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391.
Michiel Rutgers van der Loeff Manmohan M. Sarin Mark Baskaran Claudia Benitez-Nelson Ken O. Buesseler Matt Charette Minhan Dai
rjan Gustafsson Pere Masque Paul J. Morris Kent Orlandini Alessia Rodriguez y Baena Nicolas Savoye Sabine Schmidt Robert Turnewitsch Ingrid Vge James T. Waples 《Marine Chemistry》2006,100(3-4):190
The short-lived thorium isotope 234Th (half-life 24.1 days) has been used as a tracer for a variety of transport processes in aquatic systems. Its use as a tracer of oceanic export via sinking particles has stimulated a rapidly increasing number of studies that require analyses of 234Th in both marine and freshwater systems. The original 234Th method is labor intensive. Thus, there has been a quest for simpler techniques that require smaller sample volumes. Here, we review current methodologies in the collection and analysis of 234Th from the water column, discuss their individual strengths and weaknesses, and provide an outlook on possible further improvements and future challenges. Also included in this review are recommendations on calibration procedures and the production of standard reference materials as well as a flow chart designed to help researchers find the most appropriate 234Th analytical technique for a specific aquatic regime and known sampling constraints. 相似文献
392.
Vera Ẑutić Boẑena Ćosović Elena Marčenko Nevenka Bihari Frano Kršinić 《Marine Chemistry》1981,10(6):505-520
Electrochemical methods based on adsorption of organic molecules at the mercury electrode-solution interface were used to investigate surfactant production by marine phytoplankton. Six species of marine phytoplankton, representing the classes of Bacillariophyceae, Haeptophyceae, Chlorophyceae and Cryptophyceae, were studied in batch cultures.Our experimental results showed that surfactants were produced in culture media by healthy exponential growing cells. The measured response was found to depend on the particular species and the age of the culture.Total surfactant content in culture media generally increased with cell density, while surfactants per cell showed an inverse relation to cell density. However, we found that in Cryptomonas culture medium, during the exponential growth, excretion of the insoluble surfactant material per cell was independent of cell concentration.In addition to culture experiments, surfactant activity at several northern Adriatic stations was measured during various stages of phytoplankton bloom. It was concluded that a significant part of surfactant activity in a seawater column is due to phytoplankton production. 相似文献
393.
394.
A. Kröner A. P. Willner E. Hegner A. Frischbutter J. Hofmann R. Bergner 《International Journal of Earth Sciences》1995,84(3):437-456
Single zircons from two orthogneiss complexes, the Grey Gneiss and Red Gneiss, the lowermost tectonic units in the Erzgebirge, were dated. The grey Freiberg Gneiss is of igneous origin and has a 207Pb/206Pb emplacement age of 550±7 Ma. A quartz monzonite from Lauenstein contains idiomorphic zircons with a mean 207Pb/206Pb age of 555±7 Ma as well as xenocrysts ranging in age between 850 and 1910 Ma. Red gneisses from the central Erzgebirge contain complex zircon populations, including numerous xenocrysts up to 2464 Ma in age. The youngest, idiomorphic, zircons in all samples yielded uniform 207Pb/206Pb ages between 550±9 and 554±10 Ma. Nd isotopic data support the interpretation of crustal anatexis for the origin of both units. Nd(t) values for the grey gneisses are –7.5 and –6.0 respectively, (mean crustal residence ages of 1.7–1.8 Ga). The red gneisses have a wider range in Nd(t) values from –7.7 to –2.8 (T
DM ages of 1.4–1.8 Ga). The zircon ages document a distinct late Proterozoic phase of granitoid magmatism, similar in age to granitoids in the Lusatian block farther north-east. However, Palaeozoic deformation as well as medium pressure metamorphism ( 8 kbar/600–650° C) are identical in both gneiss units and distinguish these rocks from the Lusatian granitoids. The grey and red gneisses were overthrust by units with abundant high-pressure relicts and a contrasting P-T evolution. Zircon xenocryst and Nd model ages in the range 1000–1700 Ma are similar to those in granitoid rocks of Lusatia and the West-Sudetes, and document a pre-Cadomian basement in parts of east-central Europe that, chronologically, has similarities with the Sveconorwegian domain in the Baltic Shield. 相似文献
395.
