Properties of chondrules in EL3 chondrites,comparison with EH3 chondrites,and the implications for the formation of enstatite chondrites     

D. M. Schneider  S. J. K. Symes  P. H. Benoit  D. W. G. Sears 《Meteoritics & planetary science》2002,37(10):1401-1416

Abstract— The study of chondrules provides information about processes occurring in the early solar system. In order to ascertain to what extent these processes played a role in determining the properties of the enstatite chondrites, the physical and chemical properties of chondrules from three EL3 chondrites and three EH3 chondrites have been examined by optical, cathodoluminescence (CL), and electron microprobe techniques. Properties examined include size, texture, CL, and composition of both individual phases and bulk chondrules. The textures, distribution of textures, and composition of silicates of the EL3 chondrules resemble those of EH3 chondrules. However, the chondrules from the two classes differ in that (1) the size distribution of the EL chondrules is skewed to larger values than EH chondrules, (2) the enstatite in EL chondrules displays varying shades of red CL due to the presence of fine‐grained sulfides and metal in the silicates, and (3) the mesostasis of EH chondrules is enriched in Na relative to that of EL chondrules. The similarities between the chondrules of the two classes suggest similar precursor materials, while the differences suggest that there was not a single reservoir of meteoritic chondrules, but that their origin was fairly local. The differences in the size distribution of chondrules in EH and EL chondrites may be explained by aerodynamic and gravitational sorting during accumulation of the meteoric material, while differences in CL and mesostasis properties may reflect differences in formation conditions and cooling rate following chondrule formation. We argue that our observations are consistent with the formation of enstatite chondrites in a thick dynamic regolith on their parent body.  相似文献   

Multitransition study and new detections of class II methanol masers     

Dinah M. Cragg  rej M. Sobolev  Simon P. Ellingsen  J. L. Caswell  Peter D. Godfrey  Svetlana V. Salii  Richard G. Dodson 《Monthly notices of the Royal Astronomical Society》2001,323(4):939-951

We have used the ATNF Mopra antenna and the SEST antenna to search in the directions of several class II methanol maser sources for emission from six methanol transitions in the frequency range 85–115 GHz. The transitions were selected from excitation studies as potential maser candidates. Methanol emission at one or more frequencies was detected from five of the maser sources, as well as from Orion KL. Although the lines are weak, we find evidence of maser origin for three new lines in G345.01+1.79, and possibly one new line in G9.62+0.20.
The observations, together with published maser observations at other frequencies, are compared with methanol maser modelling for G345.01+1.79 and NGC 6334F. We find that the majority of observations in both sources are consistent with a warm dust (175 K) pumping model at hydrogen density ∼10⁶ cm⁻³ and methanol column density ∼ 5×10¹⁷ cm⁻². The substantial differences between the maser spectra in the two sources can be attributed to the geometry of the maser region.  相似文献   

Petrography,geochemistry, and argon‐40/argon‐39 ages of impact‐melt rocks and breccias from the Ames impact structure,Oklahoma: The Nicor Chestnut 18‐4 drill core     

Christian KOEBERL  Wolf Uwe REIMOLD  Simon P. KELLEY 《Meteoritics & planetary science》2001,36(5):651-669

Abstract— The 15 km diameter Ames structure in northwestern Oklahoma is located 2.75 km below surface in Cambro‐Ordovician Arbuckle dolomite, which is overlain by Middle Ordovician Oil Creek Formation shale. The feature is marked by two concentric ring structures, with the inner ring of about 5 km diameter probably representing the collapsed remnant of a structural uplift composed of brecciated Precambrian granite and Arbuckle dolomite. Wells from both the crater rim and the central uplift are oil‐ and gas‐producing, making Ames one of the economically important impact structures. Petrographic, geochemical, and age data were obtained on samples from the Nicor Chestnut 18‐4 drill core, off the northwest flank of the central uplift. These samples represent the largest and best examples of impact‐melt breccia obtained so far from the Ames structure. They contain carbonate rocks, which are derived from the target sequence. The chemical composition of the impact‐melt breccias is similar to that of target granite, with variable carbonate admixture. Some impact‐melt rocks are enriched in siderophile elements indicating the possible presence of a meteoritic component. Based on stratigraphic arguments, the age of the crater was estimated at 470 Ma. Previous ⁴⁰Ar‐³⁹Ar dating attempts of impact‐melt breccias from the Dorothy 1–19 core yielded plateau ages of about 285 Ma, which is in conflict with the stratigraphic age. The new ⁴⁰Ar‐³⁹Ar age data obtained on the melt breccias from the Nicor Chestnut core by ultraviolet (UV) laser spot analysis resulted in a range of ages with maxima around 300 Ma. These data could reflect processes related either the regional Nemaha Uplift or resetting due to hot brines active on a midcontinent‐wide scale, perhaps related to the Alleghenian and Ouachita orogenies. The age data indicate an extended burial phase associated with thermal overprint during Late Pennsylvanian‐Permian.  相似文献   

