Plankton research adds to controversy over cause, chronology of dinosaur extinction

A recent study led by geosciences professor Gerta Keller challenges the most commonly-held belief for the mass-extinction of dinosaurs at the end of the Cretaceous period 65 million years ago.

The prevailing extinction theory — part of an ongoing 30-year controversy — holds that the dinosaurs’ destruction was caused by the Chicxulub impact, a crater buried underneath the Yucatan Peninsula in Mexico.

However, Keller and her team dated a specific species of plankton, planktonic Foraminifera, that has trails nearly half a million years old, and found chronological links between the mass extinction and volcanic eruptions of the Deccan Plateau of western India.

Keller’s close analysis of Deccan volcanism indicates that the formation of the Deccan Traps — layers of solidified flood basalt that resulted from the eruptions — caused a huge temperature drop, deteriorating environmental conditions and acid rain.

These effects altered the climate and put a strain on biodiversity, cutting off the dinosaurs’ food source and eventually killing them, Keller said.

The species of plankton that Keller studied is “very sensitive to environmental changes in temperature oxygen, salinity, nutrients and toxins,” she said. “They not only record the environmental changes over the past 150 million years but are also our best measure for reconstructing the climatic history and the conditions that led to the [dinosaurs’] mass extinction.”

Keller also wrote a paper with her former student Brian Gertsch GS ’10 investigating the same species of plankton in Meghalaya, a region in northeastern India. Gertsch, now a postdoctoral researcher at MIT, spent several years conducting lab analyses and hypothesizing about the environmental conditions that led to the extinction of the dinosaurs.

“In Meghalaya, blooms of planktonic foraminifera and high chemical weathering indices are indicative of the very high stress conditions and correlate well with the main phase two of Deccan volcanism,” Gertsch explained. “Our study clearly shows that Deccan volcanic activity ... triggered increasingly stressful environmental conditions, both marine and continental.

As such, it has become increasingly clear that Deccan volcanism is a major factor in understanding the disappearance of dinosaurs, he said.

Keller spent over 15 years trying to confirm the commonly-held meteorite hypothesis, but instead found increasing evidence against it.

During an undergraduate field trip to Mexico in 2000, Keller’s students discovered layers of impact spherules — tiny rock glass particles that resulted from the Chicxulub meteor impact — that were over a dozen meters below the mass extinction horizon, suggesting that the meteorite hit before the mass dinosaur extinction.

Indeed, further analysis and several other trips to different regions near the Yucatan site confirmed that the meteorite hit about 300,000 years before the dinosaurs died out. Later results “spectacularly confirmed the earlier findings that the Chicxulub impact predates the mass extinction,” Keller said. The only other natural disaster that occurred during the dinosaurs’ extinction period was the Deccan volcanism on the other side of the world, which prompted Keller’s research in India.

The results of Keller’s study are “far-reaching,” she said.

“The implications go beyond the mass extinction of the dinosaurs at the end of the Cretaceous,” she said. “Large volcanic eruptions occurred at four of the five major mass extinctions in Earth’s history; our studies in India show that volcanism can cause mass extinctions, and it is now up to other researchers to conduct similar studies for all other mass extinctions.”

Keller’s project in India was recently approved to continue for another two years. She has already begun evaluating the detrimental effects of Deccan volcanism on a global scale.

She said she hopes to find the underlying causes of the climate and environmental changes that led to the mass extinction, as well as the primary “killing mechanism” for the dinosaurs and the timeline of their extinction.