ScienceDaily (July 9, 2009) — 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 journal Nature. Their paper 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."
Carbonate layers hold carbon isotope evidence of the late Precambrian greening of the Earth. These are located in the Old Dad Mountains in California. (Credit: L.P. Knauth, Arizona State University)
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Nature advance online publication 8 July 2009 | doi:10.1038/nature08213; Received 20 June 2008; Accepted 18 June 2009; Published online 8 July 2009
The late Precambrian greening of the Earth
L. Paul Knauth1 & Martin J. Kennedy2
School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA
Department of Earth Science, University of California, Riverside, Riverside, California 92557, USA
Correspondence to: L. Paul Knauth1 Correspondence and requests for materials should be addressed to L.P.K. (Email: Knauth@asu.edu).
Abstract
Many aspects of the carbon cycle can be assessed from temporal changes in the 13C/12C ratio of oceanic bicarbonate. 13C/12C can temporarily rise when large amounts of 13C-depleted photosynthetic organic matter are buried at enhanced rates1, and can decrease if phytomass is rapidly oxidized2 or if low 13C is rapidly released from methane clathrates3. Assuming that variations of the marine 13C/12C ratio are directly recorded in carbonate rocks, thousands of carbon isotope analyses of late Precambrian examples have been published to correlate these otherwise undatable strata and to document perturbations to the carbon cycle just before the great expansion of metazoan life. Low 13C/12C in some Neoproterozoic carbonates is considered evidence of carbon cycle perturbations unique to the Precambrian. These include complete oxidation of all organic matter in the ocean2 and complete productivity collapse such that low-13C/12C hydrothermal CO2 becomes the main input of carbon4. Here we compile all published oxygen and carbon isotope data for Neoproterozoic marine carbonates, and consider them in terms of processes known to alter the isotopic composition during transformation of the initial precipitate into limestone/dolostone. We show that the combined oxygen and carbon isotope systematics are identical to those of well-understood Phanerozoic examples that lithified in coastal pore fluids, receiving a large groundwater influx of photosynthetic carbon from terrestrial phytomass. Rather than being perturbations to the carbon cycle, widely reported decreases in 13C/12C in Neoproterozoic carbonates are more easily interpreted in the same way as is done for Phanerozoic examples. This influx of terrestrial carbon is not apparent in carbonates older than 850 Myr, so we infer an explosion of photosynthesizing communities on late Precambrian land surfaces. As a result, biotically enhanced weathering generated carbon-bearing soils on a large scale and their detrital sedimentation sequestered carbon5. This facilitated a rise in O2 necessary for the expansion of multicellular life.
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