Paleohydrology and paleochemistry of Lake Manitoba, Canada: the isotope and ostracode records

Abstract

Lake Manitoba, the largest lake in the Prairie region of North America, contains a fine-grained sequence of late Pleistocene and Holocene sediment that documents a complex postglacial history. This record indicates that differential isostatic rebound and changing climate have interacted with varying drainage basin size and hydrologic budget to create significant variations in lake level and limnological conditions. During the initial depositional period in the basin, the Lake Agassiz phase (∼12–9 ka), δ¹⁸O of ostracodes ranged from −16‰ to −5‰ (PDB), implying the lake was variously dominated by cold, dilute glacial meltwater and warm to cold, slightly saline water.Candona subtriangulata, which prefers cold, dilute water, dominates the most negative δ¹⁸O intervals, when the basin was part of proglacial Lake Agassiz. At times during this early phase, the δ¹⁸O of the lake abruptly shifted to higher values; euryhaline taxa such asC. rawsoni orLimnocythere ceriotuberosa, and halobiont taxa such asL. staplini orL. sappaensis are dominant in these intervals. This positive covariance of isotope and ostracode records implies that the lake level episodically fell, isolating the Lake Manitoba basin from the main glacial lake.

δ¹⁸O values from inorganic endogenic Mg-calcite in the post-Agassiz phase of Lake Manitoba trend from −4‰ at 8 ka to −11‰ at 4.5 ka. We interpret that this trend indicates a gradually increasing influence of isotopically low (−20‰ SMOW) Paleozoic groundwater inflow, although periods of increased evaporation during this time may account for zones of less negative isotopic values. The δ¹⁸O of this inorganic calcite abruptly shifts to higher values (−6‰) after ∼4.5 ka due to the combined effects of increased evaporative enrichment in a closed basin lake and the increased contribution of isotopically high surface water inflow on the hydrologic budget. After ∼2 ka, the δ¹⁸O of the Mg-calcite fluctuates between −13‰ and −7‰, implying short-term variability in the lake's hydrologic budget, with values indicating the lake varied from outflow-dominated to evaporation-dominated. The δ¹³C values of Mg-calcite remain nearly constant from 8 to 4.5 ka and then trend to higher values upward in the section. This pattern suggests primary productivity in the lake was initially constant but gradually increased after 4.5 ka.