Conventional and ion-microprobe U-Pb dating of detrital zircons of the Tentudía Group (Serie Negra,SW Spain): implications for zircon systematics,stratigraphy, tectonics and the Precambrian/Cambrian boundary     

Hans-Jürgen Schäfer  Dieter Gebauer  Thomas F. Nägler  Luis Eguiluz 《Contributions to Mineralogy and Petrology》1993,113(3):289-299

Conventional multi-grain and ion-microprobe dating of detrital zircons from a very low grade metagraywacke of the Tentudía Group (upper part of the Serie Negra, Ossa-Morena Zone, SW Spain) reveals an uppermost Vendian age for the deposition of the metagraywacke. The youngest detrital zircon grain provides a maximum depositional age of about 565 Ma. Thus, these data contradict earlier Middle to Upper Riphean (ca. 1350-850 Ma) estimates on the age of the Tentudía Group and favour a Precambrian/Cambrian boundary falling into the range of 540 to 530 Ma. The presence of about 20% of Pan-African detrital zircons ranging from about 700 to 550 Ma indicates the derivation from Gondwana. From the upper intercept ages of the fan-shaped data field defined by conventionally determined zircon fractions, it can be deduced that 2.1 Ga old zircons as well as Archean zircons existed in the provenance(s) of the Serie Negra sediments. This mixing of crustal components of different ages is in line with the Nd crustal residence age of 1.9 Ga. The latter value, as well as other model ages of the Iberian Massif, indicates unusually high amounts of ancient crust to be present in the strata. This is different to other (meta)sediments of the European Hercynides and suggests that the Iberian strata of uppermost Precambrian age may contain the detritus of more internal, older parts of Gondwana than other European strata of comparable ages. Geochemical data on the analysed sample and further metagraywackes of the Tentudía Group argue for a deposition in an arc environment. Such a scenario would conform with the syn- to post-orogenic shallow marine deposition of the studied sediments. Furthermore, an upper time limit for the pre-Lower Cambrian deformational history, including two phases of regional deformation, is given by the maximum age of deposition, implying a very short time interval for deposition and deformation of the Tentudía Group. Concerning the U-Pb systematics of detrital zircon fractions, it is probable that numerous, previously published conventional multi-grain zircon data on (meta)sedimentary rocks of the European Hercynides readily can be explained by the presence of up to 20% of Pan-African detrital zircons and later Phanerozoic lead loss during metamorphic transformation of the sedimentary protoliths. Moreover, this implies that such metasediments originated from post-Pan-African sedimentary precursors.  相似文献   

Cosmogenic radionuclides and ozone at a mountain station at 3.0 km a.s.l.     

Director Dr. R. Reiter  Dipl.-Met. K. Munzert  Dipl.-Met. Hans-J. Kanter  Dr. K. Pötzl 《Theoretical and Applied Climatology》1983,32(2-3):131-160

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Résultats préliminaires sur la sédimentation pélagique de l'Atlantique tropical au Crétacé et au Tertiaire (plateau de Demerara,Leg ODP 207)     

Taniel Danelian  Laurence Le Callonnec  Jochen Erbacher  David C. Mosher  Mitchell J. Malone  Debora Berti  Karen L. Bice  Helen Bostock  Hans-Jürgen Brumsack  Astrid Forster  Felix Heidersdorf  Jorijntje Henderiks  Thomas J. Janecek  Christopher Junium  Ken MacLeod  Philip A. Meyers  Jörg H. Mutterlose  Hiroshi Nishi  Christine Glatz 《Comptes Rendus Geoscience》2005,337(6):609-616

Five sites located on a bathymetric transect of the distal Demerara Rise were studied by ODP Leg 207. Albian sediments of essentially terrigenous nature (clay, siltstone, sandstone) are the oldest drilled stratigraphic levels and form apparently the top of the synrift sequence. They are overlain by Cenomanian to Santonian finely laminated black shales, rich in organic matter of marine origin, which accumulated on a thermally subsiding ramp. Early Campanian hiatuses are thought to be the result of final disjunction of Demerara Rise (South America) from Africa and the onset of deep water communication between the two Atlantic basins (south and central). The overlying Uppermost Cretaceous–Oligocene chalk includes rich and diversified calcareous plankton assemblages, as well as two radiolarian-rich intervals (Late Campanian and Middle Eocene). A complex erosional surface developed during the Late Oligocene–Early Miocene. Sedimentation was impeded since then on the intermediate and deep sites of Demerara Rise, possibly due to the action of deep submarine currents. To cite this article: T. Danelian et al., C. R. Geoscience 337 (2005).  相似文献   

The influence of the anomalous cosmic-ray component on the dynamics of the solar wind     

Hans-Jörg Fahr  Horst Fichtner  Stanislaw Grzedzielski 《Solar physics》1992,137(2):355-383

