A new ostracod record from Lake Simcoe, Canada, sheds light on the meltwater pathways of glacial lakes in the Laurentian Great Lakes basin

Melting of the Laurentide Ice Sheet led to a freshening of the North Atlantic Ocean that may have caused brief periods of global cooling. The pathways of this meltwater flow across North America remain contested. One proposed route is through the modern-day Georgian Bay, followed by the Kirkfield-Fenelon Falls (K-FF) outlet, Lake Ontario and the St. Lawrence River. We attempted to better constrain the timing of glacial meltwater travelling along this route by collecting a piston core (PC-5) from the Lake Simcoe basin, which is between the Georgian Bay and K-FF outlet. Ostracod species from PC-5 were enumerated and analyzed to determine their oxygen isotope composition (δ18Ovalve), which was used
to estimate δ18Olake water. The basal sections of PC-5 were likely deposited by glacial Lake Algonquin because (i) these sections were comprised of grey, laminated clays typical of glacial lakes; (ii) ostracod assemblages in this section were dominated by the cold-tolerant species Candona subtriangulata; and the mean δ18Olake water of ostracods in this section was ~ –16 ‰, consistent with previous estimates of δ18Olake water of glacial Lake Algonquin. A ~12 ‰ rise in δ18Olake water, the disappearance of C. subtriangulata, and an increase in grain size suggest that Lake Algonquin began to stop flowing through the Lake Simcoe basin ~12 050 cal yr BP. A reappearance of C. subtriangulata between ~10 699 and 10 591 cal yr BP is indicative of a final meltwater pulse. The timing of this meltwater pulse, coupled with the return to low δ18Olake water (–16.1±1.6 ‰ to –13.6±1.6 ‰), is consistent with the discharge of Early Lake Mattawa through the Lake Simcoe basin. This finding suggests that meltwaters may have passed though the K-FF corridor as late as ~10 591 cal yr BP, although additional research is needed to confirm this possibility.