Sixty-six million years ago, a 10-kilometer-wide space rock fell from the sky over what is now the Yucatan Peninsula in the Gulf of Mexico. When it hit Earth, it blew a hole in the Earth’s crust the size of Maryland, igniting global firestorms and killing about 75% of species. For the dinosaurs, this event led to extinction, effectively the end of the world. But from the ashes survivors rose – our mammalian ancestors – ushering in a vibrant new era in Earth’s history. Today this effect is considered catastrophic A cosmic act of creative destructionWithout which we humans would not exist.
However, the impact of this infamous event is not comparable to the asteroid that struck Earth 3.26 billion years ago, in the middle of what scientists call the Archean Eon of our planet’s 4.5-billion-year history. The Archean space rock in this collision, dubbed “S2,” was 50 to 200 times larger, large enough to blast at least 10,000 cubic kilometers of vaporized rock into the sky, which then condensed into molten droplets. It rained back to the ground. Unsurprisingly, those conditions were “truly catastrophic for early life,” says Nadia Drabon, a geoscientist at Harvard University. But her latest research suggests that this older, older collision also had an upside — much like the collision of space rocks killed off the most famous dinosaurs — giving the early Earth’s biosphere a boost.
“What we found was really amazing,” Drabon says. Working alongside several colleagues, her examination of rock layers in South Africa showed that besides generating large amounts of evaporated rock, the S2 impact generated a massive tsunami and boiled the upper layer of the ocean. But it also pumped phosphorus and other vital elements into the world’s nutrient-starved seas, causing life to flourish.
About supporting scientific journalism
If you enjoyed this article, consider supporting our award-winning journalism by Subscribe. By purchasing a subscription, you help ensure a future of impactful stories about the discoveries and ideas shaping our world today.
Although the dinosaur-killing impact left a wake of environmental devastation lasting millions of years, the dire after-effects of this much larger impact were too short-lived to show up in chemical analyzes of the rock layers, Drabon says.
Conditions were terrible “for a couple of years, maybe a few decades,” she says. “But then life would come back very quickly.” Her new study, Published today in Proceedings of the National Academy of Sciences of the United States of AmericaThis suggests that giant impacts had a greater impact on the early Earth’s biosphere than previously thought – and that the inhabitants of our Archean planet were more resilient to this type of shock than current life would be today.
Unrecognizable land
If you had flown over our planet just before the S2 impact more than three billion years ago, it would have looked very different than it does today.
“Earth was largely a water world,” with a few volcanoes and large islands rising above the ocean surface, says Andrew Noll, a geobiologist at Harvard University who collaborated with Drabon on the new study. The world’s oceans may contain twice as much water as they do today, because the planet’s interior has not yet cooled enough to hold the amount of moisture it now holds.
Without large continents eroding and sending minerals into rivers, the oceans were deprived of important nutrients such as phosphorus, copper, molybdenum and nickel. Both the atmosphere and oceans contain almost no free oxygen, the element that now makes up more than 20% of our planet’s air and supports its animals, plants and fungi. Noll says Earth probably harbors only 1 or 2 percent of the amount of life it has today, all in the form of single-celled microbes.
Part of that scattered biome was primitively powered by photosynthesis, in which microbes use sunlight to snatch electrons from iron dissolved in seawater, allowing them to convert carbon dioxide into sugars. But the upper layers of the oceans, where light is abundant, contain only trace amounts of iron, making it difficult for even these hardy creatures to eke out a living. Those oceans were “biological deserts,” Drabon says, which is why experts often imagine that early Earth was a quiet, boring place.
Geological discoveries changed this view dramatically in the late 1980s and 1990s. In archaeological strata in South Africa, for example, geologists Donald Lowe and Gary Burley, now at Stanford University and Louisiana State University, respectively, found sand-grain-sized mineral spheres crowded into at least eight layers of rock. These tiny “globules” turned out to be solid droplets of molten rock that fell after a barrage of massive impact events. The craters from those impacts would have been eroded long ago, but thick layers of spherules show they did happen nonetheless. With astonishing frequency. Based on their studies of the stratigraphy, Lowe and Byerly estimated that between 3.5 billion and 3.2 billion years ago, an object larger than a killer dinosaur struck Earth at least once every 15 million years, much more frequently than today. They predicted that some of these asteroids may have been 350 times larger than the dinosaur killer.
From the boiling pot to heaven
Drabon, a former graduate student at Lowe’s, wondered how these Earth-shattering impacts affected the Archean biosphere. She spent years collecting rocks from a few meters directly above and below the impact layer of one of those famous events: the aforementioned S2. The two groups of rocks were of a type formed from sediments deposited on the floor of a shallow coastal sea near some rare pieces of land. The rocks beneath the impact layer, which preceded the catastrophe, were filled with smooth, black layers of ancient organic carbon, the remains of sticky mats of microbes that thrived on the sea floor before being buried, crushed and cooked by mundane geological processes. These flat, quiet layers likely accumulated over thousands of years. What was falling directly above them happened much faster.
