Deutung diskordanter Zirkonalter der Silvrettadecke und des Gotthardmassivs (Schweizer Alpen)

U-Pb isotopic analyses are reported for zircons of nine igneous rocks and high grade metamorphic paragneisses from the Silvretta nappe and the Gotthard massif, Switzerland.Following Silver and Deutsch (1961) the zircon populations isolated from six of these rocks were devided into series of size fractions with varying uranium content. An extensive Rb-Sr investigation has been made on some of these rock samples and their minerals (Arnold and Jäger, 1965: Gotthard massif, Grauert, 1966: Silvretta nappe). Rb-Sr measurements on biotites and mineral isochrons of individual gneiss samples yield ages which indicate the end of the Alpine and the Hercynian (=Variscan) metamorphism. Other measurements give mixed ages between these events. The U-Pb investigation of zircons, however, yield criteria for an even older event, 400 to 500 m.y. ago.The isotopic relationship indicates a strong episodic disturbance of the U-Pb systems in the paragneiss zircons with high losses of radiogenic lead. In a Concordia diagram the best fit lines to the data points of the paragneiss zircons from the Gotthard massif as well as from the Silvretta nappe trend both toward the data points of the orthogneiss zircons and intersect the Concordia at approximately 440 m.y. Rb-Sr analyses of the granitic orthogneisses from the Silvretta nappe (Flüela granite gneisses) gave a well defined isochron of 428 m.y. (=1.47×10^–11y^–1). This value agrees rather well with the age given by the intersection of the best fit lines to the paragneiss zircons with the Concordia.Considering the geological and petrographical observations we interpret this disturbance of the U-Pb systems in the paragneiss zircons as the result of an influence of regional high grade metamorphism and anatectic melting together with the formation of igneous rocks during the Caledonian petrogenesis.In the Silvretta the uranium rich zircons from the Flüela granite gneisses reflect a disturbance and lead loss probably during the Hercynian metamorphism. In the Gotthardmassif the zircons from a quartz diorite and an ultrabasic inclusion in this diorite, however, show no or only a slight influence of the later Hercynian and Alpine metamorphisms. The apparent ages of the zircons in the ultrabasic inclusion are nearly concordant (Pb²⁰⁶/U²³⁸: 448 m.y., Pb²⁰⁷/ U²³⁵: 452 m.y., Pb²⁰⁷/Pb²⁰⁶: 472 m.y.). There is in all orthogneiss zircons some evidence of older inhereted U-Pb components besides new zircon growth.If we assume an episodic model for the lead loss and a disturbance 450 m.y. ago the minimum primary age of the detrital zircon components in the paragneisses would be 1500 m.y.

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Der eine von uns (B.G.) möchte den Herren Prof. Dr. M. Grünenfelder und Prof. Dr. P. Signer vom Labor für Geochronologie des Instituts für Kristallographie und Petrographie der Eidgenössischen Technischen Hochschule Zürich seinen Dank für ihr Interesse und die großzügige Unterstützung bei der Durchführung dieser Arbeit aussprechen. Herr Prof. Dr. M. Grünenfelder und Herr Dr. R. Pidgeon haben ihn in die Arbeitsweise der U-Pb-Isotopenanalyse eingeführt. Herr Dr. V. Köppel sowie alle oben genannten Herren haben durch ihre wertvollen Ratschläge und kritischen Diskussionen diese Arbeit wesentlich gefördert. Ihnen und auch allen anderen Angehörigen des Labors für Geochronologie, die zum Gelingen der Arbeit beigetragen haben, sei herzlichst gedankt.

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Herr Dr. L. Rybach hat in dankenswerter Weise von vier Zirkonfraktionen die Urankonzentration durch Gamma-Spektrometrie bestimmt.

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Wir beide möchten Frau Prof. Dr. E. Jäger und Herrn Prof. Dr. E. Niggli vom Mineralogisch-Petrographischen Institut der Universität Bern für ihr wohlwollendes Interesse und die kritische Durchsicht des Manuskriptes danken.     

Infrared visibility prediction by statistical methods

This study attempts to find statistical methods of predicting infrared visibility (IRV), as calculated from hourly meteorological observations from a North Atlantic weather ship. Simple and multiple regressions expressing IRV as a function of its component weather variables, and exponential data transformations, for time lags of 1 to 24 hours, gaveR ² values from 0.68 (1-hour lag) to 0.09 (24-hour lag). These have limited predictive power for lags up to 6 hours, almost none for longer lags. Two-category discriminant analysis, using class breaks at 2 km or 10 km is of little use, due to uneven data distribution.Possibly more promising would be an application of Machine Output Statistics (MOS), used routinely for temperature forecasts, to this problem.     

Geometry,bed topography and drainage system structure of the haut glacier d'Arolla,Switzerland

As part of an integrated study of the hydrology, meltwater quality and dynamics of the Haut Glacier d'Arolla, Switzerland, the glacier's drainage network structure was determined from patterns of dye recovery in 342 injection experiments conducted from 47 moulins distributed widely across the glacier. This structure was compared with theoretical predictions based upon reconstructed patterns of water flow governed by (a) the subglacial hydraulic potential surface, and (b) the subglacial bedrock surface. These reconstructions were based on measurements of ice surface and bedrock topography obtained by a combination of ground survey and radio-echo sounding techniques. The two reconstructions simulate the drainage system structures expected for (a) closed channels, in which water is pressurized by the overlying ice, and (b) gravity-driven, open-channel flow. The closed-channel model provides the best fit to the observed structure, even though theoretical calculations suggest that, under summer discharge conditions, open-channel flow may be widespread beneath the glacier. Possible reasons for this apparent discrepancy are discussed.     

