Sara NewellESC1000November 30, 2009CHAPTER 17 QUESTIONS3) Earth’s primary atmosphere was predominantly hydrogen and helium. After these light elements boiled off into space, bolides carried gases to create a secondary atmosphere, which was composed predominantly of carbon dioxide, with lesser amounts of water and nitrogen, and trace gases. By 4 billion years ago, outgassing, which is the release of gases from the interior of the Earth, combined with reactions of gases with rocks of the geosphere, created an atmosphere rich in hydrogen and carbon dioxide. 4) Photosynthesis by primitive cyanobacteria began producing oxygen about 2.7 billion years ago. However, for 300 million years, the oxygen concentration did not increase dramatically in the atmosphere due to two processes, banded iron formations and hydrogen concentration. As the oxygen concentration increased, it initially reacted with dissolved iron in sea water to form banded iron formations.
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