H. Krüger U. Fritze-V. Alvensleben K. J. Fricke H. -H. Loose 《Astrophysics and Space Science》1993,205(1):57-63
We present spectroscopic evolutionary synthesis calculations for starburst galaxies of various metallicities in order to model the broad emission lines HeII 4686 and CIII 4650 produced by Wolf-Rayet (WR) stars in the spectra of WR galaxies. The strengths of both lines strongly decrease with decreasing metallicity. The presence of WR emission features is a clear indicator of very recent star formation less than 4 to 7 × 106 yr ago. Bursts of duration 1 to 5 × 106 yr which lead to an increase in the total stellar mass in the galaxy by 0.1 to 10 % are compatible with the equivalent width of Hell »4686 observed in WR galaxies. 相似文献
396.
R. J. Stern A. Kröner R. Bender T. Reischmann A. S. Dawoud 《International Journal of Earth Sciences》1994,83(3):564-577
This paper provides new geochemical and isotopic data on the evolution of the western foreland to the Nubian shield of north-east Africa. There is abundant evidence for early to middle Proterozoic crust west of the River Nile, but this was severely affected by the Pan-African ( 500–900 Ma) orogenic cycle. The results are reported of Rb-Sr whole rock and zircon evaporation geochronological studies and whole rock Sm-Nd and feldspar Pb isotopic analyses for four rock units around Wadi Halfa in northernmost Sudan. These results indicate the presence of heterogeneous pre-Pan-African crustal components, preserved in mylonitic gneisses and in conglomerates that unconformably overlie the gneisses. Several episodes of crust formation, inferred from zircon ages, are preserved in the gneisses : 2.6, 2.4, 2.0, 1.7, 1.2 and 0.72 Ga. Nd model ages for the same units are invariably older than the zircon ages, yet still record a predominantly late Archaean and Palaeoproterozoic history, with depleted mantle model ages between 1.3 and 2.8 Ga. The earliest recorded Pan-African magmatic event is about 720 Ma and dates the beginning of collisional deformation. A younger Pan-African volcanic sequence ( 650 Ma) has isotopic compositions of Sr and Nd compatible with derivation from late Prote rozoic asthenospheric mantle. A 530 Ma anorogenic A-type granite also has isotopic compositions suggesting derivation from a primitive source. The inferred tectonic evolution began with rifting to form an oceanic re-entrant. This was followed by subduction leading to collision at about 700 Ma, accompanied by post-orogenic rifting at about 650 Ma. 相似文献
397.
