Artificial radionuclides in the sediments of the Rhne river: a snapshot of the situation during winter 1994/95

Artificial radionuclides (⁶⁰Co, ¹⁰⁶Ru, ¹³⁴Cs and ¹³⁷Cs) produced in the nuclear power plants along the Rhne valley have been detected in most of the 35 sediment samples collected from the shores of the Rhne river in southern France. In one sample traces of ²⁴¹Am were found, suggesting the presence of transuranic elements. The wide use of river water for irrigation and the occasional big inundations of the Rhne have spread the artificial radionuclides over huge areas, quite far behind the river shores. Although artificial radionuclides are present in fields where food crops are collected, their activity is currently considerably lower than the activity of the natural radionuclides present in the river sediment.     

Proterozoic zircon growth in Archean lower crustal xenoliths, southern Superior craton – a consequence of Matachewan ocean opening

Granulite-grade, anorthositic and mafic xenoliths recovered from a Jurassic kimberlite pipe near Kirkland Lake, Ontario are fragments of the lower crust that underlies the ca. 2.7 Ga Abitibi greenstone belt of the Superior craton. Cathodoluminescence imaging and/or backscatter electron microscopy of zircon from four individual xenoliths reveals a complex crystallization history, characterized by two main stages of zircon growth. The age of the two stages has been constrained by combining imaging results with isotope dilution U-Pb dating of grain fragments and single grains. Minimum ages for the first crystallization stage in individual xeno liths are 2584 ± 7 Ma, 2629 ± 8 Ma, 2633 ± 3 Ma, whereas an approximate crystallization age for a fourth sample is 2788 ± 57 Ma. The second main stage of growth consists of chemically and isotopically distinct metamorphic zircon overgrowths. Times of solid-state zircon growth are most broadly constrained in three samples to the interval between 2.52 Ga to 2.40 Ga, and most precisely dated in a meta-anorthosite at 2416 ± 30 Ma. These complex zircons are intergrown with garnet and clinopyroxene of the host granulite-facies assemblage, and thus the Paleoproterozoic ages of the metamorphic overgrowths are interpreted to reflect an interval of isobaric, granulite-grade metamorphism of the lower crust beneath the greenstone belt approximately 150 million years after craton formation. This interval of metamorphism is broadly coeval with the intrusion of the Matachewan dyke swarm across the southern Superior craton, and with mafic magmatism and deposition of Huronian rift-margin sediments 200 km to the south during the opening of the Matachewan ocean. It is proposed that a significant volume of magma intruded the crust-mantle interface during rifting, promoting isobaric metamorphism and zircon growth in the deep levels of the Superior craton. Subsequent major rifting events along this margin apparently failed to produce a similar lower crustal response. The results have important implications for the structure of lithosphere beneath Archean continental crust. Received: 3 October 1995 / Accepted: 11 February 1997     

Heating in the D″-layer and the style of mantle convection

Summary We have investigated the influence of heating in the D-layer on the convection dynamics for the Rayleight number Ra=10⁶. Strong heating, which may represent a local small-scale heat transfer in the D-layer, results in an increase of lateral heterogeneities near the upper and lower boundaries, the blob-like structure of upwellings and the stabilization of the convection pattern. The influence of the electromagnetic heating was found to be too weak to produce any substantial effect which is in contrast with the idea of Braginskii and Meitlis [1].Dedicated to the Memory of K. P     

Path-integral seismic imaging

A new type of seismic imaging, based on Feynman path integrals for waveform modelling, is capable of producing accurate subsurface images without any need for a reference velocity model. Instead of the usual optimization for traveltime curves with maximal signal semblance, a weighted summation over all representative curves avoids the need for velocity analysis, with its common difficulties of subjective and time‐consuming manual picking. The summation over all curves includes the stationary one that plays a preferential role in classical imaging schemes, but also multiple stationary curves when they exist. Moreover, the weighted summation over all curves also accounts for non‐uniqueness and uncertainty in the stacking/migration velocities. The path‐integral imaging can be applied to stacking to zero‐offset and to time and depth migration. In all these cases, a properly defined weighting function plays a vital role: to emphasize contributions from traveltime curves close to the optimal one and to suppress contributions from unrealistic curves. The path‐integral method is an authentic macromodel‐independent technique in the sense that there is strictly no parameter optimization or estimation involved. Development is still in its initial stage, and several conceptual and implementation issues are yet to be solved. However, application to synthetic and real data examples shows that it has the potential for becoming a fully automatic imaging technique.     

Proposal for International Agreement on Ca Notation Resulting from Discussions at Workshops on Stable Isotope Measurements Held in Davos (Goldschmidt 2002) and Nice (EGS-AGU-EUG 2003)

A proposal is made to standardise the reporting of Ca isotope data to the δ⁴⁴Ca/⁴⁰Ca notation (or δ⁴⁴Ca/42Ca) and to adopt NIST SRM 915a as the reference standard.     

