July 9, 2009

Early Greening Of Earth Fueled Massive Diversity

The Earth has experienced many turning points over its 4.5-billion-year history, such as periods of extreme temperature changes, asteroids and the arrival and disappearance of various life forms.  But one of the most monumental events was the Cambrian-era explosion of life some 540 million years ago, when complex, multi-cellular life burst out all over the planet.

Scientists point to this critical period as leading to life as we know it today, although its precise causes were not completely understood.

However, researchers now believe they have found the trigger: a massive greening of the planet by non-vascular plants.

The scientists, led by Arizona State University geologist L. Paul Knauth, say these primitive ground huggers set the stage 700 million years ago for the later explosion of life through the development of early soil that sequestered carbon, leading to the build up of oxygen that allowed higher life forms to evolve.

Knauth and Martin Kennedy of the University of California, Riverside, co-authored a report about the study, entitled: "The Precambrian greening of Earth". 

The report puts forth an alternative view of published data on thousands of analyses of carbon isotopes found in limestone formed in the Neoproterozoic period 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 said.

"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," he added, referring to the work as "isotope geology of carbonates 101."

In order to understand what happened on Earth so long ago, researchers have studied the isotopic composition of limestone that formed during that time.

However, Knauth said the focus was primarily on the carbon isotopes of Neoproterozoic limestones.

Knauth and Kennedy's research widened to scope to include the bigger picture.

"There are three atoms of oxygen for every atom of carbon in limestone," Knauth said.

"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."

In a process Knauth said took three years, he and Kennedy collected all the published measurements and carefully plotted carbon isotopic data against oxygen isotopic data to devise a very different scenario for what led to complex life on Earth.

Instead of 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 said.

"Early animals would have loved breathing it as they expanded throughout the ocean of this new world."

A critical component of this scenario is not so much what the researchers observed 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 that contained little or no data, something they dubbed 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 indicates 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 said.

"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."

The findings were published in the July 8 advanced on-line version of the journal Nature.


Image 1: This is a late Precambrian carbonate outcropping at south end of Death Valley, California. Carbon isotopes in these layers bear evidence of the first extensive greening of the Earth. Credit: L.P. Knauth, Arizona State University

Image 2: 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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