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171.
The effect of blue mussel (Mytilus edulis) presence in eelgrass (Zostera marina) beds was studied from June 2004 to July 2005 in Flensborg fjord, Denmark. The field experiments were conducted at two stations, one with only Z. marina (Eelgrass station) present and one where M. edulis were present in the Z. marina beds (Mixed station). Zostera marina parameters were measured (growth of leaves, shoot density, leaf length, and nutrient content) in combination with epiphyte cover and sediment parameters (sulphate reduction rates, sediment nutrient fluxes, organic content, C, N and P content) to examine possible positive and negative effects of the mussels on eelgrass performance. The fluxes of ammonium from the sediments were stimulated at all sampling dates at the Mixed station, and possibly stimulated epiphyte growth at this station. Further 15N signals in epiphytes from the Mixed station suggested that excretion products from the mussels were important nitrogen sources at this station. Sulphate reduction rates were enhanced at the Mixed station and also sediment sulphide concentrations increased under mussel influence, which may have resulted in sulphide toxicity and decreased growth of Z. marina at this station. The study indicates that for Z. marina beds in Flensborg Fjord the effects of M. edulis in seagrass beds are primarily negative, and raises the question whether this leads to negative effects on the stability and expansion of Z. marina beds. 相似文献
172.
Recent developments in extreme values modelling have been used to develop a framework for determining the coastal erosion hazard on sandy coastlines. This framework quantitatively reproduced the extreme beach erosion volumes obtained from field measurements at Narrabeen Beach, Australia. This encouraging finding was achieved using Kriebel and Dean's [Kriebel, D.L. and Dean, R.G., 1993. Convolution method for time-dependent beach profile response. Journal of Waterway, Port, Coastal and Ocean Engineering, 119(2): 204–226.] simple beach erosion and accretion model. The method includes allowances for joint probability between all basic erosion variates including; wave height, period and direction, event duration, tidal anomalies and event spacing. A new formulation for the dependency between wave height and period has been developed. It includes the physical wave steepness limitation. Event grouping, where significantly more erosion can occur from two closely spaced storms is handled by temporally simulating the synthetic wave climate and the resulting beach erosion and accretion. 相似文献
173.
Benthic flora can contribute significantly to gross primary production (GPP) of shallow coastal waters where light reaches
the sea bottom. We quantified and compared benthic and pelagic GPP along nutrient gradients in time and space in the shallow
estuary, Limfjorden, Denmark, based on monitoring data combined with historical information. Limfjorden experienced a shift
from a pristine, benthic-dominated clear water regime with high total GPP in the early twentieth century to a eutrophic, plankton-dominated
regime still with high total GPP in the 1980s when nutrient loadings peaked. Recent reductions in nutrient loadings reduced
pelagic GPP, particularly in spring, but water clarity and benthic GPP did not increase correspondingly, so total GPP declined.
The most nutrient-rich basins have remained plankton-dominated, with benthic vegetation constrained to shallow waters. The
results support existing evidence that total GPP of shallow coastal areas does not increase systematically with eutrophication.
Furthermore, the results suggest that total GPP may decline temporarily during oligotrophication as pelagic GPP declines,
while feedback mechanisms delay or prevent restoration to a state with benthic dominance of GPP. 相似文献
174.
Melt inclusions in high-An plagioclase from the Gorda Ridge: an example of the local diversity of MORB parent magmas 总被引:2,自引:0,他引:2
R. L. Nielsen Jennifer Crum Rene Bourgeois Kaylea Hascall Lance M. Forsythe Martin R. Fisk David M. Christie 《Contributions to Mineralogy and Petrology》1995,122(1-2):34-50
The use of ocean floor basalt chemistry as a tool to investigate mantle composition and processes requires that we work with
basalts that have been modified little since leaving the mantle. One source of such basalts is melt inclusions trapped in
primitive crystals. However, obtaining information from these melt inclusions is complicated by the fact that melt inclusions
in natural basalts are essentially always altered by post-entrapment crystallization. This requires that we develop techniques
for reconstructing the original trapped liquid compositions. We conducted a series of experiments to reverse the effects of
post-entrapment crystallization by re-heating the host crystals to their crystallization temperature. For these experiments
we used plagioclase crystals separated from a single Gorda Ridge lava. The crystallization temperature for these crystals
was determined by a set of incremental re-heating experiments to be ∼1240–1260° C. The inclusions are primitive, high Ca-Al
basaltic melts, saturated with plagioclase, olivine and Al-rich chromite at low pressure. The inclusion analyses can be linked
to the host lava composition by low pressure fractionation. The major element composition of the re-homogenized melt inclusions
within each crystal is relatively constant. However, the incompatible element analyses have extremely wide ranges. The range
of La/Sm and Ti/Zr from inclusions analyzed from a single sample from the Gorda Ridge exceeds the range reported for lavas
sampled from the entire ridge. The pyroxene compositions predicted to be in equilibrium with the melt inclusion trace element
signature cover much of the range represented by pyroxenes from abyssal peridotites. The volumetric proportions of the magmas
entering the base of the crust can be evaluated using frequency distribution of melt inclusion compositions. This distribution
suggests that the array of magmas was skewed towards the more depleted compositions, with little evidence for an enriched
component in this system. This pattern is more consistent with a dynamic flow model of the mantle or of a passive flow model
where the melts produced in the peripheral areas of the melting regime were not focused to the ridge.
