Mineral-scale Sr-isotope constraints on magma evolution and chamber dynamics in the Rum layered intrusion,Scotland   总被引：1，自引：0，他引：1  

Frank?J.?TepleyIIIEmail author  Jon?P.?Davidson 《Contributions to Mineralogy and Petrology》2003,145(5):628-641

Sr isotopic zoning within single plagioclase crystals from rocks from Unit 9 of the Rum layered intrusion is used to infer events during crystal growth in a magma undergoing contamination. The ⁸⁷Sr/⁸⁶Sr diversity among minerals and between cores and rims of plagioclase crystals increases as the boundary between unit 9 and the overlying Unit 10 peridotite is approached. Models of near-solidus interaction of the cumulate with a fluid or melt, or large scale textural re-equilibration, cannot easily account for the systematic differences in ⁸⁷Sr/⁸⁶Sr between small crystals and the rims of larger crystals.We propose a simple interpretation in which crystal growth is concentrated along the cool margins of the reservoir. Crystals are subsequently advected to a site of accumulation at the base of the reservoir, probably by episodic plume-like dense downwellings allowing mixing of isotopically zoned and unzoned crystals.If the core-rim isotope variations are inherited from primary magmatic growth, then the small distances over which they are now preserved (1–2 mm) place constraints on the minimum cooling rate of the intrusion. Although the length scale of diffusive equilibration is influenced by a number of poorly-constrained variables (starting temperature, feldspar composition, temperature-time path) cooling was clearly very rapid with cooling to effective closure (~1,000 °C) within a few thousand years.Editorial responsibility: I. Parsons  相似文献   

Microwave Remote Sensing of the Temperature and Distribution of Sulfur Compounds in the Lower Atmosphere of Venus     

Jon M. JenkinsMarc A. Kolodner  Bryan J. ButlerShady H. Suleiman  Paul G. Steffes 《Icarus》2002,158(2):312-328

A multi-wavelength radio frequency observation of Venus was performed on April 5, 1996, with the Very Large Array to investigate potential variations in the vertical and horizontal distribution of temperature and the sulfur compounds sulfur dioxide (SO₂) and sulfuric acid vapor (H₂SO₄(g)) in the atmosphere of the planet. Brightness temperature maps were produced which feature significantly darkened polar regions compared to the brighter low-latitude regions at both observed frequencies. This is the first time such polar features have been seen unambiguously in radio wavelength observations of Venus. The limb-darkening displayed in the maps helps to constrain the vertical profile of H₂SO₄(g), temperature, and to some degree SO₂. The maps were interpreted by applying a retrieval algorithm to produce vertical profiles of temperature and abundance of H₂SO₄(g) given an assumed sub-cloud abundance of SO₂. The results indicate a substantially higher abundance of H₂SO₄(g) at high latitudes (above 45°) than in the low-latitude regions. The retrieved temperature profiles are up to 25 K warmer than the profile obtained by the Pioneer Venus sounder probe at altitudes below 40 km (depending on location and assumed SO₂ abundance). For 150 ppm of SO₂, it is more consistent with the temperature profile obtained by Mariner 5, extrapolated to the surface via a dry adiabat. The profiles obtained for H₂SO₄(g) at high latitudes are consistent with those derived from the Magellan radio occultation experiments, peaking at around 8 ppm at an altitude of 46 km and decaying rapidly away from that altitude. At low latitudes, no significant H₂SO₄(g) is observed, regardless of the assumed SO₂ content. This is well below that measured by Mariner 10 (Lipa and Tyler 1979, Icarus39, 192-208), which peaked at ∼14 ppm near 47 km. Our results favor ≤100 ppm of SO₂ at low latitudes and ≤50 ppm in polar regions. The low-latitude value is statistically consistent with the results of Bézard et al. (1983, Geophs. Res. Lett.20, 1587-1590), who found that a sub-cloud SO₂ abundance of 130±40 ppm best matched their observations in the near-IR. The retrieved temperature profile and higher abundance of H₂SO₄(g) in polar regions are consistent with a strong equatorial-to-polar, cloud-level flow due to a Hadley cell in the atmosphere of Venus.  相似文献   

