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Project Progress

Highlights of the Achievements in Year 8 (2006-2007)

1. New insights for the causes of mass-independent fractionation of sulfur isotopes (MIF-S) in sedimentary rocks: Most previous researchers have believed that the presence of mass-independently fractionated sulfur isotopes (MIF-S) in some sedimentary rocks older than ~2.4 billion years (Gyr), and the absence of MIF-S in younger rocks, are the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2.4 Gyr. This is because the only previously-known mechanism to produce MIF-S was ultraviolet photolysis of volcanic sulfur dioxide gas in an oxygen-poor atmosphere. We (Ohmoto, Watanabe, Ikemi, Poulson & Taylor) reported in Nature (2006) a discovery of the absence of MIF-S throughout ~100 m sections of 2.76-Gyr lake sediments and 2.92-Gyr marine shales that were recovered from the Pilbara Craton, Western Australia by the Archean Biosphere Drilling Project (ABDP). We have proposed three possible interpretations of the MIF-S geologic record: (1) the level of atmospheric oxygen fluctuated greatly during the Archean era (i.e., a yo-yo atmosphere); (2) the atmosphere has remained oxic since ~3.8 Gyr, and MIF-S in sedimentary rocks represents times and regions of violent volcanic eruptions that ejected large volumes of sulfur dioxide gas to the stratosphere; or (3) MIF-S in rocks was mostly created by non-photochemical reactions during sediment diagenesis, thus not linked to atmospheric chemistry.

We (Watanabe, Naraoka & Ohmoto, 2006) have also demonstrated experimentally that thermochemical reduction of sulfate by certain amino acids (glycine and alanine) can generate hydrogen sulfide with distinct MIF-S signatures. This discovery supports our suggestion that MIF-S in sedimentary rocks mostly created by reactions between sulfate-bearing seawater and organic matter during early diagenesis of sedimentary rocks. Since the abundances of various amino acids changed through geologic time, the geologic record of MIF-S may be linked to the biological evolution.

2. Discovery of primary hematite crystals in 3.465 Ga jasper beds: Goethite (ferric-iron hydroxide) and hematite (ferric-iron oxide) are being formed in the near-surface oxygenated environments. In deep drill core samples of the 3.465 Ga jasper beds in ABDP #1 hole, we (Bevacqua et al., 2006; Bevacqua, 2007; Hoashi et al., in prep.) have discovered abundant, submicron-sized, euhedral hematite crystals that appear to have nucleated in the mixtures of locally-discharged iron-rich submarine hydrothermal fluids and oxygenated deep ocean water. Since the oxygen contents of deep oceans have been linked to the oxygen contents of the atmosphere, this discovery suggests that the oceans and atmosphere were already oxygenated 3.465 Ga ago.

3. Discovery of ~2.8 Ga old hematitization of basalts: The ~3.46 Ga submarine basalts in ABDP #1 core are heavily hematitized. Various geochemical analyses (e.g., O isotopes; Re-Os dating of cross cutting pyrite veinlets) of the samples suggest the hematite crystals formed by reactions O2-rich groundwater about ~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 submitted to Nature by Kato et al.).

4. Discovery of the oldest (~3.43 Ga) paleosols: ABDP #8 drill hole in Pilbara intersected the oldest (~3.43 Ga) unconformity (land surface). Detailed mineralogical and geochemical investigations of the core samples (Altinok et al., 2006) have identified the oldest paleosol that developed underneath the unconformity utilizing organic acids generated by soil organisms.

5. Discovery of the oldest (~3.43 Ga) paleolaterites: 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. Many Australian geologists previously interpreted this alateration zone as products of submarine hydrothermal alteration. During the fieldwork 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). We (Allwood, Birch, Yamaguchi, Johnson and Ohmoto) confirmed this suggestion through an extensive fieldwork in July 2006. 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 Ga. We have been conducting detailed mineralogical and geochemical investigations of a large number of samples collected in the field.