The role of bathymetry and directional wave conditions on observed crescentic bar dynamics     

Rinse L. de Swart  Francesca Ribas  Gonzalo Simarro  Jorge Guillén  Daniel Calvete 《地球表面变化过程与地形》2021,46(15):3252-3270

Nearshore sandbars are important features in the surf zone of many beaches because they strongly influence the mean circulation and evolving morphology. Due to variations in wave conditions, sandbars can experience cross-shore migration and vary in shape from alongshore uniform (shore-parallel) to alongshore rhythmic (crescentic). Sandbar dynamics have been studied extensively, but existing observational studies usually do not quantify the processes leading to crescentic bar formation and straightening. This study analyses the dynamics of crescentic bar events at the fetch-limited beach of Castelldefels (northwestern Mediterranean Sea, Spain) using 7.5 years of hourly time-exposure video images and detailed wave conditions. The results show that, despite the generally calm wave conditions, the sandbars were very dynamic in the cross-shore and longshore directions. They often migrated rapidly offshore during storms (up to 70 m in one day) and more slowly onshore during post-storm conditions. Crescentic bars were often present at the study site (48% of the time), but only when the sandbar was at least 10 m from the shoreline. They displayed a large variability in wavelengths (100–700 m), alongshore migration speeds (0–50 m/day) and cross-shore amplitudes (5–20 m). Wavelengths increased for larger bar–shoreline distances and the alongshore migration speeds were strongly correlated with the alongshore component of the radiation stresses. Crescentic patterns typically developed during low–medium energetic waves with limited obliquity ( $\theta \lesssim 20$° at 10 m depth), while bar straightening occurred during medium–high energetic waves with strong oblique angles of incidence ( $\theta \gtrsim 15$°). Overall, this study provides further proof for the important role of wave direction in crescentic bar dynamics and highlights the strong dependence of crescentic bar development on the initial bathymetric configuration.  相似文献   

    

Daniel Y. Gezari  Dana E. Backman 《Astrophysics and Space Science》1995,224(1-2):463

Mid-infrared imaging photometry of the Orion BN/KL infrared cluster at eight wavelengths between 5 and 20µm using a 58 × 62 pixel imaging array camera has revealed new compact sources and the large-scale structure of the region in diffraction-limited (1 arcsec) detail. Several new objects have been detected within a few arcsec of IRc2, widely thought to be the principal luminosity source for the entire BN/KL complex. Detailed color temperature and emission opacity images are derived from the 7.8, 12.4 and 20.0µm observations, and the 9.8µm image is used to derive an image of silicate dust extinction for the region. The color temperature, opacity, and extinction images show that IRc2 may not be the single dominant luminosity source for the BN/KL region; substantial contributions to the luminosity could be made by IRc7, BN, KL, and five new compact 10µm sources detected within a few arcseconds of IRc2. We suggest that a luminous, early-type star near IRc2, which is associated with the compact radio source I and the Orion SiO maser, is the dominant luminosity source in the BN/KL region, hidden from view by cool dust material with at least Av 60 mag of visible extinction.  相似文献   

Jupiter: New retrieved thermal profiles and ammonia distributions     

Daniel Gautier  André Marten  Jean Paul Baluteau  Anne Lacombe 《Icarus》1979,37(1):214-235

New thermal profiles of Jupiter are retrieved from recent far infrared spectral measurements and for H₂ mixing ratios varying from 0.8 to 0.94. The effective temperature corresponding to the inferred thermal profile is 123.15 ± 0.35°K. Far-infrared brightness temperature spectra computed from these profiles are compared to experimental data including measurements made at high spectral resolution in the NH₃ν₂ band at 10 μm and in NH₃ pure rotational bands between 40 and 110 μm. It is found that a strong depletion of NH₃ does occur in the Jovian stratosphere and that ammonia seems to be undersaturated in the upper troposphere.  相似文献   

Numerical simulation of the final stages of terrestrial planet formation     

Larry P. Cox  John S. Lewis 《Icarus》1980,44(3):706-721

Three representative numerical simulations of the growth of the terrestrial planets by accretion of large protoplanets are presented. The mass and relative-velocity distributions of the bodies in these simulations are free to evolve simultaneously in response to close gravitational encounters and occasional collisions between bodies. The collisions between bodies, therefore, arise in a natural way and the assumption of expressions for the relative velocity distribution and the gravitational collision cross section is unnecessary. These simulations indicate that the growth of bodies with final masses approaching those of Venus and the Earth is possible, at least for the case of a two-dimensional system. Simulations assuming an initial uniform distribution of orbital eccentricities on the interval from 0 to e_max are found to produce final states containing too many bodies with masses which are too small when e_max < 0.10, while simulations with e_max > 0.20 result in too many catastrophic collisions between bodies thus preventing rapid accretion of planetary-size bodies. The e_max = 0.15 simulation ends with a state surprisingly similar to that of the present terrestrial planets and, therefore, provides a rough estimate of the range of radial sampling to be expected for the terrestrial planets.  相似文献   

