BOX 2.2
THE RISE OF OXYGEN AND EARTH’S “MIDDLE AGE”
High-precision studies of sulfur-isotope fractionation reveal that some photochemical reactions can produce isotope variations that do not scale simply with mass. These mass-independent fractionation (MIF) reactions require ultraviolet radiation that is blocked by O3, and the preservation of their fractionated reaction products requires low atmospheric O2. Sulfur-MIF studies indicate that Earth’s atmosphere became oxygenated (the “Great Oxidation Event”) in the early Proterozoic, about 2.3 billion years ago. One possible cause is the development of oxygenic photosynthesis at that epoch; alternatively, the rise of atmospheric O2 may have been mediated by geological processes.
Access to unweathered and uncontaminated samples of the oldest and least-altered sedimentary rocks is essential for understanding the early history of life on Earth and the environments in which it may have existed. The NAI initiated the Astrobiology Drilling Program (ADP), an outgrowth of the Mission to Early Earth Focus Group, which funded drilling (primarily in Western Australia) to access fresh subsurface samples that are made available to a broad scientific community.
Initial analyses reveal that at least trace amounts of O2 may have been present hundreds of millions of years before the Great Oxidation Event. Whereas it was once thought that the Proterozoic was a mildly oxygenated version of the modern, it is increasingly believed that the rise of oxygen led, paradoxically, to intensification of anoxia in large parts of the deep ocean. The NAI was instrumental in catalyzing research that tested the broad strokes of this hypothesis as well as research into the possible evolutionary consequences of a billion years of ocean redox stratification.
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