U-Pb ages of uraniferous opals and implications for the history of beryllium,fluorine, and uranium mineralization at Spor Mountain,Utah

The U-Pb isotope systematics of uraniferous opals from Spor Mountain, Utah, were investigated to determine the suitability of such material for geochronologic purposes, and to estimate the timing of uranium and associated beryllium and fluorine mineralization. The results indicate that uraniferous opals can approximate a closed system for uranium and uranium daughters, so that dating samples as young as ～1 m.y. should be possible. In addition, the expected lack of initial²³⁰Th and²³¹Pa in opals permits valuable information on the initial²³⁴U/²³⁸U to be obtained on suitable samples of ?10 m.y. age. The oldest²⁰⁷Pb/²³⁵U apparent age observed, 20.8 ± 1m.y., was that of the opal-fluorite core of a nodule from a beryllium deposit in the Spor Mountain Formation. This age is indistinguishable from that of fission-track and K-Ar ages from the host rhyolite, and links the mineralization to the first episode of alkali rhyolite magmatism and related hydrothermal activity at Spor Mountain. Successively younger ages of 13 m.y. and 8–9 m.y. on concentric outer zones of the same nodule indicate that opal formed either episodically or continuously for over 10 m.y. Several samples of both fracture-filling and massive-nodule opal associated with beryllium deposits gave²⁰⁷Pb/²³⁵U apparent ages of 13–16 m.y., which may reflect a restricted period of mineralization or perhaps an averaging of 21?and<13?m.y. periods of opal growth. Several samples of fracture-filling opal in volcanic rocks as young as 6 m.y. gave²⁰⁷Pb/²³⁵U ages of 3.4–4.8 m.y. These ages may reflect hot-spring activity after the last major eruption of alkali rhyolite.     

Zur polarographischen Bestimmung einiger Schwermetalle in Abwasser und Abwasserrückständen

Die polarographische Bestimmung von Schwermetallen in Abwasserproben erfordert eine vorhergehende Mineralisation der organischen Inhaltsstoffe, wozu sich eine Kombination von Trocken- und Naßaufschluß eignet. Gleichzeitig werden Störungen in bestimmten Grundelektrolyten beseitigt. Die Schwermetallkonzentrationen liegen bei den untersuchten Klärschlämmen vor allem für Kupfer und Cadmium relativ hoch.     

Seismotektonische Traverse von der Nordsee bis zum Apennin

Zusammenfassung Die auf ihre Erdbebentätigkeit hin untersuchte Traverse ist 1300 km lang und erstreckt sich von der Doggerbank (54° N) bis zum nördlichen Apennin (44° N). In einem Tiefenschnitt entlang der Traverse wird die räumliche Verteilung der Erdbebenherde betrachtet. In der Zahl der Herde und ihrer Anordnung zeigen sich Unterschiede zwischen den verschiedenen Teilgebieten der Traverse (Niederrheinische Bucht, Oberrhein-Graben, Schwäbische Alb, Alpen, Apennin). Bezüglich der Herdtiefe ist ein gemeinsames Merkmal fast aller Gebiete, daß sich die Erdbebenherde auf die oberen 20–25 km der Erdkruste beschränken. Die stärkste seismische Aktivität ist im Tiefenbereich von 2–12 km festzustellen. Dieses Ergebnis ist für das Verhalten der einzelnen Krustenabschnitte bei rezenter Bruchtektonik von Bedeutung. Lediglich im Bereich der Doggerbank scheinen die Herde an der Basis der Erdkruste oder im obersten Erdmantel zu liegen, was auf besondere seismotektonische Verhältnisse in diesem Gebiet hindeutet. Demgegenüber zeigt die Herdtiefenverteilung im Alpengebiet trotz abweichender Krustenstruktur keine Besonderheiten gegenüber dem nördlichen und südlichen Vorland. Vergleicht man den aus seismographischen Messungen abgeleiteten rezenten Spannungszustand von Erdbebengebiet zu Erdbebengebiet, so ergibt sich für die Alpen und das nördlich anschließende Gebiet bis zur Nordsee ein sehr einheitliches Bild für die Orientierung der horizontalen Hauptspannungen (größte Hauptspannung NW-SE, kleinste Hauptspannung SW-NE). Im Apennin ist die Spannungsverteilung gerade umgekehrt.

