Riverside Researchers Identify Clay as Major Contributor to
Oxygen that Enabled Early Animal Life
University of California
2, 2006) RIVERSIDE, Calif. — Clay made animal life possible on
Earth, a UC Riverside-led study finds. A sudden increase in oxygen in
the Earth’s recent geological history, widely considered necessary
for the expansion of animal life, occurred just as the rate of clay
formation on the Earth’s surface also increased, the researchers
“Our study shows for the first time that the initial soils covering
the terrestrial surface of Earth increased the production of clay
minerals and provided the critical geochemical processes necessary to
oxygenate the atmosphere and support multicellular animal life,”
said Martin Kennedy, an associate professor of sedimentary geology and
geochemistry at UCR, who led the study.
Study results appear in the Feb. 2 issue of Science Express,
which provides electronic publication of selected Science
papers in advance of print.
Analyzing old sedimentary rocks, the researchers found evidence of an
increase in clay mineral deposition in the oceans during a 200 million
year period that fell between 1.1 to 0.54 billion years ago — a
stretch of time known as the late Precambrian when oxygen suddenly
increased in the Earth’s atmosphere. The increases in clay formation
and oxygen shortly preceded — in geological time — the first
animal fossils about 600 million years ago.
“This study shows how we can use principles developed from the study
of modern environments to understand the very complex origin of life
on our planet — studying a time in history that has left us only a
scanty record of its conditions,” said Lawrence M. Mayer, a
professor of oceanography at the University of Maine and a co-author
of the Science paper.
Clay minerals form in soils through biological interactions with
weathering rocks and are then eroded and flushed to the sea, where
they are deposited as mud. Because clay minerals are chemically
reactive, they attract and absorb organic matter in ocean water, and
physically shelter and preserve it.
The UCR-led study emphasizes the possibility that colonization of the
land surface by a primitive terrestrial ecosystem (possibly involving
fungi) accelerated clay formation, as happens in modern soils. Upon
being washed down to the sea, the clay minerals were responsible for
preserving more organic matter in marine sediments than had been the
case in the absence of clays. Organic matter preservation results in
an equal portion of oxygen released to the atmosphere through the
chemical reaction of photosynthesis. Thus an increase in the burial of
organic carbon made it possible for more oxygen to escape into the
atmosphere, the researchers posit.
“One of the things we least understand is why animals evolved so
late in Earth history,” Kennedy said. “Why did animals wait until
the eleventh hour, whereas evidence for more primitive life dates back
to billions of years? One of the best bets to explain the difference
is an increase in oxygen concentration in the atmosphere, which is
necessary for animal life and was likely too low through most of
To establish a change in clay abundance during the late Precambrian,
the researchers studied thick sections of ancient sedimentary rocks in
Australia, China and Scandinavia, representing a history of hundreds
of millions of years, to identify when clay minerals increased in the
sediment from almost nothing to modern depositional levels.
“We predicted we would only find a significant percentage of clay
minerals in sediments toward the end of the Precambrian, when complex
life arose, while earlier sediments would have less clay content,”
Kennedy said. “This test is easier than it sounds. Because clay
minerals make up the bulk of sediment deposited today, we are saying
that it should be largely absent in ancient rocks. And this is just
what one finds.”
The study attracted the attention of the National Aeronautics and
Space Administration during the proposal stage, and the agency helped
fund the research.
“NASA is interested in what conditions to look for on other planets
that might lead to the arrival of life,” Kennedy said. “What are
the processes? Using earth as our most detailed study site, what are
the necessary steps a planet needs to go through to enable complex
animal life to arise? If oxygen is the metabolic pathway, then we need
to know what conditions have to allow for that to happen. The geologic
record provides us with a record of these steps that occurred on
UCR’s Mary Droser and David Mrofka; and David Pevear collaborated on
the study, which was supported also by the National Science
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