Recharge estimation for transient ground water modeling   总被引：11，自引：0，他引：11  

Jyrkama MI  Sykes JF  Normani SD 《Ground water》2002,40(6):638-648

Reliable ground water models require both an accurate physical representation of the system and appropriate boundary conditions. While physical attributes are generally considered static, boundary conditions, such as ground water recharge rates, can be highly variable in both space and time. A practical methodology incorporating the hydrologic model HELP3 in conjunction with a geographic information system was developed to generate a physically based and highly detailed recharge boundary condition for ground water modeling. The approach uses daily precipitation and temperature records in addition to land use/land cover and soils data. The importance of the method in transient ground water modeling is demonstrated by applying it to a MODFLOW modeling study in New Jersey. In addition to improved model calibration, the results from the study clearly indicate the importance of using a physically based and highly detailed recharge boundary condition in ground water quality modeling, where the detailed knowledge of the evolution of the ground water flowpaths is imperative. The simulated water table is within 0.5 m of the observed values using the method, while the water levels can differ by as much as 2 m using uniform recharge conditions. The results also show that the combination of temperature and precipitation plays an important role in the amount and timing of recharge in cooler climates. A sensitivity analysis further reveals that increasing the leaf area index, the evaporative zone depth, or the curve number in the model will result in decreased recharge rates over time, with the curve number having the greatest impact.  相似文献   

The impact of freshwater discharges on the ocean circulation in the Skagerrak/northern North Sea area. Part II: energy analysis     

Jon Albretsen 《Ocean Dynamics》2007,57(4-5):287-304

We perform eddy-permitting to eddy-resolving simulations of the Skagerrak/northern North Sea with a terrain-following numerical ocean model. We demonstrate that realistic representations of freshwater input are not required when the focus is on modelling mesoscale structures such as meanders and eddies. To arrive at this conclusion, we analyze the results using a recently developed energy diagnostic scheme to study the sensitivity to realistic representations of the lateral freshwater flux provided to the area from the Baltic Sea and by the major rivers. The scheme is suitable for analysis of growth of instabilities, and it has four basic instability processes prominent. We recognize both horizontal and vertical shear instabilities. There are two processes where average potential energy is converted to eddy kinetic energy, and they are related to the mean gradient in surface elevation and the mean lateral density gradient, respectively. The latter process is known as frontal instability. We demonstrate that the change in the eddy kinetic energy field is small, despite the large variations in the hydrographic properties from experiment to experiment. Moreover, generation of eddy activity appears at the same locations and with approximately the same strength regardless of actual representations of freshwater input. Furthermore, we find that vertical shear instability dominates the energy conversion processes in the Norwegian Coastal Current. Finally, we find that the areas off the northwest coast of Denmark recognized with enhanced eddy kinetic energy level is not caused by instability processes but eddy–eddy interaction rooted in variations in the sea level.  相似文献   

Guest Editorial     

Michael Wiedenbeck  Jon Woodhead 《Geostandards and Geoanalytical Research》2007,31(4):299-299

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Modeling Forest Emissivity at L-Band and a Comparison With Multitemporal Measurements     

Della Vecchia  A. Ferrazzoli  P. Wigneron  J.-P. Grant  J.P. 《Geoscience and Remote Sensing Letters, IEEE》2007,4(4):508-512

This letter describes recent advances in modeling forest emissivity at L-band. The formulation is based on a previously developed discrete model and includes a new representation of forest litter. Comparisons with multitemporal radiometric data collected in the framework of the ldquoBray 2004rdquo experiment, which was carried out within Les Landes forest, are shown and discussed. Input variables are given by using detailed ground measurements. In general, the model reproduces both absolute values and temporal variations of measured brightness temperature. The contribution of the litter to overall emission was found to be important.  相似文献   

