Science
Discovery Marks a Sea
Change
For almost two decades, Lawrence Hardie's peers resisted his
theory that the chemistry of seawater has periodically
changed over the last 600 million years. But now Hardie and
two of his former students have found redemption in tiny,
trapped droplets of ancient seawater. Using a refined
technique to analyze water droplets smaller than the tip of
a pencil, they found evidence that our oceans have not
always had the same chemical composition.
"The ruling paradigm on seawater chemistry, its major
ions and such, was that there had been no change in the last
two billion years," says Hardie, professor of Earth and
planetary sciences in the Krieger School of Arts and
Sciences. But Hardie was increasingly bothered by mineral
patterns he observed in certain limestone and salt deposits
(potash) left by ancient evaporated seas. He found that the
mineral content of potash flip-flopped every 100 to 200
million years, between magnesium-rich and magnesium-poor, a
fluctuation that Hardie thought might be explained by
changing seawater chemistry.
The idea first occurred to Hardie in the 1970s when
scientists discovered underwater vents, or volcanoes. These
vents, found where tectonic plates spread apart, not only
support new and strange life, but also form hot brines. As
seafloors spread, hot magma rises from the Earth's interior
and transforms seawater into brines by absorbing its
magnesium and releasing calcium. The cooled magma, also
transformed chemically, forms new mountains and ocean floors
that slowly spread apart. Over time, widespread vents have
resulted in extensive underwater mountain ridges--50,000
kilometers in all--across the ocean floors. Hardie believed
that enough vents existed to create brines that might
influence seawater globally. In 1984, he gave an informal
talk about how this process also might have altered the
mineral content of ancient marine deposits, but found a cool
reception. Jokes Hardie, "All the big buffs were there and
they didn't sound too interested."
Hardie wouldn't take up his idea again until the 1990s,
when geologists better understood the age of the ocean floor
and could measure historic seafloor spreading rates. He then
published his research linking changing rock mineralogy and
seawater chemistry and their possible connection to plate
tectonics. "The fit was really quite spectacular," says
Hardie of his correlations. But his ideas threatened to
remain theory until he could demonstrate that seawater
actually had changed.
Determined, Hardie teamed up with two of his former
graduate students: Tim Lowenstein (PhD '82) and Robert
DeMicco (PhD '81), now at Binghamton University, State
University of New York. They thought that shifting ocean
chemistry might be revealed in trapped droplets of water
found in ancient marine halite. They collected this type of
salt deposit from around the world to look at densely packed
droplets. Too tiny to extract, droplets were frozen, sliced
into sections, and examined with a scanning electron
microscope.
In the November 2, 2001, issue of Science, they
report
that magnesium-calcium ratios in the droplets were the same
around the world at any given time, but varied through
history.
"I hope this will finally lay to rest the idea that
seawater was constant in composition," says Lowenstein.
"When people see that seawater chemistry has changed, I
think they will open up their eyes to try to look at how
other things changed in rhythm with seawater chemistry."
Both Hardie and Lowenstein believe that seawater
chemistry changed with seafloor spreading, which would
explain why magnesium-calcium ratios vary in the droplets.
Other known historical phenomena, like volcanism and global
sea levels, also fit their observed patterns of changing
seawater.
And now, says Steven Stanley, a Johns Hopkins
paleontologist, fossil records tie in as well.
In papers coauthored with Hardie, Stanley reported on
trends of simple organisms like coccolithophores,
free-floating microscopic algae that absorb calcium
carbonate and form chalky armor shields. During the
Cretaceous period--the great chalk age around 100 million
years ago--coccolithophores thrived in such abundance that
when they died, their armor plates formed tremendous
limestone formations like the White Cliffs of Dover in
England and those along the Brittany Coast of France. As
seafloor spreading slowed and the Cretaceous period waned,
these shields shrank in size, and one group of
coccolithophores became extinct, presumably due to changing
chemistry of the seas.
The Hopkins scientists observed similar trends in other
calcium-secreting algae, sponges, and corals. Says Stanley,
"It's exciting to connect paleontology to all of these other
areas of geology in ways that we wouldn't have suspected
before." --Carol Marzuola (MA '02)