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[en] Complete text of publication follows. Although it is known that the geodynamo has been operating for at least 3.2 Ga, it remains difficult to infer the intensity, dipolarity and stability (occurrence of reversals) of the Precambrian magnetic field of the Earth. In order to assist the interpretation of paleomagnetic data, we produce models for the long-term evolution of the geodynamo by combining core thermodynamics with a systematic scaling analysis of numerical dynamo simulations. We update earlier dynamo scaling results by exploring a parameter space which has been extended in order to account for core aspect ratios and buoyancy source distributions relevant to Earth in the Precambrian. Our analysis highlights the central role of the convective power, which is an output of core thermodynamics and the main input of our updated scalings. As the thermal evolution of the Earth's core is not well known, two end-member models of heat flow evolution at the core-mantle boundary are used, respectively terminating at present heat flows of 11 TW (high-power scenario) and 3 TW (low power scenario). The resulting models predict that the geodynamo has been active for ages younger than 3.8 Ga. From that time to the appearance of the inner core, a thermal dynamo driven only by secular cooling can produce a dipole moment of strength comparable to that of the present field, thus precluding an interpretation of the oldest paleomagnetic records as evidence of the inner core presence. The observed lack of strong long-term trends in paleointensity data throughout the Earth's history can be rationalized by the weakness of paleointensity variations predicted by our models relatively to the data scatter. Specifically, the most significant internal magnetic field increase which we predict is associated to the sudden power increase resulting from inner core nucleation, but the dynamo becomes deeper-seated at the same time, thus largely cancelling the increase at the core and Earth surface, and diminishing the prospect of observing this event in paleointensity data. Our models additionally suggest that the geodynamo has lied close to the transition to polarity reversals throughout its history. In the Precambrian, we predict a dynamo with similar dipolarity and less frequent reversals than at present times, due to conditions of generally lower convective forcing. Quantifying the typical core-mantle boundary heat flow fluctuations required to switch the geodynamo from a reversing to a non-reversing state, we find that it is unlikely that these may have caused superchrons in the last 0.5 Ga without shutting down dynamo action altogether.
[en] The purpose of this work is about the election, exploitation and modes of supply mineral raw resources used in the manufacturing of lithic and ceramic archaeological artifacts in Guayacas - Dayman - Paysandu
[en] The regularity of the deposition of main mineralization of industrial types within Semipalatinsk test site proves and here and there defines more exactly location of the ore levels in Eastern Kazakhstan. Two mega levels, namely: Cambrian-Ordovician (siliceous-basalt, island-arc) and Carboniferous (especially carbonaceous-tarragons) ones are the most perspective for localizing the leading gold mineralization in the region
[en] Precambrian large igneous provinces flourished during the Proterozoic and some, such as the 1.27 Ga Mackenzie event in North America, resemble their Phanerozoic counterparts with good evidence for the production of large volumes of mafic magma in short periods of geologic time (<10 m.y.), a specific focus of this magmatism, and regional uplift prior to the onset of volcanism. In many Precambrian examples, the flood basalts have been largely removed by erosion and only the root zone (e.g. dyke swarm, layered mafic intrusion) of these large igneous provinces is preserved. The short duration of this magmatism and the geometric relationship between linear dyke swarms and basement geology can potentially provide a robust geological pin in continental reconstructions. The extensive 2.37 Ga E-W trending Bangalore dyke swarm in the Dharwar craton, India cross-cuts N-S trending Neoarchean greenstone belts and the 2.51 Ga Closepet granite and is a good example of the potential for providing a cornerstone in Paleoproterozoic continent reconstruction. In contrast, the 1.89 Ga Southern Bastar-Cuddapah large igneous province in southern India is one of several large igneous provinces formed at this time on different continents, including the 1.89-1.88 Ga circum-Superior mafic/ultramafic magmatism in North America and the 1.88-1.87 Ga Waterberg Group sills in South Africa, and is part of a global period of enhanced mantle melting. A challenge for future attempts at Precambrian continent reconstruction is to distinguish between discrete global periods of enhanced mantle melting and a single specific focus large igneous province. (author)
