Antarctic Fish Fossils Fuel Asteroid Impact Debate
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A bed of fossilized fish bones recently discovered in Antarctica may be the first remains of direct victims from the catastrophic event 65 million years ago that wiped out the dinosaurs and 70 percent of the world's species.
Purdue University paleontologist William Zinsmeister discovered the fossil "horizon of death" dating from the boundary of the Cretaceous/Tertiary (K/T) geologic periods 65 million years ago, while on a National Science Foundation (NSF)funded expedition early this year to Seymour Island near the Antarctic Peninsula. The bone bed, covering more than 12 square kilometers of the island, rests directly above a layer of iridium, an element that is rare on Earth but is a common signature of meteorite impact. Zinsmeister is presenting his results November 9 at the Geological Society of America meeting in New Orleans.
A widely believed theory holds that a giant asteroid hit the Earth and set off the mass extinctions at the K/T boundary. The impact site most favored by scientists is a large crater in Mexico's Yucatan.
"The fish bones are an exciting discovery that should help us better understand environmental changes at a crucial time in Earth history -- the end of the Cretaceous," commented Scott Borg, director of NSF's Antarctic geology and geophysics program. "Seymour Island is an important site for understanding what happened on a global scale to the environment at that time. When the sediments containing the fish bones were deposited, the island was located in the far southern latitudes - just like today -- and very far from the probable impact site in the Yucatan. The polar region has a very different atmospheric circulation than the tropics or temperate regions. So, we can compare the excellent exposures of Seymour Island's K/T fossils to sites of the same age at other latitudes to construct a fuller picture."
In the past, Zinsmeister had argued -- based on 20 years of research on late Cretaceous marine fossils in Antarctica -- that the southernmost continent's fossil record did not support the asteroid-extinction theory. Even after finding the bed of fish bones, he thinks the picture may be more complicated, and that change was more prolonged than sudden.
"The fossil record in Antarctica suggests that the final extinction event wasn't immediate, but rather occurred over a period of time up to 500,000 years," he said. "We actually see a decrease in the global diversity of life starting between 8 and 10 million years before the impact."
He points out that two important marine animals -- the ammonites, spiralled molluscs related to the modern nautilus, and the inoceramid bivalves, a clam relative -- began disappearing from the fossil record about 10 million years before the K/T boundary.
"I think the events at the end of the Cretaceous were not due to a single catastrophic event, but represent a conjunction of events -- climatic change, maybe a period of volcanism, and then, ultimately, a major impact or catastrophic event," he said.
"The reigning idea is that the Earth had a bad day from the impact, but I think it had a series of bad days," Zinsmeister said. "The Earth's biosphere was already stressed to a critical point, and the impact could have pushed it over the edge, causing local catastrophes such as the fish kill in the Antarctic."
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