Evolution of agropastoralism around Lake Igaliku (Southern Greenland) during the last two millenia through molecular biomarkers.
Résumé
Artic is the most sensitive region to the effects of global warming. Indeed, it warms three times faster than the rest of the world. Nowadays, it is an important issue to characterize the responses of these ecosystems to a rise in temperature and the part that Human plays in this mechanism. For several millennia, agriculture and husbandry are known as one of the major anthropogenic impacts on environment. During the last two millennia, global climate warming events allowed two phases of agricultural expansion in South Greenland. 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, at the very 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 on the environment, a first molecular inventory was conducted on a sedimentary sequence retrieved from Lake Igaliku (61°00'N, 45°26'W, 15m asl). The analysis of molecular biomarkers retrieved from a well-dated core spanning the last two millennia was confronted to pollen, non pollen palynomorph and elements, allowing us discussing the evolution of agropastoralism and its impacts on the local ecosystems. When found in soils or sediments, fecal sterols and bile acids can help identifying the former presence of Humans and their livestock. In our case, only deoxycholic acid (DCA) is detected with high fluxes recorded during the two phases of agricultural expansion, coincident with high percentages of coprophilous spores. DCA is produced by all herbivores but, because it is the unique bile acid found in Igaliku sediments, we propose that it mainly derives from sheep. Thus, it could be possible to distinguish predominant livestock species raised in the catchment of Igaliku during the two phases of settlements. Sheep breeding induce an increase in trimethyl tetra hydro chrysene (TTHC) and in Ti, considered as tracers of soil erosion during the last centuries. This increase erosion is accompagnied by an increase in n-C17 alkane fluxes and mesotrophic diatoms percentages that attest to an eutrophication of the lake waters in recent times, potentially due to an important supply of fertilizers. During the last centuries as well as the medieval period, sheep breeding has also involved a change in vegetation, pointed by lows percentages of trees and shrubs pollens associated with an increase in triterpenyl acetates fluxes, and an increase of the average chain length of n-alkanes, molecular biomarkers of an open area. The combined analysis of molecular biomarkers and pollen allows tracing the compared evolution of farming activities during the two major phases of anthropisation. The impacts of these activities on local ecosystems allow us providing a new insight into the Norse history in Greenland, completing the archaeological and palaeoenvironmental knowledge.