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1.
To investigate muon-induced nuclear reactions leading to the production of radionuclides, targets made of C 9H 12, SiO 2, Al 2O 3, Al, S, CaCO 3, Fe, Ni, Cu, Gd, Yb and Tl were irradiated with 100 and 190 GeV muons in the NA54 experimental setup at CERN. The radionuclide concentrations were measured with accelerator mass spectrometry and γ-spectroscopy. Results are presented for the corresponding partial formation cross-sections. Several of the long-lived and short-lived radionuclides studied are also produced by fast cosmic ray muons in the atmosphere and at depths underground. Because of their importance to earth sciences investigations, calculations of the depth dependence of production rates by fast cosmic ray muons have been made. 相似文献
2.
Noble gases were extracted in steps from grain size fractions of microdiamonds ( < 100 μm) from the Kokchetav Massif, Northern Kazakhstan, by pyrolysis and combustion. The concentration of 4He in the diamonds proper (liberated by combustion) shows a 1/ r dependence on grain size. For grain diameters > 15 μm the concentration also decreases with the combustion step. Both results are clear evidence that 4He has been implanted into the diamonds from -decaying elements in the surrounding matrix. The saturation concentration of 4He(5.6 × 10 −4 cm 3 STP/g) is among the very highest observed in any terrestrial diamonds. Fission xenon from the spontaneous fission of 238U accompanies the radiogenic 4He; the 136Xe f/ 4He ratio of (2.5 ± 0.3) × 10 −9 agrees well with the production ratio of 2.3 × 10 −9 expected in a reservoir where Th/U 3.3. Radiogenic 40Ar is predominantly ( > 90%) set free upon combustion; it also resides in the diamonds and appears to have been incorporated into the diamonds upon their formation. 3He, on the other hand is mainly released during pyrolysis and hence is apparently carried by ‘contaminants’. The concentration in the diamonds proper is of the order of 4 × 10−12 cm3 STP/g, with a 3He/4He ratio of 1 × 10−8. Excess 21Ne, similarly, appears to be present in contaminants as well as in diamonds proper. These two nuclides in the contaminants must have a nucleogenic origin, but it is difficult to explain their high concentrations. 相似文献
3.
Cosmic-ray-produced 26Al ( t1/2 = 7.05 × 10 5 years) has been measured in the Apollo 15 long core (surface to 390 g/cm 2—218 cm) for study of galactic cosmic ray production profiles, using accelerator mass spectrometry. The results are in general accord with non-destructive counting data obtained earlier, but systematically lower, and significantly higher precision. From this experiment the half-attenuation length for 26Al production can be calculated to be 122 g/cm 2 (150–400 g/cm 2 region) after normalizing the data to average chemical composition. The 53Mn ( t1/2 = 3.7 × 10 6 years) production profile in deep cores was also compiled to date. The half-attenuation length for 53Mn production was calculated to be 123 g/cm 2 (150–400 g/cm 2 region). 相似文献
4.
Experiments [T. Irifune (1994) Nature 370, 131–133; E. Ito et al. (1998) Geophys. Res. Lett. 25, 821–824; A. Kubo, M. Akaogi (2000) Phys. Earth Planet. Int. 121, 85–102] indicate that (Mg,Fe)SiO 3 perovskite, commonly believed to be the most abundant mineral in the Earth, is the preferred host phase of Al 2O 3 in the Earth’s lower mantle. Aiming to better understand the effects of Al 2O 3 on the thermoelastic properties of the lower mantle, we use atomistic models to examine the chemistry and elasticity of solid solutions within the MgSiO 3(perovskite)–Al 2O 3(corundum)–MgO(periclase) mineral assemblage under conditions pertinent to the lower mantle: low Al cation concentrations, P=25–100 GPa, and T=1000–2000 K. We assess the relative stabilities of two likely substitution mechanisms of Al into MgSiO 3 perovskite in terms of reactions involving MgSiO 3, MgO, and Al 2O 3, in a manner similar to the 0 Kelvin calculations of Brodholt [J.P. Brodholt (2000) Nature 407, 620–622] and Yamamoto et al. [T. Yamamoto et al. (2003) Earth Planet. Sci. Lett. 206, 617–625]. We determine the equilibrium composition of the assemblage by examining the chemical potentials of the Al 2O 3 and MgO components in solid solution with MgSiO 3, as functions of concentration. We find that charge coupled substitution dominates at lower mantle pressures and temperatures. Oxygen vacancy-forming substitution accounts for 3–4% of Al substitution at shallow lower mantle conditions, and less than 1% in the deep mantle. For these two pressure regimes, the corresponding adiabatic bulk moduli of aluminous perovskite are 2% and 1% lower than that of pure MgSiO 3 perovskite. 相似文献
5.
