COLLEGE PARK, Md. - The history of life on Earth is closely linked to the appearance of oxygen in the atmosphere, which scientists think first occurred in significant amounts during a "Great Oxidation Event" some 2.4 billion years ago. However, until now little was known of environmental changes prior to this event. New findings by two teams of scientists - one led by geologists from the University of Maryland and the other by Arizona State researchers - indicate that significant oxidative changes were occurring in the oceans and atmosphere before the Great Oxidation Event.
The researchers analyzed layers of sedimentary rock in a 3000 ft-long core sample from the Hamersley Basin in Western Australia and found evidence that a small but significant amount of oxygen was present in the oceans and possibly Earth's atmosphere 2.5 billion years ago. Their data also suggest that oxygen was nearly undetectable just before that time. Two papers outlining these findings will be published in the September 28 issue of the journal Science.
"Together, these papers provide compelling evidence for a shift in the oxidation state of the surface ocean 50 million years before the Great Oxidation Event," said Alan Jay Kaufman, Associate Professor of Geochemistry at the University of Maryland. "We believe that these findings are a significant step in our understanding of the oxygenation of Earth because they link changes in the environment with that of the biosphere."
Kaufman and graduate student David Johnston led the Maryland research team, and Kaufman was also a member of the second Arizona State University team led by biogeochemist Ariel Anbar. Also involved in the companion papers were scientists from the University of Washington, the University of California in Riverside and the University of Alberta. The work was supported by the Astrobiology Drilling Program of the NASA Astrobiology Institute and by the National Science Foundation, with logistical support from the Geological Survey of Western Australia.
Prelude to a Big Jump in O2
Ancient sedimentary rocks contain evidence of oxidation and other chemical reactions that took place in the oceans and atmosphere as the rocks formed. For example, rock formed from sediments deposited in the shallow waters of an ancient ocean contains chemical clues to the conditions of that water and the air above it. Using sophisticated analytical tools called mass spectrometers scientists can detect and read these clues.
"The first difficulty to unearthing such ancient information is that there are only a few places on Earth where it's possible to find unaltered rock formed in the first half of our planet's history," Kaufman said. "The Hamersley Basin in Western Australia is one of those special places."
Kaufman, Johnston, and Maryland colleagues Associate Professor James Farquhar and Undergraduate Research Assistant Andrew Masterson, as well as other members of the University of Maryland-led team conducted high-resolution geochemical analyses of organic-rich shale and carbonate in the core drilled down through the Hamersley Basin sediments looking for evidence provided by sulfur isotopes trapped in their rock samples.
University of Maryland geologists have led the development of methods to study and document links between sulfur isotopes and the evolution of Earth's oceans and atmosphere. In the current work, the Maryland-led team made the first ever use of the rarest isotope of sulfur to find evidence of changes in both the oxidation state of the surface ocean and the composition of the atmosphere some 2.5 billion years ago, just prior to the end of the Archean Eon (~3.9 - 2.5 billion years ago), a period during which microbial life on Earth arose and diversified.
The "record of sulfur isotopes captures the widespread and possibly permanent activation of the oxidative sulfur cycle, for perhaps the first time in Earth history," they write in Science. The researchers correlated their findings in core samples from northwestern Australia with samples from equivalent geologic strata from South Africa. The findings from the different samples were consistent, suggesting that changes in the sulfur cycle - recorded in two broadly separated marine successions - were global in scope and linked to atmospheric evolution. The data suggest that oxygenation of the surface ocean preceded pervasive and persistent atmospheric oxygenation by 50 million years or more.
The papers are:
A. J. Kaufman, D. T. Johnston, J. Farquhar, A. L. Masterson, T. W. Lyons, S. Bates, A. D. Anbar, G. L. Arnold, J. Garvin and R. Buick (2007). Global biospheric oxygenation and atmospheric evolution at the close of the Archean Eon.
A. D. Anbar, Y. Duan, T. W. Lyons, G. L. Arnold, B. Kendall, R. A. Creaser, A. J. Kaufman, G. Gordon, C. Scott, J. Garvin and R. Buick (2007). A whiff of oxygen before the Great Oxidation Event"
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