Contacts: Mark Thiemens (858) 534-6882;
James Farquhar (858) 534-6053
Media Contact: Kim
McDonald (858) 534-7572,
Images and graphic available at: http://ucsdnews.ucsd.edu/newsrel/science/mcoxygen.htm
Credit for images: NASA; Credit for graphic: James Farquhar, UCSD
CHEMICAL SIGNATURES IN ROCKS PROVIDE CLUES TO ORIGIN AND
OF OXYGEN AND TERRESTRIAL LIFE ON EARTH
analyzing some of the oldest-known rocks on Earth have discovered
for the first time a way to recover from the geological record
details about the evolution of oxygen and ozone in the planet’s
early atmosphere—two key ingredients that permitted and recorded
the expansion of terrestrial life.
In the August 4 issue of Science, chemists from the
University of California, San Diego report that their analysis of
Precambrian sedimentary rocks as old as 3.8 billion years reveal a
"profound change" in the chemical reactions involving sulfur
and oxygen in the atmosphere that begins before 2.1 billion and
extends to 2.5 billion years ago, a period during which the oxygen
levels in the atmosphere are known to have increased sharply.
we found is a geochemical indicator that originated in the atmosphere
and it’s clearly a global signature," says James Farquhar,
a postdoctoral fellow at UCSD and the first author of the paper.
"It appears in samples that are older than 2 billion years,
but is most pronounced in samples older than 2.5 billion years."
"This is the first time that anyone has been able to see a
record of oxygen from the ancient atmosphere," says Mark Thiemens,
a professor of chemistry and Dean of UCSD’s Physical Sciences
Division, who led the study, which included UCSD postdoctoral fellow
Huiming Bao. "We now know it’s possible to track the evolution
on Earth of oxygen and ozone, which both coincide with the evolution
of life and the build up of the conditions on the planet that led to a
major shift in the atmosphere 2.2 billion years ago."
Geologists know from banded iron formations in 2.2 billion-year-old
rocks that significant quantities of oxygen were present at the time—enough,
at least, to oxidize the iron in the rocks in a process akin to
rusting. Some of that oxygen was presumably generated by
photosynthetic cyanobacteria, which were known to exist 3.5 billion
years ago, and some came from the chemical separation of water
molecules into oxygen and hydrogen.
But until now scientists had no way to probe what proportion each
process may have contributed to this sharp rise in oxygen and to the
development of the Earth’s ozone layer, which permitted the
expansion of terrestrial life by shielding organisms from the most
damaging effects of ultraviolet radiation.
"The banded iron formations tell you that the Earth had to
have significant quantities of oxygen then," says Thiemens.
"But you don’t know how much or where it came from. Because the
fossil record is so spotty, the period from the earliest-known rocks,
at 3.9 billion years ago, to 2.2 billion years ago is a black hole of
knowledge about the atmosphere and about life. This method provides a
way to track the record of oxygen in the atmosphere and, more
importantly, of ozone in the earliest rocks."
The technique the UCSD scientists developed to track oxygen in the
ancient atmosphere involves discerning a recognizable signature in
rocks that originated from chemical processes in the atmosphere—in
this case, from the oxidation of sulfur-bearing gases. From variations
in the four most common isotopes, or forms, of sulfur that were
incorporated into sulfide and sulfate minerals in the rocks, the
scientists were able to infer that the atmosphere 2.45 billion years
ago had limited free oxygen and was the main arena for chemical
reactions involving sulfur. That’s contrasted to the present-day
environment in which the atmosphere has significantly more free oxygen
and in which chemical reactions involving sulfur are dominated by
terrestrial processes—specifically continental oxidative weathering
and the reduction of sulfates by microbes.
Scientists had assumed for decades that the isotopic variations
used by the UCSD researchers to infer processes in the ancient
atmosphere could only be found in meteorites and other
extraterrestrial sources and were a unique byproduct of
nucleosynthesis in stars. But in a recent paper, published in the July
13 issue of Nature, the scientists demonstrated that their
presence in 20 million-year-old volcanic-ash deposits and 10
million-year-old gypsum deposits reflected chemical processes in the
Earth’s atmosphere. The UCSD team’s latest discovery pushes that
window into the ancient atmosphere back to a critical period in the
planet’s history—when oxygen and ozone were accumulating in the
atmosphere and the first terrestrial forms of life were expanding.
"It's a new discovery," says Robert N. Clayton, a
professor of chemistry and geophysical sciences at the University of
Chicago. "No one has seen anything like that before. It's another
handle on ancient atmospheric chemistry. It's surely going to be
Besides improving knowledge about the ancient atmosphere, the UCSD
finding has implications for improving the understanding of long-term
atmospheric events in the future, such as global warming.
"One always hears the argument, ‘Isn’t global warming all
part of a natural cycle?’" says Thiemens. "To answer that
question, you really want to have a large-scale record. This will give
it to us. We really need to understand the past in order to understand
the present and the future."
The study was financed by the National Aeronautics and Space
Administration and the National Science Foundation.
Caption for Images: Chemical signatures in ancient rocks hold the
key to the evolution of oxygen and ozone in the Earth’s early
atmosphere. Credit: NASA
Caption for Graphic: Variations in sulfur isotopes in the rocks
allowed UCSD scientists to infer that the atmosphere 2.45 billion
years ago had limited free oxygen and was the main arena for chemical
reactions involving sulfur. Credit: James Farquhar, UCSD.