POSTGLACIAL DEVELOPMENT OF A KETTLE-HOLE PEATLAND IN SOUTHERN ONTARIO

A kettle-hole peatland in southern Ontario was examined in a two core, multi-component paleoecological study to determine the relative impor tance of external and internal factors in its long-term development. D ated sediment stratigraphies, loss-on-ignition, macrofossils and diato m assemblages from two cores were used to infer hydroseral changes wit hin the basin, while pollen assemblages from one core were used to inf er surrounding forest succession. A circumneutral to slightly alkaline lake occupied the basin at 11 750 years BP after the melting of an ic e block. Changes in diatom assemblages and loss-on-ignition in the ear ly lake indicate fluctuating limnological conditions, but their exact nature is unclear. At 8200 years BP, limnological conditions in the ce ntre of the basin stabilized to circumneutral to slightly alkaline, mo derately hard waters following sediment accumulation and macrophyte gr owth at the edge of the basin. The water level declined 7 m by 5600 ye ars BP, and following a return of higher water levels at around 5200 y ears BP, a fen mat formed around the edges of the basin, isolating the remnant lake from alkaline influences. An uncharacterized, basin-wide shift occurred at 2800 years BP, possibly as a result of a water leve l change. At 600 years BP, the floating fen mat advanced over the cent re of the basin, and after 1830 AD, a Sphagnum-dominated floating mat was formed. Changes in basin infilling and vegetation succession coinc ide with regional vegetation succession, indicating that external forc ing factors initiated these hydroseral chances. External factors inclu de climate change at around 8200 years BP, 5600 years BP, 2800 years B P and 600 years BP and land clearance after 1830 AD. They appear to al ter hydrological characteristics in the basin, especially water level and the resultant connection to surrounding calcareaous waters. This s tudy demonstrates the sensitivity to external influences of these kett le-hole peatlands with floating mats. Paradigms on their development s hould be revised to take this sensitivity into account.