238U, 232Th, 230Th and 226Ra abundances have been measured in six samples of recent Hawaiian basalt by high precision mass spectrometry, in an attempt to compare the melting process in plumes and at spreading ridges. The data reveal a very small range in ( 230Th/ 238U) activity ratio up to a maximum value of 1.02 ± 0.01, and ( 226Ra/ 230Th) activity ratios which lie between 1.10 ± 0.015 and 1.19 ± 0.02. UTh and RaTh abundances are linearly correlated demonstrating that the disequilibria predate crystallisation and differentiation. Using recently published estimates for the bulk partition coefficients of U and Th, the results are consistent with melting rates > 10 −3 kg m −3 a −1 at porosities < 10 −3 for dynamic fractional melting in the garnet stability field. 相似文献
6.
We have developed techniques to determine 238U, 234U and 232Th concentrations in seawater by isotope dilution mass spectrometry. U measurements are made using a 233U 236U double spike to correct for instrumental fractionation. Measurements on uranium standards demonstrate that 234U/ 238U ratios can be measured accurately and reproducibly. 234U/ 238U can be measured routinely to ± 5‰ (2σ) for a sample of 5 × 10 9 atoms of 234U (3 × 10 −8 g of total U, 10 ml of seawater). Data acquisition time is 1 hour. The small sample size, high precision and short data acquisition time are superior to-counting techniques. 238U is measured to ± 2‰ (2σ) for a sample of 8 × 10 12 atoms of 238U ( 3 × 10 −9 g of U, 1 ml of seawater). 232Th is measured to ± 20‰ with 3 × 10 11232Th atoms (10 −10 g 232Th, 1 1 of seawater). This small sample size will greatly facilitate investigation of the 232Th concentration in the oceans. Using these techniques, we have measured 238U, 234U and 232Th in vertical profiles of unfiltered, acidified seawater from the Atlantic and 238U and 234U in vertical profiles from the Pacific. Determinations of 234U/ 238U at depths ranging from 0 to 4900 m in the Atlantic (7°44′N, 40°43′W) and the Pacific (14°41′N, 160°01′W) Oceans are the same within experimental error (± 5‰,2σ). The average of these 234U/ 238U measurements is 144 ± 2‰ (2σ) higher than the equilibrium ratio of 5.472 × 10 −5. U concentrations, normalized to 35‰ salinity, range from 3.162 to 3.281 ng/g, a range of 3.8%. The average concentration of the Pacific samples (31°4′N, 159°1′W) is 1% higher than that of the Atlantic (7°44′N, 40°43′W and 31°49′N, 64°6′W). 232Th concentrations from an Atlantic profile range from 0.092 to 0.145 pg/g. The observed constancy of the 234U/ 238U ratio is consistent with the predicted range of 234U/ 238U using a simple two-☐ model and the residence time of deep water in the ocean determined from 14C. The variation in salinity-normalized U concentrations suggests that U may be much more reactive in the marine environment than previously thought. 相似文献
7.
