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Project Progress

1. Understanding the causes of mass-independent fractionation of sulfur isotopes (MIF-S) in sedimentary rocks: The presence of mass-independently fractionated sulfur isotopes (MIF-S) in many sedimentary rocks older than ~2.4 billion years (Gyr), and the absence of MIF-S in younger rocks, has been considered the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2.4 Gyr. This is because the only known mechanism to produce MIF-S has been ultraviolet photolysis of volcanic sulfur dioxide gas in an oxygen-poor atmosphere. We (Ohmoto, Watanabe, Ikemi, Poulson & Taylor) report in Nature (in press) the absence of MIF-S throughout ~100 m sections of 2.76-Gyr lake sediments and 2.92-Gyr marine shales, recovered by the Archean Biosphere Drilling Project (ABDP) from the Pilbara Craton, Western Australia. We propose three possible interpretations of the MIF-S geologic record: (1) the level of atmospheric oxygen fluctuated greatly during the Archean era; (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.

From a series of laboratory experiments, we (Watanabe, Naraoka & Ohmoto) have discovered 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; 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. 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). Preliminary 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. This alateration zone was previously interpreted by some Australian geologists as the products of submarine hydrothermal alteration. During the fieldwork in July, 2005 and 2006, 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). This suggestion was confirmed by an extensive fieldwork by us (Allwood, Birch, Yamaguchi, Johnson and Ohmoto) 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.