NEWS RELEASE, 4/22/99
New evidence links mass extinction with massive
eruptions that split Pangea supercontinent and created the Atlantic 200
million years ago
By Robert Sanders, Public Affairs
BERKELEY--Hundreds of basalt outcroppings rimming the Atlantic Ocean are actually the remnants of a single huge volcanic eruption some 200 million years ago that may have triggered a large extinction of life at the end of the Triassic period, according to a report in this week's issue of Science.
A team of researchers led by Paul R. Renne, director of the Berkeley Geochronology Center and an adjunct associate professor of geology and geophysics at the University of California, Berkeley, concluded that these basalt dikes, sills and lavas, dispersed from the New Jersey Palisades and the Brazilian Amazon to Spain and West Africa, resulted from the most extensive pulse of magma eruptions known to date.
At the time these areas were near one another in the center of a supercontinent known as Pangea. The eruptions began a process that drove the land mass apart to create the Atlantic Ocean, at the same time dispersing evidence of the eruption widely on the margins of four continents.
The large flows of magma, which the researchers dubbed the Central Atlantic Magmatic Province (CAMP), came from the Earth's upper mantle and covered about seven million square kilometers over a geologically short period of a few million years.
The beginning of the event matches within 20,000 years a global extinction at the end of the Triassic period and the beginning of the Jurassic. During this mass extinction, about half of all marine species, mostly the ammonoid and bivalve mollusks, died out, while on land several families of reptiles disappeared. Paleontologists regard the extinction as one of the most deadly in the 600-million-year history of multi-celled life. Many believe these changes set the stage for the rise of dinosaurs in the Jurassic.
The close correspondence between the beginning of the CAMP eruptions and the mass extinction suggests that the global disruption caused by a long series of volcanic eruptions could have set off or at least exacerbated the die-off.
"This is the best example of flood basalts associated with an extinction," Renne said. "Some of the best stratigraphic evidence of the mass extinction occurs in exactly the same site in which you find the flood basalts - they sit right smack on top of one another."
The likely scenario is that, as the magma surfaced in the form of volcanoes over a large area of Pangea, noxious aerosols and greenhouse gases disrupted the global climate and caused the extinction of a large number of species. The basalts would have taken more than a million years to cover the area and, thus, would overlay sedimentary evidence of the extinction.
In 1995, a group led by Renne attributed another mass extinction, the one that occurred at the end of the Permian and beginning of the Triassic, to a similar magma flood in Siberia 250 million years ago. A third mass extinction, the well-known event associated with the demise of the dinosaurs at the boundary between the Cretaceous and Tertiary periods, has been linked to a large volcanic flood that produced the Deccan Traps in India.
"This is still one of the most intriguing issues in geology, the relationship between extinctions and cataclysms such as magma floods or asteroid impacts," he said. "What we now have is another piece of evidence that shows there was a relationship between flood basalts and biologic crises."
The article detailing the extent of the CAMP basalts appears in the April 23 issue of Science. Renne's coauthors include postdoctoral fellow Andrea Marzoli, formerly of the Berkeley Geochronology Center and now a research scientist at the University of Geneva, Switzerland; Marcia Ernesto, a paleomagnetist at the University of São Paulo; and geochemist Enzo M. Piccirillo of the University of Trieste, Italy.
Though many of these basalt outcrops, such as the well-studied Palisades sill, had been dated to this general time period, until now no one had realized the extent of the eruptions.
One reason the extent of the magma flow had not been recognized before is that much of the evidence for the event is in temperate or tropical forests, where outcroppings are obscured by vegetation and are heavily eroded. However, recent field studies by French geologists in West Africa and American geologists in the southeastern U.S. have provided new information on the extent of basalt dikes and sills around the Atlantic margins.
Sills are what remain after magma intrudes into horizontal underground fissures, then cools and is subsequently exposed. Dikes are the remnants of magma that flowed into vertical fractures. Lava is magma that broke through the Earth's crust and flowed on the surface before cooling.
The researchers in this study used the argon-argon (40Ar/39Ar) method to date basalt lava flows, dikes and sills in Brazil, many of them 2,000 kilometers (1,300 miles) inland from the coast, and correlated them with the ages of known formations up and down the East Coast of the United States, in southwestern Spain, throughout West Africa, and on the northern coast of South America. All proved to be from the same era 200 million years ago, suggesting they have a common origin. Paleomagnetic data from the Brazilian rocks confirmed the date.
While other episodes of flood basalts that have been correlated with mass extinctions - the Siberian Traps and the Deccan Traps, for example - are often several thousand feet thick, the preserved CAMP lava flow piles are only about 1,000 feet thick. The researchers estimate that in all the CAMP magmatism extended over 7 million square kilometers in a period of a few million years, peaking about 200 million years ago. The total volume of magma is estimated at 2 million cubic kilometers (500,000 cubic miles).
"While the volume is comparable to other flood basalt events, this one spread over an area far larger than the others," Renne said. "This system could have had many more volcanoes than the others."
As for the cause, Renne favors a theory put forward 10 years ago by UC Berkeley geophysics professor and chair Mark Richards. Richards ascribes flood basalts to a buoyant magma plume rising through the viscous mantle from a spot near the boundary between the mantle and the molten iron core, eventually cracking the surface in the middle of the continent. The resultant rift zone pushes the land masses apart to create an ocean, with an island chain forming over the hot spot and recording the continual movement of the sea floor away from the rift zone.
Though numerous hot spot island chains have been identified - the Hawaiian Islands in the Pacific, Iceland in the north Atlantic - no such chain has been associated with the break up of Pangea 200 million years ago, Renne noted.
Two other coauthors of the paper are geochemist Giuliano Bellieni of the University of Padova, Italy, and geochemical technician Angelo De Min of the University of Trieste.
The research is supported by the National Science Foundation, several Brazilian and Italian funding agencies and the Ann and Gordon Getty Foundation.
Paul Renne can be reached at (510) 644-9200 or (510) 644-1350, or via email at email@example.com.
NOTE: Renne will NOT be available between 2 p.m. PDT Thursday, April 22, and 8 a.m. Monday, April 26. Reporters seeking an interview should call before 2 p.m. Thursday.
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