Chemists at the University of California, San Diego have discovered an isotope anomaly previously thought unique to meteorites and other extraterrestrial rocks in sulfate minerals on Earth.
The surprising finding, detailed in the July 13 issue of Nature, concludes a decade-long search in terrestrial rocks and sediments for this "anomalous" oxygen-isotope signature. Such signatures had been detected before in gases on Earth. But the inability before now to detect this anomaly in terrestrial solids had forced scientists to conclude that it was unique to extraterrestrial sources and an exclusive byproduct of nucleosynthesis in stars.
Its discovery in terrestrial rocks in a form that showed it could be produced on Earth not only alters ideas among planetary scientists about the source of this anomaly. It will now give earth scientists and atmospheric chemists an important new probe to answer questions about the composition of Earth’s early atmosphere, the atmospheric processes of ancient volcanic eruptions, past ocean circulation patterns and early biological productivity.
"It will enable us to understand more about the history of the Earth and possibly climate on time scales that were out of reach before," says Mark H. Thiemens, a professor of chemistry and dean of UCSD’s Division of Physical Sciences who headed the research effort.
"It opens up this whole new area for geochemists to look at things like ancient atmospheric deposits," says Huiming Bao, a geochemist at UCSD and the first author of the paper. "Once we figure out the fundamental sulfur-oxidation processes occurring in the atmosphere, it will provide a good way to understand ancient atmospheric processes."
Bao initially discovered the "anomalous" signatures—something odd about variations of the three stable isotopes of oxygen—in gypsum deposits from the Namibian desert and in volcanic ash deposits in Nebraska and South Dakota. Also contributing to the discovery and analysis were UCSD chemists Thiemens, James Farquhar, Douglas A. Campbell and Charles Chi-Woo Lee; Klaus Heine of the University of Regensburg in Germany; and David B. Loope of the University of Nebraska. The study was financed by the National Aeronautics and Space Administration and the National Science Foundation.
The scientists knew the anomalous isotope signatures were terrestrial because they were recorded in sulfate (SO42-) minerals that had been deposited in volcanic ash beds 20 million years ago or, in the case of Namibian gypsum deposits, associated with sulfur-producing marine organisms that emitted dimethyl sulfide into the atmosphere during the past 10 million years.
"We believe that ultimately these anomalous signatures come from the Earth’s atmosphere," says Bao. "And these signatures get transferred from ozone and other atmospheric oxidants to sulfate during the oxidation of reduced sulfur gases, such as those emitted by marine microorganisms or from volcanic eruptions."
With the exception of the isotopic signatures of gases trapped in ice cores for the past 200,000 years, scientists have had little knowledge of how major components in the Earth’s ancient atmosphere—such as sulfur, carbon, and oxygen—cycled through the oceans and terrestrial rocks. The UCSD development is important because it now provides a window into some of these processes extending millions or billions of years into the Earth’s past.
"To understand how the surface of the planet works, you’d really like to understand how this cycle couples to the atmosphere," says Thiemens. "No one has been able to find a way to do it on Earth except through ice cores. Now we can go far back in time and that’s never been done before."
The UCSD researchers believe the signatures in the volcanic ash could provide geologists with additional information about the chemistry of volcanic plumes and the nature of the eruptions that produced them. "Characteristic signatures may also help to temporally correlate continental deposits among different basins, where such a correlation has been a challenging task," says Bao.
Because the coast off central Namibia is a major zone of upwelling with intense biological activity, the researchers were able to tie the anomalous sulfate deposits to the activity of nearby sulfur-producing marine microorganisms and the unique desert environment that is able to preserve the signature. However, the upwelling current may not have been constant during the past several millions of years and may be intimately tied to the change of ancient climatic conditions. "It is too early to tell," says Bao, "but if this connection can be made, we may have a way of gaining insight into past ocean circulation and biological productivity."
The UCSD discovery also suggests that planetary geologists need to be careful in interpreting the origin of oxygen-isotope anomalies on meteorites, since these signatures can occur in terrestrial as well as extraterrestrial rocks. "Our observations suggest that caution needs to be exercised when looking for these anomalies in meteorites, because some of them may have been imparted during their residence on Earth," says Bao. "Some meteorites lay on ice or in the desert for thousands of years, so the secondary minerals in these meteorites may have originated on Earth."
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