Comparison of high-resolution seismic profiles and the geoacoustic properties of Eckernförde Bay sediments

High-resolution seismic profiles of Eckernförde Bay and the adjacent Baltic Sea were collected, and the geoacoustic properties of sediments there were measured. Bulk densities averaged ~ 1.35 g cm^–3 and ranged from ~ 1.2 to ~ 1.7 g cm^–3. Compressional wave velocities in gas-free sediments averaged ~ 1460 m s^–1 and ranged from ~ 1425 to ~ 1555 m s^–1. In nongassy sediments, bulk density variations typically controlled changes of acoustic impedance. Impedance changes were usually too small and closely spaced to be resolved seismically, although, at certain sites, significant impedance changes are far apart enough that they correlate one-to-one with seismic reflectors. Where free gas is present, velocity decreases and wave energy is scattered, causing a prominent seismic reflector.     

Multigrain sedimentation/erosion model based on cross-shore equilibrium sediment distribution: Application to nourishment design

In the light of global warming and sea level rise there are many coastal beaches that suffer from erosion. Beach nourishment has become a common practice to maintain the sediment balance on a shore-face. In this paper, a three-dimensional numerical model for evaluating long-term impact of beach nourishment projects has been developed. The model addresses the longstanding complex issue of coastal morphology and sediment grain size distribution from an unconventional angle, which exploits the strong links between grain size distribution and the prevailing transport direction of each sediment constituent under ‘average’ wave and storm action. The present model predicts the redistribution of nourished sediment according to the subtle clues implied by equilibrium distribution curves and latest coastal wave transformation theories. After verification against recent field observations in Terschelling, The Netherlands, the model was used to predict long-term effects of different beach nourishment strategies. It was found that: (a) given the source sediment available in Terschelling the tactics of large volume and less frequent implementation are better than otherwise; and (b) from a pure engineering point of view, waterline nourishment outperforms offshore trough nourishment.The model offers an additional tool for coastal engineers to evaluate the feasibility, effectiveness and the optimization of dumping locations for beach nourishment projects. It is also a useful tool for stratigraphic modelling of shallow-marine sedimentation in conjunction with sea level changes.     

Banded sulphide ores: The experimental production of monomineralic sulphide bands in sediments

Summary Banded sulphide ores of the McArthur River type are characterized by large numbers of conformable, monomineralic sulphide bands. These appear to have formed prior to consolidation of the host sediments. This paper describes a series of laboratory experiments designed to test the possibility that the bands could have been generated by the migration of metal, sulphide, and possibly carbonate ions through unconsolidated sedimentary materials or slowly settling suspensions. It is concluded that ionic migration processes probably occur during the formation of these banded sulphide ores, in conjunction with the deposition of metalliferous mud layers.

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Zusammenfassung Die Schichten der Schwefelerze des Typs vom McArthur Fluß sind charakterisiert durch eine Vielzahl von Parallelen, die ausschließlich aus Schwefelschichten bestehen. Diese scheinen sich vor der Verfestigung der Sedimentenmasse gebildet zu haben. Dieser Aufsatz beschreibt eine Reihe von Laboratoriums-Experimenten, die die Möglichkeit untersuchen sollten, ob diese Schichten entstehen konnten durch die Wanderung von Metall-, Schwefeloder möglicherweise Kohlensäure-Ionen durch noch nicht verfestigtes oder sich langsam setzendes Sedimentmaterial. Es wird daraus gefolgert, daß wahrscheinlich während der Formation dieser Schwefelerz-Schichten in Verbindung mit der Ablagerung metallhaltiger Schlammlagen Ionenwanderungen stattfinden.

     

High-resolution optical spectroscopy of a newly discovered post-AGB star with a surprising metallicity in the globular cluster M79

An abundance analysis based on a high-resolution spectrum is presented for a newly discovered post-asymptotic giant branch (PAGB) star in the globular cluster M79. The surprising result is that the iron abundance of the star is apparently about 0.6 dex less than that of the cluster's red giants as reported by published studies including a recent high-resolution spectroscopic analysis by Carretta and colleagues. Abundances relative to iron appear to be the same for the PAGB star and the red giants for the 15 common elements. It is suggested that the explanation for the lower abundances of the PAGB star may be that its atmospheric structure differs from that of a classical atmosphere; the temperature gradient may be flatter than predicted by a classical atmosphere.     

