The possible answer: Ancient seawater's low magnesium-to-calcium ratio during this interval made it difficult for the marine animals -- which build their skeletons from a mineral called aragonite calcium carbonate -- to grow and flourish into vast reefs. That left few to end up in the fossil record, posits doctoral candidate Justin Ries and his advisor Steven Stanley, professor in the Morton K. Blaustein Department of Earth and Planetary Sciences at the university's Zanvyl Krieger School of Arts and Sciences.
"Scientists have grappled with this question for years, and my research shows that the answer is that the chemistry of Cretaceous seawater did not support the secretion of the aragonite mineral from which corals construct their skeleton," said Ries, who will present his research on Nov. 10 at the 116th annual meeting of The Geological Society in Denver. "What's more, my experiments suggest that corals from the Cretaceous period almost certainly built at least part of their skeletons from calcite. This is groundbreaking, because it was previously believed that organisms do not generally change their skeletal mineralogy over time. Now we know that they do."
Ries spent two months growing three species of modern Scleractinian corals (the major reef-building corals in today's seas) in seawater formulated at six different chemical ratios that have existed throughout the geologic history of corals. He created this seawater "from scratch" according to recipes provided by earth and planetary sciences Professor Lawrence Hardie, who recently discovered that the magnesium-to-calcium molecular ratio of seawater has
Contact: Lisa DeNike
Johns Hopkins University