Resistance of salt marsh substrates to near-instantaneous hydrodynamic forcing     

Helen Brooks  Iris Möller  Simon Carr  Clementine Chirol  Elizabeth Christie  Ben Evans  Kate L. Spencer  Tom Spencer  Katherine Royse 《地球表面变化过程与地形》2021,46(1):67-88

Salt marshes deliver vital ecosystem services by providing habitats, storing pollutants and atmospheric carbon, and reducing flood and erosion risk in the coastal hinterland. Net losses in salt marsh areas, both modelled globally and measured regionally, are therefore of concern. Amongst other controls, the persistence of salt marshes in any one location depends on the ability of their substrates to resist hydrodynamic forcing at the marsh front, along creek margins and on the vegetated surface. Where relative sea level is rising, marsh elevation must keep pace with sea-level rise and landward expansion may be required to compensate for areal loss at exposed margins. This paper reviews current understanding of marsh substrate resistance to the near-instantaneous (seconds to hours) forcing induced by hydrodynamic processes. It outlines how variability in substrate properties may affect marsh substrate stability, explores current understanding of the interactions between substrate properties and erosion processes, and how the cumulative impact of these interactions may affect marsh stability over annual to decadal timescales. Whilst important advances have been made in understanding how specific soil properties affect near-instantaneous marsh substrate stability, less is known about how these properties interact and alter bulk substrate resistance to hydrodynamic forcing. Future research requires a more systematic approach to quantifying biological and sedimentological marsh substrate properties. These properties must then be linked to specific observable erosion processes, particularly at the marsh front and along creek banks. A better understanding of the intrinsic dynamics and processes acting on, and within, salt marsh substrates will facilitate improved prediction of marsh evolution under future hydrodynamic forcing scenarios. Notwithstanding the additional complications that arise from morphodynamic feedbacks, this would allow us to more accurately model the future potential protection from flooding and erosion afforded by marshes, while also increasing the effectiveness of salt marsh restoration and recreation schemes. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd  相似文献   

A regional coupled approach to water cycle prediction during winter 2013/14 in the United Kingdom     

Huw W. Lewis  Simon J. Dadson 《水文研究》2021,35(12):e14438

A regional coupled approach to water cycle prediction is demonstrated for the 4-month period from November 2013 to February 2014. This provides the first multi-component analysis of precipitation, soil moisture, river flow and coastal ocean simulations produced by an atmosphere-land-ocean coupled system focussed on the United Kingdom (UK), running with horizontal grid spacing of around 1.5 km across all components. The Unified Model atmosphere component, in which convection is explicitly simulated, reproduces the observed UK rainfall accumulation (r² of 0.95 for water day accumulation), but there is a notable bias in its spatial distribution—too dry over western upland areas and too wet further east. The JULES land surface model soil moisture state is shown to be in broad agreement with a limited number of cosmic-ray neutron probe observations. A comparison of observed and simulated river flow shows the coupled system is useful for predicting broad scale features, such as distinguishing high and low flow regions and times during the period of interest but are less accurate than optimized hydrological models. The impact of simulated river discharge on NEMO model simulations of coastal ocean state is explored in the coupled modelling framework, with comparisons provided relative to experiments using climatological river input and no river input around the UK coasts. Results show that the freshwater flux around the UK contributes of order 0.2 psu to the mean surface salinity, and comparisons to profile observations give evidence of an improved vertical structure when applying simulated flows. This study represents the first assessment of the coupled system performance from a hydrological perspective, with priorities for future model developments and challenges for evaluation of such systems discussed.  相似文献   

Ground penetrating radar imaging of cap rock, caliche and carbonate strata     

S. E. Kruse  J. C. Schneider  D. J. Campagna  J. A. Inman  T. D. Hickey 《Journal of Applied Geophysics》2000,43(2-4)

Field experiments show ground penetrating radar (GPR) can be used to image shallow carbonate stratigraphy effectively in a variety of settings. In south Florida, the position and structure of cap rock cover on limestone can be an important control on surface water flow and vegetation, but larger scale outcrops (tens of meters) of cap rock are sparse. GPR mapping through south Florida prairie, cypress swamp and hardwood hammock resolves variations in thickness and structure of cap rock to 3 m and holds the potential to test theories for cap rock–vegetation relationships. In other settings, carbonate strata are mapped to test models for the formation of local structural anomalies. A test of GPR imaging capabilities on an arid caliche (calcrete) horizon in southeastern Nevada shows depth penetration to 2 m with resolution of the base of caliche. GPR profiling also succeeds in resolving more deeply buried (5 m) limestone discontinuity surfaces that record subaerial exposure in south Florida.  相似文献   

