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 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.
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-hundred-square-kilometer area of West Africa has remained quite flat and very stable. Con-sequently, 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 scallop-shaped 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,” 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 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.”
This is an excerpt from EARTH: INSIDE AND OUT, edited by Edmond A. Mathez, a publication of the New Press. © 2000 American Museum of Natural History.
More About This Resource...
This online article, from Earth: Inside and Out, reports on a museum expedition to the Sahara Desert to retrieve a stromatolite. It discusses:
- What stromatolites are and how they were created.
- What their discovery has taught us about the evolution of the ancient Earth.
- How scientists selected the stromatolite to bring back for the museum exhibit.
- The lengths the team had to go to in order to retrieve the 760-kilo (1,675-pound) boulder.
Less than 1 period
Supplement a study of geology with an activity drawn from this essay about an expedition to the Sahara Desert.
- Ask students what they know about the Proterozoic Era and whether or not life existed during it.
- Send students to this online article, or print copies of the essay for them to read.
- Have them write a one-page reaction to the article, focusing on what they learned about the significance of stromatolites.