World's End

Did a single asteroid bring down the dinosaurs?

by Charles E. Reineke

It came without warning some 65 million years ago, an unnamed and unwelcome visitor from deep in space. By the time it plowed into the Yucatan Peninsula, scientists estimate the six-mile-wide mass of rock and mineral was traveling as fast as 20 kilometers per second, more than 50 times the speed of sound.

This wasn't the first time the Earth had absorbed a major asteroid impact. But our planet had never experienced anything quite like this. According to a computer model developed by researchers at the Los Alamos National Laboratory, the asteroid's energy at touchdown would have equaled the force of 300 million nuclear weapons. "Immediate effects," the study found, "would include an eardrum-puncturing sonic boom, intense blinding light, severe radiation burns, a crushing blast wave, lethal balls of hot glass, winds with speeds of hundreds of kilometers per hour, and flash fires." Soon thereafter tsunamis, some perhaps 1,000 feet high, would have drowned huge swaths of land, even as the dry land burned. Within hours vast clouds of dust and soot would have blocked out the sun, plunging the entire planet into darkness and cold.

Welcome to the world's worst day, says Ken MacLeod, a professor of geological sciences at MU. "As you can imagine, slamming a six-mile wide rock into the Earth is going to be pretty destructive."

Just how destructive, however, remains a point of contention. The majority of scholars, MacLeod among them, believe that this cataclysmic impact drove almost every living thing on the planet to extinction. Most famous among the casualties were the dinosaurs, those magnificent beasts that had ruled the Earth for more than 165 million years.

A handful of other scientists, however, are equally convinced that no single event, not even one as devastating as the Yucatan strike, could explain the great culling of plant and animal species at the end of the Cretaceous Period. Led by Princeton paleontologist Gerta Keller, these scientists have spent the past decade arguing that only a host of factors, including perhaps multiple asteroid strikes, could explain the mass extinction phenomenon. What's more, they say, the fossil record shows the Yucatan strike likely predated the dinosaurs' demise by some 300,000 years.

Getting at the truth has proven difficult, chiefly because the destruction wrought by the asteroid has led to ambiguities in the geological record. The much-studied strata near the impact site itself, the Yucatan's 110-mile-wide Chicxulub (pronounced CHEEK-shoo-loob) crater is particularly resistant to conclusive analysis. Now, thanks to a chance discovery by MacLeod and an international team working on a shipboard drilling rig some 2,800 miles from the crater, the picture is getting clearer.

With his co-investigators, Donna L. Whitney of the University of Minnesota, Brian T. Huber of the Smithsonian's Museum of Natural History and Christian Koeberl of the University of Vienna, MacLeod examined a series of uniquely telling sediment samples recovered from deep within the Demerara Rise, a plateau located in one to 2-mile deep waters off the coasts of Surinam and French Guyana. Their analysis of the samples, published last January in the Geological Society of America Bulletin, provides compelling support for the idea that, for almost every species alive on that terrible day, a single asteroid brought near instantaneous extinction.

"The samples we found strongly support the single impact hypothesis," MacLeod said shortly after the study's publication. "Our samples come from very complete, expanded sections without deposits related to large, direct effects of the impact -- for example, landslides -- that can shuffle the record, so we can resolve the sequence of events well. What we see is a unique layer composed of impact-related material precisely at the level of the disappearance of many species of marine plankton that were contemporaries of the youngest dinosaurs. We do not find any sedimentological or geochemical evidence for additional impacts above or below this level, as proposed in multiple impact scenarios."

MacLeod, 43, is a hale, square-jawed man who, by appearances, might just as easily be moving earth as studying it. He describes the expedition that led to his Demerara discovery -- eight weeks aboard the 470-foot JOIDES Resolution, a former oil exploration ship refitted for scientific use -- with the gusto of a seasoned sailor. "It's an incredible boat," he says, one that has virtually everything an ocean-going earth scientist could want.

The Resolution, named in part for its sponsor, the Texas A&M-based Joint Oceanographic Institutions for Deep Earth Sampling project, is indeed an impressive vessel. Its drilling derrick, towering some 200 feet above the waterline, allows the ship to deploy about 30,000 feet of pipe. Sixteen side-thrusters keep the vessel on site even in heavy seas. After samples are brought onboard, the Resolution's seven stories of laboratory space allow scientists to conduct detailed examinations of rock and sediment cores. On any given journey, the Resolution provides berths and board for some 100 crew members and 30 scientists working on dozens of projects. For this particular cruise, Leg 207, nobody was thinking much about single impacts and mass extinctions. Instead, MacLeod says, the chief goal was to retrieve and examine fossil-rich black shale that is found in abundance beneath the Demerara Rise.

As he talks, MacLeod holds up a bag of the shales, each 2- to 3-inch piece looking something like a misshapen barbecue briquette. "The motivation for the leg was actually to investigate the Late Cretaceous greenhouse climate," he says, "the warmest conditions the Earth has seen for the last 150 200 million years. These are actually some samples right here. They are really organic rich, and contain micro-fossils that are beautifully preserved. So the idea was to get samples that were so well preserved that you could, in effect, do chemical analyses of the ancient ocean."