Communicating academic geography—the continuing challenge     

Jon Shaw  Judith Matthews 《Area》1998,30(4):367-372

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Recently increased tropical cyclone activity and inferences concerning coastal erosion and inland hydrological regimes in New Zealand and Eastern Australia     

Patrick J. Grant 《Climatic change》1981,3(3):317-332

For the area bounded by 0–35 °S latitude and 105 °E–105 °W longitude a significant increase of tropical cyclone frequency occurred about 1954–55 and this regime persisted throughout the 1960s and 1970s. Probably this frequency increase was accompanied by an increase of cyclonic storm intensity. It was related to an atmospheric pressure regime change over Australasia which commenced in early 1954. Tropical cyclones produce large rainfall amounts and strong winds which generate high-energy sea waves. Severe damage can result on land and at sea. However, much recently recorded damage has resulted from extra-tropical cyclones. Therefore it is postulated that extra-tropical cyclones also increased in frequency after the mid 1950s and that both types of cyclone contributed to a significant increase in total storminess. In eastern Australia and around both islands of New Zealand a major coastal regime change occurred in the 1950s and has persisted to the present;erosion has been the dominant process. In eastern Australia there was a significant increase in the magnitude of river floods after the late 1940s. After the mid 1950s most of the North Island of New Zealand experienced the wettest years of this century which caused large closed lakes to reach the highest levels for several centuries. Throughout the North Island the majority of the greatest recorded stream floods of this century have occurred during the 1950s, 1960s and 1970s. And in forested mountain areas of the North Island the average rate of erosion and alluvial sedimentation increased markedly after the mid 1950s. The regime change of each natural phenomenon is explicable in terms of increased storminess since the mid 1950s. Consequently it is hypothesised that the coastal and inland environmental regime changes discussed were either initiated or accentuated after the mid 1950s as a result of increased activity of both tropical and extra-tropical cyclones. Some further studies, necessary to test the links in the overall hypothesis, are mentioned.  相似文献   

A. Programmable device for simulation of tidal flux     

Mark L. Gleason  Jon R. DiMarco  Joseph C. Zieman 《Estuaries and Coasts》1981,4(3):220-222

An apparatus is described which permits precise, versatile control of simulated tidal flux in laboratory microcosms. A key component of the system, a programmable electronic control unit, can closely imitate any natural tidal function and is readily adaptable to other experimental applications that utilize timed switching capability. The programmable control unit is also cheaper and easier to operate than computer-based systems.  相似文献   

Plinian-type eruptions in the medicine lake highland, california, and the nature of the underlying magma     

Grant Heiken 《Journal of Volcanology and Geothermal Research》1978,4(3-4)

Contemporaneous Plinian eruptions of rhyolite pumice from Glass Mountain and Little Glass Mountain during the last 1100 years B.P. were followed by extrusion of lava flows. 1.2 km of material was erupted and 10% by volume is tephra. All of the tephra deposits consist of very poorly sorted coarse ash and lapilli that are mostly pumice pyroclasts.Eruptive sequences, chemical composition and petrographic character of the rhyolites at Little Glass Mountain and Glass Mountain suggest that they came from the same magma body. The 1:9 ratio of tephra to lavas is typical of small silicic magma chambers. Eruption from a small chamber, 4–6 km deep, at vents 15 km apart is possible if magma rose along cone sheets with dips of 45–60°. The caldera rim and arcuate lines of vents near it may represent the surface expression of several concentric cone sheets.Pumice pyroclasts erupted at Glass Mountain and Little Glass Mountain may have formed in the following manner: (1) vesicle growth and coalescence beginning at 1–2 km depths; (2) elongation of the vesicles by flow within the cone sheets; (3) disruption of the vesiculated magma when it reached the surface by an expansion wave passing down through it; and (4) eruption of comminution products as pumice pyroclasts. Plinian activity at Little Glass Mountain and Glass Mountain continued until the volatile-rich top of the magma chamber had been depleted.  相似文献