Received: 5 January 1995 / Accepted: 13 June 1995 相似文献
175.
Three‐dimensional modelling of differential railway track settlement using a cycle domain constitutive model
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A method for simulation of differential (spatially varying) track settlement in a ballasted railway track is presented. It employs a cycle domain constitutive model to determine accumulated plastic (permanent) deformation of the granular layers supporting the track. The constitutive model is adopted for both the ballast and the sub‐ballast but with different parameter sets. The proposed framework can be used to predict differential track settlement accounting for heterogeneous (space‐variant) track characteristics and loading conditions. Here, it is demonstrated for three‐dimensional continuum modelling of a railway crossing panel subjected to a large number of axle passages. Because of the design of the crossing panel and the transient character of the impact loads on the crossing, the load transferred into the track bed is not uniform along the track, and the resulting differential settlement leads to vertical irregularities in track geometry. The spatial variation of track settlement is calculated both along the sleepers and along the rails. The influences of the number of adjacent sleepers accounted for in the model and the stiffness of the subgrade on the predicted settlement at the crossing are studied. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
176.
Ilya V. Veksler Yana M. Fedorchuk Troels F. D. Nielsen 《Contributions to Mineralogy and Petrology》1998,131(4):347-363
The evolution of nephelinitic melts in equilibrium with mica-bearing liquidus assemblages and melting relations have been
studied on two silica-undersaturated joins of the KAlSiO4– Mg2SiO4– Ca2SiO4– SiO2– F system at atmospheric pressure by quench runs in sealed platinum capsules. Fluorine has been added to the batch compositions
by the direct exchange of fluorine for oxygen (2F− = O2−). The first join is the pseudo-ternary Forsterite – Diopside – KAlSiO3F2 system. Forsterite, diopside, F-phlogopite and leucite crystallisation fields and a fluoride-silicate liquid immiscibility
solvus are present on the liquidus surface of the join. Sub-liquidus and sub-solidus phases include akermanite, cuspidine,
spinel, fluorite and some other minor fluorine phases. The second system is the pseudo-binary Akermanite – F-phlogopite join
that intersects the Forsterite – Diopside – KAlSiO3F2 join. Akermanite, forsterite, diopside, F-phlogopite, leucite and cuspidine are found to crystallise on the join. Forsterite
(fo) and leucite (lc) are related to F-phlogopite (phl) by a reaction with the fluorine-bearing liquid: fo + lc + l = phl,
and the reaction proceeds until forsterite or leucite are completely consumed. The reaction temperature and resulting phase
association depend on batch composition. Thus, leucite is not stable in the sub-solidus of the Akermanite – F-phlogopite join,
but is preserved in a part of the Forsterite – Diopside – KAlSiO3F2 system where forsterite reacts out, or does not crystallise at all. The phlogopite-in reaction has an important effect on
the composition of the coexisting liquid. The liquids initially saturated in forsterite evolve to extremely Ca rich, larnite-normative
residuals. The experimental data show that larnite-normative melilitolites can crystallise from evolved melilititic melts
generated from “normal” melanephelinitic parental magmas with no normative larnite. The evolution towards melilitites requires fractionation of phlogopite-bearing assemblages under volatile pressure.
Received: 3 June 1997 / Accepted: 5 January 1998 相似文献
177.