Cosmic-ray-produced 10Be ( t1/2 = 1.6 × 10 6 years) and 36Cl ( t1/2 = 3.0 × 10 5 years) have been measured in the Apollo 15 long core for study of galactic cosmic ray production profiles using tandem accelerator mass spectrometry. From these experiments, the half-attenuation length for 10Be production and 36Cl production were calculated to be 120 g/cm 2 and 132 g/cm 2 (150–400 g/cm 2 region). The measured half-attenuation length for 10Be is slightly longer than that predicted by the Reedy-Arnold theoretical model. The flatter and somewhat deeper maximum seen in the 36Cl profile compared to the 10Be, 26Al and 53Mn profiles can be explained by production from secondary thermal neutrons on 35Cl. 相似文献
8.
High-sensitivity stepped extraction reveals two isotopically distinct forms of carbon in submarine basalt glasses: an isotopically light carbon component released by combustion from 200 to 600°C and an isotopically heavy CO 2 liberated from vesicles (magmatic carbon) from 600 to 1200°C. The δ 13C PDB of the low release temperature carbon varies from −24 to −30‰ and is believed to be surficial organic contamination. A survey of various types of oceanic glasses demonstrates that the δ 13C of magmatic CO 2 varies from −4.2 to −7.5‰ in mid-ocean ridge basalt (MORB), from −2.8 to −6.7‰ in glasses from Hawaii and Explorer Seamount and from −7.7 to −16.3‰ in glasses from the Scotia Sea and Mariana Trough. Magmatic CO 2 in back-arc basin basalts (BABB) is on average 5‰ lighter than equivalent CO 2 in MORB and can be explained by the mixing in the source regions for BABB magmas of juvenile (MORB-like) CO 2 with an organic carbon component from subducted pelagic sediments. It is inferred that significant amounts of pelagic carbonate carbon (δ 13C 0‰) must be recycled into the mantle. 相似文献
9.
Chemical and isotopic ratio (He, C, H and O) analysis of hydrothermal manifestations on Pantelleria island, the southernmost active volcano in Italy, provides us with the first data upon mantle degassing through the Sicily Channel rift zone, south of the African–European collision plate boundary. We find that Pantelleria fluids contain a CO 2–He-rich gas component of mantle magmatic derivation which, at shallow depth, variably interacts with a main thermal (100°C) aquifer of mixed marine–meteoric water. The measured 3He/ 4He ratios and δ 13C of both the free gases (4.5–7.3 Ra and −5.8 to −4.2‰, respectively) and dissolved helium and carbon in waters (1.0–6.3 Ra and −7.1 to −0.9‰), together with their covariation with the He/CO 2 ratio, constrain a 3He/ 4He ratio of 7.3±0.1 Ra and a δ 13C of ca. −4‰ for the magmatic end-member. These latter are best preserved in fluids emanating inside the active caldera of Pantelleria, in agreement with a higher heat flow across this structure and other indications of an underlying crustal magma reservoir. Outside the caldera, the magmatic component is more affected by air dilution and, at a few sites, by mixing with either organic carbon and/or radiogenic 4He leached from the U–Th-rich trachytic host rocks of the aquifer. Pantelleria magmatic end-member is richer in 3He and has a lower (closer to MORB) δ 13C than all fluids yet analyzed in volcanic regions of Italy and southern Europe, including Mt. Etna in Sicily (6.9±0.2 Ra, δ 13C=−3±1‰). This observation is consistent with a south to north increasing imprint of subducted crustal material in the products of Italian volcanoes, whose He and C (but also O and Sr) isotopic ratios gradually evolve towards crustal values northward of the African–Eurasian plate collision boundary. Our results for Pantelleria extend this regional isotopic pattern further south and suggest the presence of a slightly most pristine or ‘less contaminated’, 3He-richer mantle source beneath the Sicily Channel rift zone. The lower than MORB 3He/ 4He ratio but higher than MORB CO 2/ 3He ratio of Pantelleria volatile end-member are compatible with petro-geochemical evidence that this mantle source includes an upwelling HIMU–EM1-type asthenospheric plume component whose origin, according to recent seismic data, may be in the lower mantle. 相似文献
10.
