Evidence from Nipawin Bay in Frobisher Lake, Saskatchewan, for three highstand and three lowstand lake phases between 9 and 10 (10.1 and 11.5 cal) ka BP

Abstract

This investigation offers new insight into the drainage history of the northwest arm of Lake Agassiz basin, generally recognized as a key discharge route of glacial meltwater to the oceans. Sediment lithology, AMS radiocarbon chronology, and pollen stratigraphy from the basal 1.7 m of core in Nipawin Bay, Frobisher Lake, Saskatchewan, provide evidence for three highstand and three lowstand lake phases between 9 and 10 (10.2 and 11.5 cal) ka BP. Banded clay at the base of the core represents highstand phase 1, and a tundra pollen signal in the clay suggests deposition during the latter part of the Younger Dryas (YD) cold interval. Overlying the banded clay is stratified fine to medium coarse sand with plant detritus that is AMS-dated at 10,000 ± 35 (11,352–11,603 cal) BP. Pollen and plant macrofossils in the sand indicate spruce woodland vegetation and a climate warmer than the previous YD cold interval. The plant detritus-rich sand is interpreted as a shallow-water lagoonal deposit that accumulated during lowstand phase 1. A thin layer of clay on top of the sand equates with higher water levels (highstand phase 2), while a second stratified sand deposit above the clay layer denotes a second low-water interval (lowstand phase 2). Overlying the upper sand unit is a glaciolacustrine sequence of discontinuous laminated grey clay, laminated to faintly laminated reddish-grey clay, and grey clay rhythmites which represents a re-occupation of Nipawin Bay during highstand phase 3. Coinciding with this highstand was a reversion to a colder climate, the Preboreal Oscillation (BPO), which lasted to the end of glaciolacustrine deposition. Cessation of clay rhythmite deposition and onset of FeS deposition, when Nipawin Bay became a separate sedimentary basin for the first time, mark a major drawdown of water levels (lowstand phase 3) that corresponds with opening of new outlets east to Lake Superior basin following retreat of the 10 (11.5 cal) ka BP Marquette re-advance. Enhanced warming, at this time, would have provided optimal conditions for spruce to have taken advantage of the newly-exposed, lake bottom bordering Nipawin Bay and quickly migrate into the area. Tentative correlations are made between the highstands and lowstands and stages of the recently interpreted glacial Lake Churchill (merged proglacial lakes McMurray and Meadow with a regional lake in the Churchill River valley), including possible northwestward drainage of glacial Lake Churchill, and a possible merged glacial Lake Churchill-Lake Agassiz.