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2003 Annual Science Report

Pennsylvania State University Reporting  |  JUL 2002 – JUN 2003

Evolution of Atmospheric O2, Climate, and Biosphere - Ohmoto

4 Institutions
3 Teams
0 Publications
0 Field Sites
Field Sites

Project Progress

Our research has focused on (i) the chemistry of atmosphere and oceans and (ii) the nature of marine and terrestrial ecosystems on early Earth, especially before ~2.0 Ga (billion years ago). The research goals have been pursued by the following four approaches: (1) field investigations of geological environments (tectonics, paleogeographical, igneous, hydrothermal) for the accumulation of some key geologic formations ~3.5 – ~2.0 Ga in age; (2) biogeochemical investigations on selected rock samples from (1); (3) laboratory experiments on mineral-gas-solution experiments; and (4) thermodynamics and kinetic analyses of mineral-gas-solution reactions. Investigations in (3) and (4) have been carried out to understand the geochemical data from (2). Important achievements have been made in all phases of research, including the following:

(1). We have initiated the Archean Biosphere Drilling Project (ABDP), an international collaborating research project involving four organizations (Kagoshima University, The University of Western Australia, Geological Survey of Western Australia, and Pennsylvania State Astrobiology Research Center (PSARC), representing the NAI). The ABDP aims to: (i) recover through deep drilling “fresh” Archean-age (>2.5 Ga) rocks that were least affected by alteration since their formation, such as by metamorphism, hydrothermal mineralization, and modern weathering; and (ii) use these rocks for detailed and systematic biogeochemical investigations. We selected six drilling sites in the Pilbara district, Western Australia, during fieldwork in the summer of 2002, and began drilling in June 2003. The drilling has already produced unequivocal evidence that the abundant hematite (ferric oxide) crystals in the 3.46 Ga Marble Bar chert/jasper sequence were not formed by the modern oxidation of ferrous-rich carbonate (siderite) as postulated by many previous investigators, but by the mixing of Fe2+-bearing submarine hydrothermal fluids and oxygenated local seawater while the rocks were accumulating on the ocean floor. This is important evidence suggesting the oceans and atmosphere were already oxygenated 3.46 Ga ago. The ABDP has also recovered a large amount of black shales, containing remnants of microbes that lived in the Archean oceans.

(2). Our mineralogical and geochemical investigations (e.g., C, N and S isotopes; rare Earth elements) on Archean shales and banded iron formations from the Abitibi district, Canada, the Pilbara-Hamersley district, Australia, and other places have revealed that the redox structure of the Archean oceans was essentially identical to that of modern oceans: the oceans were globally oxic, but locally developed anoxic basins sustained complexed ecosystems. Distinct differences were found in the carbon isotopic characteristics between the microbial communities in submarine hydrothermal environments and those in normal oceans.

(3). Siderite (FeCO3) is a major constituent of carbonate-rich rocks and banded iron formations older than ~1.8 Ga. Our analyses on the formational conditions for siderite based on thermodynamic data and carbon isotope data suggest the CO2 level of the Archean atmosphere was at least 100 times greater than today, implying that CO2, rather than methane, was the major green-house gas in the Archean, as well as in later geologic time.