[en] The products of bottom current circulation around the Iberian continental margin are characterised by large erosional and depositional features formed under a variety of geological and oceanographic contexts. The Iberian margins are influenced by several water masses that mainly interact along the upper and middle con- tinental slopes, as well as along the lower slope with the abyssal plains being influenced to a lesser extent. The main depositional features occur along the Ceuta Contourite Depositional System (CDS) within the SW Alboran Sea, in the Gulf of Cadiz (the most studied so far), the western margins of the Portugal/Galician mar- gin, the Ortegal Spur and the Le Danois Bank or Cachucho. Moreover, erosional contourite features have also been recently indentified, most notably terraces, abraded surfaces, channels, furrows and moats. The majority of these features are formed under the influence of the Mediterranean water masses, especially by the interaction of the Mediterranean Outflow Water (MOW) with the seafloor. The MOW is characterized as relatively warm (13 degree centigrade) and with a high salinity (∼36.5), giving it a high density relative to the surrounding water masses, hence constituting an important contribution to the global thermohaline circulation, making it one of the most studied water masses surrounding Iberia. The development of both depositional and ero- sional contourite features does not only depend on the bottom-current velocity but also on several other important controlling factors, including: 1) local margin morphology affected by recent tectonic activity; 2) multiple sources of sediment supply; 3) water-mass interphases interacting with the seafloor; and 4) glacioeustatic changes, especially during the Quaternary, when the increasing influence of the bottom cur- rent has been observed during the cold stages. The main objective of this special volume contribution is to provide a review and description of the regional along-slope processes and their sedimentary impact around the Iberian margin. Despite the numerous examples of bottom current processes recorded, there remains a number of challenges to understanding CDSs around the Iberian margin including: 1) evidencing their important scientific implications (stratigraphy, sedimentology, palaeoceanography and palaeoclimatology); assess- ing their geological hazard and their economic potential (for mineral and energy resources); and 3) using them to create conceptual models for CDS formation. There is a lack of complete knowledge about the different oceanographic processes that may drive bottom currents, and there is also the need to document the great variety of contourite features (processes and products) and facies models, along with their evolution over time and space. Therefore advances, both in new technologies and integrated studies (Geology, Physical Oceanography and Benthic/planktonic Biology), are anticipated. (Author)
[en] Two quantitative methods were used to reconstruct paleoenvironments and vegetation in the Altai-Sayan mountains, Central Asia, during the Holocene. The 'biomization' method of Prentice et al (1996 Clim. Dyn. 12 185-96), applied to the surface pollen record, worked fairly well in the reconstructions of current vegetation. Applying this method to fossil pollen data, we reconstructed site paleovegetation. Our montane bioclimatic model, MontBioCliM, was used inversely to convert site paleovegetation into site paleoclimates. The differences between site paleo and current climates served as past climate change scenarios. The climatic anomalies for 2020, 2050, and 2080 derived from HadCM3 A1FI and B1 of the Hadley Centre, UK, served as climate change scenarios in the 21st century. MontBioCliM was applied directly to all climate scenarios through the Holocene to map past and future mountain vegetation over the Altai-Sayan mountains. Our results suggest that the early Holocene ca 10 000 BP was cold and dry; the period between 8000 and 5300 BP was warm and moist; and the time slice ca 3200 BP was cooler and drier than the present. Using kappa statistics, we showed that the vegetation at 8000 BP and 5300 BP was similar, as was the vegetation at 10 000 BP and 3200 BP, while future vegetation was predicted to be dissimilar to any of the paleovegetation reconstructions. The mid-Holocene is frequently hypothesized to be an analog of future climate warming; however, being known as warm and moist in Siberia, the mid-Holocene climate would likely impact terrestrial ecosystems differently from the projected warm and dry mid-century climate.
[en] On the southern side of the Beausset syncline (southwest Provence), lithostratigraphic and structural propositions are divergent. Since 1978, the available geological map at 1/50 000 has revealed these contradictions. During the recent revision of these maps at 1/10 000 scale and with detailed triasic lithostratigraphy, the authors have proposed a new interpretation of the relationship between the Trias and Santonian in diverse structural units of this area. These facts confirm premature tectonic phases (before Santonian), evoked since 1891 by M. Bertrand in this area (Saint-Jean oratory), where the embryonic structures are taken back in the following phases of the Paleogene. Similarly to the other hills around Toulon (Var), this premature genesis of the provençal folds can be attributed to the diapiric behaviour of the Trias, which seems to be the main engine of these local and initial structures. (Author)