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411.
Sandy shallow seas, like the North Sea, are very dynamic. Several morphological features are present on the bed, from small ripples to sand waves and large tidal sandbanks. The larger patterns induce significant depth variations that have an impact on human activities taking place in this area. Therefore, it is important to know where these large-scale features occur, what their natural behaviour is and how they interact with human activities. Here, we extend earlier research that compares the results of an idealized model of large-scale seabed patterns with data of seabed patterns in the North Sea. The idealized model is extended with a grain size dependency. The adaptations lead to more accurate predictions of the occurrence of large-scale bed forms in the North Sea. Therefore, grain size dependency and, in particular, critical shear stress are important to explain the occurrence of sand waves and sandbanks in the North Sea. Responsible Editor: Alejandro Souza  相似文献   
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413.
We point out an acceleration of the DORIS clock on-board the Jason satellite during passes over the South Atlantic Anomaly (SAA). When this effect is ignored in the current geodetic positioning of the DORIS stations, derived coordinates show almost linear trends in time, corresponding to anomalous horizontal and vertical velocities of the order of 1 m yr?1. We propose a simple scientific explanation of this physical phenomenon that is corroborated by direct Jason/TOPEX clock comparisons with respect to the DORIS master beacons in Kourou and Toulouse. To cite this article: P. Willis et al., C. R. Geoscience 336 (2004).  相似文献   
414.
The graphitization of carbonaceous material (CM) in a high-pressure metamorphic gradient is characterized along a cross section in the Schistes Lustrés formation, Western Alps. Along this 25-km cross section, both the CM precursor and the host-rock lithology are homogeneous, and the prograde evolution of the pressure-temperature metamorphic conditions from the lower blueschist-facies (13 kbar, 330 °C) to the eclogite-facies (20 kbar, 500 °C) is tightly constrained by literature data. Raman microspectroscopy shows that at the micrometre scale, this process is progressive and continuous with increasing metamorphic grade, and that the structure of CM is very sensitive to temperature variations. At the nanometre scale (HRTEM), the CM is composed of a mixture of a microporous phase and an onion-ring like phase, both known as non-graphitizing under the effect of temperature at ambient pressure. The HP-LT graphitization produces structurally and microtexturally heterogeneous CM. With increasing metamorphic grade, the graphitization of the two types of CM proceeds up to the triperiodic graphite stage because of microtextural and structural changes that are specific to each type of CM. The microporous material is progressively transformed into graphite through a macroporous transitional stage. In this case, graphitization mainly occurs on the pore walls as a result of pore growth. In the case of concentric onion-ring like material, graphitization occurs in the regions with the largest radius of curvature, i.e. on the outer part of the ring. In comparison with 1-bar experiments, pressure seems to induce microtextural changes, which allows the subsequent structural modifications of the starting material.  相似文献   
415.
This paper presents evidence for strong biochemical weathering of basaltic outcrops induced by fungal communities in a cold environment. Weathering rind formation is considered to be a consequence of the biological activity. Comparisons between in vitro experiments and in situ observations allow a characterization of fungal effects on rocks and help to define the place of these micro‐organisms in the cold environment weathering chain. It is concluded that biological weathering is chronologically the first process of weathering, probably leading to the subsequent expression of cryogenic processes. Information presented here suggests the need for reconsideration of the traditional frost‐driven morphogenetic system normally considered for subpolar areas. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   
416.
Radiocarbon data for 11 stations and tritium data for 16 stations in the North Atlantic Ocean from 74°N to 3°N are presented. For radiocarbon, normal errors inΔ14C are± 4‰, and in tritium,± 0.09TU or± 3%, whichever is larger. There is a remarkable, but not simply linear, correlation between oceanic bomb transients in14C and3H. The deep convective mixing in the Greenland Sea is reflected in substantial bomb tracer penetration to all depths, with residence time for the deep, cold core water that seems to be 20 to 30 years. The outflow in the bottom layer southward over the sills of the Denmark Strait and Faroe Passage carries significant tritium concentration, at least to 40°N. Complicated, but coherent, profile structures in the subtropical Atlantic suggest effects of large-scale lateral advection. In particular, a pronounced minimum in both14C and3H might be associated with the Antarctic intermediate water.  相似文献   
417.
Sensitivity experiments with a simple water-balance model were used to constrain the possible climatic causes of distinct Holocene patterns of lake-level variation in different regions of Europe. Lakes in S Sweden were low at 9 ka, high around 6.5 ka, low again around 4 ka and are high now. Lakes in Estonia show similar but weaker trends. Lakes in S France were highest around 9 ka, lowest around 4 ka, intermediate now. Lakes in Greece were also high around 9 ka but continued rising until 7.5 ka, then fell gradually from 5 ka with a brief high phase around 3 ka, and are low now. The model was forced with insolation anomalies deduced from orbital variations, temperature anomalies inferred from the pollen record and cloudiness anomalies derived from changes in the position of the subtropical anticyclone (inferred from reconstructed changes in the equator-to-pole temperature gradient), in order to evaluate the effects of resultant evaporation changes on catchment water balance. The resulting simulated changes in runoff (precipitation minus actual evapotranspiration) were slight, and frequently opposite to the observed trends. Larger changes in precipitation are plausible and are required to explain the data. The required precipitation increase in N Europe from 9 ka (low) to 6 ka (high) is suggested by GCM experiments to have been a consequence of interacting insolation and residual ice-sheet effects on the atmospheric circulation over the North Atlantic. The explanation of other observed changes, including the drying trend during the Holocene in S Europe, has not been provided by GCM experiments to date. Explanations may lie in changes in mesoscale circulation, sea-surface temperature patterns and the coupling between these phenomena that may not follow orbital changes in any simple way. Correspondence to: SP Harrison  相似文献   
418.
