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81.
Silicon isotope fractionation in bamboo and its significance to the biogeochemical cycle of silicon 总被引:2,自引:0,他引:2
A systematic investigation on silica contents and silicon isotope compositions of bamboos was undertaken. Seven bamboo plants and related soils were collected from seven locations in China. The roots, stem, branch and leaves for each plant were sampled and their silica contents and silicon isotope compositions were determined. The silica contents and silicon isotope compositions of bulk and water-soluble fraction of soils were also measured. The silica contents of studied bamboo organs vary from 0.30% to 9.95%. Within bamboo plant the silica contents show an increasing trend from stem, through branch, to leaves. In bamboo roots the silica is exclusively in the endodermis cells, but in stem, branch and leaves, the silica is accumulated mainly in epidermal cells. The silicon isotope compositions of bamboos exhibit significant variation, from −2.3‰ to 1.8‰, and large and systematic silicon isotope fractionation was observed within each bamboo. The δ30Si values decrease from roots to stem, but then increase from stem, through branch, to leaves. The ranges of δ30Si values within each bamboo vary from 1.0‰ to 3.3‰. Considering the total range of silicon isotope composition in terrestrial samples is only 7‰, the observed silicon isotope variation in single bamboo is significant and remarkable. This kind of silicon isotope variation might be caused by isotope fractionation in a Rayleigh process when SiO2 precipitated in stem, branches and leaves gradually from plant fluid. In this process the Si isotope fractionation factor between dissolved Si and precipitated Si in bamboo (αpre-sol) is estimated to be 0.9981. However, other factors should be considered to explain the decrease of δ30Si value from roots to stem, including larger ratio of dissolved H4SiO4 to precipitated SiO2 in roots than in stem. There is a positive correlation between the δ30Si values of water-soluble fractions in soils and those of bulk bamboos, indicating that the dissolved silicon in pore water and phytoliths in soil is the direct sources of silicon taken up by bamboo roots. A biochemical silicon isotope fractionation exists in process of silicon uptake by bamboo roots. Its silicon isotope fractionation factor (αbam-wa) is estimated to be 0.9988. Considering the distribution patterns of SiO2 contents and δ30Si values among different bamboo organs, evapotranspiration may be the driving force for an upward flow of a silicon-bearing fluid and silica precipitation. Passive silicon uptake and transportation may be important for bamboo, although the role of active uptake of silicic acid by roots may not be neglected. The samples with relatively high δ30Si values all grew in soils showing high content of organic materials. In contrast, the samples with relatively low δ30Si values all grew in soil showing low content of organic materials. The silicon isotope composition of bamboo may reflect the local soil type and growth conditions. Our study suggests that bamboos may play an important role in global silicon cycle. 相似文献
82.
A. van Geen K. Radloff Z. Aziz Z. Cheng M.R. Huq K.M. Ahmed B. Weinman S. Goodbred H.B. Jung Y. Zheng M. Berg P.T.K. Trang L. Charlet J. Metral D. Tisserand S. Guillot S. Chakraborty A.P. Gajurel B.N. Upreti 《Applied Geochemistry》2008
One of the reasons the processes resulting in As release to groundwater in southern Asia remain poorly understood is the high degree of spatial variability of physical and chemical properties in shallow aquifers. In an attempt to overcome this difficulty, a simple device that collects groundwater and sediment as a slurry from precisely the same interval was developed in Bangladesh. Recently published results from Bangladesh and India relying on the needle-sampler are augmented here with new data from 37 intervals of grey aquifer material of likely Holocene age in Vietnam and Nepal. A total of 145 samples of filtered groundwater ranging in depth from 3 to 36 m that were analyzed for As (1–1000 μg/L), Fe (0.01–40 mg/L), Mn (0.2–4 mg/L) and S (0.04–14 mg/L) are compared. The P-extractable (0.01–36 mg/kg) and HCl-extractable As (0.04–36 mg/kg) content of the particulate phase was determined in the same suite of samples, in addition to Fe(II)/Fe ratios (0.2–1.0) in the acid-leachable fraction of the particulate phase. Needle-sampler data from Bangladesh indicated a relationship between dissolved As in groundwater and P-extractable As in the particulate phase that was interpreted as an indication of adsorptive equilibrium, under sufficiently reducing conditions, across 3 orders of magnitude in concentrations according to a distribution coefficient of 4 mL/g. The more recent observations from India, Vietnam and Nepal show groundwater As concentrations that are often an order of magnitude lower at a given level of P-extractable As compared to Bangladesh, even if only the subset of particularly reducing intervals characterized by leachable Fe(II)/Fe >0.5 and dissolved Fe >0.2 mg/L are considered. Without attempting to explain why As appears to be particularly mobile in reducing aquifers of Bangladesh compared to the other regions, the consequences of increasing the distribution coefficient for As between the particulate and dissolved phase to 40 mL/g for the flushing of shallow aquifers of their initial As content are explored. 相似文献
83.
