Paper No. 13
Presentation Time: 4:45 PM
HOW AN INCREASE IN THE MG/CA RATIO OF SEAWATER AFTER THE CRETACEOUS DEPRESSED CHALK PRODUCTION AND CAUSED SOME COCCOLITHOPHORES TO SECRETE HIGH-MG CALCITE
High-Mg calcite (mole % Mg > 4) precipitates from seawater at ambient Mg/Ca ratios of 1, and is joined by aragonite at ratios above 2. The Mg/Ca ratio of Late Cretaceous seawater was below unity. Our experiments show that three modern coccolithophore species flourish in such seawater even though it is not their natural environmental medium. We predicted this pattern because coccoliths are formed of calcite, and enhanced calcification under favorable chemical conditions should fertilize photosynthesis because precipitation of calcium carbonate forms carbon dioxide as a biproduct. The three experimental species exhibited dramatic increases in exponential rate of population growth as the Mg/Ca ratio was stepped down from 5.2 (the ratio for modern seawater) to 0.5; for two of the species, the exponential rate approximately tripled. By varying each while holding the other constant, we found that both the Mg/Ca ratio and the absolute concentration of Ca influenced population growth rates. One species secreted low-Mg calcite in all treatments. The other two secreted high-Mg calcite in modern seawater and low-Mg calcite in Cretaceous seawater, displaying fractionation curves for Mg with changes in the ambient Mg/Ca ratio that resemble the curve for nonskeletal precipitation. Population growth of the species that secreted low-Mg calcite under all ambient Mg/Ca conditions was favored by the low Mg/Ca ratio and abundant Ca in Cretaceous seawater. Presumably the two species that incorporated more Mg into their coccoliths with an increase in the ambient Mg/Ca ratio simulaneously experienced reduced rates of coccolith growth rate as a result of Mg poisoning. We conclude that the chemistry of Cretaceous seawater accounts for the coccolithophores production of the massive chalk deposits that gave the Cretaceous Period its name. Conversely, the high Mg/Ca ratio and low absolute concentration of Ca appear to limit population growth of nearly all coccolithophore species today (Emiliania huxlyi is a recently evolved exception, pumping Ca at an exceptional rate). Being unable to take advantage of high concentrations of other nutrients (phosphates, nitrates, and iron) in eutrophic regions, most modern coccolithophore species are relegated to oligotrophic waters of the subtropics and tropics, where their populations are sparse.