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551.
The management of tidal inlets requires the accurate prediction of equilibrium morphologies. In areas where the flow from
rivers is highly regulated, it is important to give decision makers the ability to determine optimal flow management schemes,
in order to allow tidal inlets to function as naturally as possible, and minimise the risk of inlet closure. The River Murray
Mouth in South Australia is one such problem area. Drought and the retention of water for irrigation and urban water consumption
have limited the amount of water entering the estuary. As a result, sediment from the coastal environment is being deposited
in the mouth of the estuary, reducing the effect of further coastal interactions. Currently, situations such as this are modelled
using traditional process-based methods, where wave, current, sediment transport and sediment balance modules are linked together
in a time-stepping process. The modules are reapplied and assessed until a stable morphology is formed. In this paper, new
options for modelling equilibrium morphologies of tidal inlets are detailed, which alleviate some of the shortfalls of traditional
process-based models, such as the amplification of small errors and reliance on initial conditions. The modelling problem
is approached in this paper from a different angle and involves the use of entropy based objective functions, which are optimised
in order to find equilibrium morphologies. In this way, characteristics of a system at equilibrium can be recognised and a
stable system predicted without having to step through time. This paper also details the use of self-organisation based modelling
methods, another non-traditional model application, where local laws and feedback result in the formation of a global stable
equilibrium morphology. These methods represent a different approach to traditional models, without some of the characteristics
that may add to their limitations.
Responsible Editor: Alejandro Souza 相似文献
552.
553.
Heat‐flow in the Caribbean is poorly known and generally low in the major basins and the Greater Antilles arc, but with some high values in active zones, like in the Cayman trough or in the Lesser Antilles Arc. Here we present new heat‐flow data for offshore Haiti, which is part of the Greater Antilles arc. We obtain new heat‐flow estimates from in situ measurements and Bottom Simulating Reflector (BSR). Both methods suggest a regionally low heat‐flow, respectively 46 ± 7 and 44 ± 12 mW/m2, with locally high values exceeding 80 mW/m2. The high heat‐flow values are generally located near faults, and could be related to fluid circulations. Our study confirms a low heat‐flow pattern at the scale of the Caribbean but points out the existence of local‐scale variability with high heat‐flow along the northern faults of the Caribbean region. 相似文献
554.
555.
Karen Stamatelopoulou-Seymour Don Francis John Ludden 《Contributions to Mineralogy and Petrology》1983,84(1):6-14
A sequence of ultramafic rocks in the Lac Guyer Archean greenstone belt exhibit brecciated flow tops, pillow structures, and spinifex textures testifying to their volcanic origin. Massive, spinifex-textured and differentiated flows in the sequence have the chemical characteristics of peridotitic komatiite, with MgO ranging from 19–25 wt.%. Associated pillowed flows have compositions that straddle the conventional boundary between komatiite and komatiitic basalt with MgO contents ranging from 16 to 19 wt.% MgO and are best termed pyroxenitic komatiites. Unlike other komatiitic occurrences, the peridotitic and pyroxenitic komatiites at Lac Guyer constitute a continuous chemical spectrum with no evidence of population minimum near 18 wt.% MgO. The contrasting behaviour of highly compatible elements, such as Ni and Cr, versus incompatible elements, such as Zr, indicate that this compositional spectrum was produced by a variation in the extent of partial melting (10–40%) of a garnet lherzolite source in the Archean mantle. The pyroxenitic komatiites represent liquids produced during lower (10–20%) degrees of melting during which garnet remained in the mantle residue. However, a change in slope in the distribution of Zr vs. Y between the pyroxenitic and the peridotitic komatiites indicates that garnet was completely consumed at the more extensive degrees of melting which produced the peridotitic komatiites. The Lac Guyer volcanic rocks display a population minimum at 15 wt.% MgO separating komatiitic magmas whose compositions are controlled by partial melting from basalts whose composition is controlled by crystal fractionation. The population minimum near 18 wt.% MgO which is taken as the boundary between komatiite and komatiitic basalt may have a similar origin. 相似文献
556.
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????2007??11????????Walferdange???????ξ????????????????????????????????????????????20???????????μ?????????ECGS??15??????????????????С??2.1??10 -8 ms -2?? 相似文献
557.
