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Climate-driven changes in lake conditions during late MIS 3 and MIS 2: a high-resolution geochemical record from Les Echets, France

Abstract : Ice-core (Dansgaard et al. 1993; Johnsen et al. 2001; NGRIP members 2004) and marine sediment records (Bond et al. 1992; Moreno et al. 2004; Rasmussen & Thomsen 2004) spanning the last glacial cycle provide compelling evidence of multiple reorganizations of the climatic system triggered by changes thought to originate in the North Atlantic region (Broecker et al. 1992; Clark et al. 2002). Sudden shifts in air temperature from a cool climate to interstadial values, known as Dansgaard-Oeschger events (DO), have been active most notably during Marine Isotope Stage (MIS) 3. Abrupt and large in amplitude, DO cycles operated on a millennial to centennial time scale and are best expressed in the North Atlantic region (Dansgaard et al. 1993; Allen et al. 1999; NGRIP members 2004; Moreno et al. 2005; Grimm et al. 2006; Wohlfarth et al. 2008), though recent research suggests that these events were probably important on a global scale (e.g. Voelker et al. 2002). Iceberg surges, known as Heinrich events (H events), appear in marine records as sudden cold spells associated with a drastic reduction in sea surface temperature, a southern shift of the Polar Front, disruption of North Atlantic thermohaline circulation and substantial delivery of ice-borne detritus to the open ocean, reaching as far south as Portugal (Bond et al. 1992; Broecker et al. 1992; Bard et al. 2000; Hemming 2004). On the European mainland, lake sediments are the most promising archives for recording long-term and short-term climatic changes (Voelker et al. 2002). The few long lacustrine records from continental Europe show that, on a broad scale, long-term palaeoclimate variations are expressed clearly through changes in vegetation composition and dynamics (Woillard & Mook 1982; de Beaulieu & Reille 1984; Guiot et al. 1989; Allen & Huntley 2000; de Beaulieu et al. 2001; Guiter et al. 2003). However, in the context of abrupt climate change (e.g. for most of MIS 3), only limited palaeoecological information is available: most of the data come from sites located on the southern peninsulas of Mediterranean Europe (Voelker et al. 2002). It has been suggested that changes in plant cover varied in this region in concert with climatic fluctuations recorded in Greenland ice cores, with a diverse range of thermophilous taxa present at any time (Allen et al. 1999; Sánchez Goñi et al. 2002; Tzedakis et al. 2004). Much less floristic variability is seen, however, during MIS 3-2 in pollen records from sites located north of the main mountain ranges of central and northern Europe. Here, data from peat deposits (Behre 1989; Preusser 2004), lacustrine sediments (Woillard & Mook 1982; de Beaulieu & Reille 1984; Helmens et al. 2000; Müller et al. 2003; Sirocko et al. 2005; Engels et al. 2008) and terrestrial proxies in marine cores (Sánchez Goñi et al. 2008) reveal long cold intervals interrupted sporadically by weakly expressed interstadials marked by rises in boreal tree pollen (Allen & Huntley 2000; Guiter et al. 2003). Such significant differences in ecological responses point to strong regional and local climatic gradients associated with these events, issues that are not sufficiently considered when inferring synchronicity of events between various records. In order to improve the understanding of how the European mainland was affected by rapid climate changes, we show that important palaeoecological information can be extracted from geochemical lake records (e.g. Lallier-Vergès et al. 1993; Bernasconi et al. 1997; Meyers 1997; Dean 1999; Meyers & Lallier-Vergès 1999; Meyers & Teranes 2001; Talbot 2001; Leng et al. 2005). Les Echets (Fig. 1A, B) is situated in a region sensitive to current climate changes at the boundary between humid Atlantic air masses and Mediterranean influences. The physical and chemical properties of the former lake and catchment area underwent significant changes on millennial to centennial time scales, changes that we attempt to quantify in palaeoenvironmental terms through multi-proxy geochemical analyses of the organic fraction within sediments. We demonstrate that the region was greatly influenced by the rapid climate variability specific to MIS 3, as seen in marine sediments (Bond et al. 1992; Moreno et al. 2004; Rasmussen & Thomsen 2004; Sánchez Goñi et al. 2008) or ice cores (Johnsen et al. 2001; NGRIP members 2004) hundreds of kilometres away.
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Daniel Veres, Elisabeth Lallier-Vergès, Barbara Wohlfarth, Terri Lacourse, Didier Kéravis, et al.. Climate-driven changes in lake conditions during late MIS 3 and MIS 2: a high-resolution geochemical record from Les Echets, France. Boreas, Wiley, 2009, 38 (2), pp.230-243. ⟨10.1111/j.1502-3885.2008.00066.x⟩. ⟨insu-00343673⟩

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