The modern distribution of chironomid sub-fossils (Insecta: Diptera) in the Sierra Nevada, California: Potential for paleoclimatic reconstructions

Surface lake sediment was recovered from 57 lakes along an elevation gradient in the central, eastern Sierra Nevada of California. The surface sediment was analysed for subfossil chironomid remains in order to assess the modern distribution of chironomids in the region. The lakes sampled for the calibration dataset were between 2.0 and 40.0 m in depth, spanned an altitudinal gradient of 1360 m and a surface water temperature gradient of approximately 14 °C. Redundancy analysis (RDA) identified that five of the measured environmental variables – surface water temperature, elevation, depth, strontium, particulate organic carbon – accounted for a statistically significant amount of the variance in chironomid community composition. Quantitative transfer functions, based on weighted-averaging (WA), partial least squares (PLS) and weighted-averaging partial least squares (WA-PLS), were developed to estimate surface water temperature from the chironomid assemblages. The best model was a WA model with classical deshrinking, which had a relatively high coefficient of determination (r² = 0.73), low root mean square error of prediction (RMSEP = 1.2 °C) and a low maximum bias (0.90 °C). The results from this study suggest that robust quantitative estimates of past surface water temperature can be derived from the application of these models to fossil chironomid assemblages preserved in late-Quaternary lake sediment in this region.     

Dynamic tracking of mica breakdown reactions with electrochemical measurements

The breakdown of hydrous minerals involves multiple reactions of dehydration and often oxidation of ferric iron. Such reactions are important in mantle metasomatism, metamorphic petrology and material sciences. Here we track these reactions dynamically over the temperature range from 400° to 1100° C using Y-doped ZrO₂ sensors in a novel manner, comparable to differential thermal analyses. This contrasts to the conventional use of ZrO₂ cells in equilibrium electrochemical measurements for oxygen fugacity.     

Fold-axis parallel extension in an arcuate fold- and thrust belt: the case of the Helvetic nappes

The Helvetic nappes of western Switzerland are discussed as an example of an arcuate foreland fold- and thrust belt in which active fold-axis parallel stretching occurred. Fold-axis parallel extension is recorded by:

1. (1) Incremental strain data from pressure shadow fibres. The significance of pressure shadow fibres for the determination of the deformation history of a region is discussed. Pressure shadows are used to quantify the amount of, and to describe the distribution of fold-axis parallel extension occurring in the Helvetic nappes.

2. (2) The extension directions of conjugate systems of en échelon veins. It is shown that an analysis of the geometry of conjugate vein systems can reveal a regional deformation pattern. The relative age of the conjugate en échelon vein systems in the Helvetic deformation history can be assessed, the geometry of the conjugate sets relative to the local anisotropy plane is described, and the significance of the preferred orientation of their extension directions is discussed.

3. (3) Fold-axis parallel sections. A comparison of the regional distribution of the fold-axis parallel strain with the shape of the Helvetic nappes in fold-axis parallel sections shows that the fold-axis parallel strain cannot be related to the footwall topography of the nappes.

It is concluded that the fold-axis parallel extension in the Helvetic nappes was induced by a change of direction of overthrust shear. This change occurred late in the deformation history and was superposed on the already formed nappes. The changing direction of overthrust shear is the expression of an overall anticlockwise rotation going on in the overthrusting Alpine nappe pile, relative to the European plate, a rotation which lead to the arcuate shape of the Western Alps.     

Co-Ni-As sulphides in serpentinites of different metamorphic grade in the eastern Central Alps (Switzerland and Italy)

Summary Co-Ni-As sulfides are described from serpentinites of the Penninikum in the eastern Central Alps. They include: (1) non-stoichiometric gersdorffite in ophiolitic serpentinites which were subjected to lower greenschist facies metamorphism, from the Oberengadin, Switzerland, and (2) stoichiometric, almost pure, cobaltite in an ultramafic complex, equilibrated at greenschist facies metamorphic conditions and characterized by the silicate association of antigorite + olivine + diopside, from Val Malenco, Italy. A comparison with data for similar sulphides from the Outokumpu area, Finland, suggests that the non-stoichiometric cobaltite-gersdorffite series tends to equilibrate as solid solutions towards a cobalt-enrichment during metamorphism and that stoichiometric cobaltite is the stable end-phase.

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Co-Ni-As-Sulphide in Serpentiniten verschiedenen Metamorphosegrades aus den östlichen Zentralalpen (Schweiz und Italien)

Zusammenfassung Co-Ni-As-Sulphide in Serpentiniten des Penninikums der östlichen Zentralalpen wurden untersucht: (1) nicht-stöchiometrische Gersdorffite in ophiolithischen Serpentiniten der unteren Grünschieferfazies im Oberengadin, Schweiz, und (2) stöchiometrische, reine Kobaltite in einem Ultramafit-Komplex mit einer Gleichgewichts-Silikat Paragenese der Grünschieferfazies, Antigorite + Olivine + Diopsid, im Val Malenco, Italien. Ein Vergleich mit Daten ähnlicher Sulphide des Outokumpu Gebietes in Finnland legt nahe, daß nicht-stöchiometrische Kobaltit-Gersdorffit-Mischkristalle während der Metamorphose equilibrieren, wobei Kobalt anreichert wird, und stöchiometrischer Kobaltit die stabile Endphase des Equilibrierungsprozesses ist.