What killed the dinosaurs?
Scientists believe asteroid theory may not tell the whole story of KT extinction
Posted September 12, 2008
Dinosaurs, asteroids and death — mass extinctions don’t get more exciting than the most recent one of 65 million years ago. That’s when an asteroid slammed into the Earth, wiping out non-avian dinosaurs, but assuring them a starring role in a string of Hollywood epics.
Known as the Alvarez hypothesis for the father-and-son team who published the paper in the journal Science in 1980, the theory suggests that the impact sent up a cloud of particles into the atmosphere, drastically cutting down solar energy and hence photosynthesis. Plants and marine algae withered and died, followed by the herbivores that munched on them, and reverberating up the food chain to the predators that depended on the plant-loving animals.
Makes a great movie, and case closed, right? Maybe not.
A science team led by paleontologist Peter Ward with the University of Washington in Seattle doesn’t dispute that an asteroid scored a lucky shot on this planet at the end of the Cretaceous Period. The evidence is overwhelming, with sedimentary rock layers at the 65-million-year mark revealing a concentration of iridium many times greater than normal — the only source possible being from an outer space rock.
What Ward and his colleagues do suggest is that the planet was already on a downward spiral, an overheated greenhouse that was snuffing out life before the asteroid arrived.
In boxing parlance, the asteroid didn’t deliver a first-round knockout but instead arrived at the match in round 12, after the planet’s biological life had taken a series of environmental blows. The extraterrestrial rock merely offered the final knockout punch, the coup de grace.
“The evidence for there being an impact near the K-T boundary is extremely good,” conceded Joe Kirschvink, a geobiologist at the California Institute of Technology and principal investigator for an expedition with Ward and Eric Steig of the University of Washington to the Antarctic Peninsula this coming field season. “But that’s not the whole story.”
A murder mystery
The scientists instead point to a major volcanic event that occurred roughly around the same time as the so-called Cretaceous-Tertiary extinction event, also known as the K-T extinction boundary.
The Deccan Traps in India represent one of the largest volcanic features on Earth. Formed between 60 and 68 millions years ago from a series of volcanic eruptions, the Deccan Traps consist of multiple layers of solidified flood basalt, a large area covered by basalt lava that today is about 2,000 meters thick, encompassing an area of 500,000 square kilometers. (The term traps comes from the Swedish word for stairs, referring to the step-like hills forming the landscape of the region.)
The massive release of volcanic gases, particularly carbon dioxide, from the eruptions may have created a greenhouse effect, heating up the planet and stressing the giant dinosaurs and many other species to extinction.
“It may have been one of those weird instances where the environment was deteriorating drastically because of the flood basalts, and then this impact bopped it all and did them in,” Kirschvink said. “In science, you try to find underlying principles that can explain many things. The impact hypothesis was once thought to be a universal explanation for all of these mass extinctions.
“It’s one of these murder mysteries that scientists love to get tangled up in,” he added.
Five major extinctions have occurred in the past 500 million years — the Ordovician, the Devonian, the Permian, the Triassic and the Cretaceous — as well as many minor ones. All of the big five except the K-T extinction appear to be greenhouse extinctions, according to Ward.
“How can we say all of these other mass extinctions are associated with flood basalts — big flood basalts — and then right across K-T we have one of the biggest flood basalts in history of the planet, the Deccan Traps, but this one we say had no effect,” he said.
“In other words, flood basalts cause extinctions every time except during the K-T. Hmm. Something is not right about this picture.”
Clues at high latitudes
Ward has devoted the last 15 years of his career to studying greenhouse extinctions. He recently published a book, “Under a Green Sky,” that details the competing hypotheses for these massive die offs. The Alvarez hypothesis held sway for many years, and Ward’s own work in the lower latitudes studying fossilized ammonites, shelled cephalopods that thrived prior to 65 million years ago, backed up the impact theory.
To prove that K-T extinction was already under way before the Alvarez asteroid hit, the scientists need evidence that species diversity was already in decline. Such evidence of extinctions leading up to the K-T boundary may exist at the higher and colder latitudes, which are more sensitive to climate change, according to the scientists.
Noted Tom Wagner, program manager of Earth Sciences at the National Science Foundation’s Office of Polar Programs, “The Earth sciences advance through discovery, and with so little of it explored, Antarctica consistently offers new finds.
“But its polar position is important, too. In this case, it will help characterize the extinction process,” he added. “Did extinction spread gradually across the globe? Were creatures in polar oceans affected differently than in the tropics? Are the poles a refuge or the first to go? Questions like this can only be answered by research in Antarctica.”
“You really need to know what’s going on in the Southern Ocean to really understand what’s happening globally, to tie the rocks there with the rest of the world,” Kirschvink explained.
And that’s the plan. The scientists will join colleagues from Argentina at James Ross and Seymour islands to collect samples for analysis in the lab. Kirschvink visited Seymour last year on something of a reconnaissance mission at the invitation of Eduardo Olivero, a paleontologist with the Argentine Antarctic program.
“Seymour Island section [of exposed rock] has the best time resolution I’ve ever seen for this K-T extinction boundary,” Kirschvink said. One meter represents about 4,000 years in geologic time.
“It gives us the most incredible ability to go in and get high-resolution data with what was going on with this extinction than anywhere else I’ve seen,” he explained. “Not only that, the rocks are completely unheated, almost unburied, almost no alteration. … It’s an incredibly exciting sequence to work through. And fossils? You just whack on a few of these things, and boom, out come ammonites.”
The scientists, along with graduate students from both countries, will spend about a month from mid-February to March digging out those fossils for analysis in this first field season. “We’re going to do some of the most horribly boring work on the face of the continent, any continent,” Ward said.
Using a diamond-tipped coring drill, driven by a modified chainsaw, the scientists will extract small cores of rock, about 10 centimeters deep and about three centimeters in diameter, Ward explained. They will measure the spatial orientation of the cores with sun and magnetic compasses, and a bubble level, for the paleogmagnetic work. They will also record the cores’ position in the rock sequence relative to the mass extinction layer to understand the stratigraphy.
Back at the lab, the scientists will measure the direction of its ancient magnetism, which can tell them what position the continent was in 65 million years ago. “More importantly, the Earth’s magnetic field reverses back and forth in a random pattern, and, as the history of these ‘geomagnetic reversals’ is well known during this time interval, the pattern of magnetic reversals can provide accurate age constraints on the sediments,” Kirschvink said.
It will take at least a year before there are any results, Ward said. “The field work is the fun part and the lab work is slow.”
Aside from Kirschvink’s trip to Seymour, both scientists are new to the U.S. Antarctic Program. “One of the USAP’s goals is broadening participation,” Wagner said.
The project is not only about solving a murder mystery for Ward. He sees disturbing parallels between the past greenhouse extinctions and today’s climate change. Previous studies have suggested as much as 25 percent of the world’s species today may vanish before the end of the century unless greenhouse gases are curtailed.
“I’m personally worried about sea level change,” Ward said. “Even a one-meter rise is going to be a disaster in the world. … It’s a good time for me to go back to the first [mass extinction] I ever studied after seeing what the greenhouse [extinctions] can do.”
NSF-funded research in this story: Peter Ward and Eric Steig, University of Washington in Seattle, and Joseph Kirschvink, California Institute of Technology, Award No. 0739541.
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