Several techniques are available to examine the isotopic composition of historic lake waters, providing data that can subsequently
be used to examine environmental changes. A recently-developed technique is the stable oxygen isotope analysis of subfossil
chironomid (Diptera: Chironomidae) head capsules (mostly chitin) preserved in lake sediments. This technique involves a high
Temperature Conversion Elemental Analyzer (TC/EA), which has been a relatively recent addition to the suite of online peripherals
for analyzing the stable isotopic composition of organic samples. The highly precise and accurate 18O/16O and D/H measurements obtainable using the TC/EA with samples in the microgram range make this instrumentation suitable for
studying geochemical and biological processes. Preparation of organic samples for isotopic analysis typically requires first
weighing each sample into silver/tin capsules. These capsules can introduce oxygen and hydrogen contamination (a “blank effect”),
which is especially problematic for analysis of small organic samples (e.g. less than 100 μg). Here we tested tin and silver
capsules from two manufacturers and a range of sizes to assess contamination to small organic samples on the TC/EA. We also
assessed how a method for cleaning silver capsules affected our analysis of commercial chitin. In general, capsules made of
silver have less detectible oxygen than those made of tin, and capsules from the two manufacturers varied in their detectible
oxygen. There was no detectable H contamination from silver capsules. In addition to our empirical findings, we present a
model demonstrating the influence that contaminant oxygen can have on the δ18O of small organic samples. Sample mass becomes an important issue for such analyses. In light of our findings, we recommend
a minimum sample mass ≥50 μg (approximately 120 whole chironomid head capsules) on a TC/EA-IRMS (Deltaplus XP system). Finally, we present a detailed protocol for preparing and transferring chironomid head capsules into silver capsules
that minimizes the influence of contaminant oxygen. This protocol provides the paleo-community with another potential method
for reconstructing paleoenvironments. 相似文献
558.
Vincent P. Milesi Didier Jzquel Mathieu Debure Pierre Cadeau Franois Guyot Grard Sarazin Francis Claret Emmanuelle Vennin Carine Chaduteau Aurlien Virgone Eric C. Gaucher Magali Ader 《Sedimentology》2019,66(3):983-1001
The volcanic crater lake of Dziani Dzaha in Mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of Mg‐phyllosilicates associated with carbonate rocks. The Dziani Dzaha is characterized by intense primary productivity, volcanic gases bubbling in three locations and a volcanic catchment of phonolitic/alkaline composition. The lake water has an alkalinity of ca 0·2 mol l?1 and pH values of ca 9·3. Cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon–hydrogen–nitrogen elemental analyzer, X‐ray fluorescence spectrometry and X‐ray powder diffraction. In surface sediments, the content of total organic carbon reaches up to 20 weight %. The mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment. Below 30 cm depth, X‐ray diffraction analyses of the <2 μm clay fraction indicate the presence of a saponite‐like mineral, a Mg‐rich smectite. The saponite‐like mineral accumulates at depth to reach up to ca 30 weight %, concurrent with a decrease of the contents of hydromagnesite and organic matter. Thermodynamic considerations and mineral assemblages suggest that the evolution of the sediment composition resulted from early diagenetic reactions. The formation of the saponite‐like mineral instead of Al‐free Mg‐silicates resulted from high aluminum availability, which is favoured in restricted lacustrine environments hosted in alkaline volcanic terrains commonly emplaced during early stages of continental rifting. Supersaturation of the lake water relative to saponite is especially due to high pH values, themselves derived from high primary productivity. This suggests that a genetic link may exist between saponite and the development of organic‐rich carbonate rocks, which may be fuelled by the input of CO2‐rich volcanic gases. This provides novel insights into the composition and formation of saponite‐rich deposits under a specific geodynamic context such as the Cretaceous South Atlantic carbonate reservoirs. 相似文献
559.
560.
When a stellar wind bubble expands into an homogeneous medium, there are two possible outcomes. This is due to the fact that the self-gravity of the swept-up shell acts in two orthogonal directions: tangentially , to promote fragmentation of the shell, and radially , to decelerate expansion of the shell. The outcome depends on whether self-gravity works faster in the tangential or the radial direction.
If the wind power ℒo is large and the effective isothermal sound speed a o in the swept-up gas is small – approximately – tangential self-gravity works faster. A thin dense shell is swept up and fragments while it is still expanding supersonically. This is the scenario often invoked to explain sequentially self-propagating star formation.
However, if ℒo is small and/or a o is large, radial self-gravity works faster. Expansion of the bubble stalls before the shell can fragment. The expansion speed ceases to be supersonic, the outer shock dissipates, and the shell is neither thin nor dense. Under this circumstance, the shell is unlikely to fragment and star formation will not propagate sequentially.
These conclusions are probably not altered significantly when the medium into which the wind blows is inhomogeneous, provided that the mean density on opposite sides of the bubble does not differ by many orders of magnitude. 相似文献
If the wind power ℒ
However, if ℒ
These conclusions are probably not altered significantly when the medium into which the wind blows is inhomogeneous, provided that the mean density on opposite sides of the bubble does not differ by many orders of magnitude. 相似文献