News: Explosive growth of life on Earth fueled by early greening of planet
- From: "Robert Karl Stonjek" <rstonjek@xxxxxxxxxxxxxx>
- Date: Fri, 10 Jul 2009 13:11:36 -0400 (EDT)
Explosive growth of life on Earth fueled by early greening of planet
July 8th, 2009 in Space & Earth / Earth Sciences
Earth's 4.5-billion-year history is filled with several turning points when
temperatures changed dramatically, asteroids bombarded the planet and life
forms came and disappeared. But one of the biggest moments in Earth's
lifetime is the Cambrian explosion of life, roughly 540 million years ago,
when complex, multi-cellular life burst out all over the planet.
While scientists can pinpoint this pivotal period as leading to life as we
know it today, it is not completely understood what caused the Cambrian
explosion of life. Now, researchers led by Arizona State University
geologist L. Paul Knauth believe they have found the trigger for the
Cambrian explosion.
It was a massive greening of the planet by non-vascular plants, or primitive
ground huggers, as Knauth calls them. This period, roughly 700 million years
ago virtually set the table for the later explosion of life through the
development of early soil that sequestered carbon, led to the build up of
oxygen and allowed higher life forms to evolve.
Knauth and co-author Martin Kennedy, of the University of California,
Riverside, report their findings in the July 8 advanced on-line version of
Nature. Their paper, "The Precambrian greening of Earth," presents an
alternative view of published data on thousands of analyses of carbon
isotopes found in limestone that formed in the Neoproterozoic period, the
time interval just prior to the Cambrian explosion.
"An explosive and previously unrecognized greening of the Earth occurred
toward the end of the Precambrian and was an important trigger for the
Cambrian explosion of life," said Knauth, a professor in Arizona State's
School of Earth and Space Exploration.
"During this period, Earth became extensively occupied by photosynthesizing
organisms," he added. "The greening was a key element in transforming the
Precambrian world - which featured low oxygen levels and simple, bacteria
dominant life forms - into the kind of world we have today with abundant
oxygen and higher forms of plant and animal life."
Knauth calls the work "isotope geology of carbonates 101."
In order to understand what happened on Earth such a long time ago,
researchers have studied the isotopic composition of limestone that formed
during that period. Researchers have long studied these rocks, but Knauth
said many focused only on the carbon isotopes of Neoproterozoic limestones.
Knauth and Kennedy's study looked at a bigger picture.
"There are three atoms of oxygen for every atom of carbon in limestone,"
Knauth says. "We looked at the oxygen isotopes as well, which allowed us to
see that the peculiar carbon isotope signature previously interpreted in
terms of catastrophes was always associated with intrusions of coastal
ground waters during the burial transformation of initial limestone muds
into rock. It's the same as we see in limestones forming today."
Brave new world
By gathering all of these published measurements and carefully plotting
carbon isotopic data against oxygen isotopic data, a process Knauth said
took three years, the researchers began to formulate a very different type
of scenario for what led to complex life on Earth. Rather than a world
subject to periods of life-altering catastrophes, they began to see a world
that first greened up with primitive plants.
"The greening of Earth made soils which sequestered carbon and allowed
oxygen to rise and get dissolved into sea water," Knauth explained. "Early
animals would have loved breathing it as they expanded throughout the ocean
of this new world."
A key element to this scenario is not so much what the researchers saw in
the data, but what was missing. When they plotted the data for various areas
from which it was derived they kept noticing an area on the plots that
contained little or no data. They dubbed it the "forbidden zone."
"If previous interpretations of carbon isotope data were correct, there
would be no forbidden zone on these cross plots," Knauth said. "The
forbidden zone would be full of Neoproterozoic data."
"These zones show that the isotopic fingerprints in limestone we see today
started in the late Precambrian and must have involved the simultaneous
influx of rain water that fell on vegetated areas, infiltrated into coastal
ground waters and mixed with marine pore fluids. During sea level drops,
these coastal mixing zones are dragged over vast geographic regions of the
flooded continents of the Neoproterozoic," Knauth said. "Vast areas of
limestone can form in these mixed pore fluids."
All of which points to an environmental trigger of the Cambrian explosion of
life.
"Our work presents a simple, alternative view of the thousands of carbon
isotope measurements that had been taken as evidence of geochemical
catastrophes in the ocean," Knauth explained. "It requires that there was an
explosive greening of Earth's land surfaces with pioneer vegetation several
hundred million years prior to the evolution of vascular plants, but it
explains how a massive increase in Earth's oxygen could happen, which has
been long postulated as necessary for animals to evolve big time."
"The isotopes are screaming that this happened in the Neoproterozoic," he
added.
Source: Arizona State University (news : web)
http://www.physorg.com/news166288435.html
Posted by
Robert Karl Stonjek
.
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