| Summary: | Présentation orale International audience During the last two millennia, global climate warming events allowed two phases of agricultural expansion in Southwestern Greenland (Massa et al., 2012a). The first phase coincided with the medieval Norse colonization between 986 AD and the mid-fifteenth century; the second, corresponds to the modern reestablishment of farmers since 1920, after the end of the Little Ice Age. This context appears as an exceptional study model to examine the transition from a pristine to an anthropogenic landscape. To this respect, lake deposits provide a sensitive recorder of environmental changes due to land use. In order to assess the history and impacts of grazing activities, a first molecular inventory was conducted on a sedimentary sequence retrieved from Lake Igaliku (61°00'N, 45°26'W, 15m asl). A well-dated core spanning the last two millennia was analyzed in order to identify biomarkers that attest to pastoralism such as fecal sterols (Bull et al., 2001) and bile acids (Tyagi et al., 2008). 40 samples were extracted, separated into neutral, acid and polar fractions. The neutral fraction was further separated into aliphatics, aromatics, ethers ketones and alcohols by flash chromatography. After addition of an internal standard, fractions were analysed by Gas Chromatography-Mass Spectrometry. 205 compounds were identified and quantified including n-alkanes, n-alkan-2-ones, mid-chain ketones, wax esters, sterols, hopanoids, fernenes, triterpenyl acetates, polycyclic aromatic hydrocarbons (PAH), tricyclic diterpenes, fatty acids, bile acids, diacids and hydroxy-acids. Principle Components Analysis of the data allowed us distinguishing three main groups of distinct behaviour: (i) fatty acids, bile acids, diacids and hydroxy-acids; (ii) tricyclic diterpenes; (iii) PAH and sterols. The evolution of biomarker concentrations were compared to other parameters such as total organic carbon, total nitrogen and sulfur contents, sediment accumulation rate and non-pollens palynomorph ...
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