Deposition of Subglacial Lake Sediments underneath the Laurentide Ice Sheet? – Sediment Cores from Lake Melville, Canada, provide first Evidence

Lake Melville is a fjord-type lake located in Labrador, Eastern Canada, as part of the Hamilton Inlet System. It is significantly characterised by its eastern connection to the Labrador Sea through a shallow sill at the entrance to the Rigolet Narrows. In contrast, the western part of the lake receives freshwater input from several major rivers. Previous studies assumed that Lake Melville was fully excavated by the Laurentide Ice Sheet (LIS) during the last glacial. Thus, the retreat of the ice mass within the area from ~10,000 to 8,000 years ago caused the deposition of a typical glacial sediment sequence in the lake (King, 1985; Syvitski and Lee, 1997). However, up to 400 m of sediment have been accumulated in the deepest basins of Lake Melville. Since this shows a resemblance to other Canadian sites (e.g. Great Slave Lake; Christoffersen et al., 2008), we assume that Lake Melville potentially persisted as a subglacial lake underneath the LIS and that its sediments might preserved pre-deglacial paleoclimate history. During expedition MSM84 in 2019, we were able to collect new hydroacoustic data and several sediment cores. Initial sediment-physical measurements and lithological investigations were conducted on three selected sediment cores until now. Two of them (~14 m and 16 m in length) indicate a significant change within the lowermost two metres. Additionally, a preliminary age model of core MSM84_18-1 based on first 14C-dates (bivalve and gastropod remains) provided an extrapolated basal age of ~11.6 ka cal BP, pre-dating the local deglaciation. Furthermore, XRF data was used to make a first correlation attempt between the three cores resulting in extrapolated basal ages of ~9,200 yrs BP (MSM84_9-2) and 12,000 yrs BP (MSM84_26-1) for the two remaining cores. Therefore, we assume that these initial findings, in accordance with sediment echosounder data and microscopic analysis, indicate sediment accumulation in a subglacial lake environment. However, recently conducted pore water analyses of the lowermost section of MSM84_26-1 suggest that the hypothesized subglacial Lake Melville might have been already connected to the Labrador Sea. This new result so far only relies on a few measurements and thus has to be verified by further analyses. However, it already indicates that the original hypothesis of an isolated subglacial lake has to be modified. Therefore, we aim at conducting further multi-proxy analyses as well as significantly improving the age model by considering other dating techniques