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501.
Os and Os enrichments and high-He/He sources in the Earth’s mantle: Evidence from Icelandic picrites
Alan D. Brandon David W. Graham Bjarni Gautason 《Geochimica et cosmochimica acta》2007,71(18):4570-4591
Picrites from the neovolcanic zones in Iceland display a range in 187Os/188Os from 0.1297 to 0.1381 (γOs = + 2.1 to +8.7) and uniform 186Os/188Os of 0.1198375 ± 32 (2σ). The value for 186Os/188Os is within uncertainty of the present-day value for the primitive upper mantle of 0.1198398 ± 16. These Os isotope systematics are best explained by ancient recycled crust or melt enrichment in the mantle source region. If so, then the coupled enrichments displayed in 186Os/188Os and 187Os/188Os from lavas of other plume systems must result from an independent process, the most viable candidate at present remains core-mantle interaction. While some plumes with high 3He/4He, such as Hawaii, appear to have been subjected to detectable addition of Os (and possibly He) from the outer core, others such as Iceland do not.A positive correlation between 187Os/188Os and 3He/4He from 9.6 to 19 Ra in Iceland picrites is best modeled as mixtures of 1 Ga or older ancient recycled crust mixed with primitive mantle or incompletely degassed depleted mantle isolated since 1-1.5 Ga, which preserves the high 3He/4He of the depleted mantle at the time. These mixtures create a hybrid source region that subsequently mixes with the present-day convecting MORB mantle during ascent and melting. This multistage mixing scenario requires convective isolation in the deep mantle for hundreds of million years or more to maintain these compositionally distinct hybrid sources. The 3He/4He of lavas derived from the Iceland plume changed over time, from a maximum of 50 Ra at 60 Ma, to approximately 25-27 Ra at present. The changes are coupled with distinct compositional gaps between the different aged lavas when 3He/4He is plotted versus various geochemical parameters such as 143Nd/144Nd and La/Sm. These relationships can be interpreted as an increase in the proportion of ancient recycled crust in the upwelling plume over this time period.The positive correlation between 187Os/188Os and 3He/4He demonstrates that the Iceland lava He isotopic compositions do not result from simple melt depletion histories and consequent removal of U and Th in their mantle sources. Instead their He isotopic compositions reflect mixtures of heterogeneous materials formed at different times with different U and Th concentrations. This hybridization is likely prevalent in all ocean island lavas derived from deep mantle sources. 相似文献
502.
A two-dimensional vertically integrated hydrodynamic finite-element model of the west coast of Britain is used to examine
the response of the region to extreme meteorological forcing. The extent to which tide–surge interaction modifies the computed
surge elevation and current distributions is examined in detail. The nature of the finite-element model with its ability to
refine the mesh in nearshore regions is ideal for examining the influence of non-linear effects upon surges in these regions.
Calculations using spatially uniform orthogonal wind stresses show that the surge elevation and current in shallow water are
particularly sensitive to the method used to remove the tide as a result of the highly non-linear nature of the tide–surge
interaction in these regions. The most accurate means of de-tiding the solution is by subtracting a tide derived by harmonic
analysis of the tide and surge time series at the time of the surge. Subtracting a tide-only solution (the usual approach)
leads to tidal energy leaking into the surge solution. Calculations show that this arises because the surge modifies the tidal
amplitude and phase in shallow-water regions to such an extent that they are appreciably different to those found in the tide-only
calculation. Results suggest that this problem becomes more important, as nearshore meshes are refined in an attempt to improve
surge prediction. This suggests that in the future, highly accurate fine-mesh models will be required to compute total water
levels without the present linear separation into tidal and surge signal used in operational surge prediction. 相似文献
503.
A finite element model of the Irish and Celtic Sea regions with a range of grid resolutions is used to examine the influence of resolution upon the higher harmonics of the tide in the region. Comparisons are also made with published results from finite difference models of the area, and observations. Calculations using fine near-shore elements with non-zero water depths in coastal regions were found to be more accurate and less time consuming than those using a zero coastal water depth. A detailed examination of the spatial variability of the higher harmonics in near-shore regions of the eastern Irish Sea particularly the Solway and Morecambe Bay showed significant small-scale variability. This together with the variation in higher harmonics in the eastern Irish Sea and adjacent estuaries, clearly shows the need for an unstructured grid model of the region that can include the estuaries. To match the high resolution of the model in near-shore regions accurate high-resolution topography is required. 相似文献
504.
Climate and Rural Income 总被引:3,自引:0,他引:3
Robert Mendelsohn Alan Basist Pradeep Kurukulasuriya Ariel Dinar 《Climatic change》2007,81(1):101-118
This paper tests whether climate has an impact on per capita rural income. The study finds that total (agricultural and nonagricultural)
income in rural counties and municipios in the US and Brazil are affected by climate. The study demonstrates that this income
effect is due to changes in the net value of agriculture. Regions with better climates for agriculture support higher rural
incomes and regions with poor climates have more rural poverty. The results also suggest that global warming will likely increase
rural poverty. 相似文献
505.