Gas is extracted from large (6–31 kg) Antarctic ice samples to obtain sufficient CO 2 for 14C measurements with small low-level proportional counters. The 14C ages of Byrd core ice are in accord with glaciological estimates ranging from (2.2 −1.1+1.4)×10 3 yr at 271 m depth to more than 8 × 10 3 yr at 1071 m depth. The CO 2 abundances in gas extracted from Byrd core ice range from 0.0216 to 0.051%, with below present-day atmosphere CO 2 abundances for ice from 1068 and 1469 m depths. The CO 2 abundance in gas from Allan Hills surface ice samples ranges between four and six times the atmospheric value and the CO 2 had a specific activity three times that of contemporary carbon. A possible explanation for the anomalously high specific activity is surface melting with the incorporation into CO 2 of 14C produced by cosmic ray spallation of oxygen in ice. The CO 2 abundance in gas extracted from subsurface Allan Hills ice ranged from 0.030 to 0.065%, and the specific activities are below contemporary carbon, indicating ages greater than 5×10 3 yr. The 18O/ 16O ratio of oxygen in the trapped gas is the same as that of atmospheric oxygen and differs markedly from the 18O/ 16O ratio in the ice. The O 2, N 2, and Ar abundances and isotopic compositions are similar to those in contemporary air, except for positive 15N/ 14N ratios in a few samples. 相似文献
11.
A series of Co-60 compounds, considered as potentially useful tracers of groundwater movement, were prepared and evaluated in soil column and batch studies. The compounds were: potassium hexacyanocobaltate (III), K 3Co(CN) 6; potassium ethylenediamminetetraacetato-cobaltate (III), KCo(EDTA); ammonium 12-tungstodicobaltoate (III), (NH 4) 8[Co(II)Co(II)W 12O 42] · 20 H 2O; chloraquotetrammine cobalt (III) chloride, [Co(NH 3) 4(H 2O)Cl]Cl 2; and sodium hexanitrocobaltate (III), Na 3Co(NO 2) 6. The performance of K 3 60Co(CN) 6 and K 60Co EDTA were dependent on their environment and more especially on the type and amount of clay present. The EDTA cobalt complex cannot be used in soils that are basic or where a large fraction of the clay is saturated with Na + and/or Ca ++. Of the other radioactive tracers tested, none were suitable for groundwater tracing because of instability in aqueous solution (Na 3Co(NO 2) 6), variable anionic structure under acid conditions ((NH 3) 8[Co(II)Co(II)W 12O 42] · 20 H 2O), or preferential uptake by the clay fraction ([Co(NH 3) 4(H 2O)Cl]Cl 2). 相似文献
12.
This study examined the effect of copper on the settlement success of planula larvae of the reef-building coral Acropora tenuis during 1994 and 1996 at Magnetic Island, Great Barrier Reef. Copper concentrations of 2, 10, 20 μg l −1 did not inhibit larval settlement after 48-h exposure. However, copper concentrations of 42 μg l −1 and 81 μg l −1 significantly reduced settlement success of A. tenuis larvae after 48-h exposure compared with controls using normal seawater. At 200 μg l −1 copper, all larvae died. EC 50 values for the effect of copper on A. tenuis larval settlement were calculated from the 1996 results using measured copper concentrations. The 48-h EC 50 was 35 μg l −1 with an upper and lower 95% confidence limit of 37 μg l −1 and 32 μg l −1, respectively. The 48-h NOEC value for both experiments was 20 μg l −1 copper. These experiments provide some of the first data on sub-lethal effects of trace metals on tropical marine organisms, and demonstrate that relatively low copper concentrations impair or inhibit settlement of coral larvae. 相似文献
13.