M. El Ghorfi F. Melcher T. Oberthür A. E. Boukhari L. Maacha A. Maddi M. Mhaili 《Mineralogy and Petrology》2008,92(1-2):59-80
Summary The Neoproterozoic Bou Azzer ophiolite complex hosts numerous, small lenticular bodies of massive and disseminated chromite.
Metallurgical-grade high-Mg and high-Cr spinels (cores with 48–62 wt% Cr2O3) reveal complex alteration patterns of successive Cr and Mn enrichment and loss of Al towards the rims, while the Mg# ratios
[(Mg/(Mg + Fe2+)] remain almost constant. Concentration patterns of platinum-group elements are typical for ophiolitic chromitite poor in
sulfides, with predominance of the IPGE, variable Rh, and low Pt and Pd. The most abundant platinum-group mineral is Rh-bearing
laurite that occurs either included in spinel or in silicate matrix, whereas Os-Ir-Ru alloy is always included in spinel.
Laurite inclusions reveal complex intergrowth textures with Rh-Ru-Pt rich alloy, and with Rh-rich sulfide. Most laurites display
trends to sulfur-poor compositions leading to local formation of very fine-grained Ru-Os-Ir alloy phases. Ni-Co-Fe sulfides,
arsenides and sulfarsenides devoid of PGE are associated with the alteration of chromite. Textural position and chemical composition
of the base metal inclusions, as well as comparison of alteration features between chromite and accessory chromian spinel
in the Co-Ni-As ores of the Bou Azzer ophiolite indicate a close connection. It is suggested that hydrothermal fluids percolated
through the marginal zones of the ophiolite belt during greenschist facies metamorphism and deposited Ni-Co-Fe arsenides,
sulfarsenides and minor sulfides as accessories within altered chromitites, and also in structurally favourable zones as Ni-Co-As
ores.
Author’s address: Dr. Frank Melcher, Federal Institute for Geosciences and Natural Resources, Stilleweg 2, 30655 Hannover, Germany 相似文献
84.
National flood discharge mapping in Austria 总被引:5,自引:0,他引:5
This article presents the approach and the results of a study in which 30, 100 and 200 year return period flood discharges
were estimated for 26,000 km of Austrian streams. Three guiding principles were adopted: combination of automatic methods
and manual assessments by hydrologists to allow speedy processing and account for the local hydrological situation; combination
of various sources of information including flood peak samples, rainfall data, runoff coefficients and historical flood data;
and involvement of the Hydrographic Services to increase the accuracy and enhance the acceptance of results. The flood discharges
for ungauged catchments were estimated by the Top-kriging approach with manual adjustment to the local flood characteristics.
The adopted combination approach proved to be very efficient both in terms of the project time required and in terms of the
accuracy and acceptability of the estimated flood discharges of given return periods. 相似文献
85.
B. Bühn 《Mineralogy and Petrology》2008,92(3-4):453-470
Summary Two bimodal carbonatite complexes in Namibia of Cretaceous age are explored as to the presence and composition of a coexisting
carbonatitic fluid. The Kalkfeld and Ondurakorume complexes contain both Ca- and Mg/Fe-carbonatites, composed of calcite alone
or calcite with ferroan dolomite, fluorapatite and strontianite. The major element evolution in the bulk rocks from s?vites
to beforsites is due to crystallization of calcite and fluorapatite. All carbonatites show a negative Y anomaly in normalised
REE plots. Fractionation is accompanied by successively lower HREE contents between Tb and Yb, expressed by the ratios Nd/Ho
and Ho/Lu. The evolution of this downward-facing hump goes along with decreasing Y contents in bulk rocks and minerals. All
this requires an additional phase coexisting with the carbonate liquid during fractionation. Comparison between the bulk rocks
and the expelled fluid shows that the latter had preferentially accumulated the HREE and Y. Further evidence for this process
are hydrothermal, HREE, Y-rich fluorites in other carbonatite complexes which reflect the composition of the expelled fluid.
The high strength of fluoride complexes suggests that fluoride complexing in the carbonatitic fluid is the process responsible
for extracting HREE and Y from the carbonatite magma, leaving fractionated carbonatite rocks depleted in these elements. The
geochemical evolution of carbonatite magmas along fractionation has therefore to be considered in a melt-mineral-fluid system.
Correspondence: B. Bühn, Instituto de Geociências, Universidade de Brasilia, Campus Universitário Darcy Ribeiro, Brasilia
70910-900, Brazil 相似文献
86.
87.
