supersaturated solution flows over an obstruction, leading to a growth instability that causes the formation of terraces. At relatively high flow rates, the most important mechanism for this behaviour seems to be hydrodynamic advection of dissolved species either towards or away from the calcite surface, depending on location relative to the obstruction, which deforms the concentration gradients. At lower flow rates, steepening of diffusion gradients around protrusions becomes important. Enhanced degassing over the obstruction due to shallowing and pressure drop is not important on small scales. Diffusion controlled transport close to the calcite surface can lead to a fingering-type growth instability, which generates porous textures. Our results are consistent with existing diffusive boundary layer theory, but for flow over non-smooth surfaces, simple calcite precipitation models that include empirical correlations between fluid flow rate and calcite precipitation rate are inaccurate. 相似文献
2.
Three outdoor plastic tanks were filled with North Sea water. The water was filtered free of algae and inoculated with the diatom Thalassiosira rotula. Under nearly natural nutrient supply an exponential plankton growth occurred with realistic cell concentrations. The flexible tanks (3–4 m3, 4–5 m deep) were exposed in the German Bight at a station in the outer harbour of Helgoland. The concentration changes of dissolved free amino acids (DFAA) were determined. The direct determination method applied permitted an analysis in picomole range without loss and with good reproducibility. The preliminary concentration of DFAA amounted to 0.2 μmol dm−3 in all three cultures, increasing during the development of T. rotula to 0.9–1.3 μmol dm−3 and decreasing in the stationary phase back again to smaller values. A high degree of temporal variation in DFAA was observed in all three cultures. In spite of these rapid changes in concentration, a good parallel development could be shown for all cultures, which extended as far as the spectra of the amino acids. Relationships between DFAA and other parameters of the culture-systems were examined. 相似文献
3.
Pockmarks in the inner Oslofjord,Norway 总被引:5,自引:3,他引:2
Multibeam bathymetric surveys of the Inner Oslofjord, Norway have revealed a high density of pockmarks in the 179-km2 inner fjord area, which contains over 500 pockmarks of varying size, typically 20–50 m in diameter and 2–10 m deep. These
pockmarks have been investigated with a variety of techniques, including acoustic subbottom profiling, sedimentological and
geochemical analyses of cores, remotely operated vehicle observation, and morphometry. Both the distribution and shapes of
the pockmarks suggest that they are related to structures in the bedrock underlying relatively thin (<50 m) unconsolidated
glacial and postglacial sediments. The data provide no direct indication of a particular mode of pockmark formation, but release
of large amounts of biogenic, shallow methane seems unlikely. Several lines of evidence point to a continuous process of pockmark
formation followed by inactivity, with some pockmarks recently active whereas others have been inactive for a considerable
time. Some pockmarks are characterised by coarse sediment in their centres. The density, variety and easy access make this
pockmark field an ideal model area for pockmark research.
John S. Gray is deceased. 相似文献
4.
Molly C. McCanta Malcolm J. Rutherford Julia E. Hammer 《Journal of Volcanology and Geothermal Research》2007,160(3-4):263-284
A series of experiments and petrographic analyses have been run to determine the pre-eruption phase equilibria and ascent dynamics of dacitic lavas composing Black Butte, a dome complex on the flank of Mount Shasta, California. Major and trace element analyses indicate that the Black Butte magma shared a common parent with contemporaneously erupted magmas at the Shasta summit. The Black Butte lava phenocryst phase assemblage (20 v.%) consists of amphibole, plagioclase (core An77.5), and Fe–Ti oxides in a fine-grained (< 0.5 mm) groundmass of plagioclase, pyroxene, Fe–Ti oxides, amphibole, and cristobalite. The phenocryst assemblage and crystal compositions are reproduced experimentally between 890 °C and 910 °C, ≥ 300 MPa, XH2O = 1, and oxygen fugacity = NNO + 1. This study has quantified the extent of three crystallization processes occurring in the Black Butte dacite that can be used to discern ascent processes. Magma ascent rate was quantified using the widths of amphibole breakdown rims in natural samples, using an experimental calibration of rim development in a similar magma at relevant conditions. The majority of rims are 34 ± 10 μm thick, suggesting a time-integrated magma ascent rate of 0.004–0.006 m/s among all four dome lobes. This is comparable to values for effusive samples from the 1980 Mount St. Helens eruption and slightly faster than those estimated at Montserrat. A gap between the compositions of plagioclase phenocryst cores and microlites suggests that while phenocryst growth was continuous throughout ascent, microlite formation did not occur until significantly into ascent. The duration of crystallization is estimated using the magma reservoir depth and ascent rate, as determined from phase equilibria and amphibole rim widths, respectively. Textural analysis of the natural plagioclase crystals yields maximum growth rates of plagioclase phenocryst rims and groundmass microlites of 8.7 × 10− 8 and 2.5 × 10− 8 mm/s, respectively. These rates are comparable to values determined from time-sequenced samples of dacite erupted effusively from Mount St. Helens during 1980 and indicate that syneruptive crystallization processes were important during the Black Butte eruptive cycle. 相似文献
5.
