Biologically induced calcite and its isotopic composition in Lake Ontario

Abstract

We studied water, sediment trap, and core samples from eastern Lake Ontario to reconstruct the factors controlling the biologically induced production and sedimentation of calcite during so‐called whiting events. Calcite accumulation and its isotopic composition are controlled by a complex set of interrelated factors, including temperature, primary productivity, and the abundance of pica‐cyanobacteria during the stratified period. Calcite precipitation is highly correlated to lake temperature, because physical and biological factors interact to produce conditions favorable for whitings during warm years when the lake stratifies early in the seasonal cycle. Carbonate stratigraphies in multiple cores from eastern Lake Ontario revealed similar patterns of historical variation in percent carbonate. An exponential rise in carbonate accumulation occurred in nine cores after 1930, culminating in peak values in the early 1980s. This rise was related to historic increases in primary productivity resulting from increased phosphorus loading to Lake Ontario. Superimposed upon this rise were four peaks (centered on 1940–1942, 1957–1961, 1971–1977, and 1983) that correlate with maxima in summer air‐temperature anomalies from the Great Lakes region and with strong El Niiio events. These peaks are also associated with maxima in δ13C values and minima in δ18O values of carbonate, lending support to our model that more calcite is precipitated with higher δ13C values during warm years when thermal stratitication occurs early in the seasonal cycle. Beginning in the mid‐1980s calcite accumulation and its δ13C ratio began to decrease, suggesting a reduction in primary productivity in surface waters, probably related to lower phosphate concentrations in epilimnetic waters of Lake Ontario during the stratified period. Reduced summer P loading may be explained either by a lagged response to P abatement measures that began in the late 1970s or by decreased P loading from upstream Lake Erie beginning in the late 1980s as a result of the establishment of filter‐feeding zebra mussels.