AN EARLY LAKE ONTARIO PALEO-BARRIER BEACH BUILT AT SEA LEVEL 12.8-12.5 CAL KA BP

An overstepped, concave-eastward, barrier beach beneath Holocene mud in western Lake Ontario has been delineated by acoustic and seismic reflection profiles and piston cores, and related to Early Lake Ontario (ELO). The average ELO barrier depth below present mean lake level is 77.4 m to 80.6 m, or about -6 to -2.8 m above present sea level (asl). Trend surface analysis of Champlain Sea (Atlantic Ocean) marine limits defined the contemporaneous marine water surface in valleys of the upper St. Lawrence, and Ottawa rivers, and Lake Champlain. Projections of this surface pass ~25 m above the outlet sill of the Lake Ontario basin and extend to the ELO paleo-barrier, a unique sand and gravel deposit beneath western Lake Ontario. ELO was connected to the Champlain Sea above the isostatically rising outlet sill for up to 3 centuries after about 12.8 cal ka BP, while the glacio-isostatically-depressed St. Lawrence River Valley was inundated by the Atlantic Ocean. During the period of this connection, ELO level was confluent with slowly rising sea level, and the lake constructed a transgressive beach deposit with washover surfaces. ELO remained fresh due to a high flux of meltwater inflow. The marine water level connection stabilized water level in ELO relative to its shore and facilitated shore erosion, sediment supply, and barrier construction. Glacio-isostatic uplift of the outlet sill, faster than sea-level rise, lifted ELO above the Champlain Sea about 12.5 cal ka. Shortly after, a hydrological deficit due mainly to a combination of diverted meltwater inflow and dry climate, well known from regional pollen studies, forced the lake into a lowstand. The lowstand stranded the barrier, which remains as evidence of sea level, the farthest inland in eastern North America north of the Gulf of Mexico at the time. The highest paleo-washover surface provides a sea level index point ~12.5 cal ka BP with elevation -3.1±3.9 m asl, referenced to modern transgressive beach washover surfaces in southwestern Gulf of St. Lawrence with elevations of 2.5±1.2 m asl. Climatic change and increasing meteoric water supply began raising Lake Ontario over the barrier about 3 millennia later, and subsequently buried it with offshore mud.