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.


Bibliography


A.D. Anbar and A.H. Knoll, “Proterozoic Ocean Chemistry and Evolution: A Bioinorganic Bridge?” Science 297: 1137-1142, 2002.

A.D. Anbar, Y. Duan, T.W. Lyons, G.L. Arnold, B. Kendall, R.A. Creaser, A.J. Kaufman, G. Gordon, C. Scott, J. Garvin, and R. Buick, “A Whiff of Oxygen Before the Great Oxidation Event?” Science 317: 1903-1906, 2007.

H. Ohmoto, Y. Watanabe, H. Ikemi, S.R. Poulson, and B.E. Taylor, “Sulphur Isotope Evidence for an Oxic Archaean Atmosphere,” Nature 442: 908-911, 2006.

S. Ono, B. Wing, D. Johnston, D. Rumble, and J. Farquhar, “Mass-dependent Fractionation of Quadruple Stable Sulfur Isotope System as a New Tracer of Sulfur Biogeochemical Cycles,” Geochimica et Cosmochimica Acta 70: 2238-2252, 2006.

Y. Shen, A.H. Knoll, and M.R. Walter, “Evidence for Low Sulphate and Anoxia in a Mid-Proterozoic Marine Basin, Nature 423: 632-635, 2003.

ecosystems), combined with greenhouse cultures, reveal a complex layered symbiotic ecology with more than 1,000 species and substantial diurnal fluxes of nutrients and of both reduced and oxidized gases. Ancient mats may have been a significant contributor to long-term atmospheric oxygenation.13-16 For additional details see Box 2.4.

  • Discovery of the “rare biosphere.” Using novel biotechnology that permits detection of almost all members in a microbial community, the NAI team at the Marine Biological Laboratory have discovered that the microbial diversity in the deep ocean is up to 100 times greater than expected within a population that is more than a million-fold depleted relative to the primary microbiota. This “rare biosphere” gene pool could serve as reserve of genetic diversity for repopulation of a habitat should conditions change dramatically.17-20 For additional details see Box 2.5.

  • Sub-seafloor life. NAI investigators from the University of Rhode Island, Woods Hole Oceanographic Institution, and the University of North Carolina led the first ocean-drilling expedition focused on exploration of subsurface life and habitability. Their results demonstrated that deep sub-seafloor communities are metabolically



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