Implications of geochemical investigations of sedimentary rocks within and around the McArthur zinc-lead-silver deposit, Northern Territory

The McArthur Zn---Pb---Ag deposit occurs within a belt of Middle Proterozoic sediments which extends discontinuously from the northwest of Queensland to the north of the Northern Territory. The similar Mount Isa and Hilton deposits, as well as many smaller orebodies, occur in the same belt, and there is every reason to believe that the full mineral potential of this region is far from realized at the present time.The McArthur deposit is effectively unmetamorphosed and has undergone a minimum of structural deformation. Therefore, it presents an ideal opportunity for investigating the genesis of this kind of stratiform ore, and establishing guidelines for prospecting for other such deposits. With these aims in mind, we analysed a large number of drill core samples of sediments from within and around the McArthur deposit, and assessed these data in the light of salient geological features.We conclude that favourable areas for locating deposits of this type should contain all of the following features: (1) pyritic, high-potash, carbonaceous shales which have Zn contents of around 0.5% or more, and significant Pb, As and Hg anomalies; (2) dolomites with moderate to high Fe and Mn contents; (3) vitroclastic tuffite bands.Detailed investigations in suitable areas should concentrate on the basal parts of thick carbonaceous shale sequences, proximal to major fault zones.     

The use of values of gravity in the adjustment of the triangulation of Europe

Sans résumé

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The use of values of gravity in the adjustment of the triangulation of Europe

     

Combining geomorphological and geodetic data to determine postglacial tilting in Manitoba

Estimates of postglacial rebound in central North America from Laurentide deglaciation to the present time are uncertain as a result of lack of data from the continental interior. A more precise knowledge of postglacial tilt history will assist studies of the evolution of the major lakes in Manitoba and will facilitate the engineering and environmental management of the present-day hydrological system. This paper explores the benefits of combining geomorphological data with high-precision, real-time geodetic data (GPS positioning and absolute gravity) and lake-gauge tilt data now being collected for postglacial rebound studies in Manitoba and adjacent regions in the US. Presently-available data sets representing these data types are (1) tilting of the 9.5 kyr B.P. Campbell strand line south and west of Lake Winnipeg, (2) the rate of decrease in absolute gravity values measured from 1987 to 1995 at Churchill, Manitoba, and (3) the present-day regional tilt rate derived from water-level gauges in southern Manitoba lakes. These data are compared to theoretical predictions based on the published ICE-3G loading history and on a model of Earth rheology characterized by a 1066B density and elastic structure, an upper-mantle viscosity of 10 ²¹Pa s, a lower-mantle viscosity of 2 × 10 ²¹Pa s, and a lithosphere thickness of 120 km (Tushingham & Peltier, 1991). All three data types show disagreement in Manitoba with ICE-3G and the standard Earth model. ICE-4G does better but could not be investigated in any detail. The constraints on model parameters provided by the different data types were investigated by varying, one at a time, three key parameters, (1) the thickness of the lithosphere in excess of 120 km, (2) the lower mantle viscosity, and (3) the thickness of Laurentide ice over the Prairies, to obtain better fits to the data. The present data do not appear to constrain lithosphere thickness in excess of 120 km very well. While both the Campbell strand line data and the Churchill absolute gravity data are consistent with an increase in lower-mantle viscosity, the present-day, lake-gauge data are not. All three data types are consistent with a thinning of the Laurentide ice-sheet over the Prairies relative to the ICE-3G model. Simultaneous adjustment of model parameters with the advantage of anticipated new data in Manitoba and adjacent regions in the US will lead to better understanding of the trade-offs between Earth rheology and ice sheet history and hence to an improved Laurentide postglacial rebound model.