Summary A human energy balance model is calculated using the homogeneous data set of Vienna, Austria (48°N and 16° E). Variations in the different stages of comfort since 1873 are discussed. The historical data are then used to generate scenarios of a changing climate and calculate the impact of these variations on thermal comfort.With 5 Figures 相似文献
6.
Torsional waves supported by magnetic flux tubes have long been thought to bear a high potential for supplying energy and momentum to the upper solar atmosphere, thereby contributing to its heating and to the driving of dynamic events like spicules. This hope rested on the belief that their propagation is not impeded by cutoff restrictions, unlike longitudinal and kink waves. We point out that this applies only to thin, isothermal tubes. When they widen in the chromosphere, and as a result of temperature gradients, cutoff restrictions arise. We compare them to recent observational reports of such waves and of vortex motions and find that their long period components are already affected by cutoff restrictions. An observational strategy is proposed that should permit the derivation of better information on vortex flows from off‐center observations with next generation telescopes (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
7.
Experimental investigation of the hydrodynamics in pockmarks using particle tracking velocimetry 总被引:1,自引:1,他引:0
Water tank experiments were performed in order to investigate the behaviour of currents in pockmarks. A particle-seeded flow was visualised and quantified with the aid of the particle tracking velocimetry technique. The employed analogue pockmark is a 1:100 idealised scale model of a natural pockmark, while the highest Reynolds number in the experiments was one order of magnitude smaller than in nature. Interaction of the flow with the pockmark geometry resulted in an upwelling current downstream of the pockmark centre, along with enhanced water turbulence in the depression. Scaling-up the experimental measurements, it is found that the upwelling would be capable of preventing the settling of particles as large as very fine sand. Furthermore, the increased turbulence would support the suspended fine material, which can thus be transported away before settling. The net effect for a variable-direction near-bed current over long periods of time would be to winnow the settling sediments and reduce the sedimentation rate in pockmarks. These mechanisms may be responsible for the observed lack of sediment infill and the typical presence of relatively coarser sediments inside pockmarks compared to the surrounding bed. In contrast, sediments transported as bedload are likely to be deposited in pockmarks because of the weakening of near-bed currents as well as lateral flow convergence associated with the upwelling. Bedload, however, may not be the dominant mode of sediment transport in areas covered by cohesive sediments, where pockmarks are found. 相似文献
8.
9.
Dr. W. Hammer 《International Journal of Earth Sciences》1912,3(5-6):449-451
Ohne Zusammenfassung 相似文献
10.
Degassing and microlite crystallization during pre-climactic events of the 1991 eruption of Mt. Pinatubo, Philippines 总被引:2,自引:2,他引:0
Dacite tephras produced by the 1991 pre-climactic eruptive sequence at Mt. Pinatubo display extreme heterogeneity in vesicularity,
ranging in clast density from 700 to 2580 kg m–3. Observations of the 13 surge-producing blasts that preceded the climactic plinian event include radar-defined estimates
of column heights and seismically defined eruptive and intra-eruptive durations. A comparison of the characteristics of erupted
material, including microlite textures, chemical compositions, and H2O contents, with eruptive parameters suggests that devolatilization-induced crystallization of the magma occurred to a varying
extent prior to at least nine of the explosive events. Although volatile loss progressed to the same approximate level in
all of the clasts analyzed (weight percent H2O=1.26-1.73), microlite crystallization was extremely variable (0–22%). We infer that syn-eruptive volatile exsolution from
magma in the conduit and intra-eruptive separation of the gas phase was facilitated by the development of permeability within
magma residing in the conduit. Correlation of maximum microlite crystallinity with repose interval duration (28–262 min) suggests
that crystallization occurred primarily intra-eruptively, in response to the reduction in dissolved H2O content that occurred during the preceding event. Detailed textural characterization, including determination of three-dimensional
shapes and crystal size distributions (CSD), was conducted on a subset of clasts in order to determine rates of crystal nucleation
and growth using repose interval as the time available for crystallization. Shape and size analysis suggests that crystallization
proceeded in response to lessening degrees of feldspar supersaturation as repose interval durations increased. We thus propose
that during repose intervals, a plug of highly viscous magma formed due to the collapse of vesicular magma that had exsolved
volatiles during the previous explosive event. If plug thickness grew proportionally to the square root of time, and if magma
pressurization increased during the eruptive sequence, the frequency of eruptive pulses may have been modulated by degassing
of magma within the conduit. Dense clasts in surge deposits probably represent plug material entrained by each subsequent
explosive event.
Received: 4 December 1997 / Accepted: 13 September 1998 相似文献