The layer of pellets, in places as high as several stacked bed mattresses, was filled with sand and gravel, indicating a series of tsunami waves that swept and mixed with the seafloor in the hours after the impact. Thick layers of fossilized clay overlain this impact debris, and were supposed to have formed over days or weeks as the fine silt was kicked up by waves and settled on the sea floor. Above that mud was something that astonished Drabon: small hexagonal salt crystals that had been deposited as a result of the sudden evaporation of salty seawater. The crystals were a sure sign that the impact “had actually heated up the surface and started boiling some of the (ocean) water,” Drabon says.
She, Noll, and other co-authors (including Lowe) argue that anywhere from a few meters to a few tens of meters of water were heated to steam. If that had actually happened, Noll says, it would have killed “a whole lot of bacteria.” Debris thrown into the atmosphere would have blocked out the sun for months or years, making life more difficult for any surviving photosynthetic microbes.
But things would calm down quickly.
Several meters above the impact layer, the rocks are once again filled with black, carbon-rich microbial layers, perhaps denser than the layers below, indicating that “life may have flourished after the impact,” says Drabon.
She and her team suggest that such blooms were driven by several factors. The layers of rock above the impact contain high levels of phosphorus, an important nutrient used in biology to manufacture everything from DNA to cell membranes. They estimate that asteroid S2 could have transported 360 billion metric tons of extraterrestrial phosphorus into Earth’s hungry oceans. More of the element would have flowed into the seas through huge amounts of rock and silt eroded from islands hit by the tsunami.
The microbial layers above the impact are also filled with a rusty red iron mineral called siderite, which was supposedly formed from iron-rich water stirred up from the depths by turbulent tsunami waves. This flow would have contained iron-dependent photosynthetic bacteria that were already filled with phosphorus from the impact, further fueling the blooms.
Drapon also checked Ratios of heavy and light carbon isotopesor carbon atoms of different atomic masses, in the dark microbial layers above and below the collision. This can provide clues about what types of organisms were present, because different types of life absorb heavy and light carbon isotopes at different rates. You have revealed something important.
“We see a shift in carbon isotopes,” says Drabon, noting that the mix of microbes changed after the impact. “We have a new dominant metabolism” in the ocean, she says, and it likely reflects an increase in microbes that use iron to generate energy, either through photosynthesis or other pathways.
Microbe versus mammoth
This new evidence that life flourished after the S2 impact “is a really interesting discovery,” says Alexandra Davatzis, a geologist at Temple University who studies the effects of the Archean impact. She points out that other major perturbations to Earth’s environment have also improved the biosphere, such as “Earth snowballEvents believed to have occurred between 700 million and 635 million years ago. During those events, glaciers spread across much of the world’s surface and probably wiped out much of life. But when the ice finally retreated, it dumped huge amounts of nutrient-rich, glacially crushed rock into the ocean to stimulate biological recovery.
Eva Stocken, a geobiologist who studies ancient Earth at the University of St Andrews in Scotland, believes Drabon’s research could lead to further discoveries.
“There are definitely a lot of (impact) events that we missed,” she says. After all, no earth pits from that time are known to have continued to this day. The spherical layers scattered on our planet by these collisions are not guaranteed to be preserved in such ancient rocks. But as evidence of more previously unknown impacts is found, it may increase our appreciation of how these events not only severely damaged Earth’s biosphere, but also helped heal those wounds.
Stocken wonders whether S2 and other giant impacts might have fertilized life in another way, beyond what Drabon suggested. The fiery fallouts of incoming asteroids may have pulled a crucial additional nutrient, nitrogen, from the atmosphere, delivering it to the ocean in chemically reactive forms that can be absorbed by microbes. “This is something I would be excited to explore,” she says.
Simon Marchi, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, sees an important lesson in S2’s impact. He says there is an “interesting interaction” between an asteroid impact and the type of life present when it occurs. Microbes, unlike brontosaurs or mammoths, could survive extreme heat, dehydration and radiation by forming cysts or spores that persist for years. Together, microorganisms have a superior ability to resist environmental disturbances in countless other ways. If one billion microbes survive, they can replenish the entire population because they grow and divide very quickly.
“Life at the time could take the hit” from the impact of S2, Marchi says. But what if this larger asteroid had struck Earth 66 million years ago, after flowers, trees, dinosaurs, mammals, fish, and other complex life forms had evolved?
“In this kind of event, it was only possible to survive on a simple life,” he says. Instead of eliminating the dinosaurs while leaving the mammals, birds, and fish intact, the impact would have wiped out all plants and animals. “It would be a complete reset of life, going back to the bacterial level,” Marchi says.