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Retrieving a Stromatolite
from the Sahara Desert
How do earth scientists prepare for a two-week
expedition to Saharan Africa? “First of all, we
got vaccinated,” replies Heather Sloan ruefully.
A Research Scientist and Exhibition Coordinator
for the Gottesman Hall of Planet Earth, Dr.
Sloan became a geologist because, “I was
always curious about the world around me.
I loved getting information that I could use to
answer questions like, why is that rock here?
Or why is the land shaped the way it is?”
Curiosity, a Ph.D. in marine geophysics, many
shots, and twenty hours in the air landed Sloan
in Nouakchott, Mauritania in February 1998.
There, she and the Museum team, which
included Edmond A. Mathez, head of the
Department of Earth and Planetary Sciences,
were met by Dr. Janine Sarfati of France’s
Université de Montpellier 2, who knew the
region well. The purpose of the two-week
expedition was to bring back a rock called
a stromatolite for display in the Museum’s
Gottesman Hall of Planet Earth. This is a new
exhibit built around dramatic rock samples that
exemplify the dynamic processes that shaped
the Earth.
Stromatolites are “organo-sedimentary”
structures—basically slimy masses—built by
microbes, the only life to exist on Earth until
about a billion years ago. “They are the
earliest evidence of life from the Proterozoic
Era, which was supposed to be ‘azoic’ ”—
without life—points out Dr. Sarfati. In addition,
stromatolites played a tremendously significant
part in the evolution of the ancient Earth and
its atmosphere. Originally, the atmosphere
contained very little oxygen. “But about 2.6
billion years ago there was a global expansion
in the number of stromatolite colonies,” explains
Sloan, possibly because that Era’s shallow
Case Study
Left to right: Janine Sarfati, Heather Sloan, Ed Mathez
and two helpers work to lift the sample high enough so
that the straps that will lift it into the truck can be placed
underneath it.
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coastal and inland seas provided an optimum
environment for these colonies of algae. “The
fact that these organisms photosynthesize
totally changed the chemistry of Earth’s
atmosphere, because it contributed to a very
rapid increase in the concentration of oxygen,”
she continues. “It shaped the type of life which
followed, because life could become aerobic”—
that is, able to consume oxygen for survival.
(Steve Mojszis’s essay in this section describes
this process in detail.)
Stromatolites also provide information about the
climate conditions under which they formed.
Because most stromatolites need sunlight, their
growth is generally restricted to the ocean’s
photic zone, typically water less than 150
meters deep, “so we know that wherever they
were growing the sea must have been relatively
shallow.” Different stromatolite shapes reflect
different water conditions as well. “If they were
agitated in surf zones, stromatolites show
protective ‘walls’ around the structures and
pieces of rock in the buildup,” says Sarfati.
Others stand high above the sediment, which
indicates that they developed in quiet water
beneath the wave action.
Sarfati began studying ancient stromatolites
along a 1200-kilometer long outcrop in Algeria
and Mauritania in the 1960s. When French
soldiers on camelback discovered the
structures in the 1920s, they at first mistook
them for petrified forests. Unlike the many
places where stromatolites have been buried by
tectonic activity, this two- to three-hundredsquare-kilometer area of West Africa has
remained quite flat and very stable. Consequently, it’s home to unique formations of
almost twenty-five different types of
stromatolites—"some cone-shaped, some with
branches, some shaped like big loaves of
bread, called bioherms,” says Sloan. The
formations are composed of layers built up over
many millions of years, and are between ten
and thirty meters thick and up to ten meters
high. In the Sahara the formations are exposed
by the outcrop and free of vegetation, so, says
Sarfati, “in one glance you can trace the story
occurring around you."
After a grueling drive inland to the town of Atar,
it was up to Sarfati to explain the story behind
different formations to the American team.
“Janine has this tremendous store of knowledge
and field experience about stromatolites,
and along with all that, she is absolutely
indomitable,” recounts Sloan admiringly. “At
seventy she was scampering up and down rock
faces in 115°F heat. And afterwards, while most
of the younger members of the party collapsed,
she would drag me out into the market to look
for bargains.” It took three days to explore the
outcrops and pick out the right stromatolite for
the exhibit. Scientific, aesthetic, and logistical
criteria all had to be satisfied. “We wanted one
which showed the structure as completely as
possible, because a lot were badly weathered.
And we wanted as large a rock as possible, but
we didn’t have any heavy machinery, and the
roads are terrible.”
The boulder selected, a Jacutophyton that Sloan
describes as “a central conical structure with
beautiful branches coming off all around it. They
grew quite close together, a bit like a submarine
forest.” The stone had a beautiful, iridescent
desert patina, caused by “the combination of
windblown minerals and the action of bacteria
that live on whatever the wind brings them.” It
also had a delicate texture of beautiful scallopshaped ridges built up by algae and bacteria
and exposed by weathering. Fortunately, the
stromatolite was already free of the outcrop and
lying by the road. But it weighed 760 kilos, and
it was quite fragile.
“So Janine and I went to the market and we
started buying things to wrap a rock up in,”
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CASE
recounts Sloan, grinning at the memory. Since
many commodities are used and reused in the
desert, rope was available only in short pieces.
They found cargo nets, and rice sacks to keep
the rock dry, and twine, and purchased a
handmade, six-inch needle from an ironmonger.
“By this time we’d acquired quite a following of
children, because nobody could figure out what
on Earth we wanted these things for. We also
had to communicate to the guy at the garage
that we wanted the totally ruined tires, the ones
that simply couldn’t be patched any more, to
use as padding. Then we headed back to the
hotel and started sewing sacks together.”
A local merchant lined them up with a big truck
and giant frame with a big pulley on it, but
halfway to their destination the ancient vehicle
broke an axle in the sand. “So a little pick-up
truck passing by, the local bus in effect, agreed
to take this big iron frame, and drive it out to
STUDY:
RETRIEVING
the site—along with all of the passengers. They
were all willing to come out and watch because
they simply couldn’t believe we were going to all
this trouble to pick up a rock,” recalls Sloan
with a laugh. “And of course everybody
participates, so we suddenly had a crew of
eighteen or nineteen people setting up this huge
frame. And thank goodness, because it was
heavy.” The team had already laid out the
packing material in their little Toyota truck,
the stromatolite was loaded onto the truck bed,
and they trussed the whole thing up. “It
took about twelve hours to get back to
Nouakchott,” recounts Sloan, “and it was a
very exciting trip.”
Left to Right: Heather Sloan, Janine Sarfati, and Ed Mathez
discussing the most promising sample locations while
studying a local geologic map.
A
STROMATOLITE