"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 Earth's history."
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 Earth."
UCR's Mary Droser and David Mrofka; and David Pevear collaborated on the study, which was supported also by the National Science Foundation.
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