Water levels in Lake Ontario 4230?2000 years B.P.: evidence from Grenadier Pond, Toronto, Canada

Abstract

Lake Ontario water levels have been rising for the past 11 500 years due to differential isostatic rebound of the St. Lawrence outlet. Small scale fluctuations in water level superimposed on this general trend have received little study, with the exception of the 'Nipissing Flood'.The transgression of a Grenadier Pond was studied from cores along a transect from the bar that separates the pond from Lake Ontario to the marsh on the north shore. Radiocarbon dates of the transition from swamp peat to pond marl in five cores provide estimates of the rate of water level rise since 4230 years B.P. These estimates are supported by changes in sediment type and in abundance of pollen and seeds of aquatic plants. There were three short intervals of accelerated water level rise in Grenadier Pond, around 4200, 3000, and 2000 years B.P., when water levels rose up to 2 m instantaneously, within the resolution of radiocarbon dating. Sedimentological and paleobotanical data suggest that Grenadier Pond was an open embayment of Lake Ontario until 1970-1850 years B.P., when it was isolated by the bar, and therefore sediments deposited prior to this time reflect water levels in Lake Ontario.Short term departure of up to 2 m from the average rate of water level rise over the past 4000 years, as observed in the record at Grenadier Pond, is of the same range as historically observed departures from the mean lake stage of Lake Ontario. This implies that a threshold discharge exists above which broadening of the outflow channel occurs to accommodate further increase in discharge with little rise in lake level. The intervals of accelerated water level rise in Lake Ontario broadly coincide with periods of cool, wet climate, suggesting that increased moisture may have caused the short term fluctuations in water level.

Water-level fluctuations in Lake Ontario over the last 4000 years as recorded in the Cataraqui River lagoon, Kingston, Ontario

“…Because of its proximity (1 13 km) to the controlling sill at Galop Rapids, it has experienced a slower rate of water-level rise than the lagoons studied by Flint et al (1988) and McCarthy and McAndrews (1988) (Fig. As a result, it is expected that the small fluctuations (*I m) in water level proposed by Flint et al (1988) and McCarthy and McAndrews (1988) would be more clearly recorded in the lagoon stratigraphy. The rate of sediment accumulation (see below) has also been much lower and water depths have remained shallower (<I m prior to 1500 BP) here than in the previously studied areas.…”

“…I). The purpose of this study is, therefore, to critically test the ideas proposed by Flint et al (1988,1989) and McCarthy and Methods McAndrews (1988), which are based on studies at the west end Eight cores with lengths of 0.8-2.4 m were collected from the open-water lagoon in 1987 and 1988 (Fig. As a result, it is expected that the small fluctuations (*I m) in water level proposed by Flint et al (1988) and McCarthy and McAndrews (1988) would be more clearly recorded in the lagoon stratigraphy.…”

“…,1989 show, however, that the lake-level history is more complex, with a number of abrupt steps or several-hundred-year oscillations about the longer term trend. The correlation with temperature is more controversial, with high levels having been linked to both warmer (Flint et al 1988(Flint et al , 1989 and cooler (McCarthy and McAndrews 1988) periods. The correlation with temperature is more controversial, with high levels having been linked to both warmer (Flint et al 1988(Flint et al , 1989 and cooler (McCarthy and McAndrews 1988) periods.…”

Pre-settlement Snail Fauna on the Sandbanks Baymouth Bar, Lake Ontario, Compared with Nearby Contemporary Faunas

“…Overall the level of Lake Ontario has risen since 11 500 years BP when the lake was formed in postglacial times (McCarthy and McAndrews 1988). However, there is much evidence for fluctuation superimposed on this general trend.…”

“…However, there is much evidence for fluctuation superimposed on this general trend. Fluc tuation occurred in the early postglacial time (Jackson et al 2000), and, much later (3000-2100 years BP), there were also periods of rapid rise in water level related to episodes of relatively cool, wet climate (McCarthy and McAndrews 1988). Also superimposed on the general rising trend, Flint et al (1988) reported oscillations over the past 3300 years, with an amplitude in the order of 1 m over several hundred years.…”