Cosmic-ray production of deuterium,He3, lithium,beryllium, and boron in the Galaxy

The production of deuterium, He³, lithium, beryllium, and boron by galactic cosmic rays in the interstellar medium, over the life of the Galaxy, is calculated. It is found that high-energy - reactions contribute in an essential way to the observed lithium. When allowance is made for the interchange of material between stars and the interstellar gas and for the change of cosmic-ray intensity with time, the Li⁶, Be⁹, and boron produced turn out to be ample to explain the observed abundances, and with remarkable internal consistency. Deuterium and He³ arenot produced in significant amounts, nor Li⁷ in sufficient amount, however. To explain the Li⁷/Li⁶ ratio measured terrestrially and in chondritic meteorites, we invoke cosmological production of Li⁷. This implies the production of deuterium, He³, and He⁴ as well, in amounts consistent with observation. The theory in its present form cannot explain a solar-system Li⁷/Li⁶ ratio of 12and stellar ratios as low as 3, but additional processes can be adduced to reconcile them. The consistency of the numbers when cosmological production is included lends additional support to the big-bang hypothesis. An incidental result is that the mean luminosity of the Galaxy over its lifetime has been about 3 times its present luminosity.     

From late Visean to Stephanian: pinpointing a two-stage basinal evolution in the Variscan belt. A case study from the Bosmoreau basin (French Massif Central) and its geodynamic implications

Post-convergence evolution of the Variscan belt is characterized by the development of intramontane coal-bearing basins containing volcano-sedimentary successions. In the French Massif Central, K––Ar ages on clay particles from fine-grained sediments of the Bosmoreau basin (Limousin area), help pinpoint the evolution of the basin. In the lower part of the sedimentary pile, illite in a siltstone underlying a volcanic layer previously dated at 332±4 Ma by the U––Pb method on zircon, yields a consistent K––Ar age of ca. 340 Ma. Upward in the sedimentary succession, illite yields Stephanian K––Ar ages, which can be combined to provide a mean deposition age of 296.5±3.5 Ma. The Bosmoreau basin, albeit mainly filled with Stephanian deposits, was initiated during the late Visean, i.e. ca. 30 Ma earlier than inferred from biostratigraphical constraints. During the Stephanian, the same structure was reactivated and late Visean deposits were eroded and subsequently blanketed by thick clastic sediments. These results emphasise a two-stage evolution for the Bosmoreau basin, which is closely related to extensional tectonics identified on basement country rocks, and they are used to propose a geodynamic evolution of the studied area.     

An <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar,Rb/Sr,and stable isotope study of micas in low-grade fold-and-thrust belt: an example from the Swiss Helvetic Alps

White micas in carbonate-rich tectonites and a few other rock types of large thrusts in the Swiss Helvetic fold-and-thrust belt have been analyzed by ⁴⁰Ar/ ³⁹Ar and Rb/Sr techniques to better constrain the timing of Alpine deformation for this region. Incremental ⁴⁰Ar/ ³⁹Ar heating experiments of 25 weakly metamorphosed (anchizone to low greenschist) samples yield plateau and staircase spectra. We interpret most of the staircase release spectra result from variable mixtures of syntectonic (neoformed) and detrital micas. The range in dates obtained within individual spectra depends primarily on the duration of mica nucleation and growth, and relative proportions of neoformed and detrital mica. Rb/Sr analyses of 12 samples yield dates of ca. 10–39 Ma (excluding one anomalously young sample). These dates are slightly younger than the ⁴⁰Ar/ ³⁹Ar total gas dates obtained for the same samples. The Rb/Sr dates were calculated using initial ⁸⁷Sr/ ⁸⁶Sr ratios obtained from the carbonate-dominated host rocks, which are higher than normal Mesozoic carbonate values due to exchange with fluids of higher ⁸⁷Sr/ ⁸⁶Sr ratios (and lower ¹⁸O/ ¹⁶O ratios). Model dates calculated using ⁸⁷Sr/ ⁸⁶Sr values typical of Mesozoic marine carbonates more closely approximate the ⁴⁰Ar/ ³⁹Ar total gas dates for most of the samples. The similarities of Rb/Sr and ⁴⁰Ar/ ³⁹Ar total gas dates are consistent with limited amounts of detrital mica in the samples. The d ¹⁸O values range from 24–15‰ (VSMOW) for 2–6 µm micas and 27–16‰ for the carbonate host rocks. The carbonate values are significantly lower than their protolith values due to localized fluid-rock interaction and fluid flow along most thrust surfaces. Although most calcite-mica pairs are not in oxygen isotope equilibrium at temperatures of ca. 200–400 °C, their isotopic fractionations are indicative of either 1) partial exchange between the minerals and a common external fluid, or 2) growth or isotopic exchange of the mica with the carbonate after the carbonate had isotopically exchanged with an external fluid. The geological significance of these results is not easily or uniquely determined, and exemplifies the difficulties inherent in dating very fine-grained micas of highly deformed tectonites in low-grade metamorphic terranes. Two generalizations can be made regarding the dates obtained from the Helvetic thrusts: 1) samples from the two highest thrusts (Mt. Gond and Sublage) have all of their ⁴⁰Ar/ ³⁹Ar steps above 20 Ma, and 2) most samples from the deepest Helvetic thrusts have steps (often accounting for more than 80% of ³⁹Ar release) between 15 and 25 Ma. These dates are consistent with the order of thrusting in the foreland-imbricating system and increase proportions of neoformed to detrital mica in the more metamorphosed hinterland and deeply buried portions of the nappe pile. Individual thrusts accommodated the majority of their displacement during their initial incorporation into the foreland-imbricating system, and some thrusts remained active or were reactivated down to 15 Ma.     

