A Time-transgressive Two-phased Hypsithermal Warm Period Across the Boreal Region of Northwest Ontario: A Multiproxy Study From Deep-water and Near-shore Cores
A Time-transgressive Two-phased Hypsithermal Warm Period Across the Boreal Region of Northwest Ontario: A Multiproxy Study From Deep-water and Near-shore Cores
Previously Published Material: The pollen that will be presented has not been published or submitted. The information on changes in water levels from the nearshore core was just published in The Holocene (2015) Vol. 25. It is the time-transgressive nature of the change in the pollen and the linkage to the hydrological balance of these lakes that is unique, which together provides evidence for two-phased climatic optimum, or Hypsithermal Period.
Abstract ID#: 35864
English Abstract:
Climate warming is expected to have major effects on boreal ecosystems situated near the prairie-forest boundary in central North America. To investigate the impact of past climate change on boreal ecosystems in northwest Ontario over the Holocene, we developed a network of deep-water and near-shore cores from three headwater lakes that spanned a distance of over 250-km from boreal regions near the forest-prairie boundary to spruce-dominated forests to the east (i.e. northwest of Kenora to north-east of Sioux Lookout). Center cores were used to reconstruct large-scale changes in climate based on changes in pollen assemblages in the cores. The lake closest to the forest-prairie boundary exhibited a prolonged period of early-to-mid Holocene aridity, as indicated by a decline in spruce and increases in shrubs and grasses, from approximately 10,000 to 3,000 cal yr BP. Sites located further into the boreal forest had a much shorter period of enhanced aridity that extended from approximately 8,500 to 4,500 cal yr BP. We used near-shore sediment cores to track changes in water levels in these three lakes over the Holocene based on changes in diatom assemblages to reconstruct past water balance. In all cores, planktonic diatom assemblages increased during the later phase of the Hypsithermal Period (as indicated by the pollen), with changes in the lake closest to the forest-prairie boundary showing the longest and most pronounced changes. Taken together, this study supports a two-phased Hypsithermal Period: a warm-dry phase followed by a warm-wet phase. Further, the extent of the Hypsithermal was substantially longer closer to the forest-prairie boundary, with an earlier onset and later termination, in comparison to the sites that are today well within the boreal region. This study also suggests that high water levels in northwest Ontario have occurred only over the last four-to-five thousand years, and that many boreal lakes were more productive during the Hypsithermal Period.