It is well known that both the galactic and anomalous cosmic rays show positive intensity gradients in the outer heliosphere which are connected with corresponding pressure gradients. Due to an efficient dynamical coupling between the solar wind plasma and these highly energetic media by means of convected MHD turbulences, there exists a mutual interaction between these media. As one consequence of this scenario the enforced pressure gradients influence the distant solar wind expansion. Here we concentrate in our theoretical study on the interaction of the solar wind only with the anomalous cosmic-ray component. We use the standard two-fluid model in which the cosmic-ray fluid modifies the solar wind flow via the cosmic-ray pressure gradient. Then we derive numerical solutions in the following steps: first we calculate an aspherical pressure distribution for the anomalous cosmic rays, describing their diffusion in an unperturbed radial solar wind. Second, we then consider the perturbation of the solar wind flow due to these induced anomalous cosmic-ray pressure gradients. Within this context we especially take account of the action of a non-spherical geometry of the heliospheric shock which may lead to pronounced upwinddownwind asymmetries in the pressures and thereby in the resulting solar wind flows. As we can show in our model, which fits the available observational data, radial decelerations of the distant solar wind by between 5 to 11% are to be expected, however, the deviations of the bulk solar wind flow from the radialdirections are only slightly pronounced.  相似文献   

An incentive mechanism for reducing emissions from conversion of intact and non-intact forests   总被引：1，自引：0，他引：1  

Danilo Mollicone  Frédéric Achard  Sandro Federici  Hugh D. Eva  Giacomo Grassi  Alan Belward  Frank Raes  Günther Seufert  Hans-Jürgen Stibig  Giorgio Matteucci  Ernst-Detlef Schulze 《Climatic change》2007,83(4):477-493

This paper presents a new accounting mechanism in the context of the UNFCCC issue on reducing emissions from deforestation in developing countries, including technical options for determining baselines of forest conversions. This proposal builds on the recent scientific achievements related to the estimation of tropical deforestation rates and to the assessment of ‘intact’ forest areas. The distinction between ‘intact’ and ‘non intact’ forests used here arises from experience with satellite-based deforestation measurements and allows accounting for carbon losses from forest degradation. The proposed accounting system would use forest area conversion rates as input data. An optimal technical solution to set baselines would be to use historical average figures during the time period from 1990 to 2005. The system introduces two different schemes to account for preserved carbon: one for countries with high forest conversion rates where the desired outcome would be a reduction in their rates, and another for countries with low rates. A ‘global’ baseline rate would be used to discriminate between these two country categories (high and low rates). For the hypothetical accounting period 2013–2017 and considering 72% of the total tropical forest domain for which data are available, the scenario of a 10% reduction of the high rates and of the preservation of low rates would result in approximately 1.6 billion tCO₂ of avoided emissions. The resulting benefits of this reduction would be shared between those high-rate countries which reduced deforestation and those low-rate countries which did not increase their deforestation over an agreed threshold (e.g., half of “global” baseline rate).  相似文献   

Barite concretions as evidence of pauses in sedimentation in the Marnes Bleues Formation of the Vocontian Basin (SE France)     

Jean-G. Brhret  Hans-J. Brumsack 《Sedimentary Geology》2000,130(3-4):205-228

Several intervals rich in barite nodules occur in the mid-Cretaceous marls of the Vocontian Trough (SE France). These concretions, which grew on heterogeneities in the sediment, are arranged as horizons parallel to the bedding, and are often associated with carbonate and phosphate nodules. Detailed sedimentological and stratigraphical observations suggest that they represent the consequence of short-term pauses in sedimentation during early diagenesis, but the most important occurrences of barite are below prominent discontinuities. This may be explained by the frequency of pauses in sedimentation caused by a progressive starvation, or an increase in the energy of bottom-water currents. The high number of barite occurrences on the basin margins is directly linked to the recurrence of sedimentary breaks. The geochemical data show that the host sediment corresponds to a pelagic setting, far from hydrothermal influences, but generally in a restricted, dysoxic to suboxic, depositional environment. The sulphur isotopic composition of the barite sulphate (+16 to +76‰ rel. CDT) supports the hypothesis of an early diagenetic origin of the barite nodules. Their genesis may be explained by the fixation of a diagenetic front corresponding to the penetration of a sulphate reduction zone from the seawater–sediment interface down in the sediment which is in contact with barium-rich porewaters expelled upwards as a consequence of compaction. The barium of biogenic origin, initially stored in the underlying black shale, was mobilized under strongly reducing conditions. Such fixation is normally induced by a pause or a stop in sedimentation due to starvation or renewed bottom-water circulation. Together with other concretions, the barite nodules mark condensed stratigraphic intervals and often represent major breaks. As such they shed light on variations in sedimentary activity and their importance in the sedimentology and stratigraphy of pelagic marly sequences cannot be ignored.  相似文献   

A 3-D lithospheric model of the Caribbean-South American plate boundary     

Javier Sanchez  Hans-Jürgen Götze  Michael Schmitz 《International Journal of Earth Sciences》2011,100(7):1697-1712

