A MULTI-PROXY RECORD OF HOLOCENE PALEOCLIMATE AND PALEOLIMNOLOGICAL CHANGE FROM NORTH RAFT LAKE, HUDSON BAY LOWLANDS, NORTHERN ONTARIO, CANADA

The Hudson Bay Lowlands (HBL) is an extensive peatland extending over 372,000 km² in northern Manitoba, Ontario and Quebec, Canada. Over the Holocene, > 31 Pg of carbon have accumulated in the bog and fen habitats across the region. The region is presently subject to significant warming and permafrost thaw, and paleoenvironmental records are critical for understanding the fate of peatland carbon under different climatic regimes. While a variety of paleoecological studies of peat cores from the region are contributing to an emerging picture of peatland development in response to climatic, hydrological and ecological processes, paleolimnological records derived from lake sediments are a critical source of paleoclimatic records independent from peatland processes. While lakes are abundant in the HBL region, the majority are shallow (Z<1 m) and do not preserve whole-Holocene sediment sequences. North Raft Lake (54.53°N; -84.76°W; 113 m asl) is situated in the vicinity of the Sutton Ridge in the northeastern sector of Ontario’s Hudson Bay Lowlands. With a depth of ~11 m, North Raft Lake contains a continuous middle to late Holocene sedimentary sequence 1.5 m in length. Initial sediments are carbonate-rich, low in pollen, and contain marine shells and abundant marine dinoflagellate cysts, suggesting inundation by the Tyrell Sea (the proto-Hudson Bay). Pollen taxa present include Chenopodiaceae, Shepherdia canadensis and shrub Betula and Salix, indicative of tidally influenced salt marsh vegetation. Isostatic uplift gradually reduced marine influence, and by the middle Holocene, sediment organic matter increased above 5%, pollen concentrations increased and assemblages indicate establishment of a closed-basin freshwater lake. Tree pollen assemblages suggest local presence of boreal-type forested peatlands dominated by Picea and Larix, with Alnus, Cupressaceae, Ericaceae and Populus present as well. This assemblage was relatively stable through to present day, with few shifts in the pollen stratigraphy. Chironomid head capsules were also well preserved in the North Raft sediments; assemblage changes suggest mid-Holocene warming across the HBL, with increases in lake water temperatures and/or lake littoral habitat.