MacLeod freely admits his chief area of expertise is not directly involved with asteroid strikes and their geological imprints. He's more into a group of giant clams, an ancient mollusk that, MacLeod discovered early in his career, actually faded from the fossil record well before the mass extinction, an episode known to geologists and paleontologists as the "K/T boundary event."

The boundary represents the 65-million-year-old line in the geological record indicating a dramatic break between an abundance of plant and animal species living during the Cretaceous (K) Period, and the disappearance of most of these same species during Tertiary (T) Period. It was the anomalous abundance of a rare element, iridium, at the K/T boundary that led a father-and-son team of scientists to surmise that an asteroid seeded the Earth with iridium during a mass extinction event.

"I was asked to sail because, to get to these rocks that are 90 million years old, you need to drill through the rocks that are 75 to 65 million years old, which goes right through my area of expertise and interest," MacLeod says. "There is a rule on these expeditions that, if an area hasn't been cored before, you have to core it and describe it. You're not allowed to just blow off everybody else's interests to get to the rocks you are interested in."

Because sediment cores from an earlier journey to Demerara had failed to show the K/T boundary, organizers of Leg 207 didn't bother to invite researchers specializing in the boundary event. The shock was palpable, MacLeod recalls, when the Resolution's drills yielded what is arguably the most pristine K/T boundary core ever recovered.

"We thought there would be a 3- to 5-million-year missing interval," MacLeod says. "As the core kept coming up we were all saying, 'Boy, we should be hitting that hiatus pretty soon.' Then it was, 'Oh, my goodness, we're within a million years of the boundary,' and then, 'It should be in the next core!'"

And so it was. The demarcation of the boundary was obvious even before researchers began preparing the sample for analysis, MacLeod says. "It was just gorgeous."

Thanks to the lack of K/T boundary specialists on board, the first opportunity to analyze the samples fell to MacLeod, the researcher whose area of expertise most closely matched the K/T boundary time frame.

"Getting the boundary was, well, I wouldn't say luck, it was always there, and when you drill through exciting intervals you might get exciting samples," he says. "But it was unexpected."

Because of their remarkable clarity, the cores provide an unusually clear picture of the events at the time of the mass extinction. "With our samples, there just aren't many complications to confuse interpretation," MacLeod says, holding up for view the key sample itself. It's a narrow strip of sedimentary rock clearly delineated by distinct fields of color, now encased in protective plastic.

"The key fossil taxa, those that are used to identify the youngest Cretaceous and the oldest Tertiary, are present in the section right here, bracketing on either side a single layer that can be convincingly tied back to an impact."

MacLeod next explains how, at a point some two-and-a-half meters below this line, one can see the first appearance of a specific type of foraminifera, tiny organisms that he describes as "essentially an amoeba with a shell." The leader of the multi-impact hypothesis camp, he says, has argued that this species of foraminifera appeared around 300,000 years before the boundary event.

"We've now got a continuous record, from before this foraminifera appeared all the way up to the impact," MacLeod says. "The multi-impacters say that there was a first impact, one that they tie to the Chicxulub crater in the Yucatan, some 300,000 years before the boundary event. We can say, 'Look, we've got the fossil that tells us this two and half meters of core was deposited over the last 300,000 years of the Cretaceous. And you can see there is absolutely no evidence of impact until you get to this [boundary] level. And then there is a wholesale extinction.'"

MacLeod goes on to describe how the core sample from Leg 207 shows a complete deposition; that is, an uninterrupted record of sediment deposits, during the time that the alternative hypothesis says multi-impacts should have occurred. No interruptions, he says, means no place to "lose" or "hide" evidence of other impacts. There is, on the other hand, clear evidence of a colossal event where the single impact hypothesis predicts one should be. "Here is a unique layer in these sections; here is the first few thousand years after that impact," MacLeod says, once again tracing his fingers along lines in the core. "Now, going on up, we see there is no other evidence for impacts, but we have wholesale extinction right at the level of the one impact deposit in this section."

Might this be enough to convince the doubters? Initial indications are no. In a Reuters story following publication of the journal article by MacLeod's team, Gerta Keller called their findings "hyper-inflated," saying they "do not withstand close examination."

MacLeod says he's not bothered by the statement, particularly since it has not been backed by any scholarly refutation. He says he has met Keller at conferences, and finds her to be "plenty nice, a very intelligent arguer."

In an e-mail last January, one of MacLeod's coauthors, Christian Koeberl of the University of Vienna, was less sanguine. "Let me just reiterate that the 'single impact theory' story is the consensus of the scientific community after three decades of research; the only ones who are sniping are Keller and her co-workers. In the 1980s, Keller did not believe in any impact. In the 1990s, she said Chicxulub is not an impact crater. Wrong on both counts. And now she wants multiple impacts, for which there is no evidence in the stratigraphic record."

For his part, MacLeod seems content to let the boundary experts have their say, though he acknowledges he's entered something of an academic minefield.

"I guess you could say passions run pretty high," MacLeod says with a shrug. "But so far, thankfully, I haven't gotten any envelopes with white powder in them."