Acoustic characteristics of fish bombing: potential to develop an automated blast detector     

Woodman GH  Wilson SC  Li VY  Renneberg R 《Marine pollution bulletin》2003,46(1):99-106

The use of explosives to catch fish has caused extensive damage to coral reefs throughout Southeast Asia, but the frequency with which they are used is largely unknown. The aim of this work is to develop a detection system capable of distinguishing underwater explosions from background noise, and locating their origin by triangulation. Blast signals have been recorded over a range of distances and the key features that differentiate them from background noise have been determined. For small charges the effective range of such a detector is more than 12 km and may extend up to 50 km depending on the mass of the charges being used. Such a system would help to determine the scale of the problem, identify areas at greatest risk and quantify the effectiveness of management intervention designed to control destructive fishing practices. It may assist with fisheries enforcement in some areas.  相似文献   

Rapid response flood detection using the MSG geostationary satellite     

Simon Richard Proud  Rasmus FensholtLaura Vang Rasmussen  Inge Sandholt 《International Journal of Applied Earth Observation and Geoinformation》2011

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Influence of grass soil cover on water runoff and soil detachment under rainfall simulation in a sub‐humid South African degraded rangeland     

Pascal Podwojewski  Jean Louis Janeau  Séraphine Grellier  Christian Valentin  Simon Lorentz  Vincent Chaplot 《地球表面变化过程与地形》2011,36(7):911-922

In most regions of the world overgrazing plays a major role in land degradation and thus creates a major threat to natural ecosystems. Several feedbacks exist between overgrazing, vegetation, soil infiltration by water and soil erosion that need to be better understood. In this study of a sub‐humid overgrazed rangeland in South Africa, the main objective was to evaluate the impact of grass cover on soil infiltration by water and soil detachment. Artificial rains of 30 and 60 mm h^?1 were applied for 30 min on 1 m² micro‐plots showing similar sandy‐loam Acrisols with different proportions of soil surface coverage by grass (Class A: 75–100%; B: 75–50%; C: 50–25%; D: 25–5%; E: 5–0% with an outcropping A horizon; F: 0% with an outcropping B horizon) to evaluate pre‐runoff rainfall (Pr), steady state water infiltration (I), sediment concentration (SC) and soil losses (SL). Whatever the class of vegetal cover and the rainfall intensity, with the exception of two plots probably affected by biological activity, I decreased regularly to a steady rate <2 mm h^?1 after 15 min rain. There was no significant correlation between I and Pr with vegetal cover. The average SC computed from the two rains increased from 0·16 g L^?1 (class A) to 48·5 g L^?1 (class F) while SL was varied between 4 g m^?2 h^?1 for A and 1883 g m^?2 h^?1 for F. SL increased significantly with decreasing vegetal cover with an exponential increase while the removal of the A horizon increased SC and SL by a factor of 4. The results support the belief that soil vegetation cover and overgrazing plays a major role in soil infiltration by water but also suggest that the interrill erosion process is self‐increasing. Abandoned cultivated lands and animal preferred pathways are more vulnerable to erosive processes than simply overgrazed rangelands. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

On the role of tides and strong wind events in promoting summer primary production in the Barents Sea     

Vincent Le Fouest  Clare PostlethwaiteMiguel Angel Morales Maqueda  Simon BélangerMarcel Babin 《Continental Shelf Research》2011,31(17):1869-1879

Tides and wind-driven mixing play a major role in promoting post-bloom productivity in subarctic shelf seas. Whether this is also true in the high Arctic remains unknown. This question is particularly relevant in a context of increasing Arctic Ocean stratification in response to global climatic change. We have used a three-dimensional ocean-sea ice-plankton ecosystem model to assess the contribution of tides and strong wind events to summer (June-August 2001) primary production in the Barents Sea. Tides are responsible for 20% (60% locally) of the post-bloom primary production above Svalbard Bank and east of the Kola Peninsula. By contrast, more than 9% of the primary production is due to winds faster than 8 m s⁻¹ in the central Barents Sea. Locally, this contribution reaches 25%. In the marginal ice zone, both tides and wind events have only a limited effect on primary production (<2%). Removing tides or winds faster than 8 m s⁻¹ promotes a regime more sustained by regenerated production with a f-ratio (i.e. the proportion of nitrate-based “new” primary production in the total primary production) that decreases by up to 26% (east of the Kola Peninsula) or 35% (central Barents Sea), respectively. When integrated over all Barents Sea sub-regions, tides and strong wind events account, respectively, for 6.8% (1.55 Tg C; 1 Tg C=10¹² g C) and 4.1% (0.93 Tg C) of the post-bloom primary production (22.6 Tg C). To put this in context, this contribution to summer primary production is equivalent to the spring bloom integrated over the Svalbard area. Tides and winds are significant drivers of summer plankton productivity in the Barents Sea.  相似文献