26 resultados para Picot, Michel Joseph Pierre, 1770-

em Publishing Network for Geoscientific


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Paleomagnetic inclination, declination and relative paleointensity were reconstructed from the sediments of Laguna Potrok Aike in the framework of the International Continental scientific Drilling Program (ICDP) Potrok Aike maar lake Sediment Archive Drilling prOject (PASADO). Here we present the u-channel-based full vector paleomagnetic field reconstruction since 51.2 ka cal BP. The relative paleointensity proxy (RPI) was built by normalising the natural remanent magnetisation with the anhysteretic remanent magnetisation using the average ratio at 4 demagnetisation steps part of the ChRM interval (NRM/ARM10e40 mT). A grain size influence on the RPI was removed using a correction based on the linear relationship between the RPI and the median destructive field of the natural remanent magnetisation (MDFNRM). The new record is compared with other lacustrine and marine records and stacks from the mid- to high-latitudes of the Southern Hemisphere, revealing consistent millennial-scale variability, the identification of the Laschamp and possibly the Mono Lake geomagnetic excursions, and a direction swing possibly associated to the Hilina Pali excursion at 20 ka cal BP. Nonetheless, a global-scale comparison with other high-resolution records located on the opposite side of the Earth and with various dipole field references hint at a different behaviour of the geomagnetic field around southern South America at 46 ka cal BP.

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Subcontinuously cored early(?)-middle Miocene to recently deposited sediments from ODP Site 645 were studied texturally, mineralogically, and geochemically. The entire sequence contains minerals and associated chemical elements that are chiefly of detrital origin. In particular, the clay minerals, which include smectite, kaolinite, chlorite, and illite, are detrital. No obvious evidence of diagenesis with depth, of burial, of volcanism, or of hydrothermal alteration was observed. The sedimentary textures, clay mineralogy, and <2-µm fraction geochemistry of the early middle Miocene sediments (630 to 1147 mbsf) suggest the pronounced but variable influence of a southward bottom current. Two clay facies are defined. The lower one, Cj (780 to 1147 mbsf), is characterized by the great abundance of discrete smectite (with less than 15% illite interlayers), probably detrital in origin, and reworked older, discrete, smectite-rich sediments. The upper clay facies, C2 (630 to 780 mbsf), shows a net decrease of the fully expandable clay abundances, with a great abundance of mixed-layer, illite-smectite clays (60 to 80% of illite interlayers). Such clay assemblages can be inherited from paleosoils or older sedimentary rocks. An important change occurs at 630 mbsf (clay fraction) or 600 mbsf (sedimentary texture), which may be explained by the beginning of continental glaciation (630 mbsf, ~9 Ma) and the onset of ice rafting in Baffin Bay (600 mbsf, ~8 Ma). Above this level, the characteristics and modifications of the clay assemblages are controlled climatically and can be explained by the fluctuations of (1) ice-rafting, (2) speed of weak bottom currents, and (3) some supply by mud turbiditic currents. Three clay facies (C3, C4, and C5) can be defined by the abrupt increases of the inherited chlorite and illite clays.

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