Recent crustal subsidence at Yellowstone Caldera,Wyoming     

Daniel Dzurisin  James C Savage  Robert O Fournier 《Bulletin of Volcanology》1990,52(4):247-270

Following a period of net uplift at an average rate of 15±1 mm/year from 1923 to 1984, the east-central floor of Yellowstone Caldera stopped rising during 1984–1985 and then subsided 25±7 mm during 1985–1986 and an additional 35±7 mm during 1986–1987. The average horizontal strain rates in the northeast part of the caldera for the period from 1984 to 1987 were: ₁ = 0.10 ± 0.09 strain/year oriented N33° E±9° and ₂ = 0.20 ± 0.09 strain/year oriented N57° W±9° (extension reckoned positive). A best-fit elastic model of the 1985–1987 vertical and horizontal displacements in the eastern part of the caldera suggests deflation of a horizontal tabular body located 10±5 km beneath Le Hardys Rapids, i.e., within a deep hydrothermal system or within an underlying body of partly molten rhyolite. Two end-member models each explain most aspects of historical unrest at Yellowstone, including the recent reversal from uplift to subsidence. Both involve crystallization of an amount of rhyolitic magma that is compatible with the thermal energy requirements of Yellowstone's vigorous hydrothermal system. In the first model, injection of basalt near the base of the rhyolitic system is the primary cause of uplift. Higher in the magmatic system, rhyolite crystallizes and releases all of its magmatic volatiles into the shallow hydrothermal system. Uplift stops and subsidence starts whenever the supply rate of basalt is less than the subsidence rate produced by crystallization of rhyolite and associated fluid loss. In the second model, uplift is caused primarily by pressurization of the deep hydrothermal system by magmatic gas and brine that are released during crystallization of rhyolite and them trapped at lithostatic pressure beneath an impermeable self-sealed zone. Subsidence occurs during episodic hydrofracturing and injection of pore fluid from the deep lithostatic-pressure zone into a shallow hydrostatic-pressure zone. Heat input from basaltic intrusions is required to maintain Yellowstone's silicic magmatic system and shallow hydrothermal system over time scales longer than about 10⁵ years, but for the historical time period crystallization of rhyolite can account for most aspects of unrest at Yellowstone, including seismicity, uplift, subsidence, and hydrothermal activity.  相似文献   

Libration effects for retrograde satellites in the restricted three-body problem     

Daniel Benest 《Celestial Mechanics and Dynamical Astronomy》1976,13(2):203-215

Numerical explorations of the restricted problem have shown that for stable large nonperiodic retrograde satellite orbits, the motion can be decomposed into a fast reference motion and a slow libration aroundB ₂ We study here this libration in the circular plane Hill's case, for which the reference motion is elliptic. We establish the equations of motion for the coordinates of the centre of this ellipse. We find two integrals of motion: the first is the semi-major axis of the ellipse; the second is essentially Jacobi's integral, translated into the new coordinates. We give a formula for the period of the libration and we find its limiting value for small libration amplitudes. A numerical verification gives very good agreement for all these results.  相似文献   

Layered mantle structure beneath the western Guyana Shield, Venezuela: Evidence from diamonds and xenocrysts in Guaniamo kimberlites     

Daniel J. Schulze  Dante Canil  Felix V. Kaminsky 《Geochimica et cosmochimica acta》2006,70(1):192-205

Mantle xenoliths and xenocrysts from Guaniamo, Venezuela kimberlites record equilibration conditions corresponding to a limited range of sampling in the lithosphere (100-150 km). Within this small range, however, compositions vary considerably, but regularly, defining a strongly layered mantle sequence. Major and trace element compositions suggest the following lithologic sequence: highly depleted lherzolite from 100 to 115 km, mixed ultra-depleted harzburgite and lherzolite from 115 to 120 km, relatively fertile lherzolite from 120 to 135 km, and mixed depleted harzburgite and relatively fertile lherzolite from 135 to 150 km. Based on comparison with well-documented mantle peridotites and xenocrysts from elsewhere, we conclude that the Meso-proterozoic Cuchivero Province (host to the Guaniamo kimberlites) is underlain by depleted and ultra-depleted shallow Archean mantle that was underplated, and uplifted, by Proterozoic subduction, perhaps more than once. These Proterozoic subduction events introduced less-depleted oceanic lithosphere beneath the Archean section, which remains there and is the source of the abundant Guaniamo eclogite-suite diamonds that have ocean-floor geochemical signatures. Although diamond-indicative low-Ca Cr-pyrope garnets are abundant, they are derived primarily from the shallow depleted layer within the field of graphite stability, and the rare peridotite-suite diamonds are either metastably preserved at these shallow depths, or were derived from the small amount of depleted lithosphere sampled by these kimberlites that remains within the diamond stability field (the mixture of Archean and Proterozoic mantle in the depth range 135-150 km).  相似文献   

