ABSTRACT: Reconstruction of hydro-climatic and land-use changes using high resolution sedimentary records during the last 400 years in the Sept-Îles area, Gulf of St. Lawrence.

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A sedimentological multiproxy approach was performed on five sediment boxcores sampled on the North Shore of the Gulf of St. Lawrence in order to better understand the impacts of recent climatic or land-use changes along the coast. Multibeam sonar and subbottom profiler surveys allowed to carefully select the coring sites. Cores 56BC, 58BC and 63BC were collected onboard the RV Coriolis II in 2006. Non destructive and continuous analyses included magnetic susceptibility measurements and computerized axial tomodensitometry (CAT-Scan). The cores were then split, described and photographed. X-Ray images and micro-fluorescence-X measurements were also performed on the cores. Subsamples were taken for geochemical (carbon & nitrogen contents) and grain size analyses. 210Pb measurements allowed to derive recent sedimentation rates of 0.12, 0.25, 0.09 cm yr-1 for cores 56BC, 58BC and 63BC, respectively. The results reveal a major shift in all the sedimentological parameters between 1630 and 1872 AD. The origin of this shift is still under investigation. In addition, an extreme sedimentological event that occurred at ~1843 ą 4 AD was recorded in the grain size distribution of cores 56BC and 63BC located near the Moisie and Ste-Marguerite river mouths, respectively. This shift most likely reflects a major flood. The small temporal lag of this event between the two cores supports the accuracy of both cores age model. In a more recent perspective, an inflexion point was found in many parameters (relatively lower grain size, density, magnetic susceptibility and Fe content) and dated about AD1984. This inflexion could be related to relatively lower inputs of detrital sediments due to reduced coastal and fluvial erosion. Hence, from 1981 to 1987, the number of storms blowing northeastward (which are the most common in the Sept-Îles area) decreased dramatically, culminating in an important negative anomaly in snowfall precipitations in 1987 and in total annual precipitations. These conditions led to reduced discharges of the Moisie and Ste-Marguerite rivers, which were particularly low in 1984. Finally, two significant decreasing shifts in the amplitude of the grain size variations in core 056BC are dated ~1900 and ~1988 AD and could be related to the retention of coarse sediments in the river following the construction of dams on the Ste-Marguerite River.

Bibliography of Canadian Geomorphology