A high resolution authigenic 10Be/9Be record of geomagnetic moment variations over the last 300 ka from sedimentary cores of the Portuguese margin.

A high resolution study of authigenic Be isotopes (10Be and 9Be) combined with continuous relative paleointensity records has been performed along the same marine sedimentary sequences from the Portuguese margin (N.E. Atlantic) covering the past 300 ka in order to assess relationships between geomagnetic moment variations and 10Be production rate variations. A carefull examination of the various ways of taking into account environmental disturbing effects on the authigenic 10Be concentration leads to the conclusion that the most reliable proxy of cosmonuclide production rates is presently the authigenic 10Be/9Be ratio. Eight intervals of significant authigenic 10Be/9Be enhancement evidence geomagnetic moment drops related to global paleomagnetic excursions, some being already admitted, others being proposed as new geomagnetic features. Since, contrarily to sedimentary magnetic remanence, the authigenic 10Be/9Be records dipole moment variations without significant acquisition delay, it provides better constraints on their timing. Comparison of 10Be/9Be and benthic δ18O records from the same cores suggests that dipole moment lows preferentially occured during or at the end of interglacial episodes, with a quasi-period of 100 ka

Sensibilité de la sédimentation organique aux variations climatiques du Tardi-Wuerm et de l'Holocène; le lac du Bouchet (Haute-Loire, France)

Deux carottes de sédiments superficiels (2 à 3 m) prélevées dans le lac du Bouchet (Haute Loire, France) ont été étudiées du point de vue de leur composition minérale et organique. Les corrélations établies à partir des données concernant lamatière organique ainsi que celles de la palynologie, ont permis de replacer les deux premiers mètres de la série sédimentaire dans la logique climato-stratigraphique du passage Tardi-Würm-Holocène. Nous montrons ici que des variations climatiques de faible amplitude comme celles de l'Holocène, engendrent des changements importants dans la nature, la quantité et le mode de préservation des constituants organiques sédimentés

An integrated palaeoenvironmental investigation of a 6200 year old peat sequence from Ile de la Possession, Iles Crozet, sub-Antarctica

International audienceA 6200 year old peat sequence, cored in a volcanic crater on the sub-Antarctic Ile de la Possession (Iles Crozet), has been investigated, based on a multi-proxy approach. The methods applied are macrobotanical (mosses, seeds and fruits) and diatom analyses, complemented by geochemical (Rock-Eval6) and rock magnetic measurements. The chronology of the core is based on 5 radiocarbon dates. When combining all the proxy data the following changes could be inferred. From the onset of the peat formation (6200 cal yr BP) until ca. 5550 cal yr BP, biological production was high and climatic conditions must have been relatively warm. At ca. 5550 cal yr BP a shift to low biological production occurred, lasting until ca. 4600 cal yr BP. During this period the organic matter is well preserved, pointing to a cold and/or wet environment. At ca. 4600 cal yr BP, biological production increased again. From ca. 4600 cal yr BP until ca. 4100 cal yr BP a “hollow and hummock” micro topography developed at the peat surface, resulting in the presence of a mixture of wetter and drier species in the macrobotanical record. After ca. 4100 cal yr BP, the wet species disappear and a generally drier, acidic bog came into existence. A major shift in all the proxy data is observed at ca. 2800 cal yr BP, pointing to wetter and especially windier climatic conditions on the island probably caused by an intensification and/or latitudinal shift of the southern westerly belt. Caused by a stronger wind regime, erosion of the peat surface occurred at that time and a lake was formed in the peat deposits of the crater, which is still present today

Variations of the relative palaeointensity of the geomagnetic field in western Europe in the interval 25-10 kyr BP as deduced from analyses of lake sediments

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The rock magnetic signal of climate change in the Maar lake sequence of Lac St Front (France)

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Variations of the relative palaeointensity of the geomagnetic field in western Europe in the interval 25-10 kyr BP as deduced from analyses of lake sediments

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Evidence for a geomagnetic excursion recorded in the sediments of Lac St. Front, France: A link with the Laschamp excursion?

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The rock magnetic signal of climate change in the Maar lake sequence of Lac St Front (France)

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