Before the end
Scientists find evidence of extinctions prior to impact event that killed the dinosaurs
Posted August 12, 2011
Few topics in science are as hotly debated as the extinction of the dinosaurs 65 million years ago.
However, evidence continues to pile up in favor of the theory first put forth more than 30 years ago that an asteroid impact on Earth dealt a geologically swift blow to the Earth’s dominant fauna.
But a team of National Science Foundation -funded scientists suggests there was at least one “pre-cursor” extinction under way more than 100,000 years before a giant space rock hit the planet — an event known as the K-Pg extinction for when the Cretaceous period ended and the Paleogene began.
The claim comes after four expeditions in as many years to islands off the Antarctic Peninsula, where researchers recovered well-preserved fossils from a stretch of sediments that afforded them a high-resolution look back in time. [See previous article: What killed the dinosaurs?]
“A number of different species all went extinct at one interval prior to the actual boundary,” said Thomas Tobin, a PhD student at the University of Washington in Seattle and lead author on a recently submitted paper about the pre-cursor extinction.
The creatures that went extinct were invertebrates such as snails and clams, according to Peter Ward , with the University of Washington in Seattle and co-principal investigator on the project.
“We’re seeing two very different types of extinctions going on,” Ward said, explaining that the pre-cursor extinction appeared limited to benthic organisms, critters living on the ocean floor. He believes the world underwent a rapid global warming event that eventually affected ocean circulation so that the waters stratified, making the bottom layers anoxic. No oxygen — no life.
They say the link is a major volcanic event — or series of events — that occurred roughly around the same time in an area known as the Deccan Traps in India. Formed between 60 and 68 million years ago from a series of volcanic eruptions, the Deccan Traps consist of multiple layers of solidified volcanic basalt. The massive release of volcanic gases, particularly carbon dioxide, from the eruptions may have created a greenhouse effect.
Ward said he hopes the idea of a pre-cursor extinction cools down the debate between the camps that argue greenhouse effect versus impact for the K-Pg extinction.
“I think what’s so wonderful about our finding is that it defuses this sense of how come there’s no effect from these flood basalts around this particular time when other times in Earth’s history, around other flood basalts, there’s always an effect,” Ward said.
“I think both camps should be very pleased with this result,” he added. “It finally demonstrates something predicted to have happened did happen, but it also shows that it was minor compared to how devastating the impact was.”
Tobin said it would be important, though challenging, to look for evidence of the pre-cursor extinction in similarly aged rocks elsewhere to see just how widespread the die-off might have been.
“We don’t have great evidence that this is a global phenomenon, because we’re looking at a very narrow window of time that’s difficult to resolve in a lot of places,” he said, noting that the outcrop of rocks on James Ross Island for the targeted time period was unusually expanded.
“This is the highest resolution section that you can actually collect fossils from effectively,” he said.
Nailing down the timeline was the job of Joe Kirschvink , a geobiologist at the California Institute of Technology and an expert in magnetostratigraphy, a technique that dates the layers of rocks by measuring the remnant magnetization — the polarity of the Earth’s magnetic field when the sediments were deposited.
The Earth’s magnetic field has two stable states, one when the compass points north and one when it points south. Numerous times throughout Earth’s history, the magnetic field has reversed. Though the pattern of reversals is random, scientists have very accurately mapped dozens of reversals over the last several billion years.
A particularly long period of stability occurred from about 120 to 83 million years ago, about the age of the rocks that the team was examining.
“If we can find where that long interval ends, we can start putting down pretty accurate timelines and the actual age,” Kirschvink said.
The technique relies on retrieving small rock cores with a modified, diamond-tipped chainsaw. The fine-grained magnetic minerals that end up in the sediments orient themselves with the Earth’s magnetic field at the time of deposition. It’s a bit like finding a compass buried in the rock.
“If you can do the magnetics, it suddenly gives you a very clear time picture of the age of the sediments. We haven’t had that in Antarctica before,” Kirschvink said.
The K-Pg extinction wiped out about 75 percent of species, including all non-avian dinosaurs. It is one of five major extinctions that scientists have identified over the last half-billion years, though some believe the planet is in the midst of its sixth major extinction, caused largely by climate change and degradation of ecosystems by humans.
Ward said that while the pre-cursor extinction that apparently occurred 100,000 years or so before the K-Pg boundary was relatively minor, it might have weakened the ecosystem so that the effects from the impact were more severe than they might have been. Ward has argued that the other four mass extinctions were all caused by climate change — three by global warming and one by global cooling.
The team has recently submitted a new proposal to the National Science Foundation, hoping to continue their research into the pre-cursor extinctions using the excellently exposed outcrops and preserved fossils on James Ross, Snow Hill, Vega, and Seymour islands. One question before the group is the mystery of an apparent earlier extinction — about 10 million years before the K-Pg boundary — of particular ammonites and bivalves in Antarctica relative to anywhere else in the world.
“It’s certainly exciting and interesting to resolve these sorts of questions on this timeframe,” Tobin said.
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