Rocks Tell Tales of Earth Billions of Years Ago

January 21, 2003
By KENNETH CHANG 
 

From just a few sulfur atoms trapped inside some diamonds
from Botswana, scientists have inferred that there was
almost no oxygen in the air until about 2.4 billion years
ago. Meanwhile, a biology experiment looking at starved
bacteria suggests that methane was in the air, perhaps
explaining why the planet did not freeze over. 

When scientists try to figure out what the earth was like
billions of years ago, they do not have much to work with,
mostly just rocks. But a rock can tell much about itself.
Limestone is a remnant of ancient sea sediment. Basalt is a
hardened lava flow. Granite is magma that slowly cooled
underground into a hodgepodge of crystalline minerals. 

And over the earth's history, the rocks have changed. Some
mineral deposits like vast banded iron formations from
ancient times cannot form today, because the oxygen in
today's atmosphere and oceans now prevents iron from
dissolving. 

Scientists find even subtler clues in the weight of
individual atoms within rocks. The earliest evidence of
life on the earth is not a fossil - microbes rarely
fossilize and when they do, they look like cracks or air
bubbles - but rather, deposits of carbon dating back 3.8
billion years that almost completely lack a heavier version
of carbon. 

Ninety-nine percent of carbon atoms are carbon 12,
containing six protons and six neutrons in their nuclei.
Carbon 13, a rarer, heavier version, or isotope, contains
seven neutrons. Living things prefer to be lazy if they
can, eating compounds containing the lighter carbon 12 and
leaving carbon 13 behind. Thus, clumps of graphite
containing higher-than-normal levels of carbon 12 are
believed to be the remains of ancient microbes. 

To deduce when oxygen entered the atmosphere, Dr. James
Farquhar, a professor of geology at the University of
Maryland, plays a similar game of chemical isotopes with
sulfur. As sulfur compounds, spewed out by volcanoes, float
in the air, ultraviolet light hits them and breaks them
apart, producing a ratio of isotopes later preserved in
rocks. 

In work reported three years ago, Dr. Farquhar found that
these sulfur ratios changed between 2.4 billion and 2.1
billion years ago, which he attributes to the advent of
oxygen, in the form of ozone, that blocked out the
ultraviolet radiation. 

Dr. Farquhar has now found the same sulfur isotope signal
in tiny specks, about one ten-thousandth of an inch wide,
within diamonds. "It tells us it was sulfur from a
volcano," he said. The new findings appeared in the Dec. 20
issue of the journal Science. 

Besides confirming his earlier result, the findings also
indicate that parts of the earth's crust were being pushed
down and reheated and brought back up, much as they are
today. "You can reconstruct this whole cycle," Dr. Farquhar
said. "Volcano to atmosphere to sediments to mantle, back
to surface." 

Dr. Donald E. Canfield, a professor of ecology at the
University of Southern Denmark, has taken an unusually
modern approach in investigating the ancient earth. He and
his colleagues grew bacteria, ones that eat sulfur
compounds known as sulfates and produce another group of
compounds known as sulfides as waste. The bacteria, lazy as
always, tend to eat sulfates containing the lighter sulfur
isotopes. "It's easier for them eat, basically," Dr.
Canfield said. 

Thus, the sulfates contain more of the heavier sulfur left
behind by the bacteria, and their sulfide waste is richer
in the lighter isotopes. But in rocks older than 2.5
billion years old, both sulfate and sulfide compounds
contain roughly the same ratio of sulfur isotopes. 

That, Dr. Canfield said, means that so little sulfate was
in the oceans then that bacteria ate any sulfate they
happened to find. In their experiments, they starved
sulfate-eating bacteria to see at what point they stopped
being picky eaters. Their conclusion, reported in the same
issue of Science, is that sulfate levels then were at most
one-hundredth of current levels. 

With sulfate-eating bacteria limited by their food supply,
that would have opened up an ecological niche for competing
bacteria that produce methane, possibly solving the "faint
sun paradox." The sun back then was about 20 percent
dimmer, yet the earth was just as warm, so presumably the
planet back then had a thicker blanket of so-called
greenhouse gases. 

Some scientists believe that high concentrations of carbon
dioxide warmed the planet, but others believe that methane,
which is more efficient than carbon dioxide at trapping
heat, was the reason. "You have a possibility of making a
methane greenhouse," said Dr. Uwe Wiechert, a geochemist at
the Swiss Federal Institute of Technology in Zurich. "I
like this idea very much." 

Not everyone agrees with the stories deduced from the
isotope data. Dr. Hiroshi Ohmoto, a professor of
geochemistry at Penn State University, still believes that
oxygen accumulated in the atmosphere soon after the advent
of oxygen-producing bacteria at least 2.7 billion years
ago. 

Dr. Ohmoto points out that most of Dr. Farquhar's rocks and
diamonds show an excess of the sulfur 34 isotope while
experiments emulating the early atmosphere produce
lower-than-average amounts of sulfur 34. "Until they come
up with experimental data, it's awfully difficult for me to
accept the explanation," he said. 

Nonetheless, all the scientists agree that big changes were
occurring in the earth's oceans and atmosphere about 2.4
billion years ago. 

"We're just starting to understand how all these pieces
correlate to this time interval," Dr. Canfield said. 

If they can figure out what the rocks are telling
them.

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