HAMILTON-HARBOR, ONTARIO - 8300 YEARS OF LIMNOLOGICAL AND ENVIRONMENTAL-CHANGE INFERRED FROM MICROFOSSIL AND ISOTOPIC ANALYSES

Limnological changes in Hamilton Harbour, Lake Ontario, over the Holoc ene were investigated by using proxy evidence from diatoms and other s iliceous microfossils in a radiometrically dated sediment core (HH26co mp), together with environmental data derived from sediment pollen and oxygen and carbon isotope analyses. The evidence demonstrates that th e site of Hamilton Harbour has changed over the past 8300 y from a sha llow, separate waterbody, to a deep embayment of Lake Ontario. The ear liest evidence, from 8300 BP to 7000 BP, is of a mesotrophic pond of m oderate alkalinity, warmer than present, and probably with an extensiv e marginal wetland. An initial transitory connection with the rising w ater level of Lake Ontario was established at c. 7000 BP, possibly via a deep outlet channel. This connection is 2000 y earlier then previou sly estimated. Permanent confluence with Lake Ontario was established at c. 6200 BP, causing a decline in inferred trophic level and water t emperatures. Microfossils reach a minimum at 4400 BP coincident with t he Nipissing Flood. Decreased mixing of Lake Ontario water from about 4000 BP following the Nipissing Flood highstand is evidenced in isotop ic and diatom data. Three isolated shifts in the diatom spectrum at c. 4900 BP, 4500 BP, and 3500 BP may be associated with extreme turbidit y or storm deposit events. Between 3200 BP and 280 BP, Hamilton Harbou r was evidently a moderately alkaline embayment of Lake Ontario, oligo trophic to mesotrophic, and relatively cooler than present. The final 280 y sedimentary record reveals the magnitude of anthropogenically in duced changes to the harbour, including eutrophication and organic pol lution.