July 12, 2000
Media Contact: Kim McDonald (858)
UCSD CHEMISTS FIND
EXTRATERRESTRIAL ‘ANOMALY’ IN EARTH’S ROCKS
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
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
"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
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
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."