Rethinking early Earth's atmosphere

Two popular hypotheses about Earth's early atmosphere are being called into question, according to new reports. Researchers say the atmosphere of early Earth may have had quite high levels of oxygen (Nature 2006, 442, 908) and rising methane levels during the past ice age were produced by plants and wetlands, not a sudden belching of gas from ocean bottom clathrates (Science 2006, 313, 1109). Scientists had believed that atmospheric oxygen didn't arise in appreciable quantities until 2.4 billion years ago. Now, studies of sulfur isotopes in 2.7 billion- to 2.9 billion-year-old rocks in Australia (shown) by Pennsylvania State University geochemistry professor Hiroshi Ohmoto and colleagues indicate the rocks resemble their younger cousins, which were produced in an oxygen-rich atmosphere. It's also well-known that atmospheric methane levels rose steadily during the end of the last glacial period, contributing to global warming. Hinrich Schaefer at Oregon State University, Corvallis, and colleagues found that the carbon isotopic signatures in methane from ice cores show that the gas was likely steadily released from tropical wetlands and plants.