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The seismic activity has been studied along a profile reaching from the North Sea to the northern part of the Apennine mountains covering 1300 km in length. A cross section of the Earth's crust along the profile forms the basis for this study. There are differences in number and spatial distribution of the foci between the different structure units of the studied region as the Lower Rhine Basin, the Upper Rhinegraben, the Swabian Jura, the Alps and the Apennines. It is a common feature of all epicentral regions that nearly all earthquake foci are situated inside the upper crust. This result is important regarding the behavior of different parts of the Earth's crust under the conditions of recent fracture tectonics. It seems that only in the North Sea region the foci reach depths near the Moho indicating the special seismotectonic situation of this area. On the other hand there could not be found any remarkable differences in the distribution of focal depths between the Alps and their northern and southern forelands although the crustal structure is quite different. Comparing the recent stress distribution as deduced from seismograph records for the different focal regions there results a very uniform picture for the orientation of the principal stress directions.

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Résumé Le présent travail étude l'activité séismique d'un profil qui s'étend sur 1300 km de longeur du Doggerbank (54° N) jusqu'à 1' Apennin septentrional et dans lequel on considère la distribution spatiale des foyers séismiques. Il existe, tant dans le nombre des foyers que dans leur distribution, des différences selon les régions traversées par le profil (Bassin du Rhin inférieur, le fossé rhénan, le Jura souabe, les Alpes et les Apennins). Caractéristique commune à presque toutes les régions, les foyers des séismes sont confinés aux quelque 20–25 km supérieurs de la croûte terrestre. Il faut signaler que l'activité séismique la plus marquée se trouve dans les profondeurs de 2 à 12 km. Ce résultat est important quand on regarde le comportement de chacun des secteurs de la croûte terrestre au cours de la tectonique récente. C'est seulement dans la région du Doggerbank que les foyers semblent se trouver à la base de la croûte terrestre ou dans la partie tout à fait supérieure du Manteau, ce qui indique des conditions séismoteetoniques particulières dans cette région. Par contre, la distribution des hypocentres dans les Alpes ne montre rien de particulier par rapport à l'avant-pays du sud et du nord, malgré la constitution différente de la croûte terrestre. Si l'on compare, en partant des mesures séismographiques, l'état récent de tension de région à région, on trouve pour les Alpes et la région attenant au Nord jusqu'à la Mer du Nord, une image très homogène concernant l'orientation des tensions principales suivant l'horizontale (tension principale la plus forte, direction NW-SE; tension la plus faible, direction SW-NE). Dans les Apennins, la répartition des tensions est inverse.

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Domestic Combustion of Biomass Fuels in Developing Countries: A Major Source of Atmospheric Pollutants

Biomass burning has important impacts on atmospheric chemistry and climate. Fires in tropical forests and savannas release large quantities of trace gases and particulate matter. Combustion of biofuels for cooking and heating constitutes a less spectacular but similarly widespread biomass burning activity. To provide the groundwork for a quantification of this source, we determined in rural Zimbabwe the emissions of CO₂, CO, and NO from more than 100 domestic fires fueled by wood, agricultural residues, and dung. The results indicate that, compared to open savanna fires, emissions from domestic fires are shifted towards products of incomplete combustion. A tentative global analysis shows that the source strength of domestic biomass burning is on the order of 1500 Tg CO₂–C yr^–1, 140 Tg CO–C yr^–1, and 2.5 Tg NO–N yr^–1. This represents contributions of about 7 to 20% to the global budget of these gases.     