Rare earth distributions in clay minerals and in the clay-sized fraction of the Lower Permian Havensville and Eskridge shales of Kansas and Oklahoma

The REE (rare earth element) content of a wide variety of clay mineral groups have been analyzed using radiochemical neutron activation and have been found to be quite variable in absolute REE content (range of ∑REE = 5.4–1732) and less variable in relative REE content (range of chondritenormalized La/Lu = 0.9–16.5). The variable REE content of the clay mineral groups is probably determined by the REE content of the source rock from which the clay mineral was derived and not from the separate minerals in the rock.The clay-sized fractions of the Havensville and Eskridge shales of Kansas and Oklahoma have similar relative REE distributions and identical negative Eu anomaly size as the composite of NAS (N. American shales), but an absolute REE content (range of ∑REE = 46–348) that may differ significantly from the composite of NAS. The clay-sized fraction of samples from any given outcrop did not vary much in absolute or relative REE content, but samples from northern Oklahoma, probably composed of continental to near-shore marine sediments, have higher absolute REE contents and higher La/Lu ratios than samples of marine deposits in Kansas (e.g. mean ∑REE in Oklahoma = 248; mean ∑REE in Kansas = 69–116). The differencess in the REE content between samples in Oklahoma and Kansas may be caused by chemical weathering processes in the source area, exchange reactions in the environment of deposition, or diagenesis and do not appear to be a result of the different clay minerals.Most samples have Eu anomalies relative to chondrites (range of Eu/Sm ratios of samples = 0.035–1.17; chondrites = 0.35). Some montmorillonites and kaolinites are anomalous in Eu relative to the NAS (range of Eu/Sm ratios of samples = 0.056–0.21; NAS = 0.22). These anomalies may be inherited from source rocks with Eu anomalies originally produced by igneous processes, or they may be produced by chemical weathering processes in the source area.     

Reconstruction of the last deglaciation: deconvolved records of δ18O profiles,micropaleontological variations and accelerator mass spectrometric14C dating

Bioturbation acts as a low-pass filter in displacing and reducing the amplitudes of stratigraphic signals. This often leads to a loss of high-frequency events in the stratigraphic record. In addition, when considering an isotopic signal ¹⁸O,¹⁴C measured in stratigraphic carriers, such as foraminifera, bioturbation and carrier abundance changes can create artifacts which may be falsely interpreted as leads or lags in the paleoclimatic record.We presenthere a model in which bioturbationis treated as a time-invariant filter whose impulse response function (IRF) is like that of a first-order system. The method involves first deconvoluting the abundance curves of the carriers and then the isotopic signals using the restored carrier abundances. This analysis was initially used to artificially generate ideal curves, with the aim of qualitatively modelling the effects of bioturbation. Following this, deconvolved curves were obtained using data from the core CH73-139C using ¹⁸O, A. M. S. C-14 ages, and abundances of two planktonic foraminifera:G. bulloides andN. pachyderma left-coiling. A comparison of the data with the unmixed curves enables separation of the bioturbation artifacts and the construction of a common deglaciation curve based on the restored signals.Importantly, the model emphasizes some severe limitations of mathematical analysis of stratigraphic signals.     

Acoustic tomography and conventional meteorological measurements over heterogeneous surfaces

Summary Conventional micro-meteorological measuring methods are not particularly suitable for the investigation of the energy exchange under heterogeneous surface conditions. To consider the influence of the different surface properties, area-covered and spatially averaged meteorological measurements in combination with highly resolved simulations are necessary. In this context, the method of acoustic travel time tomography is introduced to provide information about the horizontal temperature and wind field.To check the applicability of the tomographic method for this problem and to provide data for the model initialisation a field experiment was carried out.The tomographic system and conventional meteorological equipment were utilised inside the investigation area with an extension of several hundred meters (300×700m²), which was arranged over areas with different surface properties: grassland and bare soil.The results of the field experiment show that differences between the measuring systems (in-situ and remote sensing) exist and the heterogeneity of the underlying surface is visible in the near surface temperature and wind field at a scale, which can be resolved with highly resolved numerical models. Depending on the incoming solar radiation and the local advection regional distinctions in the air temperature and wind field as well as in the vertical sensible heat fluxes were observed.The investigations demonstrate that the sensitivity of the acoustic tomography is sufficient to verify gradients in the meteorological fields even when the horizontal differences in sensible heat flux are small.     

On the date of construction of Lechaion,western harbor of ancient Corinth,Greece

Lechaion, the western harbor of ancient Corinth, was an artificial harbor excavated in a marshy area and connected to the open sea through a channel with revetment walls. It was probably the most important harbor of this type in antiquity, and one of the most important harbors in Greece for more than one millennium. Yet, the date of construction of this harbor is a matter of debate; 600 B.C., ca. 44 B.C., and A.D. 350 are the most probable dates. Geomorphological and biological investigations, in combination with AMS radiocarbon dating of exposed marine shells found in the walls of the channel leading to the inner basin of the harbor indicate that the channel was open to the sea before a seismic land uplift that probably occurred sometime in the fifth to third century B.C. These data indicate that the construction of the harbor began in ca. 600 B.C., the period of Corinthian expansion to the Ionian Sea and southern Italy. © 1996 John Wiley & Sons, Inc.