| Summary: | Organic matter stored in wetlands constitutes a globally significant terrestrial carbon pool interacting with atmospheric carbon via unique ecological processes that release CH4 but take in CO2. Efforts are ongoing to quantify the role of wetlands in terrestrial-atmosphere carbon exchanges, but observational data are unavoidably limited. Wetland drainage and losses have been profound in many countries, particularly in temperate latitudes; these losses took place prior to any eld measurements of carbon stocks or fluxes. Temperate wetlands have often been assumed to accumulate little to no peat, and thus, have had minimal importance in wetland carbon inventories. This thesis aims to point out the underestimated role that the temperate wetlands have played in the terrestrial carbon cycle, by conducting three case studies during the Anthropocene, the Holocene, and the late glacial period. First, the distribution of pre-settlement wetlands in southern Ontario was reconstructed using geospatial map overlays—the locations, peat depths and carbon densities from extant wetlands were used to assign pre-settlement wetlands to wetland types (bog, fen, swamp or marsh). More than half of the pre-settlement wetlands disappeared with potential carbon release up to 2 Pg since the early 19th century. Second, southern Ontario's largest swamp was investigated for paleoecological history using peat coring and pollen counting. This analysis indicated that the swamp established by about 8000 years ago, perhaps in response to the intensification of lake-effect snow, and it has accumulated thick organic-rich deposits since that time. Third, a model-simulated peatland expansion in the American Midwest was linked spatially to the distribution of enigmatic "no-analog" pollen assemblages during the last deglaciation. Estimates of early settlement era forest composition were used in conjunction with wetland simulations and machine learning algorithms to reconstruct natural wet forests in the deglacial landscape. These results show that wetland extent exceeded that of uplands, and this explains the key components of the no-analog pollen assemblages. During the Bølling-Allerød period, rivers transporting ice-sheet meltwater may have increased wetland cover across the region, causing the unprecedented dominance of wet forests, which potentially contributed to the northern hemispheric CH4 source enhancement. Ph.D.
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