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Variabilités climatiques et océaniques du dernier cycle glaciaire-interglaciaire. Propriétés magnétiques et géochimiques des sédiments de la marge Nord-Ouest Américaine subtropicale.

Abstract : The magnetic and geochemical properties of four marine sediment cores from the North-Western American margin allowed reconstructing the dynamics of the terrigenous sedimentation and the post-depositional processes, related to climatic and oceanological changes during the last glacial-interglacial cycle (0-120,000 yrs). The cores have been retrieved in three sites located on the Californian and Mexican coasts: Santa Barbara Basin (35°N), margin of Baja California (23°N) and Gulf of Tehuantepec (15°N). Terrigenous and biogenic components of the sediments are deposited following a seasonal dynamic and accumulate rapidly (35 to 150 cm/ka). The degradation of the organic matter and weak ventilation of the waters lead to low dissolved oxygen contents in the water masses between 300 and 800 m depth, called the Eastern Pacific oxygen minimum zone.
The magnetic parameters are used to trace the concentration, nature or grain sizes changes of the magnetic fraction. Two types of ferromagnetic minerals can be detected: the terrigenous iron oxides (magnetite, hematite, goethite) and the authigenic iron sulfides (greigite, Fe3S4 and pyrrhotite, Fe7S8). The relative contents of major and trace elements, measured by X-ray fluorescence (XRF) on key interval of the cores, help to improve the interpretations. Isotopic ratios of oxygen and carbon measured on planktonic and benthic foraminifera in the core from Gulf of Tehuantepec allowed the reconstitution of the physical and chemical characteristics and of the structure of the water column.
The sedimentary sequences, dated by correlation of magnetic susceptibility profiles, calibrated 14C ages and identification of paleomagnetic excursions, cover the glacial-interglacial transition (0-40 ka) and one of them covers the last climatic cycle (0-120 ka).
The high concentration of iron oxides in the glacial sediments in the three sites suggests strong terrigenous inputs on the NW American margin and notably strong aeolian inputs during the last glacial maximum (20-26 ka BP). Iron sulfides species of different reduction levels, detected in the sediments of Santa Barbara Basin, have been interpreted as proxies of bottom waters oxygenation conditions. The succession of anoxic (pyrite, FeS2) and dysoxic (less anoxic, ferrimagnetic sulfides) phases is in good accordance with the bioturbation index (Behl and Kennett, 1996) for the last 15 ka and revealed dysoxic phases during abrupt cooling of North Atlantic. In the Gulf of Tehuantepec, the thermocline was less deep during the deglaciation (16-10 ka BP) than during the Holocene (10-0 ka BP) and the presence of authigenic gypsum (CaSO4), formed by reoxidation of pyrite, in the sediments suggests a better oxygenation of the bottom waters during the glacial periods.
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https://tel.archives-ouvertes.fr/tel-00133129
Contributor : Cécile Blanchet <>
Submitted on : Friday, February 23, 2007 - 4:21:46 PM
Last modification on : Wednesday, October 14, 2020 - 3:50:36 AM
Long-term archiving on: : Tuesday, April 6, 2010 - 10:49:48 PM

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  • HAL Id : tel-00133129, version 1

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Cécile Blanchet. Variabilités climatiques et océaniques du dernier cycle glaciaire-interglaciaire. Propriétés magnétiques et géochimiques des sédiments de la marge Nord-Ouest Américaine subtropicale.. Minéralogie. Université de droit, d'économie et des sciences - Aix-Marseille III, 2006. Français. ⟨tel-00133129⟩

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