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Project Progress

A variety of research was carried out by Ohmoto and his group (Yumiko Watanabe, Research Associate; Katya Bazilevskaya and Tsubasa Otake, graduate students; Denny Walizer, John McGrorey, and David Bevacqua, technicians) to increase our understanding of the evolutionary history of the biosphere, hydrosphere, and atmosphere of early Earth. Our accomplishments during the past year include the following:

1. Geochemical investigations of the Archean Biosphere Drilling Project (ABDP) cores: In summer 2003, under my direction, the ABDP drilled six deep holes in the Pilbara district of Western Australia to recover modern-weathering-free sedimentary and igneous rocks of 3.5-2.7 Ga in ages. In the first drill core, which intersected a 3.46 Ga succession of jasper-chert-pillow basalt, we discovered abundant hematite (Fe³⁺-oxide) crystals. Results from microscopic observations and chemical mapping of these samples using a Horiba X-ray microscope strongly suggest that the hematite crystals formed as a result of mixing of Fe²⁺-rich submarine hydrothermal fluids with O₂-rich deep ocean water. This discovery is significant because the presence of free O₂ molecules in deep ocean water is an indication that the atmospheric pO₂ level was already more than 50% of the present level at 3.46 Ga. We also discovered that shales (both marine and lacustrine) in all six drill cores contained abundant organic carbon and pyrite (FeS₂), indicating that microbes, including probably cyanobacteria, fermentaters, methanogens, and sulfate reducers, flourished in the Archean oceans and lakes. We initiated C and S isotope analyses of these shale samples to better understand the nature of the Archean biosphere.

2. Geochemical investigations of banded iron formations (BIFs): Siderite (Fe²⁺-rich carbonate) frequently occurs as huge massive beds in pre-1.8 Ga marine sedimentary rock sequences. From thermodynamic analyses of the formational conditions of siderite and analyses of C & O isotope ratios of more than 300 siderite samples from Australia and Canada, we (Ohmoto, Watanabe & Kumazawa, 2004) recently published a paper in Nature suggesting that the pre-1.8 Ga atmosphere was CO₂-rich (pCO₂ >10^-1.4±0.2 atm, i.e., >100 times the present level) and CH₄-poor (<<100 ppm). Kato, Yamaguchi, and Ohmoto submitted a paper for a GSA Special Volume, suggesting that the presence of negative Ce anomalies in almost all BIFs is strong evidence that the Archean atmosphere-ocean systems were already oxygenated.

3. Geochemical investigations of pre-2.0 Ga paleosols and so-called “detrital pyrite pebbles”: Our analyses of organic carbon and its host-paleosols (2.7 — 2.35 Ga in ages) from South Africa suggest that land surfaces were fully colonized by microbes by 2.7 Ga ago (Watanabe et al., 2004). Rounded “pyrite pebbles” in some pre-2.2 Ga quartz pebble conglomerate beds have been thought by many previous investigators to be detrital pyrite, and sited as important evidence for an anoxic atmosphere because pyrite is not stable under an oxic atmosphere. Chemical mapping of over 100 “pyrite pebbles” using a Horiba XGT, however, has revealed that the “pyrite pebbles” formed by reactions between rounded pebbles of hematite-rich chert (i.e., oxide BIF) and H₂S-rich fluids long after the sedimentation of the conglomerate beds. Thus, the “detrital pyrite pebbles” are not evidence of an anoxic atmosphere.