Chemical diffusion of four rare-earth elements (La, Nd, Dy and Yb) has been measured in natural calcite under anhydrous conditions, using rare-earth carbonate powders as the source of diffusants. Experiments were run in sealed silica capsules along with finely ground calcite to ensure stability of the single-crystal samples during diffusion anneals. Rutherford backscattering spectroscopy (RBS) was used to measure diffusion profiles. The following Arrhenius relations were obtained over the temperature range 600–850°C: DLa =2.6×10 −14 exp(−147±14 kJ mol −1/ RT) m 2 s −1, DNd =2.4×10 −14 exp(−150±13 kJ mol −1/ RT) m 2 s −1, DDy =2.9×10 −14 exp(−145±25 kJ mol −1/ RT) m 2 s −1, DYb =3.9×10 −12 exp(−186±23 kJ mol −1/ RT) m 2 s −1. In contrast to previous findings for refractory silicates (e.g. zircon), differences in transport rates among the REE are not pronounced over the range of temperature conditions investigated in this study. Diffusion of the REE is significantly slower than diffusion of the divalent cations Sr and Pb and slower than transport of Ca and C at temperatures above 650°C. Fine-scale zoning and isotopic and REE chemical signatures may be retained in calcites under many conditions if diffusion is the dominant process affecting alteration. 相似文献
14.
The impact of domestic sewage effluent (SE) on the dynamics of phytoplankton assemblages from Bedford Basin was evaluated in the laboratory. Phytoplankton production and chlorophyll a increased proportionally with SE enrichment. Phytoplankton species composition also changed. The potentially harmful diatoms, Pseudonitzschia spp., present initially in small numbers (600 cells 1 −1) in Bedford Basin seawater, and became dominant (3–5×10 6 cells 1 −1) when the seawater was enriched with 0.5–5% untreated SE. With higher proportions of SE, other harmful species such as Fragilaria spp. and Euglena spp. became dominant (7−15×10 6 and 2.2×10 4 cells 1 −1, respectively). Treatment of SE with UV light or activated charcoal seems to favour growth of benign species, such as Chaetoceros socialis, Skeletonema costatum and Thalassiosira spp., but not harmful species such as Pseudonitzschia spp. Further research on UV treatment of domestic sewage is recommended. 相似文献
15.
Carbon 14 activity measurements made by Accelerator Mass Spectrometry on two modern stalagmites from the Han-sur-Lesse cave (Belgium) and from the Postojna Cave (Slovenia) permit the construction of 14C activity ( a14C) time series over the last 50 years. A high precision chronology is given by annual laminae in the first stalagmite and by a specific mark (explosion in the Postojna Cave in 1944) in the second one. In both stalagmites, 14C activity increase due to nuclear tests in the atmosphere is remarkable. However, instead of a sharp peak like the one observed in the atmosphere around 1963–1964, the 14C activities of the stalagmite CaCO 3 show an abrupt increase, with an offset of 1–10 years, followed by a high activity plateau for the Han-sur-Lesse sample and a slight decrease for the Postojna sample. For both stalagmites, the variation of the a14C amplitude between pre- and post-bomb period is much lower than the atmospheric record, which demonstrates the damping effect of the soil carbon reservoir. We have modeled the CaCO 3 activities using fractionation processes between atmosphere CO 2, soil CO 2 and organic matter (OM), dissolved inorganic carbon and stalagmite CaCO 3. In both cases studied, the model and former soil studies suggest that CO 2 from soil organic matter (SOM) decomposition, which has a slow turnover (i.e. >1 y), is of major importance in winter, when the development of speleothem is the most important. Combined with the fact that 80–90% of the stalagmite carbon comes from soil CO 2, this produces a damping effect on the speleothem a14C. Consequently, the ‘geochemical time resolution', at least for speleothem carbon, is much lower than the structural resolution given by annual laminae alternations and is mainly controlled by soil carbon dynamics: a14C and δ 13C are smoothed over several years. Differences between the 14C time series of the Han-sur-Lesse and Postojna stalagmites are likely to be due to the double amount of precipitation in Postojna, which produces a faster soil OM turnover and thus a ‘system' which is more sensitive to atmospheric changes. 相似文献
16.