Seasonal metal remobilization in the sediments of Great Bay,New Hampshire

Concentrations of dissolved iron, manganese, molybdenum, copper, and organic carbon (DOC) were measured in the pore waters from surficial sediments of a temperate estuary to delineate seasonal metal remobilization from 1978 through 1980. Iron and DOC data were collected for 31 months and covaried inversely and exponentially. Iron dissolution occurred during the spring and during periods of active bioturbation with concentrations as high as 18 mg 1^?1. Iron values were low during winter due to oxidation to ferric oxides. The lack of active bioturbation during the summer of 1978 allowed for the nearly complete removal of iron as a monosulfide precipitate. However, bioturbation resumed during the summer of 1979 and 1980 and dissolved iron concentrations as high as 10 mg 1^?1 were observed at those times. The iron and DOC data were a qualitative measure of bioturbation activity. Dissolved manganese, molybdenum, and copper data were collected for 18 months during 1978 and 1979. All three metals displayed spring maxima covariate with iron, suggesting that they behaved chemically like iron and/or were associated with iron- or manganese-rich phases during this time of the year. In general, manganese and molybdenum varied temporally with iron while copper concentrations mimicked iron variations only during the spring.     

Holocene evolution of the crater lake at Malha,Northwest Sudan

A reconstruction of the hydrological and environmental evolution of the crater lake at Malha (Northern Darfur, Sudan) resulted from the mineralogical and biological study of a 9.21 m section of lake sediments, representing an uninterrupted sequence of lacustrine deposition since 8 290 ¹⁴C years BP.Important changes in water supply and conditions of sedimentation are reflected in the nature of the sediments and the morphology and stratigraphical distribution of various salt minerals. Additional information on lake circulation patterns and salinity conditions are obtained from associated benthic paleocommunities, represented by ostracods and dipterid larvae. Combining both lines of evidence, the studied sequence can be divided in six distinct sections, which correspond to six successive periods in the lake's Holocene history. The first three periods, generally characterized by high lake levels, represent three generations of a meromictic lake, two of which have ended with a complete desiccation of the lake basin. Meromixis was stable during Period I, due to wind shelter and pronounced density stratification. In the course of Periods II and III stratification was repeatedly interrupted. During Period II, the disruptions were accompanied by important water budget fluctuations; a superimposed gradual decrease in net water supply eventually resulted in holomictic conditions terminating this period. Evidence of turbulence periodically affecting profundal waters is recorded in the sediments of Period III, suggesting that disruptions of stratification were now initiated by very strong winds. Between Period I and Period III, the littoral mixolimnion gradually evolved from near fresh to mesosaline. In Periods IV to VI, lake level was intermediate to low. The lake was holomictic for most of the time and meso- to hypersaline; during Period V, it repeatedly shrunk to a shallow brine pool.The Holocene evolution of Malha Crater Lake illustrates the progressive increase in aridity over most of North Africa following a well-established, early- to mid-Holocene major humid episode. The uninterrupted sedimentary sequence lends itself for detailed reconstruction of Holocene climatic evolution in arid Northeast Africa, a region where records of continuous lacustrine deposition are extremely scarce. As the chronology of critical events in the lake's history remains as yet unsupported by radiocarbon dates, correlation with other Holocene sequences in the eastern Sahara is highly tentative at this point.     

Direct analysis of microaggregates shrinkage for drying: Application to microaggregates from a Brasilian clayey Ferralsol

Subrounded microaggregates 100 to 500 μm in diameter were collected in a Ferralsol located in the state of Goiàs. Observation of these microaggregates in dry conditions in scanning electron microscopy showed morphological characteristics that are representative of microaggregates described earlier in many Ferralsols. The shrinkage for drying of initially water saturated microaggregates was studied using an environmental scanning electron microscope. Results showed very small shrinkage for the microaggregates usually studied. However, the shrinkage varies since the volume of dried microaggregates was found to be 93 to 99% of their volume when they are saturated. To cite this article: N. Volland-Tuduri et al., C. R. Geoscience 336 (2004).     

