(SGER): Experiments on the Effects of the Magnesium-Calcium Ratio of Seawater on Carbonate Skeletal Secretion

Johns Hopkins University, Baltimore MD

Investigators

Abstract

There is now powerful evidence that changes in the Mg/Ca ratio of seawater have produced the oscillations during Phanerozoic time between intervals when nonskeletal carbonates precipitating from seawater have consisted of low-Mg calcite and intervals when these chemical precipitates have consisted of high-Mg calcite and aragonite. In previous work that focused on patterns in the geologic record, we showed that throughout Phanerozoic time biologically simple carbonate-secreting organisms, especially those that hypercalcify (produce massive skeletons or generate sediment rampantly) have adhered to the 'calcite sea'/'aragonite sea' pattern. This correspondence implies that simple organisms hypercalcify only when the Mg/Ca ratio of ambient seawater favors their mineralogy. Thus far, PI's research has been based on patterns in the geologic record. They are now embarking on an entirely new approach: they plan to conduct laboratory experiments to test the effects of the Mg/Ca ratio on the calcification of living organisms. Specifically, they plan three kinds of controlled aquarium experiments that they anticipate will be particularly tractable. They will determine whether, when grown in water having low and very low ('Permian' and 'Cretaceous') Mg/Ca ratios, (1) specimens of coralline algae, which secrete high-Mg calcite in modern seawater, incorporate less Mg into their skeletons than when they are grown in modern seawater; (2) specimens of aragonitic algae grow at lower rates than when grown in modern seawater; and (3) calcareous nannoplankton, which secrete calcite, experience more rapid population growth than comparable populations cultured under identical conditions but with an ambient Mg/Ca ratio equal to that of modern seawater. (in other words, when extant species of nannoplankton are grown in 'Cretaceous' seawater, in which the Mg/Ca ratio is close to unity and there is a large total concentration of dissolved Ca, will they bloom as their ancestors did in the 'chalk seas' of the Cretaceous?).