A ∼14 000-year record of environmental change from Lake Simcoe, Canada

While the evolution of the Laurentian Great Lakes has been researched intensively, fewer studies have focused on reconstructing the history of Lake Simcoe, the fourth largest lake wholly within southern Ontario. This study uses proxy data, including ostracod valves, contained in lake sediments to reconstruct a ∼14 000-year history of paleoenvironmental change in the Lake Simcoe basin. First, oxygen-isotope compositions of ostracod valves (δ18Ovalve) are used to estimate the oxygen-isotope composition of ancient lake water, δ18Olake water. In Lake Simcoe, δ18Olake water is insensitive to changes in lake level but sensitive to shifts in temperature, capturing periods such as the early Holocene warming. This δ18Olake water record demonstrates that the delivery of meltwater from Lake Algonquin to Lake Simcoe began to slow at ∼12 050 cal yr BP, supporting previous research based on geophysical data. It also suggests that an isolated pulse of 16O-rich water from Early Lake Mattawa may have occurred from ∼10 699 to 10 591 cal yr BP. Next, stable carbon-isotope compositions of ostracod valves (δ13Cvalve) are used to estimate the stable carbon-isotope composition of dissolved inorganic carbon (δ13CDIC) in Lake Simcoe. These data, combined with ostracod assemblages, grain size, magnetic susceptibility (MS) and sediment mineralogy, support our interpretation of temperature-sensitive variation in δ18Olake water and provide further insights into in-lake conditions. A pronounced increase in accumulation rate, and subtler increases in grain size and detrital carbonate contents, between ∼8300 and ∼8000 cal yr BP may reflect the abrupt transition from cold and dry to wet and warm conditions resulting from the collapse of the Laurentide Ice Sheet (LIS) at ∼8200 cal yr BP. In summary, this study refines our understanding of the evolution of Lake Simcoe and provides a holistic picture of environmental change over the last ∼14 000 years. •Ostracods reveal paleoenvironmental history of Lake Simcoe, Canada.•Glacial meltwater input to Lake Simcoe began to slow at ∼12 050 cal yr BP.•Early Lake Mattawa may have inundated Lake Simcoe ∼10 600 cal yr BP.•Proxies from Lake Simcoe reveal relative Holocene temperature change.•Physical properties of Lake Simcoe sediment record the 8.2 ka event.