We report precise 207Pb/206Pb single zircon evaporation ages for low-grade felsic metavolcanic rocks within the Onverwacht and Fig Tree Groups of the Barberton Greenstone Belt (BGB), South Africa, and from granitoid plutons bordering the belt. Dacitic tuffs of the Hooggenoeg Formation in the upper part of the Onverwacht Group yield ages between 3445 +/- 3 and 3416 +/- 5 Ma and contain older crustal components represented by a 3504 +/- 4 Ma old zircon xenocryst. Fig Tree dacitic tuffs and agglomerates have euhedral zircons between 3259 +/- 5 and 3225 +/- 3 Ma in age which we interpret to reflect the time of crystallization. A surprisingly complex xenocryst population in one sample documents ages from 3323 +/- 4 to 3522 +/- 4 Ma. We suspect that these xenocrysts were inherited, during the passage of the felsic melts to the surface, from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite-trondhjemite plutons along the southern and western margins of the BGB, and units predating any of the exposed greenstone or intrusive rocks. Several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 Ma. coeval with or slightly older than Onverwacht felsic volcanism, while the Kaap Valley pluton along the northwestern margin of the belt is coeval with Fig Tree dacitic volcanism. These results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites. Some of the volcanic plutonic units contain zircon xenocrysts older than any exposed rocks. These indicate the existence of still older units, possibly stratigraphically lower and older portions of the greenstone sequence itself, older granitoid intrusive rocks, or bodies of older, unrelated crustal material. Our data show that the Onverwacht and Fig Tree felsic units have distinctly different ages and therefore do not represent a single, tectonically repeated unit as proposed by others. Unlike the late Archaean Abitibi greenstone belt in Canada, which formed over about 30 Ma. exposed rocks in the BGB formed over a period of at least 220 Ma. The complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones. A surprising similarity in rock types, tectonic evolution, and ages of the BGB in the Kaapvaal craton of southern Africa and greenstones in the Pilbara Block of Western Australia suggests that these two terrains may have been part of a larger crustal unit in early Archaean times. 相似文献
398.
One of the major current global environmental challenges is the conservation of forest biodiversity. Deforestation and forest degradation continue despite international governmental agreements on forest conservation. In recent years private regulation in the international forest governance system has increased. Numerous partnerships between governments, business and/or civil society have been developed. Most of them focus on a single threat to forest biodiversity. This private regulation has had a limited positive impact. The most valuable contribution has been filling the gap of lack of implementation by governments. The forest governance system can become more effective if metagovernance is strengthened. 相似文献
399.
Ingrid Berrodier François Farges Marc Benedetti Gordon E. Brown Jr Michel Deveughèle 《Geochimica et cosmochimica acta》2004,68(14):3019-3042
Gold adsorption products on powdered ferrihydrite, goethite, and boehmite samples, prepared by reacting Au(III)-Cl solutions ([Au] = 4.2 × 10−5-9.0 × 10−3 M; [Cl] = 0.017-0.6 M) with these adsorbents at pH values of 4 to 9 and Au adsorption densities ranging from 0.046 to 1.53 μmol/m2 were characterized using Au-LIII XAFS spectroscopy. The solutions (before and after uptake) were investigated by Raman scattering to determine speciation and by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) to determine solution composition. We present an analysis of several effects that are observed in the Au LIII-edge XAFS spectra, including X-ray beam-induced photo-reduction, multi-electronic excitations, disorder effects, and multiple scattering, that would complicate interpretation of the spectra if not accounted for. A combination of methods (spectral deconvolution, principal component analysis, spectral inversion, and wavelet analysis) was used to identify and quantify these effects, to characterize the nature of mixed ligands around gold, and to distinguish between multiple-scattering features and features due to next-nearest neighbors in the XAFS spectra.Analysis of the Au-LIII XAFS spectra showed that Au(III) is present as square-planar Au(III)(O,Cl)4 complexes in the aqueous solutions and on the surfaces of the Al/Fe-(oxy)hydroxide adsorption samples with dominantly O ligands at pH > 6 and mixed O/Cl ligands at lower pH values. The EXAFS-derived Au-O and Au-Cl distances are 2.00(2) and 2.28(2) Å, respectively, and the magnitudes of the Debye-Waller factors and third cumulants from anharmonic analyses indicate very little thermal or positional disorder around Au(III) in the adsorption samples. Iron second neighbors are present around Au in the Au(III)/ferrihydrite and Au(III)/goethite adsorption samples, with Au-Fe distances of 3.1(1) and 3.3(1) Å. In boehmite, two sets of Au-Al distances were detected at 3.0(1) and 3.2(1) Å. A reverse Monte Carlo study of the XAFS spectroscopic data suggests the presence of a continuum of edge-shared AuO4-FeO6 distances, which cannot be described correctly by a classical model of these data in which only a mean distance (although severely under-estimated) is derived. 相似文献
400.