Published by the American Geological Institute
of the Earth Sciences
In back-to-back reports, University of California, San Diego, scientists have shown it is possible to wring out the chemical signatures of Earth’s atmosphere from Precambrian stone. As they hunt down isotopic anomalies never before seen in Earth’s rock record, the scientists are excited about the future prospects of their technique.
“People have been looking for a way to track the evolution of oxygen
and the origin of life for at least 50 years and this is the first time
there’s ever been a quantitative way to do that,” says Mark Thiemens, dean
of physical sciences at UCSD.
Until now, analysis of ancient atmospheric signatures depended on the depth of glacial ice. The oldest known ice core comes from Vostok, Antarctica, where ice 3,000 meters deep provides some 400,000 years of history. But a core extending beyond 300,000 years becomes highly compressed at its base, making information difficult to extract, the scientists say.
|Turning to terrestrial rocks offers an easier, and sometimes warmer,
method for reading the longer story about Earth’s air. Reporting in the
Aug. 4 Science, James Farquhar, Huiming Bao and Thiemens pried atmospheric
secrets out of 3.8 billion-year-old rocks from Greenland; 3.4 billion-year-old
rocks from Australia and South Africa; and 700 million-year-old rocks from
Australia, China, Africa, Canada and the United States.
Farquhar and colleagues found multiple isotope variations of a kind only recently considered possible in terrestrial solids: anomalous sulfur ratios incorporated into the rocks after the rocks oxidize in air. Their discovery pointed to a profound change in the sulfur cycle between 2.1 billion and 2.5 billion years ago, a time when Earth’s atmosphere is known to have shifted from primitive, oxygen-poor conditions to an oxygen-rich environment more like today’s.
Huiming Bao hard at work at the University of California,
San Diego. Photo courtesy of James Farquhar.