Deep sea impact
Underwater geology provides answers to mass extinction questions
What do dinosaurs have in common with microscopic life forms in the ocean? One giant meteorite.
The “single impact” theory states that a massive meteorite crashed into Earth 65 million years ago and caused mass extinction on land and in the sea. Now, researchers led by Ken MacLeod, associate professor of geological sciences at Mizzou, have found new evidence to bolster that theory.
The group collected sediment samples from the Demerara Rise in the Atlantic Ocean. The samples contain a layer representing the Cretaceous-Tertiary (K-T) boundary, which separates the eras before and after the meteorite hit earth. Before the impact, the sediment contains anywhere from 60 to 90 percent seashell fossils. After the K-T boundary, that number drops to low single digits. The theory is that the meteorite’s impact — plus the giant tsunamis (hundreds of meters high) and atmospheric pollution that followed — wiped out life on an unprecedented scale. MacLeod’s samples fit that theory perfectly.
“It dramatically illustrates how truly catastrophic the event was,” he says. "It’s simple, Discovery Channel layer-cake geology.”
Science and serendipity
In the late 1970s, Nobel Prize winner Louis Alaverez and his son, Walter, went to Italy looking for geological evidence of a supernova. Instead, they found high levels of iridium, an element found in abundance in meteorites. In 1980, the researchers proposed the impact theory, which was met with skepticism until the discovery of the 110-mile-wide Chicxulub crater on Mexico’s Yucatan peninsula.
Likewise, MacLeod and his colleagues came across this new evidence through serendipity while taking part in an Ocean Drilling Program cruise in 2002. They weren’t looking for it, but when they saw core samples from deep beneath the sea, the unique band that marks the K-T boundary was clear to the naked eye. "We found five copies of it," MacLeod says. "It’s always right at the same level.”
Sediment tells a story in time. Each band represents a different era in the Earth’s history. The K-T boundary shows limited signs of living things. When combined with the discovery of the crater and the high levels of iridium, it paints a picture of a wholesale turnover of life.
Support in the sediment
“This layer has been found throughout the world: in New Zealand, the central Pacific, multiple European sites, in dinosaur-bearing rocks in North Dakota, Alaska and New Mexico,” MacLeod says. “But this is an unusually clean copy of it.”
The sediment samples from the Demerara Rise are so clean in part because of their moderate distance from the impact crater, about 2,800 miles. Samples closer to the site were affected by the tsunamis, landslides and water sloshing in the Gulf of Mexico. However, if you get too far away, the K-T layer becomes too thin to study. The samples from Demerara are the ideal size for analysis. Additionally, the site’s location around the bend of the Yucatan may have shielded it from tsunamis.
Most K-T researchers agree about the single impact theory, MacLeod says. However, one research group at Princeton disputes the timing of the event and also whether the extinctions came from one impact or several. Likewise, some paleontologists doubt the tie between sea and land events.
However, that’s the healthy spirit of skepticism in science, and MacLeod’s research adds new, clear and quantifiable evidence in support of the single impact theory. As for the debate, MacLeod says it has one added benefit: “As long as there’s a controversy, the introductions to papers practically write themselves.”
The sediment sample at left shows evidence of mass extinction, says MU geologist Ken MacLeod. Below the unique band in the middle, life was abundant in the form of microfossils (shown above). In the band, which represents the time after a giant meteorite hit Earth, life is much more scarce.