Abstract The Taiwan orogen has been the focus of a number of models of mountain building processes, but little attention has been paid to high‐pressure (HP) metamorphic rocks that are found as exotic blocks intermingled within the deepest units of the mountain belt. In this study, we re‐appraise from updated petrological and thermodynamic databases the physical conditions of HP metamorphism in Taiwan, and we combine our findings with available geochronological data to estimate the thermal history of these rocks. Our results indicate that peak metamorphic conditions of ~550 °C and 10–12 kbar have been followed by a rapid isothermal decompression, with exhumation possibly as rapid as burial. These units have subsequently been stored at a pressure of ~3 kbar for ~4–5 Myr, before their final exhumation, probably facilitated by the accretion of passive margin sequences during the Late Cenozoic collision. Therefore, HP units in Taiwan maintain a record of processes at depth from the early stages of oceanic subduction to the present arc‐continent collision. 相似文献
88.
A numerical analysis of cyclotron instabilities is carried out by computing the dispersion relation for a three component cold plasma-beam system. Rates of growth and damping for various values of the stream density are calculated from the dispersion relation. The rates of growth and damping increase monotonically as the number density of the proton stream increases. It is found that the frequencies at the rates of maximum growth and the damping decrease slightly to lower frequencies and a sharp peak at these frequencies becomes blunt. The minimum e-folding times of an ion cyclotron wave for (a) σs = 10−4, σi = 10−2 and (b) σs = 10−1, σi = 10−2 are about 3·84 and 0·16 sec respectively in the vicinity of the equatorial plane at 6 Re, where σs and σi are the ratios of the beam density Ns and the helium ion (H6+) density Ni to the total positive ions in the plasma-beam system. 相似文献
89.
Zusammenfassung Die Davoser Windverh?ltnisse werden in ihrem tages- und jahreszeitlichen Ablauf sowie in ihrer Abh?ngigkeit von verschiedenen
typischen Wetterlagen untersucht. Neben den ungest?rten Sch?nwetterlagen mit ihrem ausgepr?gten Talwind, der wie der Malojawind
talabw?rts weht, werden die F?hnstr?mung aus Südwest und die Nordoststr?mung bei Staulagen und Bise kurz besprochen. Gegenüber
dem benachbarten Dischmatal zeigt sich ein sp?terer Beginn des Talwindes bei Sch?nwetterlagen, w?hrend im Vergleich zu anderen
T?lern, z. B. zum Wallis, ein früheres und schw?cheres nachmitt?gliches Windmaximum auftritt.
Mit 6 Textabbildungen
Herrn Dr.W. M?rikofer zu seinem 70. Geburtstag gewidmet. 相似文献
Summary The wind at Davos is analysed in his diurnal and annual variation and in the depence on the different weather types. The fair and undisturbed weather shows a typical valley breeze during the day, but not in the normal direction towards the higher part of the valley, but downstream, similar to the Malojawind in the Engadine. The opposite wind direction, from southwest, characterises the foehn-situation. Northeasterly winds develop in the case of higher pressure north of the Alps (e. g. in situations of barrage). The valley-breeze begins later than in the adjacent Dischma Valley, but it is weaker and reaches the highest intensity earlier than in other valleys, as e. g. in the Valais.
Résumé Les vents de Davos sont analysés selon leurs variations diurnes et annuelles et selon des types de temps caractéristiques. La situation de beau temps montre un vent de vallée assez accentué, soufflant du haut en bas, donc dans une direction anormale, mais analogue au vent de la Maloja en Engadine. En cas de ?foehn? le vent souffle du sud-ouest tandis qu'un vent du nord-est se manifeste par situations de surpression au nord des Alpes. Le vent de vallée se met à souffler plus tard à Davos que dans la vallée de Dischma toute proche. Il y atteint par contre sa vitesse maximale plus t?t que les vents correspondants d'autres vallées p. ex. du Valais. Le maximum y est cependant moins élevé.
Mit 6 Textabbildungen
Herrn Dr.W. M?rikofer zu seinem 70. Geburtstag gewidmet. 相似文献
90.
Dozent Dr. Siegfried Haussühl Dozent Dr. German Müller 《Contributions to Mineralogy and Petrology》1963,9(1):28-39
Summary Argillaceous sediments of jurassic and lower cretaceous age in NW-Germany contain in several localities authigenous ZnS-polytypes. The crystals with a maximum height of 2 mm exhibit hexagonal-pyramidal and trigonal-pyramidal habit.With X-ray rotation and precession technique the following polytypes could be identified: Sphalerite (3 R = 3 C) 211, 411, 6H 9R, 12R, 15R and 21R.The 9 R-, 12 R- and 21 R-polytypes have not until now been described elsewhere.The structures of the new polytypes were determined as 9R=(21), 12R=(13) and 21R=(3112).Oriented intergrowths of R- and H-polytypes were frequently observed. From the intergrowth-phenomena arguments for the environmental conditions of formation can be deducted.[/p] 相似文献