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Project Progress

Evolution of the Atmosphere, Oceans, and Biosphere on Early Earth:
Geological, Geochemical, and Biological Approaches

Hiroshi Ohmoto, Yumiko Watanabe, Tsubasa Otake, David C. Bevacqua, Dennis Walizer, and Alice Klarke

Highlights of the Achievements in Year 7

1. The presence of mass-independent fractionation of sulfur isotopes (MIF-S) in some sedimentary rocks older than ~2.45 billion years (Gyr), and absence of MIF-S in sediments younger than ~2.32 Gyr, have been used to suggest a dramatic rise of atmospheric pO₂ from <10^-5 PAL (present atmospheric level) to >10^-5 PAL around 2.35 Gyr. However, during the course of geochemical investigations of deep drill core samples from the Archean Biosphere Drilling Project (ABDP), we have discovered the absence of MIF-S through a 63 m section of 2.76 Gyr lake sediments and through a 170 m section of ~2.92 Gyr marine shales in the Pilbara Craton, Western Australia. Based on these and other data, we suggest the following three possibilities to connect the evolutionary history of atmospheric O₂ to MIF-S: (I) The atmospheric pO₂ level greatly fluctuated during the Archaean era: from anoxic (before ~3.0 Gyr), to oxic (between ~3.0 and ~2.75 Gyr), to anoxic (between ~2.75 and ~2.45 Gyr), and to oxic (after 2.45 Gyr); (II) The atmosphere remained oxic since ~3.8 Gyr, and the MIF-S signatures represent periods when volcanic eruptions ejected large volumes of SO₂ to the stratosphere; or (III) The atmospheric pO₂ history was not linked to the geologic record of MIF-S. (I) and (II) are possible scenarios if the only mechanisms to create MIF-S in geologic samples are photochemical reactions of volcanic SO₂ by UV light in the absence of an ozone shield; (III) is possible if other mechanisms existed to create MIF-S in geologic samples. Future research should be directed to evaluate these possibilities (manuscript submitted to Nature by Ohmoto et al., June, 2005).

2. We have discovered abundant hematite (ferric oxide) crystals in a deep drill core (ABDP #1) of Archaean greenstones from the Pilbara Craton, Western Australia. The greenstones, which erupted mostly as basaltic lavas and tuffs on the seafloor ~3.46 Gyr ago, have emerged near the land surface several times since ~3.43 Gyr ago. We obtained a rhenium-osmium (Re-Os) isotopic age of 2.762 ± 0.016 Gyr for pyrite (iron sulfide) veinlets that clearly cross-cut (i.e., are younger than) the haematized greenstones. These data, together with mineralogical and geochemical data, suggest that reactions with O₂-rich ground-water formed the hematites before ~2.76 Gyr ago. Therefore, an oxygenated atmosphere most likely developed more than 400 million years before the currently accepted ~2.35 Gyr date (manuscript to be submitted to Nature by Kato et al., August, 2005).

3. We have discovered that the abundant micro crystals of hematite in the Marble Bar Chert in the ABDP #1 core most likely formed by the mixing of Fe²⁺-rich submarine hydrothermal fluids with O₂-rich deep ocean water 3.46 Gyr: evidence for the early development of an oxygenated atmosphere (pO₂ > 40% PAL) (Ohmoto, et al., abstract for the Ann. Mtgs. of the American Geophysical Union, 2005).

4. An alteration zone with strong enrichment of Al and depletion of Fe is developed underneath the oldest unconformity (3.4 Gyr land surface) over a very large area (>100, 000 km²) in the North Pole area of the Pilbara Craton, Western Australia. This alteration zone has previously been interpreted by some Australian geologists as the products of submarine hydrothermal alteration. During the field work in July, 2005, Watanabe and Ohmoto discovered a strong possibility that this alteration zone represents an extensive lateritic soil (characterized by hematite-rich zone that is sandwiched by Fe-depleted soil zones). Since the development of a lateritic soil requires organic acids and an O₂-rich atmosphere, our discovery strongly suggests that the terrestrial biosphere and an oxygenated atmosphere had developed by 3.4 Gyr.

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