BIOLOGICALLY INDUCED CALCITE AND ITS ISOTOPIC COMPOSITION IN LAKE-ONTARIO

We studied water, sediment trap, and core samples from eastern Lake On tario to reconstruct the factors controlling the biologically induced production and sedimentation of calcite during so-called whiting event s. Calcite accumulation and its isotopic composition are controlled by a complex set of interrelated factors, including temperature, primary productivity, and the abundance of pico-cyanobacteria during the stra tified period. Calcite precipitation is highly correlated to lake temp erature, because physical and biological factors interact to produce c onditions favorable for whitings during warm years when the lake strat ifies early in the seasonal cycle. Carbonate stratigraphies in multipl e cores from eastern Lake Ontario revealed similar patterns of histori cal variation in percent carbonate. An exponential rise in carbonate a ccumulation occurred in nine cores after 1930, culminating in peak val ues in the early 1980s. This rise was related to historic increases in primary productivity resulting from increased phosphorus loading to L ake 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 wi th strong El Nino events. These peaks are also associated with maxima in delta(13)C values and minima in delta(18)O values of carbonate, len ding support to our model that more calcite is precipitated with highe r delta(13)C values during warm years when thermal stratification occu rs early in the seasonal cycle. Beginning in the mid-1980s, calcite ac cumulation and its delta(13)C ratio began to decrease, suggesting a re duction 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 explaine d either by a lagged response to P abatement measures that began in th e late 1970s or by decreased P loading from upstream Lake Erie beginni ng in the late 1980s as a result of the establishment of filter-feedin g zebra mussels.