Oxygen in Ancient Atmosphere Rose Gradually to Modern Levels (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 2 Dec 2005 16:27:40 -0500 (EST)
Office of University Communications
University of Maryland
Contacts:
Lee Tune, 301 405 4679
For Immediate Release: December 1, 2005
Oxygen in Ancient Atmosphere Rose Gradually to Modern Levels
COLLEGE PARK, Md. -- The history of life on Earth is closely linked to the
appearance of oxygen in the atmosphere. The current scientific consensus
holds that significant amounts of oxygen first appeared in Earth's
atmosphere some 2.4 billion years ago, with a second large increase in
atmospheric oxygen occurring much later, perhaps around 600 million years
ago.
However, new findings by University of Maryland geologists suggest that
the second jump in atmospheric oxygen actually may have begun much earlier
and occurred more gradually than previously thought. The findings were
made possible using a new tool for tracking microbial life in ancient
environments developed at Maryland. Funded by the National Science
Foundation and NASA, the work appears in the December 2 issue of Science.
Graduate researcher David Johnston, research scientist Boswell Wing and
colleagues in the University of Maryland's department of geology and Earth
System Science Interdisciplinary Center led an international team of
researchers that used high-precision measurements of a rare sulfur
isotope, 33S, to establish that ancient marine microbes known as sulfur
disproportionating prokaryotes were widely active almost 500 million years
earlier than previously thought.
The intermediate sulfur compounds used by these sulfur disproportionating
bacteria are formed by the exposure of sulfide minerals to oxygen gas.
Thus, evidence of widespread activity by this type of bacteria has been
interpreted by scientists as evidence of increased atmospheric oxygen
content.
"These measurements imply that sulfur compound disproportionation was an
active part of the sulfur cycle by [1.3 million years ago], and that
progressive Earth surface oxygenation may have characterized the [middle
Proterozoic]," the authors write.
The Proterozoic is the period in Earth's history from about 2.4 billion
years ago to 545 million years ago.
"The findings also demonstrate that the new 33S-based research method can
be used to uniquely track the presence and character of microbial life in
ancient environments and provide a glimpse of evolution in action," said
Johnston. "This approach provides a significant new tool in the
astrobiological search for early life on Earth and beyond."
The Air That We Breathe
When our planet formed some 4.5 billion years ago, virtually all the
oxygen on Earth was chemically bound to other elements. It was in solid
compounds like quartz and other silicate minerals, in liquid compounds
like water, and in gaseous compounds like sulfur dioxide and carbon
dioxide. Free oxygen -- the gas that allows us to breath, and which is
essential to all advanced life -- was practically non-existent.
Scientists have long thought that appearance of oxygen in the atmosphere
was marked by two distinct jumps in oxygen levels. In recent years,
researchers have used a method developed by University of Maryland
geologist James Farquhar and Maryland colleagues to conclusively determine
that significant amounts of oxygen first appeared in Earth's atmosphere
some 2.4 billion years ago. Sometimes referred to as the "Great Oxidation
Event," this increase marks the beginning of the Proterozoic period.
A general scientific consensus has also held that the second major rise in
atmospheric oxygen occurred some 600 million years ago, with oxygen rising
to near modern levels at that time. Evidence of multicellular animals
first appears in the geologic around this time.
"There has been a lot of discussion about whether the second major
increase in atmospheric oxygen was quick and stepwise, or slow and
progressive," said Wing. "Our results support the idea that the second
rise was progressive and began around 1.3 billion years ago, rather than
0.6 billion years ago."
In addition to Johnston, Wing's Maryland co-authors on the Dec. 2 paper
are geology colleagues James Farquhar and Jay Kaufman. Their group works
to document links between sulfur isotopes and the evolution of Earth's
atmosphere using a combination of field research, laboratory analysis of
rock samples, geochemical models, photochemical experiments with
sulfur-bearing gases and microbial experiments.
"Active microbial sulfur disproportionation in the Mesoproterozoic" by
David T. Johnston, Boswell A. Wing, James Farquhar and Alan J. Kaufman,
University of Maryland; Harald Strauss, Universität Münster; Timothy W.
Lyons, University of California, Riverside; Linda C. Kah, University of
Tennessee; Donald E. Canfield, Southern Denmark University: Science, Dec.
2, 2005.
.
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