Quantifying conditions required for varve formation in meromictic Crawford Lake, Ontario, Canada: important process for delimiting the Anthropocene epoch

Varved sediments in meromictic Crawford Lake consist of dark–light couplets of organic matter (primarily phytoplankton and amorphous organic matter) capped by calcite crystals. The crystals precipitate in the alkaline epilimnion between spring and fall turnover, consistent with Langelier Saturation Index calculations that predict calcite precipitation when pH and temperature exceed 7.76 and ~ 15 °C, respectively. Climate, primary production, and the pH of the epilimnion control lamina thickness: acid rain primarily affects the precipitation and accumulation of calcite crystals, whereas both endogenic calcite and authigenic organic matter are affected by climate and primary production. Thin varves, often with barely perceptible light-coloured calcite laminae were deposited between the late 1940s and mid-1970s, when the pH of the epilimnion fell slightly in response to deterioration in air and water quality associated with rapid industrialization. Conditions required for precipitation of calcite laminae were absent during the sixteenth to mid-nineteenth centuries, an interval corresponding to the Little Ice Age when no human impact affected the catchment. Varves dating from 1867 CE onwards (the Canadian Zone) facilitate the candidacy of the deep basin sediments of Crawford Lake to define the Anthropocene epoch.