A Johns Hopkins doctoral student may have solved a
problem that has been baffling marine biologists and
paleontologists for years: Why did coral reefs disappear
from the fossil record during the beginning of the
Cretaceous period — 120 million years ago —
only to reappear after its end 35 million years ago?
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 adviser, Steven Stanley, professor in
the Morton K. Blaustein Department of
Earth and Planetary Sciences in the 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 presented 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
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 oscillated between 1-to-0 and 5-to-2 over the past 540
million years due to chemical reaction between rising magma
and seawater brine along various parts of the ocean
"The artificial seawaters were created by adding
different concentrations of salts as calculated by Lawrence
Hardie," Ries said. "I specifically wanted to test how
modern corals respond to the ancient levels of magnesium
and calcium because these chemicals, along with carbon and
oxygen, are the building blocks of their skeletons. More
important, however, is that the ratio of these two
chemicals determines whether the aragonite or calcite
mineral will form."
Into 10-gallon tanks filled with these mixtures went
coral fragments replete with colonies of polyps —
tiny animals, a few millimeters in size, from which larger
corals and, eventually, reefs grow. Ries prepared the
polyps for the experiment by having them spend a one-month
"adjustment period" in tanks filled with modern seawater.
Gradually, Ries adjusted the tanks' chemistry until their
contents were in line with the prescribed "ancient"
"To prevent the corals from experiencing chemical
shock in the unfamiliar seawaters, I learned that they must
be acclimated gradually, in stages," Ries said. "This was
actually one of the most challenging aspects of the
project. There were many failed attempts before I was able
to keep the corals alive so that I could observe their
growth and calcification in the ancient seawaters."
The corals were grown under special lights called
PowerCompacts, which simulated true daylight by emitting a
wavelength commensurate to sunrise and sunset, as well as
normal sunlight during the rest of the day. Ries fed the
growing corals with plankton particles and monitored each
tank's pH level — and level of chemicals such as
strontium, iodine and manganese, as well as vitamins
— several times a week.
Ries credits his experiments with leading to "two very
important discoveries about corals."
First, the skeletons of the corals cultivated in the
ancient seawaters had a different mineral composition from
those grown in modern seawater. Those in the so-called
Cretaceous seawater began building skeletons of 35 percent
calcite mineral, as opposed to modern corals, which built
them from 100 percent aragonite. This suggests that the
skeletons of corals have been changing along with seawater
throughout the geologic past.
"This is astounding, given that most scientists have
long believed that the mineral composition of a group of
organisms' skeletons is fixed over time," Ries said.
Secondly, the experiment was important because it
proved corals cultivated in Cretaceous seawater grew more
slowly than their counterparts raised in modern
"This solves, experimentally, the long-standing
question of why the Scleratinian corals stopped making
reefs during the Cretaceous period — because the low
magnesium/calcium ratios in the oceans at that time
inhibited the growth of the aragonite mineral that they
used to build their skeletons," Ries said.
Ries' research was funded by the National Science
Foundation, the Petroleum Research Fund and a J. Brien Key
Grant. Moody Gardens Aquarium in Galveston, Texas, provided
coral fragments for the study.