High-resolution seismic sub-bottom reflection record of low hypsithermal lake levels in Ontario lakes

Recently published work in mid-continent North America based on coring of Lake Huron sediments and paleo-environmental analysis of their included pollen and microorganisms has revealed low water levels during the warmth of the mid-Holocene hypsithermal, suggesting the Great Lakes were largely disconnected from each other sometime shortly before 8000 years ago. We have supported this hypothesis by interpreting a large database of high-resolution seismic reflection track lines (~ 2000 km) from sixteen lakes in Ontario found principally on the Canadian Shield and within the limits of former glacial Lake Algonquin. Geophysical data show a common seismo-stratigraphy of late and postglacial sediment sequences bounded by distinct erosion surfaces that are regionally correlative. These can be correlated with published data on cored and dated seismic sequence boundaries from the French River area of Georgian Bay in northeastern Lake Huron together with other subsurface data from the central basin of Lake Huron. A consistent regional paleo-environmental picture emerges of lowered water levels when glacial Lake Algonquin drained approximately 11,000 ybp, fluctuating lake levels thereafter (the Mattawa-Stanley phase) accompanied by the cutting of a distinct erosion surface subaerially or in shallow water across older lacustrine sediments between approximately 9,000 and 8,000 ybp that identifies a regional lacustrine response to a much dryer mid-continent hypsithermal climate. Younger postglacial sediments deposited as lake levels recovered during more humid Neoglacial climates are spatially discontinuous across lake floors occurring as mounds and sheets similar to marine contourite deposits (“drifts”) reflecting modern sediment-starved conditions and strong wind-driven bottom currents.