The Washington Post

Earth a hothouse for 100,000 years after dinosaur calamity, study says

- BY JOEL ACHENBACH joel.achenbach@washpost.com More at washington­post.com/ news/ speaking-of-science

On a very bad day 66 million years ago, a mountain-sized object from space slammed into the Earth, initiating a cascade of calamities that eradicated threefourt­hs of the species on the planet, including the non-avian dinosaurs. The buried remnants of the 125-mile-wide crater have been identified on the Yucatán Peninsula and in the Gulf of Mexico. Scientists have long theorized that an initial pulse of heat was followed by a devastatin­g global winter. After that, as carbon dioxide in the atmosphere surged, the planet became a hothouse.

A new study published Thursday in the journal Science has produced hard data to support that global warming hypothesis, and it may have unnerving implicatio­ns for the world we live in today. The effects of the Chicxulub impact, named for a Yucatan town, produced 5 degrees Celsius (9 degrees Fahrenheit) average warming in a subtropica­l sea, and this heating persisted for 100,000 years, the researcher­s concluded.

“This is crocodiles at the poles and large areas of the tropics uninhabita­ble on land,” explained lead author Ken MacLeod, a University of Missouri paleontolo­gist.

The study suggests that even a relatively brief pulse of CO2 can have a lingering effect. That is relevant today, given many countries’ massive greenhouse-gas emissions, which are creating a spike in atmospheri­c carbon dioxide and associated global warming.

“The cascading implicatio­n of our finding is that carbon dioxide loading would have occurred for just maybe a decade, and the greenhouse warming persisted for 100,000 years,” MacLeod said. “Even if we go back to 1850 levels of CO2 emissions today, we’re locked into 100,000 years of the Earth responding to the CO2 we’ve already put in.”

The research is based on fish debris — bones, teeth, scales — retrieved from an outcroppin­g in Tunisia known as El Kef. It’s a famous site, featuring a geological formation with sedimentar­y layers from the end of the Cretaceous period (when dinosaurs roamed the Earth) and the start of the Paleogene period. This is known now to scientists as the K/Pg boundary.

The fish debris serves as a kind of thermomete­r, said Page Quinton, who began the work as a doctoral student with MacLeod and is now a professor at the State University of New York at Potsdam. The sand-sized fragments of fish contain isotopes of oxygen — atoms that have different numbers of neutrons and different atomic weights. Water temperatur­e affects the relative abundance of those isotopes. When the fragments show a shift in the isotopic ratio, that signals a change in temperatur­e, Quinton said.

Preliminar­y investigat­ion of some samples from El Kef produced a “wow” moment four years ago, with clear indication of long-term global warming, she said. The researcher­s then obtained more samples and continued scrutinizi­ng the debris, and the pattern initially detected held up over time.

“We’re providing the first empirical evidence that there’s actually warming after the impact,” she noted.

It appears the dinosaurs and much of life on Earth died out in a triple whammy, or maybe a quadruple whammy, depending on how you’re counting. First came the impact itself, with shock waves and tsunamis. In the minutes and hours that followed, the frictional heating from debris falling back into the atmosphere was so intense that “the sky became an oven,” MacLeod said. Wildfires broke out globally.

Then came years of cold and darkness as sulfates, dust and soot in the upper atmosphere blocked light from the sun.

“The first six months, it was almost a blackout,” said NASA planetary scientist Adriana Ocampo, who was not involved in the new study. If not for the warmth retained in Earth’s vast oceans, she said, “our planet would have frozen.”

MacLeod painted a bleak picture: “Anything that’s not killed by the thermal heat pulse likely had to deal with years of very little, if any, vegetation, and anything that survived that then had to survive 100,000 years of quite substantia­l greenhouse conditions.”

This is a contentiou­s scientific field, and the new paper quickly generated pushback from Gerta Keller, a Princeton geologist who has long argued that the end-Cretaceous mass extinction was triggered by volcanism in India — a huge flood of basaltic lava that created a vast geological formation known as the Deccan Traps.

Keller, who read the study in advance of publicatio­n, said her interpreta­tion of the El Kef formation and the sedimentat­ion rate that created it suggests a much longer period of greenhouse warming, about 500,000 years. That could not have been caused by a single injection of carbon dioxide, she said, but would be consistent with a protracted volcanic era.

Paul Renne, a geologist with the Berkeley Geochronol­ogy Center who has argued that the shock wave from the Chicxulub impact may have intensifie­d the Deccan Traps volcanism, said in an email: “It is most extraordin­ary that the authors don’t even mention volcanism. That is really bizarre.”

But Brian Huber, a research geologist at the Smithsonia­n's National Museum of Natural History, said he was impressed by the new report. “It’s a pretty tight study that’s telling us a pretty important story about the longevity of CO2,” he said. “The lesson is here for us with regard to future warming, and what burning fossil fuels at the rate we’re doing is doing to the atmosphere.”

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