Dinosaur Extinction Event Wasn't The First
September 6, 2012

Dinosaurs Were Not The First To Die Out 65 Million Years Ago

April Flowers for redOrbit.com - Your Universe Online

Sixty-five million years ago, the most studied mass extinction in Earth's history happened and the dinosaurs were wiped off the planet. A new study from the University of Washington, published in Palaeogeography, Palaeoclimatology, Palaeoecology, indicates that a separate extinction came shortly before that, triggered by volcanic eruptions that warmed the planet and killed life on the ocean floor.

A six-mile wide asteroid slamming into Mexico's Yucatan Peninsula is thought to have caused the dinosaur extinction event. New evidence shows that by the time of that impact, life on the seafloor — mostly species of clams and snails — was already dying out because of huge volcanic eruptions along the Deccan Plateau in what is now India.

“The eruptions started 300,000 to 200,000 years before the impact, and they may have lasted 100,000 years,” said Thomas Tobin, a UW doctoral student in Earth and space sciences.

The volcanic eruptions would have filled the prehistoric atmosphere with fine particles, called aerosols, that initially cooled the planet. More importantly, they would also have spewed carbon dioxide and other greenhouse gases to produce long-term warming that led to the first of the two mass extinctions.

“The aerosols are active on a year to 10-year time scale, while the carbon dioxide has effects on a scale of hundreds to tens of thousands of years,” Tobin said.

During the first extinction, it was primarily life on the seafloor that died, unlike the second extinction event that appeared to kill many more free-swimming species. The species in the first event, according to the study, are all species that would be recognizable on a modern beach.

The study was conducted in a fossil-rich area on Seymour Island off the Antarctic Peninsula because that area has very thick sediment deposits. The deposits, for a given interval of time, might contain 10 times more sediment as the well-known Hell Creek Formation in Montana. This gives scientists much greater detail as they try to determine what was happening at that time.

Small surface core samples from rocks and fossils in the Antarctic sediment were taken, and then a method called magnetostratigraphy, employing known changes over time in Earth´s magnetic field to determine when the fossils were deposited. The thicker sediment allowed dating to be done more precisely.

“I think the evidence we have from this location is indicative of two separate events, and also indicates that warming took place,” Tobin said.

Though there is no direct evidence yet that the first extinction event had any effect on the second, the team believes it is possible that surviving species from the first event were compromised enough that they were unable to survive the long-term environmental effects of the asteroid impact.

“It seems improbable to me that they are completely independent events,” Tobin said.

Image 2 (below): Thomas Tobin clears sand from around the fossil of a giant ammonite he found in 2009 on James Ross Island in Antarctica. Credit: UW/Tobin