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The red hematite mined for iron on Brazil's Urucum plateau formed a cap that held off erosion for millions of years.


Dinosaur-age landscapes lurk in Southern Hemisphere

Climbing to the top of the Urucum plateau, a shock of rust-red land thrust 1 kilometer above the Brazilian savanna, is a journey into Earth's deep past. Despite the region's heavy, erosive rainfall, the surface of the plateau has remained largely unchanged for some 70 million years, making it Earth's oldest known landscape. Walk along it and you're only a few meters below the surface that dinosaurs once trod.

That startling picture emerges from a study published this month in Earth and Planetary Science Letters by a team led by Paulo Vasconcelos, a geochemist at the University of Queensland in Brisbane, Australia. Until recently, scientists could estimate erosion only by looking at the sediment sloughed off of a surface. But new geochemical tools developed by, among others, Vasconcelos and his colleagues measure erosion from rock that's left behind. "They all converge to the same story," Vasconcelos says. "Though it's taken some time to convince people."

Earth scientists say ancient landscapes could exist atop other inselbergs, a German term for the isolated plateaus that dot geologically quiet regions in the Southern Hemisphere that have not been reshaped by plate tectonics or planed away by ice sheets. Geologists had suspected that these inselbergs, found in Brazil, Australia, and southern Africa, are old—enduring while erosion stripped away the surrounding landscape. Now, that history is emerging. "It's almost like the movies," says William Dietrich, a geomorphologist at the University of California, Berkeley. "You expect to see exotic animals wandering around."

For decades, geomorphologists have fixated on regions where plate tectonics accelerates geologic change, thrusting up mountains, opening rifts, and creating traps for oil and gas. "Where do most geologists go?" asks Paul Bierman, a geologist at the University of Vermont in Burlington. "They go to the mountains. Or to oil." But the new geochemical tools are turning geoscientists on to the charms of the planet's slow parts.

Vasconcelos and his team used four different geochemical dating systems to flesh out the story of the Urucum plateau and its neighbor, the Santa Cruz. One took advantage of grains of the mineral manganese oxide that formed when the surface was first exposed to rain. The grains incorporated trace amounts of radioactive potassium, which has steadily decayed into argon ever since, providing a clock that shows the landscape formed 60 million to 70 million years ago. Another gauge of erosion comes from a set of isotopes—of helium, beryllium, and aluminum—that form when cosmic rays bombard surface rocks. The high abundances of these isotopes suggest the plateaus shed only 1 meter of material every 10 million years; the surrounding landscape, meanwhile, likely eroded at 100 times that rate, the group concludes. "They put together an erosional history that's very compelling and exceptional," Dietrich says. "I don't know something so cleverly dated."

The results raise a new question, says Jane Willenbring, a geomorphologist at the Scripps Institution of Oceanography in San Diego, California: "What makes landscapes persist for millions of years?" Until now, the oldest known surfaces had been found in parched regions like the Atacama Desert in Chile or the dry valleys of Antarctica, where water-driven erosion is slow. Paradoxically, the longevity of the Brazilian plateaus depends on water, says Ken Farley, a geochemist at the California Institute of Technology in Pasadena and a co-author of the paper. They are rich in a type of iron oxide called hematite, which reacts with quartz dissolved in rainwater to form tough blocks of rock that armor the soil. "It's just iron oxide, nothing else there," Farley says.

Similar plateaus protected by iron or silica probably exist throughout the slow lands. "Vasconcelos's finding has the potential to motivate other researchers to come back to ancient and slow landforms," says Fabiano Pupim, a geomorphologist at the Federal University of São Paulo in Diadema, Brazil.

One lure is the long histories they hold. "This surface has seen a lot of geochemical processes," Vasconcelos says. "If something very drastic happened on the entire planet, a signature should be left." He's developing a technique to tease out rainfall and temperature histories from oxygen isotopes in goethite, an iron oxide that caps inselbergs in Brazil and Australia. These surfaces can also help scientists judge how often rare, powerful intraplate earthquakes strike a region. If the ancient cap is unbroken, any faults found in the rock below it must be associated with even older earthquakes, says Bierman, who recently used an isolated ancient plateau in South Africa to judge the seismic hazard to a nuclear power plant.

Inselbergs hold another gift: deposits of concentrated iron ore, protected from washing away by the impermeable surface. Left alone, the Urucum might exist for another 30 million years. But now, humanity is doing the job much faster. When the team visited earlier this decade, most of the plateau's surface had been lost to mining, Farley says. "If we went back there, I'm not sure this material would be left."