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Séminaire


Validation and use of the micro X-ray fluorescence to characterize clastic varves and paleoclimate quantification of the late Holocene in the Canadian High Arctic

Date
November 22, 2010  10:45 am

Lieu
Salle Stendhal, DGO, Bâtiment B18

Intervenant(s)
Stéphanie CUVEN, GEOLAB, CNRS UMR 6042, Clermont-Ferrand, France

Résumé
Recent environmental changes in the Arctic highlight the need for a better understanding of long-term climatic variability of this sensitive environment. Presently, few investigations exist in continental High Arctic because of the difficulties to reach this remote area. Moreover, obtaining high resolution reconstructions requires to tackle the technological challenge of analysing finely laminated sediments because of the low mean sedimentation rate (≤ 1mm/yr) in this environment. Using the ItraxTM Core Scanner, we obtained micro X-ray fluorescence (μ-XRF) measurements with a 0.1 mm resolution from a long-term clastic-clay varved sequence from Cape Bounty East Lake (unofficial name) on Melville Island (74°55N; 109°30W). The sequence contains a high resolution proxy record of paleohydrological variability spanning the last 4200 years. Varves (annual laminations) are composed of silt-clay couplets which have been frequently interrupted by sand layers since the last 1760 years. Every sedimentary facies corresponding to seasonal deposits are characterized by grain-size variations and show variability in the chemical elements abundance. In this study, we compared the μ-XRF data with direct methods in order to validate their use for paleoclimate reconstructions at very high resolution. Moreover, μ-XRF analysis and sediment fabric variability along the long-term sedimentary sequence reflect changes in sediment sources and lacustrine conditions through time.
Finally, two paleoclimatic reconstructions has been provided in this study, using varve thickness (VT) series (classical method) and μ-XRF series (new method). The VT record exhibits the signature of the Atlantic Multidecadal Oscillation (AMO) signature for several periods. We focused here on the new method and we demonstrate that μ-XRF data permit to characterize seasonal hydrological variability through time. In the case of Cape Bounty sediment, the ratio Zr/K provides a reconstruction of extreme events at seasonal scale for the last 1760 years. The occurrence of these coarse grain deposits seemed to indicate that volcanic and solar forcing played a role in seasonal climate changes during the pre-anthropogenic period. Moreover, a pronounced coarsening of the sediment grain-size starting in ~1840 AD is synchronous with increase of Greenhouse gases concentration in the atmosphere, low volcanism and high solar irradiance activities. Comparison of summer thermal conditions between East and West part of the Canadian High Arctic for the last 1600 years showed opposite climate trends variability during the Little Ice Age (LIA) period whereas it showed a synchronous warming during the Medieval period and since the 1840s. Finally, the comparison between the two paleoclimatic reconstructions at Cape Bounty highlighted that Sept-May period were colder than usual during LIA, whereas summers are warmer and wetter than usual.
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