Tracking long-term Holocene climate trends in lake 239 (experimental lakes area, NW Ontario) using diatoms, pollen, and charcoal

The boreal forest region of Canada is climatically sensitive and may be impacted by anthropogenic-induced climatic changes. The results of this multi-proxy paleolimnological study contribute detailed information on changes to the boreal forest in northwestern (NW) Ontario, showing unequivocal changes in lake and forest structure due to climate during the warmer mid-Holocene. This study uses diatoms, pollen, and charcoal reconstructions to better understand the timing and extent of climate-related changes in Lake 239, Experimental Lakes Area (ELA), in NW Ontario throughout the Holocene with an emphasis on the warmer mid-Holocene.;Diatom analysis revealed changes in water quality based on analysis of species assemblages and quantitative inferences of total phosphorus from a deep central core. Lake levels at least 8-m lower than today were inferred from a near-shore core and were concurrent with an increase in nutrient-rich diatom assemblages, an increase in diatom accumulation, and a decrease in chrysophytes relative to diatoms in the central core. A concurrent increase in pollen such as Cupressaceae and Ambrosia indicate more open boreal forest between ∼4500-8000 cal yr BP. Pollen-based inferences of temperature suggest an increase on average of 1-2°C warmer than today with winter temperatures up to 4°C warmer. The pollen inferences also suggest enhanced precipitation, but with increased evaporation/evapotranspiration resulting in reduced moisture availability overall. A transect of cores surrounding ELA was synthesized using pollen-based reconstructions of temperature and precipitation to assess regional changes. All sites show shifts in pollen assemblages indicating a warmer mid-Holocene; prairie sites to the west show mid-Holocene decreases in precipitation relative to today, whereas sites near or east of ELA show consistent increases in precipitation, but with enhanced evaporation. Charcoal analysis shows an increase in charcoal accumulation during the mid-Holocene warm period compared to the early and late Holocene, suggesting a more active fire regime. Fire return intervals based on type-M charcoal show a pronounced decrease during the early-to-mid Holocene period and a corresponding increase in fire frequency, whereas fire frequency derived from total charcoal was virtually unchanged over the Holocene. This study helps fill a knowledge gap in NW Ontario noted by several large regional assessments.