THE LATE HOLOCENE ENVIRONMENTAL HISTORY OF LAKE MANICOUAGAN (QUÉBEC, CANADA, 51°N)

Lake Manicouagan, Manikuakan in the local Innu language, was a 320-m deep and 60-km long lacustrine basin that lied within the eastern trough of Manicouagan impact crater, in the boreal hinterland of Québec (51°N). This fjord lake was drowned by 140 m of water during the completion of a large hydroelectric dam during the 1960s, thereby forming Manicouagan Reservoir. Multibeam swath bathymetry and acoustic stratigraphy paired with sediment cores document the nature of sedimentation processes and environmental changes of the drowned lake for the past 2 ka. Prior to the reservoir filling, the lacustrine sedimentation was dominated by: (1) coarse-based turbidites in the upper arm of the lake fed by two large river deltas; (2) thick mass-wasting deposits (i.e., slumps, megaturbidites) within the central sector, a deep canyon surrounded by steep slopes; and (3) finely laminated deposits interrupted by turbidites in basins located downstream of a mid-lake sill. Cross-core correlations supported by dating methods (²¹⁰Pb, ¹³⁷Cs, ¹⁴C) provide a record of synchronous and lake-wide sedimentary events between 1650 CE and the onset of reservoir sedimentation around 1970 CE. Potential triggers for these events include natural and man-made lake-level variations and/or long-distance seismic sources. The reservoir sedimentation is characterized by the cessation of event sedimentation and the onset of organic-rich rhythmites, much likely biogenic varves, resulting from the drowning of clastic sources. Multi-proxy analysis of sediment cores reveals that the organic fraction of lake sedimentation has been increasing over the last two millennia, with tipping points around 650 CE and 1650 CE. The high-resolution sedimentary record of Lake Manicouagan has few equivalents in the boreal land of Québec and provides the opportunity to study the interplay between geohazards, hydroclimate, water-levels, and human impacts across a range of spatial and temporal scales.