Temperature and its control of isotope fractionation by a sulfate-reducing bacterium     

Donald E. Canfield  Claus A. Olesen  Raymond P. Cox 《Geochimica et cosmochimica acta》2006,70(3):548-561

A synthesis of previous results, which we dub the “standard model,” provides a prediction as to how isotope fractionation during sulfate reduction should respond to physiological variables such as specific rate of sulfate reduction and environmental variables such as substrate availability and temperature. The standard model suggests that isotope fractionation should decrease with increasing specific rates of sulfate reduction (rate per cell). Furthermore, the standard model predicts that low fractionations should be found at both high and low temperatures whereas the highest fractionations should be found in the intermediate temperature range. These fractionation trends are controlled, as a function of temperature, by the balance between the transfer rates of sulfate into and out of the cell and the exchange between the sulfur pools internal to the organism. We test this standard model by conducting experiments on the growth physiology and isotope fractionation, as a function of temperature, by the sulfate-reducing bacterium Desulfovibrio desulfuricans (DSMZ 642). Our results contrast with the “standard model” by showing a positive correlation between specific rates of sulfate reduction and fractionation. Also by contrast with the standard model, we found the highest fractionations at low and high temperatures and the lowest fractionations in the intermediate temperature range. We develop a fractionation model which can be used to explain both our results as well as the results of the “standard model.” Differences in fractionation with temperature relate to differences in the specific temperature response of internal enzyme kinetics as well as the exchange rates of sulfate in and out of the cell. It is expected that the kinetics of these processes will show strain-specific differences.  相似文献   

Source(s) and cycling of the nonhydrolyzable organic fraction of oceanic particles     

Jeomshik Hwang  Ellen R.M. Druffel  Daniel J. Repeta 《Geochimica et cosmochimica acta》2006,70(20):5162-5168

A major fraction of particulate organic carbon (POC) in the deep ocean remains molecularly uncharacterized. In an effort to determine the chemical characteristics and source(s) of sinking POC, we studied a nonhydrolyzable fraction of sinking POC using ¹³C NMR (nuclear magnetic resonance) spectroscopy and analytical pyrolysis. ¹³C NMR spectra and products from analytical pyrolysis of the nonhydrolyzable fraction exhibit a strongly aliphatic character that is distinct from that of bulk POC. The aliphatic nature of this fraction is consistent with its low stable carbon isotope values. We hypothesize that the nonhydrolyzable fraction derives to a significant extent from a refractory component of organisms that selectively accumulates, resulting in its manifestation as a major part of POC sinking to the deep ocean and in underlying sediments.  相似文献   

Review: Microbial biocenoses in pristine aquifers and an assessment of investigative methods   总被引：2，自引：6，他引：2  

Nico Goldscheider  Daniel Hunkeler  Pierre Rossi 《Hydrogeology Journal》2006,14(6):926-941

The current knowledge of microbial biocenoses (communities) in pristine aquifers is presented in a review, which also discusses their relevance for questions of groundwater protection. Aquifers are heterogeneous on all scales and structured in a variety of habitats. The void spaces in many aquifers are small. The biocenoses are thus predominantly composed of microorganisms and, often, microinvertebrates. Larger voids and macroorganisms occur in karst cavities. Due to the absence of light, the biocenoses depend on chemical energy resources, which are, however, scarce in non-contaminated groundwater. The microorganisms thus show small cell sizes, low population densities and reduced activity; they developed specific strategies to survive oligotrophic conditions. The review also discusses the impact of contamination on the biocenoses, and the potential use of the biocenoses or specific organisms as indicators for groundwater quality, and the limits of this approach. Bacteria are either planktonic or attached to aquifer material, which requires both fluid and solid phase sampling. Most groundwater bacteria are viable but non-culturable. Consequently, cultivation techniques give an incomplete picture of the biocenoses, while methods from molecular microbiology provide genetic fingerprints of the entire community. Different analytical methods are available to count microorganisms, identify species, characterise microbial diversity, and measure activity.  相似文献