Recent changes in productivity and climate of Lake Ontario detected by isotopic analysis of sediments

Abstract

Cores from Lake Ontario provide a record of lake responses to increased phosphorus loading that began after early forest clearance in the mid‐1800s. We measured 13C/12C ratios of organic and inorganic C (calcite) in the sediments to infer recent trends in primary production. The δ13C of organic C increased in response to historic increases in P enrichment until the mid‐1970s and then decreased in response to reduced P loading. The δ13C of calcite increased after 1940 but did not decrease in response to reduced P loading. We conclude that the results differ because δ13C of organic C is determined by primary production early in the seasonal cycle when flux of organic C to the sediments is greatest, whereas δ13C of calcite depends on the isotopic composition of dissolved inorganic C during late summer when calcite is precipitated.Calcite accumulation and its δ13C ratio oscillated after 1940 with peaks estimated at 1948, 1966, 1977, and 1983. These peaks correspond to minima in δ18O values, indicating calcite precipitation at higher temperature, and to years of early thermal stratification. We hypothesize that calcite accumulation and its stable isotopic ratios are controlled by interannual variability in onset of thermal stratification of the lake. Our results thus suggest a direct link from climate (i.e. thermal stratification) to interannual variation in lake productivity and to carbonate precipitation and its δ13C signature.