Résumé Le filon polymétallique (Sb, Pb, Zn, Ag, Cu) des Borderies (Puy de Dôme, France) résulte de la succession de quatre stades de minéralisations possédant chacun ses propres caractéristiques minéralogiques, géochimiques et isotopiques (Pb). Le premier stade est représenté par la berthiérite dont l'habitus et la composition isotopique du plomb (206Pb/204Pb 17,76) révèle un épisode métallogénique ancien. Malgré une nette évolution géochimique du dépôt, le second stade à mispickel, jamesonite I, andorite, stibine, zinkénite, etc. présente des compositions isotopiques du plomb homogènes (206Pb/204Pb=17,96), qui démontrent son appartenance à un même épisode métallogénique. Cet épisode est daté à 313±5 Ma par la méthode K/Ar. La source du plomb présent dans ces deux stades est à rechercher dans un socle métamorphique de type Limousin, inconnu à l'affleurement dans le district. Enfin, la nature des minéraux du troisième stade (zinkénite II, fülöppite, semseyite) et les compositions isotopiques du plomb (206Pb/204Pb=18,21) attestent d'une remobilisation des sulfures déjà présents dans le filon sous l'action d'un fluide tardif plombifère. Ce fluide est à rattacher aux distensions liasiques et emprunte le plomb aux roches du district (granite et métamorphites). Le dernier stade (plagionite, jamesonite II, zinkénite III) est un simple prolongement du troisième, comme le démontrent la nature des sulfures et les similitudes isotopiques (206Pb/204Pb=18,21). L'étude de filon illustre la complémentarité des observations minéralogiques et des analyses isotopiques du plomb sur phases minérales bien identifiées. Cette double approche est nécessaire pour appréhender la source des métaux et les conditions de genèse de gîtes polyphasés à paragenèses complexes.
The polymetallic vein (Sb, Pb, Zn, Ag, Cu) of Les Borderies (Puy de Dôme, France) results from a succession of four stages of mineralization, each with its own mineralogical, geochemical and isotopic (Pb) characteristics. Both residual aspect and lead isotope composition of berthierite (206Pb/204Pb=17.76) point out that the first stage (composed only by berthierite) is related to an old metallogenic event. The second stage minerals (arsenopyrite, jamesonite I, andorite, stibnite, zinkenite etc), with their homogeneous lead isotope compositions (206Pb/204Pb=17.96), depend on a single metallogenic event, characterized by an obvious evolution of geochemical conditions during deposition. This event is dated at 313±5 Ma by the K/Ar method. The lead source has been identified as a Limousin-type metamorphic basement, unknown in the district. Both mineralogical studies and lead isotope analyses (206Pb/204Pb=18.21) show that the third stage (zinkenite II, fülöppite, semseyite) result from the remobilization of the early vein minerals by a later lead-rich fluid. This fluid, probably of Liasic age, leached lead from all regional rocks including the metamorphic basement and Hercynian granites. The fourth stage extends the third one, according to the paragenesis (plagionite, jamesonite II, zinkenite III) and similar lead isotopes (206Pb/204Pb=18.21). This study shows that mineralogical studies combined with lead isotope analysis for specific sulfides allow to determine the genesis of complex ore deposits such as LesBorderies vein.
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419.
420.
Zusammenfassung Eine Abschätzung der im Gewitter zum Umsatz kommenden Energiebeträge zeigt, daß die in den elektrischen Phänomenen verbrauchte Energie im Gesamt-Energie-Umsatz nicht ins Gewicht fällt. Zieht man jedoch den Vergleich nur zu dem Teilbetrag des mechanisch-thermodynamischen Energieumsatzes, der sich über den normalen Energie-Umsatz des Wetter- bezw. Austauschgeschehens erhebt, und nimmt versuchsweise für diesen den aus dem Niederschlag allein abgeleiteten Betrag an, so ergibt sich, daß die elektrischen Phänomene etwa 1/4 dieses Betrages beanspruchen. — Die Abschätzung des elektrischen Energie-Umsatzes erlaubt es, die mittlere in einem Erdblitz umgesetzte Energie abzuleiten: Sie ergibt sich zu rund 2000 kWh. Eine noch nicht lösbare Diskrepanz ergibt sich für die Feldstärke unterhalb der Gewitterwolken.
Summary By an estimate of the energy budget of the thunderstorm is shown that the energy of electrical activity is entirely negligible in considerations of the whole budget. But if the energy of electrical activity is only compared with that amount of the whole energy which exceeds the normal energy amount of the «Austausch» it can by shown that the electrical phenomena need about 1/4 of this amount (this amount is estimated only from the precipitation). — By the estimation of the electrical energy budget it is possible to give the mean energy of a lightning: about 2000 kWh. In considerations of the potential-gradient under thunderstorm clouds a contradiction is given which can not be solved until now.
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