Ariel Anbar set out to study the geochemical properties of the rarest stable element in seawater. But now the scope of the University of Rochester scientist's work has grown to offer new insights into the link between history's largest mass extinctions and chemical irregularities found in parts of the earth's crust. His research appears in the September 13 issue of Science.
Anbar's findings demonstrate that most of the sedimentary layers of the earth's crust that contain elevated levels of the rare element iridium -- layers which have been linked by several researchers to periods of mass extinction -- can be explained by normal weathering of the continents and geochemical cycling in the world's oceans rather than collisions with iridium-rich meteorites. But the one iridium-rich sediment that has attracted the most attention -- the one linked to the extinction of the dinosaurs 65 million years ago -- has been found to contain 1,000 times more iridium than all of the world's oceans.
"It's hard to imagine a terrestrial source so large," says Anbar, an assistant professor of earth and environmental sciences and chemistry at Rochester. "This strongly suggests that this particular band of iridium has extraterrestrial origins in the form of an exceptionally large meteorite -- a finding that is consistent with theories linking a meteorite impact to the extinction of the dinosaurs."
Anbar completed the research while a Ph.D. student at the California Institute of Technology with faculty members G.J. Wasserburg and Dmitri Papanastassiou and the Swedish Museum of Natural History's Per Andersson. Armed with a sensitive new method he developed for measuring iridium in natural materials and a novel theory that the thick iridium-enriched layer in the earth's crust could be the result of delayed settling from seawater, Anbar collected water from the Pacific Ocean and the Baltic Sea for the daunting task of studying the properties of iridium dissolved in natural waters -- a task that the extreme scarcity of the element in the world's oceans had previously made prohibitively difficult. (The ratio of iridium to water in the ocean is about the same as the ratio of a single tablespoonful of water to all of the water in Lake Erie.) Among Anbar's eventual findings was that iridium's "residence time" -- a measure of the rate of an element's natural removal from the oceans, much as half-life is a measure of the rate at which radioactive materials decay -- is 2,000 to 20,000 years.
"If a large amount of iridium from a meteorite dissolved in the oceans, it could very well have taken as long as 100,000 years for the contamination to wash out, and the sediments deposited during that time would reflect the oceans' elevated concentrations," Anbar says.
For over 15 years, scientists have regarded the inch-thick layer of unusually iridium-rich sediments in the earth's crust as the key to understanding the extinction of the dinosaurs, but they have often disagreed over whether the unusual deposit points to meteorites or volcanoes as the main cause of the mysterious mass extinction 65 million years ago. Anbar's finding that it can take many thousands of years for the iridium from a large meteorite to fall out of seawater robs proponents of the volcano theory of one of their key arguments: that the iridium-rich layer, which was laid down over a period of 100,000 years, is too thick to be the result of a geologically brief meteorite impact.
The unusual iridium deposit in the earth's crust lies in a layer of sediments known as the K-T boundary, which represents the chronological boundary between the Cretaceous and Tertiary eras, when the dinosaurs died out. With an average concentration of only 50 parts per trillion, iridium is exceedingly rare throughout most of the earth's crust -- except at the K-T boundary, where concentrations are roughly 200 times greater.
The connection between the iridium anomaly at the K-T boundary and the extinction of the dinosaurs was first made in 1980 by a group at Berkeley led by Nobel laureate Luis Alvarez and his son Walter. Since iridium is rare in the earth's crust but abundant in meteorites, the Alvarez group took the presence of the element at the K-T boundary as evidence that the impact of a massive meteorite led to the death of the dinosaurs.
The Alvarez theory was initially derided because there was no mechanism proposed to explain how a meteorite could cause such a mass extinction and little was known about elements like iridium, which Anbar describes as "the final frontier of the periodic table." A competing theory arose, which held that since iridium is also present in high concentrations deep in the earth's interior, the iridium at the K-T boundary resulted from an outburst of volcanic activity that killed the dinosaurs.
But Anbar says that more recent discoveries have tended to discredit the volcano theory in favor of the meteorite theory. Shocked quartz, which is generally thought to be indicative of a forceful impact, has been found at the K-T boundary, and a 125- mile-wide impact crater found in Mexico that dates to 65 million years ago provided further compelling support. Anbar believes his findings are likely to further thin the ranks of scientists still resisting the evidence of a major impact at the K-T boundary.
Anbar intends to continue studying the geochemistry of iridium and events at the K-T boundary using a new generation of ultra-sensitive analytical tools that will soon be available at Rochester. His previous research was funded by NASA, the National Science Foundation, the U.S. Department of Energy and the Swedish Meteorological and Hydrological Institute.
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