Geochemistry of Archean–Paleoproterozoic Black Shales: The Early Evolution of the Atmosphere, Oceans, and Biosphere
Open Access
- Author:
- Yamaguchi, Kosei
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 11, 2002
- Committee Members:
- Lee Kump, Committee Member
Raymond Gabriel Najjar Jr., Committee Member
Hiroshi Ohmoto, Committee Chair/Co-Chair
Michael Allan Arthur, Committee Member - Keywords:
- N2 fixation
Carbon
Photosynthesis
Sulfur
Sulfate reduction
Stable Isotopes
Proterozoic
Continental Crust
Black Shales
Paleoproterozoic
Anoxic
Pilbara
Hamersley
Iron
Australia
Nitrogen
Archean
Oxic
Pyrite
Thorium
Nitrification
Denitrification
Uranium
Molybdenum
Methanogenesis
Griqualand West
Witwatersrand
Barberton
South Africa - Abstract:
- When did the Earth's surface environment become oxic? The timing and mechanism of the rise of atmospheric pO2 level in the early Precambrian have been long debated but no consensus has been reached. The oxygenation of the atmosphere and oceans has significant impacts on the evolution of the biosphere and the geochemical cycles of redox-sensitive elements. In order to constrain the evolution of the atmosphere, oceans, biosphere, and geochemical cycles of elements, a systematic and multidisciplinary study of inorganic geochemistry and stable isotope geochemistry was conducted using Archean– Paleoproterozoic black shales, graywackes, and red shales. The samples were collected from unweathered drillcores of the Swaziland Supergroup (3.25 Ga Sheba Formation of Fig Tree Group), the Witwatersrand Supergroup (2.96 Ga Parktown Formation of the West Rand Group), the Ventersdorp Supergroup (2.71 Ga Rietgat Formation of the Platberg Group), and the Transvaal Supergroup (2.64 Ga Black Reef Formation of the Wolkberg Group, 2.56 Ga Oak Tree Formation of the Chuniespoort Group, 2.22 Ga Timeball Hill Formation of the Pretoria Group, and ~2.2 Ga Mapedi Formation of the Olifantshoek Group) in South Africa and the Mt. Bruce Supergroup (2.72 Ga Pillingini Tuff Formation and 2.69 Ga Jeerinah and Lewin Shale Formations of the Fortescue Group and >2.60 Ga Marra Mamba Iron Formation, 2.60 Ga Wittenoom and Carawine Dolomite Formations of the Hamersley Group) in Australia. The above objective was pursued from an array of inter-related studies: (1) the systematics of organic C - pyrite S - ferric Fe - ferrous Fe - P contents and stable isotopes of organic C and pyrite S; (2) the Mo geochemistry; (3) the N isotope geochemistry; and (4) the U-Th geochemistry. For comparison, data from modern sediments and Phanerozoic sedimentary rocks were complied from the literature. Based on these data, the geochemical cycles of C, S, N, Fe, P, Mo, and U in the Archean–Paleoproterozoic surface environments were compared with those of Phanerozoic through modern environments. From (1), I suggest that the contents and stable isotopic compositions of organic C and pyrite S in sediments have been primarily controlled by redox / biological processes in diverse sedimentary redox environments involving oxygenic photosynthesis, aerobic recycling of organic matter, bacterial sulfate reduction, and methanogenesis throughout geologic time. Iron has been oxidized during weathering and reduced during diagenesis, and the P-mediated redox stabilization of the atmosphere and oceans has been operating throughout geologic time. The geochemical cycles of C-S-Fe-P-O would have been essentially the same as today, and they were already in full operation at least 3.25 Ga ago. From (2), based on the kinetics of metal sulfide dissolution, I suggest that Mo-bearing minerals were quantitatively oxidized during weathering and transported to the oceans if the pO2 was higher than 10-6 atm, i.e., >0.0005 % PAL (present atmospheric level). The dissolved Mo was fixed by organic matter and S in locally anoxic environments. The geochemical cycle of Mo during the Archean–Paleoproterozoic time was essentially the same as today. From (3), I suggest that the microbially mediated redox cycling of N involving biological N2-fixation, nitrification, denitrification, and ammonification was already in operation during the Archean, based on the characteristics of the N isotopic compositions of organic-bound N and clay-bound N observed in marine sediments throughout the geologic ages. High Fe content and a positive correlation between Mo and Corg contents of the shales support (although is not an evidence for) the operation of enzymes (FeMo cofactor) responsible for microbial N2-fixation (nitrogenase) and denitrification (dinitrogenase and dinitrogenase-reductase) in the Archean oceans. From (4), I suggest that the continental weathering flux of U in the Archean was probably the same as today because of the enhanced weathering rate of U-bearing silicate minerals (feldspars) under a high pCO2 atmosphere. The weathering of uraninite is considered to be minor in the total continental weathering flux of U. Generally low U contents of the Archean-Paleoproterozoic shales (< 10 ppm) are explained by extensive submarine hydrothermal activity at mid-oceanic ridges as a major sink for the oceanic U and the lack of significant enrichment of organic matter in the shales. The sedimentary enrichment of U by organic matter and the secular increase of the Th/U ratios suggests an importance of the decoupling of U and Th and tectonic recycling of U throughout geologic time. The geochemical cycle of U in the Archean–Paleoproterozoic surface environments was essentially the same as it is today. The most important discovery of this study is the early development of the present day redox environments, microbial activity, and geochemical cycles of redox-sensitive elements in the Archean. In the Archean, N2-fixers, photosynthesizers, nitrifyers, denitrifyers, sulfate-reducers, and methane-producing / consuming microorganisms had already formed complex ecosystems, like those of today, in the globally oxic world where the atmosphere and the oceans were oxic with local anoxic environments such as mid-depth O2-minimum zone and anoxic / euxinic basins. This study has implications for the early evolution of an oxic atmosphere, oxic oceans, and complex microbial biosphere not only on the Earth but also on the other Earth-like planets distributed in the universe. Such astrobiological implications expand the possibilities for the discovery of extraterrestrial biosignatures in future space missions.