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Paleohydrological changes during the last deglaciation in Northern Brazil

Abstract : We here report a reconstruction of hydrological balance variations in Northern Brazil for the last 20 ka deduced from the ΔD values of aquatic and land plant molecules extracted from the sediment infill of Lake Caçó. Our reconstructed precipitation, lake water isotope ratio and evaporation-evapotranspiration isotope effect allow us to obtain an estimate of moisture balance, and, to a lesser extent, precipitation amount and seasonality changes. During the end of the Last Glacial Maximum (LGM, between ca. 20 and 17.3 ka), high ΔD values and smaller fractionation of leaf waxes indicate an arid to semi-arid climate with a long lasting dry season. An abrupt change towards much wetter conditions occurred within ca. 500 years from 17.3 to 16.8 ka, as shown by a 50 ‰ decrease in D/H ratios and a marked increase in H isotopic fractionation of leaf waxes. This abrupt isotopic change coincides with a major transformation from savanna-dominated vegetation to humid rain forest around the lake, based on pollen data. Comparisons with other paleo-precipitation records from South American sites indicate that late glacial humid conditions were controlled by intensification of the ITCZ and/or a southward shift of its mean position across our study site. Our isotope data show only a small rise in aridity during Younger Dryas event (13 to 11.5 ka). Although the Holocene was not screened in details, D/H ratios of terrestrial and aquatic compounds show near constant offsets, suggesting stable and relatively humid climate conditions during this period.
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Submitted on : Wednesday, December 6, 2006 - 10:03:05 AM
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Jérémy Jacob, Yongsong Huang, Jean-Robert Disnar, Abdelfettah Sifeddine, Mohammed Boussafir, et al.. Paleohydrological changes during the last deglaciation in Northern Brazil. Quaternary Science Reviews, Elsevier, 2007, 26 (7-8), pp.1004-1015. ⟨10.1016/j.quascirev.2006.12.004⟩. ⟨hal-00118667⟩



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