Gehlenite (Ca 2Al 2SiO 7) has been found to transform to a new phase at pressures greater than 100 kbar and at about 1000°C, using a diamond-anvil pressure cell coupled with laser heating. The atoms of the new phase appear to be arranged in a perovskite-related structure similar to that described for Na 2Ti 3O 7. The structure probably consists of layers of (Al 2SiO 7) 4−, which are built up from blocks of edge-sharing (Al, Si)O 6 octahedra and these blocks are joined by common octahedra corners. A small cubic unit cell with a = 3.719 ± 0.004 Å indexes completely the strong lines of the powder diffraction pattern, and a superlattice with a = 14.88 ± 0.02 Å satisfies all the observed weak lines in addition to the strong ones. However, the cell may be pseudocubic. The small cell contains a half of the gehlenite formula while the large cell contains 32 gehlenite formulae. Hence the molar volume for the new phase of Ca 2Al 2SiO 7 is calculated to be 61.96 ± 0.20 cm 3 at atmospheric pressure and room temperature. The new sodium titanate-type structure is probably more closely packed than an ordinary perovskite-type structure in which all octahedral corners are shared. This view is strongly supported by the very great density of this new phase, which is about 8% denser than the equivalent mixture of CaAl 2O 4 (calcium ferrite type) plus CaSiO 3 (cubic perovskite type). The new phase is probably the most closely packed silicate known. Mg 2SiO 4 (spinel) was found to transform to an assemblage containing MgSiO 3 (perovskite) plus MgO (periclase) at P-T conditions equivalent to the upper part of the lower mantle. By reacting with MgO, the perovskite modification of both MgSiO 3 and MgSiO 3 · xAl 2O 3 may adopt the sodium titanate structure at the still greater depths of the lower mantle. If the sodium titanate structures of Mg 2(Al 2Si)O 7 and Mg 2(MgSi 2)O 7 are present in the deep part of the lower mantle, MgO does not exist as a separate phase at the mantle-core boundary. This might be an obstacle to the possibility of dissolving these oxides (specifically the FeO component) in the molten Fe in the outer core as suggested by geophysical and geochemical studies of the earth's interior. The mechanism for developing the chemical plumes in the deep mantle proposed by Anderson does not appear to be consistent with studies of phase transformations in Ca-Al-rich compounds as outlined in this paper. 相似文献
17.
Four species of marine calcifying algae, the coccolithophores Calcidiscus leptoporus, Helicosphaera carteri, Syracosphaera pulchra and Umbilicosphaera foliosa were grown in laboratory cultures under temperatures varying between 14 and 23 °C, and one species, C. leptoporus, under varying [CO 32−], ranging from 105 to 219 μmol/kg. Calcium isotope compositions of the coccoliths resemble in both absolute fractionation and temperature sensitivity previous calibrations of marine calcifying species e.g. Emiliania huxleyi (coccolithophores) and Orbulina universa (planktonic foraminifera) as well as inorganically precipitated CaCO 3, but also reveal small species specific differences. In contrast to inorganically precipitated calcite, but similar to E. huxleyi and O. universa, the carbonate ion concentration of the medium has no statistically significant influence on the Ca isotope fractionation of C. leptoporus coccoliths; however, combined data of E. huxleyi and C. leptoporus indicate that the observed trends might be related to changes of the calcite saturation state of the medium. Since coccoliths constitute a significant portion of the global oceanic CaCO 3 export production, the Ca isotope fractionation in these biogenic structures is important for defining the isotopic composition of the Ca sink of the ocean, one of the key parameters for modelling changes to the marine Ca budget over time. For the present ocean our results are in general agreement with the previously postulated and applied mean value of the oceanic Ca sink (Δ sed) of about − 1.3‰, but the observed inter- and intra-species differences point to possible changes in Δ sed through earth history, due to changing physico-chemical conditions of the ocean and shifts in floral and faunal assemblages. 相似文献
18.