Anette Reenberg Trine Louring Nielsen Kjeld Rasmussen 《Global Environmental Change》1998,8(4):309-327
The paper addresses the dynamic relationship between the human use of land and alterations in the biophysical environment, demographic pressure or socio-economic conditions. An empirical study from the Sahelian zone in northern Burkina Faso illustrates the dynamics of cultivation pattern at the village level and the changing priorities given to different landscape units over time. Field measurements, aerial photos and satellite images from seven successive years provide information on land use pattern changes from 1945 to 1995. A household survey illustrates how socio-economic and cultural parameters enable and constrain land use strategies at the farm level. 相似文献
178.
A numerical approach to boundary layer fractionation: application to differentiation in natural magma systems 总被引:1,自引:1,他引:0
With few exceptions, geochemical models of igneous differentiation have assumed that crystals form homogeneously throughout the cooling magma chamber. This is in spite of field evidence and physical models suggesting for natural systems, that heat loss, and thus most crystallization, occurs primarily along the walls and roof of magma chambers. Such systems would produce aphyric, evolved composition lavas. Existing models of boundary layer crystal fractionation suggest that the products of such a process are significantly different from the products of homogeneous crystallization. To investigate this process further, we have developed a model using a numerical approach to boundary layer crystal fractionation based on new phase equilibria and trace element constraints. In addition to olivine, augite and plagioclase, this new model includes the ability to calculate the effects of oxide (magnetite and ilmenite), apatite and low-Ca pyroxene fractionation. This is critical if we are to effectively evaluate the geochemical signature of boundary layer fractionation. This is because the products of boundary layer fractionation are most different from homogeneous fractionation for high degrees of crystallization in the solidification zone. These are the same conditions where oxides, low-Ca pyroxene and apatite are saturated in most natural, differentiated mafic systems. The results of this model suggest that boundary layer fractionation can help to explain such phenomena as phantom crystallization, variation in incompatible element ratios, and the decoupling of major and trace element systematics. In addition, many of the geochemical patterns that we use to distinguish between the products of low pressure and high pressure differentiation are obscured by the fact that the liquid line of descent can be controlled by the removal of phases not in equilibrium with the entire magma chamber. 相似文献
179.
We have studied the effect of light acclimation on photosynthetic responses and growth during Cu2+ exposure (0–0.84 μM) in the brown seaweed Fucus serratus. Measurements of chlorophyll fluorescence parameters showed that Cu2+ exposure amplified ETR, reduced the chlorophyll content at the cellular level and that there was no effect of light adaptation on the Cu2+ resistance of the algae. In contrast to the inhibitory effects of Cu2+ on chlorophyll fluorescence, O2 evolution and the total content of chlorophyll and carotenoid of the algae was unaffected by Cu2+. We conclude that photoinhibition and perhaps pigment degradation in the meristoderm was compensated for by cells deeper in the thallus with the result that the overall photosynthetic fitness of the algae was maintained. The pronounced inhibitory effects of Cu2+ on algae growth was not a consequence of photoinhibition and could be attributed to direct inhibitory effects on the growth process. 相似文献
180.
Melt inclusion formation mechanisms and compositional effects in high-An feldspar and high-Fo olivine in anhydrous mafic silicate liquids 总被引:1,自引:1,他引:0
Edward?KohutEmail author Roger?L.?Nielsen 《Contributions to Mineralogy and Petrology》2004,147(6):684-704
Important aspects of melt inclusion formation and potential compositions effects have been addressed through a series of experiments using anorthite/fosterite saturated anhydrous mafic liquids. Experimental charges were cooled from 1,300 to 1,230 and 1,210°C at rates of 1–10°/min followed by 0–24 h isothermal periods. Hopper and skeletal crystal morphologies with variable degrees of completeness developed during the cooling period. Planar overgrowth of these textures during isothermal periods led to the formation of inclusions, the majority of which formed after 6 h of isothermal run time. We suggest that the change in morphologies is related to a decrease in growth rates and changes in dominant growth mechanisms. In general, inclusion compositions were uniform and similar to the host glass, indicating that with the isothermal times required for most inclusions to form, a boundary layer was not entrapped that could be detected within the limits of our analyses.
相似文献
Edward KohutEmail: Phone: +1-541-7373023Fax: +1-541-7371200 |
Roger L. NielsenEmail: Phone: +1-541-7371235 |