THE LATE MATTAWA FLOOD (CA. 9,100 CAL YR BP) IN THE UPPER LAURENTIAN GREAT LAKES

There is a profoundly negative oxygen isotope excursion between 9,400 and 9,000 cal yr BP in the sediment records of Lakes Michigan and Huron (the late Mattawa phase in the Huron basin.) At around 9,050 ± 200 cal yr BP, there is a sequence of thirty-six anomalously thick glacial varves within Lake Superior's sediments. These varves are up to 14-cm thick in the Isle Royale sub-basin, and represent at least a 5-fold increase in overall sediment fluxes. A recently recovered sediment core from Fenton Lake (ON) documents an episodic rise in Superior basin lake levels at around this same time. This lake level rise could have been as great as 60 feet, and probably resulted from hydraulic damming at the St. Mary's River outlet. Dating resolution in the Michigan and Huron records does not preclude the possibility that the negative oxygen isotope excursion is directly connected to Superior's thick varves, and lasted only a few decades rather than 400 years. Catastrophic flooding from Lake Agassiz drawdown from the Stonewall level is the most likely source of this floodwater; however, Agassiz drawdown events have been proposed to have lasted only 1-2 years, rather than 36 years. Hydrologic modeling efforts are complicated by a lack of dating resolution in the Huron and Michigan records and uncertainty regarding the oxygen isotope composition of overflow from the Superior basin. Nevertheless, this is likely the best direct evidence for catastrophic flooding through the Laurentian Great Lakes to the North Atlantic Ocean, and these records should serve as a model for identifying older Lake Agassiz flood events. The Greenland ice core records exhibit a 1‰ negative oxygen isotopic excursion between 9,250 and 9,350 cal yr BP (second in magnitude during the Holocene to the well known 8,200 cal yr BP event), which may be connected to this flood.