Dated sediment cores from Cayuga Lake, New York State, document that biologically mediated precipitation of calcite has been controlled by environmental change, both natural and anthropogenic, over the past 10 000 yr. During the Holocene Hypsithermal (∼9–4 ka [14C]), Milankovitch forcing of summer insolation in the Northern Hemisphere resulted in a broad increase (to 55%), then decrease (to <5%), of calcite content in bottom sediment. Warmer summers resulted in earlier onset of thermal stratification of the water column, which increased the duration of primary production as well as the abundance of picoplankton, which in turn increased the amount of calcite precipitated. At the end of the Hypsithermal ca. 3500 yr ago, global cooling greatly reduced the amount of calcite precipitated. However, since A.D. 1940, calcite contents in Cayuga Lake sediments have risen up to ∼20%. One hypothesis is that this recent increase in calcite is the result of cultural eutrophication (nutrient loading). However, this rise in calcite also closely tracks the anthropogenic rise of atmospheric carbon dioxide, suggesting a possible link to global environmental change. Further research on hard-water lake basins will be needed to test which of these two hypotheses is correct.