BIOMARKER INSIGHTS INTO THE EVOLUTION OF LAND USE IN SOUTHERN GREENLAND DURING THE LAST TWO MILLENIA
Résumé
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 (Gauthier et al., 2010; Massa et al., 2012b; Perren et al., 2012). The evolution of deoxycholic acid, bile acid that can be attributed to sheep (Tyagi et al., 2008) matches the evolution of spores of coprophilous fungi (Gauthier et al., 2010) and that of the C/N, established as an erosion marker (Perren et al., 2012). Indeed, deoxycholic acid concentration is weak before the Norse settlement and reaches a peak at 1100 AD when other indicators attest to a stronger Human impact on lands. Deoxycholic acid concentrations, standardized to the ration C/N showed the same trends which indicate that variations are not only due to the erosion rates but also to the importance of pastoralism in the watershed. These preliminary results provide a new insight into the Norse history in Greenland, completing the archaeological knowledge, and a first quantified molecular inventory for biomarkers in the lacustrine sediments in Southern Greenland. REFERENCES Bull, I. D., Betancourt, P. P., Evershed R. P., 2001. An organic geochemical investigation of the practice of manuring at a Minoan site on Pseira Island, Crete. Geoarchaeology 2, 223-242. Gauthier, E., Bichet, V., Massa, C., Petit, C., Vannière, B., Richard, H., 2010. Pollen and non-pollen palynomorph evidence of medieval farming activities in southwestern Greenland. Vegetation History and Archeobotany 19, 427-438. Massa, C., Bichet, V., Gauthier, E., Perren, B. B., Mathieu, O., Petit, C., Monna, F., Giraudeau J., Losno, R., Richard, H., 2012a. A 2500 year record of natural and anthropogenic soil erosion in South Greenland. Quaternary Science Review, 32, 119-130. Massa, C., Perren, B. B., Gauthier, E., Bichet, V., Petit, C., Richard, H., 2012. A multiproxy evaluation of Holocene environmental change from Lake Igaliku, South Greenland. Journal of paleolimnology 48, 241-258. Perren, B. B., Massa, C., Bichet V., Gauthier, E., Mathieu, O., Petit, C., Richard, H., 2012. A paleoecological perspective on 1450 years of Human impacts from a lake in Southern Greenland. The Holocene, 22, 1025-1034. Tyagi, P., Edwards, D. R., Coyne M. S., 2008. Use of sterols and bile acids biomarkers to identify domesticated animal sources of fecal pollution. Water Air Soil Pollution 187, 263-274.