Clay paved the way for evolution of complex animals

Feb 3, 2006 13:40 GMT  ·  By

In the beginning, Earth's atmosphere had no oxygen. But around 550 million years ago, when the first complex organisms, such as the trilobites, entered the picture, large quantities of oxygen had been already building up for around 50 million years.

However, why oxygen started building up in the atmosphere had remained a mystery. Photosynthesis had been going on for hundreds of millions of years, so, what happened? Scientists knew this must have been related to a certain mineral process but had only vague ideas regarding the details. Oxygen is a highly reactive substance, and something must have happened with the minerals that made them stop reacting with oxygen - thus allowing it to build up in the atmosphere. Some argued for example that at a certain point all the iron on Earth had been oxidized and oxygen molecules were then able to remain free.

A new study, conducted by geologist Martin Kennedy and his colleagues from the University of California in Riverside, has now given more insight on what happened.

They have realized that one of the most important reasons why oxygen did not accumulate was that it reacted with all sorts of carbon compounds. Thus, the most important issue was not with the minerals, but with the organic materials, which could interact with oxygen in much more numerous ways.

Trapping organic materials

Kennedy's team made the following hypothesis: Clay minerals in marine sediments attract organic carbon and absorb it, basically trapping it and preventing it from bonding with oxygen. Such clay minerals form in soil when organisms such as microbes or fungi interact with tiny bits of weathered rock. The resultant clay then washes down to the sea and settles on the bottom, where it attracts and traps the organic carbon.

The scientists reasoned that this so-called clay mineral factory might have produced the sharp rise in oxygen availability that preceded the flowering of complex life forms.

"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 explains. "Because clay minerals make up the bulk of sediment deposited today, we are saying that it should be largely absent in ancient rocks."

In order to verify the prediction they went to see the record of ancient sedimentary rock. Such ancient records have been found in places like Australia, China and Norway. The Australian oldest layers date from around 850 million years ago and are largely composed of mud or rock bits that haven't undergone many chemical changes.

However, the Australian rock record changes dramatically around 600 million years ago, as clay makes its appearance. The records in China and Norway also support the same rough chronology.

Thus, a very interesting process seems to have taken place: microbes and maybe even fungi had spread over the surface of the Earth, and marked the beginnings of a modern soil system. What began than never stopped: - soil mixes with organic material forming clay - clay gets eroded down to the sea - where it traps organic carbon - which allows oxygen to exist in free form and to build up into the atmosphere.

The team writes in a paper published today by the Science journal: "The resulting six-fold increase in oxygen would have significantly influenced biogeochemical cycling of [oxygen] sensitive elements such as [iron] and [sulfur] and ultimately increased the oxygen concentration of the atmosphere. The evolutionary innovation and expansion of land biota could permanently increase [chemical] weathering intensity and [clay] formation, establishing a new level of organic carbon burial and oxygen accumulation."

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