A 3-D structural model of the Caribbean-South American plate boundary was constructed by gravity modeling. The model was constrained by four wide-angle seismic refraction sections, Moho depth estimations from receiver functions, and additionally seismological hypocenters, surface geology, and geodynamic information. Density values were calculated from empirical velocity-density functions, and mineralogical-chemical composition considering specific P/T conditions. We tested different structural models for Western and Eastern Venezuela. In the final model, the fit of the measured and modeled gravity fields for a long Caribbean slab in Western Venezuela was better than the fit obtained for a short one. This interpretation is consistent with the constraining data. The slab is interpreted to extend further to the south beneath Northern Colombia and culminates in the area of the seismic cluster of the Bucaramanga nest. The modeling estimates a slab dip angle under Maracaibo and Mérida Andes of 15°, which increases to 32° below 100 km depth. The dip direction of approx. N150°E ± 5 increases lightly eastward. In Eastern Venezuela, considering its short wavelength, lineaments analyzed from gravity data (by curvature methods and Euler deconvolution) seem to be related to shallow structures and density contrast in the Serranía del Interior and not from a deep detached slab beneath the continental crust. It is deduced from modeling results that this slab configuration has a very small influence on the gravity field. The slab was modeled according to the subduction-transform propagation model with purely westward subduction and a slab break off along a vertical dip-slip tear through the lithosphere.  相似文献   

Intraplate volcanism in the Danube Basin of NW Hungary: 3D geophysical modelling of the Late Miocene Pásztori volcano     

Pánisová  Jaroslava  Balázs  Attila  Zalai  Zsófia  Bielik  Miroslav  Horváth  Ferenc  Harangi  Szabolcs  Schmidt  Sabine  Götze  Hans-Jürgen 《International Journal of Earth Sciences》2018,107(5):1713-1730

International Journal of Earth Sciences - Three-dimensional geophysical modelling of the early Late Miocene Pásztori volcano (ca. 11–10 Ma) and adjacent area in the Little...  相似文献   

Geochronological (U–Pb,U–Th–total Pb,Sm–Nd) and geochemical (REE, <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr, δ<Superscript>18</Superscript>O, δ<Superscript>13</Superscript>C) tracing of intraplate tectonism and associated fluid flow in the Warburton Basin,Australia     

Alexander?W.?MiddletonEmail author  I.?Tongu??Uysal  Suzanne?D.?Golding  Hans-Jürgen?F?rster  Charlotte?M.?Allen  Yeuxing?Feng  Dieter?Rhede  Vicki?J.?Marshall  Jaco?van?Zyl 《Contributions to Mineralogy and Petrology》2014,168(3):1058

The Warburton Basin of central Australia has experienced a complex tectonic and fluid-flow history, resulting in the formation of various authigenic minerals. Geochemical and geochronological analyses were undertaken on vein carbonates from core samples of clastic sediments. Results were then integrated with zircon U–Pb dating and uraninite U–Th–total Pb dating from the underlying granite. Stable and radiogenic isotopes (δ¹⁸O, Sr and εNd), as well as trace element data of carbonate veins indicate that >200 °C basinal fluids of evolved meteoric origin circulated through the Warburton Basin. Almost coincidental ages of these carbonates (Sm–Nd; 432 ± 12 Ma) with primary zircon (421 ± 3.8 Ma) and uraninite (407 ± 16 Ma) ages from the granitic intrusion point towards a substantial period of active tectonism and an elevated thermal regime during the mid Silurian. We hypothesise that such a thermal regime may have resulted from extensional tectonism and concomitant magmatic activity following regional orogenesis. This study shows that the combined application of geochemical and geochronological analyses of both primary and secondary species may constrain the timing of tectonomagmatic events and associated fluid flow in intraplate sedimentary basins. Furthermore, this work suggests that the Sm–Nd-isotopic system is surprisingly robust and can record geologically meaningful age data from hydrothermal mineral species.  相似文献   

Vulnerability of water resources in northern Cameroon in the context of climate change     

Ambe Emmanuel Cheo  Hans-Jürgen Voigt  Roy Lyonga Mbua 《Environmental Earth Sciences》2013,70(3):1211-1217

Water resources in northern Cameroon have continuously been reducing over the past years. Many studies have suggested two principal causes: (1) human activities such as poor farming practices, unsustainable use of water resources, increased demand of water, deforestation, land-use change, etc., (2) human-induced climate change. Northern Cameroon in this study includes: the Adamawa, North and Far North regions located closer to the Sahel regions of Africa. These regions are already water stressed because of their location and any further change in climate with rising temperature would impact water resource either positively or negatively. Time series analysis and a 12-month standardized precipitation index (SPI12) with digital data between 1957 and 2006 were used to investigate the variation of water resources in northern Cameroon. Results obtained varies between the different regions with an increased annual trend in temperature and precipitation for Ngaoundere (Adamawa region) and Garoua (north region), whereas Maroua (far north region) had a decreased annual trend in both precipitation and temperature. Further variability results obtained from a SPI12 show that wetter period out number drought period in all three regions. The study concluded that water resources vary with the changing climatic condition and the severity of the impact varies from region to region. Furthermore, water deficiency in northern Cameroon might not be due to climate change. The reasons might be a combination of poor water management and other factors such population growth, the environmental condition, etc.  相似文献