An incentive mechanism for reducing emissions from conversion of intact and non-intact forests 总被引:1,自引:0,他引:1
Danilo Mollicone Frédéric Achard Sandro Federici Hugh D. Eva Giacomo Grassi Alan Belward Frank Raes Günther Seufert Hans-Jürgen Stibig Giorgio Matteucci Ernst-Detlef Schulze 《Climatic change》2007,83(4):477-493
This paper presents a new accounting mechanism in the context of the UNFCCC issue on reducing emissions from deforestation
in developing countries, including technical options for determining baselines of forest conversions. This proposal builds
on the recent scientific achievements related to the estimation of tropical deforestation rates and to the assessment of ‘intact’
forest areas. The distinction between ‘intact’ and ‘non intact’ forests used here arises from experience with satellite-based
deforestation measurements and allows accounting for carbon losses from forest degradation. The proposed accounting system
would use forest area conversion rates as input data. An optimal technical solution to set baselines would be to use historical
average figures during the time period from 1990 to 2005. The system introduces two different schemes to account for preserved
carbon: one for countries with high forest conversion rates where the desired outcome would be a reduction in their rates,
and another for countries with low rates. A ‘global’ baseline rate would be used to discriminate between these two country
categories (high and low rates). For the hypothetical accounting period 2013–2017 and considering 72% of the total tropical
forest domain for which data are available, the scenario of a 10% reduction of the high rates and of the preservation of low
rates would result in approximately 1.6 billion tCO2 of avoided emissions. The resulting benefits of this reduction would be shared between those high-rate countries which reduced
deforestation and those low-rate countries which did not increase their deforestation over an agreed threshold (e.g., half
of “global” baseline rate). 相似文献
506.
Alan Penny 《Astronomy& Geophysics》2002,43(6):6.30-6.31
Asteroseismology and extrasolar planets are the main science goals of the Eddington mission, now approved by ESA for a 2007 launch. Alan Penny presents a summary of the January 2002 RAS meeting that discussed the sciences of this wide-field high-precision photometric space telescope. Since the date of this meeting, ESA has decided to implement the mission in the framework of a 2007–08 launch. 相似文献
507.
Citrate released by plants, bacteria, and fungi into soils is subject to abiotic oxidation by MnO2(birnessite), yielding 3-ketoglutarate, acetoacetate, and MnII. Citrate loss and generation of products as a function of time all yield S-shaped curves, indicating autocatalysis. Increasing the citrate concentration decreases the induction period. The maximum rate (rmax) along the reaction coordinate follows a Langmuir-Hinshelwood dependence on citrate concentration. Increases in pH decrease rmax and increase the induction time. Adding MnII, ZnII, orthophosphate, or pyrophosphate at the onset of reaction decreases rmax. MnII addition eliminates the induction period, while orthophosphate and pyrophosphate addition increase the induction period. These findings indicate that two parallel processes are responsible. The first, relatively slow process involves the oxidation of free citrate by surface-bound MnIII,IV, yielding MnII and citrate oxidation products. The second process, which is subject to strong positive feedback, involves electron transfer from MnII-citrate complexes to surface-bound MnIII,IV, generating MnIII-citrate and MnII. Subsequent intramolecular electron transfer converts MnIII-citrate into MnII and citrate oxidation products. 相似文献
508.
Phosphonoformic acid, oxalic acid, glyoxylic acid, and 10 additional organic compounds that are structurally related to them have been reacted with synthetic MnO2 (birnessite), consisting of 22% MnIII and 78% MnIV, and synthetic MnOOH (manganite), consisting solely of MnIII. Significant concentrations of dissolved MnIII were detected in reactions of phosphonoformic acid with MnOOH, indicating that ligand-assisted dissolution took place. Reaction of phosphonoformic acid with MnO2, and reaction of all other organic reactants with either MnOOH or MnO2, yielded only MnII, indicating that reductive dissolution was predominant. As far as reductive dissolution reactions are concerned, MnO2 yields a range of reactivity that is nearly 20-times greater than that of MnOOH. Oxidation converts phosphonoformic acid into orthophosphate ion, glyoxylic acid into formic acid, pyruvic acid into acetic acid, and 2,3-butanedione into acetic acid. When differences in surface area loading are accounted for, oxalic acid, pyruvic acid, and 2,3-butanedione yield virtually the same dissolution rates for the two (hydr)oxides. At pH 5.0, glyoxylic acid reacts 14-times faster with MnO2 than with MnOOH. MnO2 reacts more slowly than MnOOH by a factor of 1/16th with oxamic acid, 1/20th with lactic acid, and 1/33rd with dimethyl oxalate. Adsorptive, complexant, and reductant properties of the 13 organic reactants are believed responsible for the observed reactivity differences. 相似文献
509.
John T. WASSON Alan E. RUBIN Hisayoshi YURIMOTO 《Meteoritics & planetary science》2004,39(9):1591-1598
Abstract— Several recent studies have shown that materials such as magnetite that formed in asteroids tend to have higher Δ17O (=δ17O ? 0.52 × δ18O) values than those recorded in unaltered chondrules. Other recent studies have shown that, in sets of chondrules from carbonaceous chondrites, Δ17O tends to increase as the FeO contents of the silicates increase. We report a comparison of the O isotopic composition of olivine phenocrysts in low‐FeO (≤Fa1) type I and high‐FeO (≥Fa15) type II porphyritic chondrules in the highly primitive CO3.0 chondrite Yamato‐81020. In agreement with a similar study of chondrules in CO3.0 ALH A77307 by Jones et al. (2000), Δ17O tends to increase with increasing FeO. We find that Δ17O values are resolved (but only marginally) between the two sets of olivine phenocrysts. In two of the high‐FeO chondrules, the difference between Δ17O of the late‐formed, high‐FeO phenocryst olivine and those in the low‐FeO cores of relict grains is well‐resolved (although one of the relicts is interpreted to be a partly melted amoeboid olivine inclusion by Yurimoto and Wasson [2002]). It appears that, during much of the chondrule‐forming period, there was a small upward drift in the Δ17O of nebular solids and that relict cores preserve the record of a different (and earlier) nebular environment. 相似文献
510.