Uranium-series dating of pedogenic silica and carbonate, Crater Flat, Nevada

A ²³⁰Th-²³⁴U-²³⁸U dating study on pedogenic silica-carbonate clast rinds and matrix laminae from alluvium in Crater Flat, Nevada was conducted using small-sample thermal-ionization mass spectrometry (TIMS) analyses on a large suite of samples. Though the ²³²Th content of these soils is not particularly low (mostly 0.1-9 ppm), the high U content of the silica component (mostly 4-26 ppm) makes them particularly suitable for ²³⁰Th/U dating on single, 10 to 200 mg totally-digested samples using TIMS. We observed that (1) both micro- (within-rind) and macro-stratigraphic (mappable deposit) order of the ²³⁰Th/U ages were preserved in all cases; (2) back-calculated initial ²³⁴U/²³⁸U fall in a restricted range (typically 1.67±0.19), so that ²³⁴U/²³⁸U ages with errors of about 100 kyr (2σ) could be reliably determined for the oldest, 400 to 1000 ka rinds; and (3) though 13 of the samples were >350 ka, only three showed evidence for an open-system history, even though the sensitivity of such old samples to isotopic disruption is very high. An attempt to use leach-residue techniques to separate pedogenic from detrital U and Th failed, yielding corrupt ²³⁰Th/U ages. We conclude that ²³⁰Th/U ages determined from totally dissolved, multiple sub-mm size subsamples provide more reliable estimates of soil chronology than methods employing larger samples, chemical enhancement of ²³⁸U/²³²Th, or isochrons.     

Concentrated flow erosion in cultivated catchments: Influence of soil surface state

Concentrated flow erosion is the dominant form of winter erosion in northern France. This study correlates the ephemeral rill and gully volumes measured in 20 cultivated catchments (4–95 ha) for three consecutive winters with the size of the potential runoff-contributing areas. These areas were identified by characterizing soil surface state through crust development stage, importance of surface wheel tracks and roughness grade. A single and significant relationship was found between the size of runoff-contributing areas, estimated by this criterion, and the rill and gully volumes. This identified the proportion of the catchment area occupied by fields with a degraded surface structure as the main factor controlling the variability of erosion in a context of concentrated flow erosion on cultivated land. The extension of degraded areas was shown to be controlled by dynamic interactions between weather, land occupation and soil physical properties. This criterion accounts for the uneven distribution of rainfall in space and time. Morphological factors, such as talweg length and slope, are believed to determine part of the residual variability.     

Berriasian (Early Cretaceous) radiometric ages from the Grindstone Creek Section, Sacramento Valley, California

The Grindstone Creek Section, Glenn County, Northern California is a sequence of hemipelagic mudstone, siltstone and sandstone interbedded with concretionary limestone and a few thin tuffs and bentonites. Two tuffs have been collected from a narrow interval of this sequence and subjected to mineralogical and isotopic analyses. U&z.sbnd;Pb isotopic analyses of zircon fractions from these volcanic horizons indicate an age of 137.1 + 1.6/−0.6 Ma. A detailed investigation has been conducted on the calcareous nannofossil stratigraphy of this section based on numerous samples with moderately preserved assemblages. The nannoflora is largely of Tethyan affinity, and allows direct correlation with the Berriasian stratotype section, with sections with published magnetostratigraphies and with a DSDP site drilled between known magnetic anomalies. The dated tuffs lie in the lower part of the upper BerriasianCretarhabdus angustiforatus Zone (Assipetra infracretacea Subzone) and within the narrow range ofRhagodiscus nebulosus. At three different sections, this subzone can be correlated with M-sequence Polarity Zones M16 and M16n. An independent magnetostratigraphic correlation is provided at DSDP Site 387, drilled between anomalies M15 and M16, where basal sediments containR. nebulosus. Buchia collected within a meter of the lower tuff lie within theB. uncitoides Zone which is Berriasian in age. The upper tuff level, which occurs 65 m above the lower tuff, is situated within the overlyingB. pacifica Zone. This zone had previously been correlated with the early Valanginian, but is clearly also partly of Berriasian age based on nannofossil stratigraphy. Our results allow an estimate of the age of the Berriasian-Valanginian and Jurassic-Cretaceous boundaries of 135.1 Ma and 141.1 Ma, respectively, and these fall within the range of, but differ significantiy from, several published time-scales.     

Stimulated comption effect in very compact radio sources

Expressions for the stimulated Compton effect are derived that are complete to order /ge/, where is the photon energy in the laboratory system, and =m ₀ C ² is the electron energy. Explicit formulas are given for the energy flow between a relativistic electron and a radiation field that obeys a power law so that the number of photons is proportional to ^–m–1. The amount of energy gained by an electron per second is then numerically calculated for conditions suggested by very compact radio sources as a function of the width of the spectrum, the spectral index, and the electron energy.The National Center for Atmospheric Research is sponsored by the National Science Foundation.