What happened when a meteorite the size of fo

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Nadja Drabon, Assistant Professor of Earth and Planetary Sciences at Harvard.

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Credit: Bryant Troung

Billions of years ago, long before anything resembling life as we know it existed, meteorites often hit the planet. One such space rock crashed about 3.26 billion years ago, and even today it reveals secrets about Earth’s past.

Nadja Drabon, an early earth geologist and assistant professor at the Department of Earth and Planetary Sciences, is insatiably curious about what our planet was like in ancient eons filled with meteoritic bombardment, when only single-celled bacteria and archaea ruled – and when that reigned everything started to change themselves. When did the first oceans form? What about continents? Plate tectonics? How did all these violent influences affect the development of life?

A new study in Proceedings of the National Academy of Sciences sheds light on some of these questions in relation to the inauspiciously named “S2” meteorite impact over 3 billion years ago, for which geological evidence exists in the Barberton Greenstone Belt in South Africa today. Through the painstaking work of collecting and examining rock samples at centimeter intervals and analyzing the sedimentology, geochemistry and carbon isotope compositions they leave behind, Drabon’s team paints the most compelling picture yet of what happened the day a meteorite the size of four Mount Everests paid Earth has visited.

“Imagine you’re standing off the coast of Cape Cod on a shelf of shallow water. It’s a low-energy environment with no strong currents. Then all of a sudden you have a huge tsunami that shoots by and rips up the sea floor,” Drabon said.

The S2 meteorite, estimated to have been up to 200 times larger than the one that killed the dinosaurs, triggered a tsunami that stirred the ocean and washed debris from the land into coastal areas. Heat from the impact caused the upper layer of the ocean to boil off, while warming the atmosphere. A thick cloud of dust covered everything and shut down any photosynthetic activity that was taking place.

But bacteria are hardy, and after the impact, according to the team’s analysis, bacterial life quickly returned. With this came sharp spikes in populations of single-celled organisms that feed on the elements phosphorus and iron. Iron was probably stirred up from the deep ocean into shallow water by the aforementioned tsunami, and phosphorus was delivered to Earth by the meteorite itself and from an increase in weathering and erosion on land.

Drabon’s analysis shows that iron-metabolizing bacteria would thus have flourished in the immediate aftermath of the impact. This shift toward iron-favoring bacteria, however short-lived, is a key piece depicting early life on Earth. According to Drabon’s study, meteorite impact events—while known to kill everything in their wake (including the dinosaurs 66 million years ago)—carried a silver lining for life.

“We think of impact events as being catastrophic for life,” Drabon said. “But what this study highlights is that these impacts would have had benefits for life, especially early on … these impacts could have actually allowed life to flourish.”

These findings come from the ground-breaking work of geologists like Drabon and her students, where they hiked into mountain passes that contain sedimentary evidence of early sprays of rock that embedded themselves in the ground and were preserved over time in the Earth’s crust. Chemical signatures hidden in thin layers of rock help Drabon and her students gather evidence of tsunamis and other catastrophic events.

The Barberton Greenstone Belt in South Africa, where Drabon concentrates most of his current work, contains evidence for at least eight impact events, including S2. She and her team plan to study the area further to probe even deeper into Earth and its meteorite-activated history.


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