Foraminifera and Arcellacea from Porters Lake, Nova Scotia: modern distribution and paleodistribution

Abstract

Assemblages of foraminifera and Arcellacea examined from surficial sediment samples collected in Porters Lake, Nova Scotia, reflect the strong gradient from freshwater to marine water in the four basins of this lake. The only marine connection is a channel 3 m below higher high water. The transition from fully freshwater to marine is abrupt, with no "intermediate" arcellacean fauna, as we have observed in cores from other areas of the Maritimes, Given the present connection is 3 m below higher high water, it has been almost 1500 years since this sill first became tidal (based on 20 cm/100 year sea-level rise from nearby Chezzetcook Met over the last 3000 years); however, the upper fjord-like part of Porters Lake, although connected to the ocean, is still completely freshwater, suggesting at least a 1500 year time lag for marine conditions to penetrate upper reaches of the lake. For the first time, we are provided with a time frame of how long it takes for marine conditions to extend into remote parts of an estuary, which could have implications for interpreting records from other similar fjord-type estuaries, such as Bedford Basin, Nova Scotia. Evolution of the lake through time was studied from three cores: one from the present most marine conditions, one from an intermediate area, and one from the uppermost freshwater part of the lake. Porters Lake has experienced marine transgression for at least the last 3000 years, with one closure between 1500 and 2000 BP. Bottoniset beds from a glaciogenic gravel delta at the head of the lake were dated at 11 750 BP; this is similar to an age on a delta offshore that is now in 70 m of water.