The Development, Configuration and Chronology of the Ice-Dammed Lakes of the Ungava Peninsula (Northern Quebec, Canada) during the Last Deglaciation

The retreat of the Laurentide Ice Sheet over the Ungava Peninsula led to the formation of several large ice-dammed lakes that have left an extensive record of raised shorelines in the main river valleys and low-lying basins. These glacial lakes drained through Ungava Bay and into the Labrador Sea, a critical area of the North Atlantic where deepwater formation occurs. However, assessing the impact of the drainage of these lakes on the ocean system is prevented by the lack of data on their configuration, volume, and chronology. Furthermore, the development of these lakes is intimately linked to the configuration of the ice margin and its position throughout the deglaciation, which are still poorly constrained, as is the chronology of ice retreat for this sector. Here, we reconstruct the extent and bathymetry of 3 major glacial lakes of the Ungava Peninsula (Nantais, Klotz and Payne) through detailed mapping of raised shorelines and associated landforms. We also document the pattern of ice withdrawal through systematic mapping of eskers and moraines using aerial-photographs, high-resolution satellite imagery (RapidEye) and DEMs. Observations were validated through fieldwork that allowed elevation measurements of the main shoreline sequences using high-precision DGPS. The lake development chronology was constrained through Terrestrial Cosmogenic Nuclide (10Be) dating of boulders sampled from the most representative shorelines. Our mapping results indicate that these lakes formed along a NW-SE oriented ice margin that was retreating towards the west-southwest. The pattern of ice withdrawal caused the opening of a series of southward-flowing outlets formed by the Lepellé, Lestage and Vachon river valleys, which allowed the meltwater bodies to successively spread into lower-elevation basins. The shoreline sequences show a progressive lowering of lake surfaces and a temporary connection between Lakes Nantais and Klotz. Ultimately, ice retreat allowed the final drawdown of the lakes through the Payne River, a major E-W depression linking the Peninsula interior to Ungava Bay. Preliminary 10Be ages indicate that the sequential development of these glacial lakes occurred in a narrow timeframe centered around 8.7-9.7 ka. The context of ice withdrawal also implies that the lakes likely drained over a short time interval, suggesting that these meltwater discharges and those from other glacial lakes of the Ungava-Labrador region may have been an important component of the overall freshwater forcing(s) that altered the late deglacial and early Holecene climate. Altogether, these results refine our understanding of the development of glacial lakes in north-central Ungava and improve paleogeographic reconstructions by providing important constraints on the position and timing of the ice margin during the deglaciation of the Labrador Sector in this region.