Holocene regional climate change and formation of southern Ontario's largest swamp inferred from a kettle-lake pollen record

Abstract

Greenock Swamp wetland complex is one of few remaining natural wetlands in the Great Lakes region and, at 89 km²in areal extent, is currently the largest hardwood swamp in southern Ontario, Canada. We present here pollen and sediment records from a kettle hole (Schmidt Lake) and adjacentThuja occidentalisswamp to reconstruct regional paleoclimate and vegetation history, and to assess the timing and development of the swamp ecosystem and associated carbon stocks. Pollen-inferred paleoclimate reconstructions show the expected warming in the Early Holocene, and indicate the Mid-Holocene initiation of lake-effect snow. This enhanced snowfall may have maintained high water tables in the adjacent wetland since ca. 8300 years ago, promoting the establishment of a swamp dominated byThuja occidentalis. Carbon accumulation rates in a >2-m-long peat core collected from aThuja occidentalisstand adjacent to Schmidt Lake are 30–40 g C/m²/yr, which is higher than the average of northern high-latitude peatlands. Using topographic and hydrological parameters, we estimated that mean swamp peat thicknesses could exceed 2 m. Thus, this study encourages future investigations on temperate swamps from the perspective of hitherto underestimated Holocene carbon sinks and shows the importance of regional hydroclimate in supporting swamp ecosystems.