"The absence of siderite in some ancient soils has been linked to low carbon dioxide levels in the atmosphere, levels that would be too low to compensate for the cooler sun 2.2 billion years ago," says Dr. Hiroshi Ohmoto, professor of geochemistry and director of the Penn State Astrobiology Research Center. "The absence of siderite in these soils, however, does not constrain atmospheric carbon dioxide, but occurred because the oxygen and acidity of well-aerated soils caused iron to form into other minerals."
Previous researchers suggested that the greenhouse gas methane compensated for the low carbon dioxide levels, making the Earth warm enough for water to flow.
Ohmoto; Yumiko Watanabe, research associate Penn State, and Kazumasa Kumazawa, Oyo Corp. Miyazaki City, Japan, report in today's (May 27) issue of the journal Nature, that the abundance of large, massive siderite-rich beds in pre-1.8 billion year old sedimentary sequences and their carbon isotope ratios indicates that the atmospheric carbon dioxide concentration was more than 100 times greater than today, causing the rain and ocean water to be more acidic than today. This high carbon dioxide content, without methane provided the necessary greenhouse effect to maintain liquid oceans in the early Earth.
Previous research recognized that siderite forms in soils when the carbon dioxide pressure in the atmosphere is above a certain level. No siderite in some ancient soils therefore implied lower levels of carbon dioxide.
Ohmoto and his associates, however, looked at four, rather than one, parameters involved in siderite formation. They investigated the effects of carbon dioxide, oxygen, acidity and amounts of iron in solution. The acidity of rainwater in the past was greater than it is now, and they found that the combination of greater acidity and oxygen in the atmosphere, not a low content of carbon dioxide, was the reason for the absence of siderite in some ancient soils.
At the same time that siderite is missing from soils, it is abundant in ocean sediments before 1.8 billion years ago and formed thick bands of mineable ores. The high abundance of siderite in marine sediments and their carbon isotope ratios can be explained only if the carbon dioxide pressure in the atmosphere was more than 100 times, perhaps as much as 1000 times, higher than today's level. If the methane content was as high as the level suggested by previous researchers, the combination of high carbon dioxide and methane could have made the early Earth too hot for life. But geologic evidence suggests the life flourished in the oceans and land since at least 3.5 billion years ago. Therefore, the atmospheric methane content must have been very low.
"The atmosphere cannot have high contents of both methane and oxygen," says Ohmoto. "If one is high, the other must be low, or both must be low. So, the combination of high oxygen, high carbon dioxide and low methane was a perfectly happy scenario on Earth prior to about 1.8 billion years ago.
"How the Earth's atmosphere changes through time is related to how it maintains habitable conditions on the planet," adds the Penn State researcher. "If we understand geological history better, we may understand how life evolved and may evolve in the future. Perhaps it will make us realize how humans should or should not interfere with the Earth's systems." Understanding of the Earth's early atmospheric history will also tell us what to look for when looking for life on extra-solar plants.
The National Science Foundation and NASA supported this work.