Climatic Regionalization and the Spatio-Temporal Occurrence of Extreme Single-Year Drought Events (1500–1998) in the Interior Pacific Northwest, USA

Tree-ring records from western juniper (Juniperus occidentalis var. occidentalis Hook.) growing throughout the interior Pacific Northwest identify extreme climatic pointer years (CPYs) (i.e., severe single-year droughts) from 1500–1998. Widespread and extreme CPYs were concentrated in the 16th and early part of the 17th centuries and did not occur again until the early 20th century. The 217-yr absence of extreme CPYs may have occurred during an extended period of low variance in the Pacific Decadal Oscillation. We mapped climatic boundaries for the interior Pacific Northwest based on the location of sites with similar precipitation variability indices. Three regions, the Northwest (based on chronologies from nine sites), the Southwest (four sites), and the East (five sites) were identified. Our results suggest that western juniper radial growth indices have substantial interannual variability within the northwestern range of the species (central Oregon), particularly when compared with western juniper growing in its eastern range (eastern Oregon, southeastern Idaho, and northern Nevada) and southwestern range (southern Oregon and northeast California). We suspect that the substantial differences in the variability of western juniper radial growth indices are linked to the influence of ENSO events on winter/spring precipitation amounts.     

Gravity-induced stresses in stratified rock masses

Summary This paper presents closed-form solutions for the stress field induced by gravity in anisotropic and stratified rock masses. These rocks are assumed to be laterally restrained. The rock mass consists of finite mechanical units, each unit being modeled as a homogeneous, transversely isotropic or isotropic linearly elastic material. The following results are found. The nature of the gravity induced stress field in a stratified rock mass depends on the elastic properties of each rock unit and how these properties vary with depth. It is thermodynamically admissible for the induced horizontal stress component in a given stratified rock mass to exceed the vertical stress component in certain units and to be smaller in other units; this is not possible for the classical unstratified isotropic solution. Examples are presented to explore the nature of the gravity induced stress field in stratified rock masses. It is found that a decrease in rock mass anisotropy and a stiffening of rock masses with depth can generate stress distributions comparable to empirical hyperbolic distributions previously proposed in the literature.     

Rare earths in barites: distribution and effects on aqueous partitioning

A large variety of barites collected from marine and continental environments was analyzed by neutron activation for the rare-earth elements (REE) La, Ce, Sm, Eu and Dy. Relative to chondrites, all barites show a decrease of the lighter REE from La toward Eu. The abundance and distribution of rare earths in barites show a distinction of barite types. Deep-sea barites have large REE concentrations as do other authigenic deep-sea minerals and display the chondrite normalized Eu minimum, but not the negative Ce anomaly, of sea water. Other barites, mostly on land, some hydrothermal, and others of shallow marine origin, display lower total Ree concentrations. Chondrite normalized positive Eu anomalies are displayed by those varieties of reducing sedimentary and metamorphic origin.Distribution of REE in barite can be attributed both to crystallographic constraints of substitution, and to solution complexing of REE in the precipitating medium. Plots of rare earth partitioning versus effective ion size suggest that the decreasing enrichment toward Eu for all barite types is caused by crystallographic constraints due to contraction of the substituting REE ion sizes relative to the size of the host Ba ion. Solution effects on REE substitution in barite can be evaluated by writing solid solution distribution equations based on mass action of REE and Ba sulfates and the lanthanide (Ln) solution species Ln (CO₃)^?5₄), LnSO⁺₄, LnCl⁺² and LnF⁺². Under normal sea water conditions, solution complexing plays a minor role. However, increased alkalinities of reducing sediments and increased brine chlorinities could cause significant complexing and deplete REE heavier than Eu. Besides Dy in barites, this could be true for aqueous precipitation of REE in general.     

North American Precambrian history recorded in a single sample: high-resolution UPb systematics of the Potsdam sandstone detrital zircons,New York State

Zircons separated from the Cambrian Potsdam sandstone of New York yield four distinct populations which can be defined by a number of analytical techniques. U-Pb isotopic analyses of small samples and monozircons of each population reveal a fine chronology not apparent in milligram-sized sample analysis, and define source area ages of 1180, 1320, 2100 and 2700 m.y. for the Cambrian detrital suite. These ages correspond to well-defined sources in the Superior and Grenville Provinces of the Canadian Shield (2700 and 2100 m.y.) and the well established Grenville age rocks of the Adirondack Mountains (1180 m.y.). The 1320-m.y. age appears to be derived from the Adirondacks, and suggests the existence of pre-Grenville basement in that massif. Our techniques allow the interpretation of the Precambrian history of a large portion of eastern North America from a single sample, and thus should be valuable in the definition of source areas in paleogeographic reconstruction, and in studies of continental crustal evolution.