Highly vesicular, microporphyritic basaltic rocks have been dredged from the slow-spreading Spiess Ridge segment of the Southwest Indian Ridge. All the samples recovered are hyalocrystalline with plagioclase, clinopyroxene and olivine as phenocryst and microphenocryst phases. Titanomagnetite occurs as euhedral microphenocrysts in some of the more evolved samples. In terms of bulk rock and quench glass chemistry the lavas are characterised by highly evolved compositions(e.g. FeO*=10.3−14.2%;TiO 2=2.0−3.4%;K 2O=0.50−1.1%;MgO=6.0−3.5%;Zr=160−274ppm;Nb=14−32ppm) and can be classified as ferrobasalts. Isotopic and incompatible element ratios of the lavas(e.g. 87Sr/ 86Sr=0.70325−0.70333;Zr/Nb=8.4−11.3;Y/Nb=2.3−1.4) indicate their strongly “enriched” nature (see also Dickey et al. [6]). Quantitative major and trace element modelling indicates that most of the compositional variations observed can be attributed to low-pressure fractional crystallisation of plagioclase, clinopyroxene and minor olivine and titanomagnetite. The range in composition can be accounted for by up to 65% fractional crystallisation. We suggest that the extreme differentiation of the Spiess Ridge lavas is related not to spreading rate, but to rate of magma supply. The basaltic melts appear to have evolved in a newly established zone of magmatic activity, associated with the most recent northward jump of the Bouvet triple junction, where they were effectively isolated from significant admixture of primitive magmas. 相似文献
19.
This study aimed to determine whether the δ 13C levels in the foliage and twigs of four Eucalyptus grandis clones were related to their water use efficiency (WUE). This relationship has previously been demonstrated in a number of herbaceous species but not in mature trees. The study involved accurate measurements of tree trunk growth and water use over a period of 4 months, with subsequent isotopic analysis of mature foliage from the north and south side of the canopy, and young leaves from the top of the canopy. The water use efficiencies were found to vary from 5.97 × 10−3 to 12.3 × 10−3 m3 m−3. Significant differences were observed between clonal-mean water use efficiencies averaged over six sampling periods. The average δ13C of the mature and young foliage was found to be significantly correlated with WUE. However, the correlation was weak, suggesting that the relationship between δ13C and WUE is more complex in trees than suggested in the literature on crop plants. It is suggested that differences between sample trees in carbon allocation and leaf-to-air vapour pressure deficits may account for the poor correlation between δ13C and WUE in the four E. grandis clones studied. 相似文献
20.
A new method is described for estimating: (a) the meridional electric current density, jθ, (b) the vertical growth rate of the zonal magnetic field, ∂ Bφ/∂ r, or its scale-height, Bφ/∂ Bφ/∂ r) and (c) the vertical growth rate of the vertical current density, ∂ jr/∂ r, at a few isolated points on the top surface of the Earth's core from observations of the internal geomagnetic field at the Earth's surface. The theoretical technique rests on combining unaccelerated, gravitationally-driven Boussinesq fluid dynamics of the core with frozen-flux electromagnetism, the mantle being treated as a spherically symmetric insulator. Insertion into this theory of main field models for epochs 1965, 1975 leads to preliminary values for these quantities of magnitude: (a) jθ 1 A/m2, (b) ∂Bφ/∂r 10−6 T/m or Bφ/(∂Bφ/∂r) 10 m, (c) ∂jr/∂r 10−6 A/m3. Some geophysical implications of these estimates are discussed. 相似文献
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