Sedimentary record of glacial Lake Mackenzie, Northwest Territories, Canada: Implications for Arctic freshwater forcing

The global oceanographic impact of large volumes of freshwater entering the Arctic Ocean from North America during Laurentide Ice Sheet deglaciation is the subject of much discussion. The model of lateglacial runoff-forced changes in ocean thermohaline circulation (THC) is constrained in part due to a lack of detailed study of the relevant terrestrial glacial sedimentary and geomorphic record. This paper reports and interprets sedimentary facies that accumulated in the former glacial Lake Mackenzie, which formed a long (~ 600 km) and deep (max: 200 m) lateglacial basin ponded along the lower Mackenzie Valley in Canada's Northwest Territories. This is a key study area because the lake was dammed between approximately 10,500 14C yr bp and 9100 14C yr bp when most of the runoff from glacial Lake Agassiz was routed northward through the Mackenzie Valley to the Arctic Ocean. Detailed outcrop descriptions and geomorphic mapping in the glacial Lake Mackenzie basin fails to identify a clear record of abrupt shortlived flood outbursts but a regionally extensive deposit of massive and laminated mud at least 55 m thick, records trapping of large volumes of suspended fine sediment. Recent work shows the importance of suspended sediment content in freshwater runoff in influencing THC; too much sediment and freshwater enters the ocean hyperpycnally with little effect on surface waters. The role of Lake Mackenzie may have been to trap suspended sediment that otherwise would have been released to the Arctic Ocean, thereby acting to increase the effect of freshwater runoff on thermohaline circulation.