Varve formation in meromictic Crawford Lake, Ontario, Canada: important process for characterizing 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 acid precipitation control lamina thickness: acid precipitation 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 lamina were deposited between the late 1940s and mid-1970s, when air and water quality were impacted by rapid industrialization. Conditions required for precipitation of calcite laminae were absent during the 16th to mid-19th centuries, an interval corresponding to the Little Ice Age when no human impact occurred in 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.