大本 洋 ペンシルヴァニア州立大学, 地球科学科, 教授 (60194304)
NEDACHI Yoko Kagoshima Immaculate Heart College, Associate Professor, 生活学科, 助教授 (80290659)
NARAOKA Hiroshi Tokyo Metropolitan University, Faculty of Science, Associate Professor, 理学部, 助教授 (20198386)
KAKEGAWA Takeshi Tohoku Univ., Faculty of Science, Assistant Professor, 理学部, 助手 (60250669)
HAYASHI Kenichiro Tohoku University, Associate Professor, 理学部, 助教授 (40124614)
|Budget Amount *help
¥24,200,000 (Direct Cost : ¥24,200,000)
Fiscal Year 1999 : ¥11,400,000 (Direct Cost : ¥11,400,000)
Fiscal Year 1998 : ¥12,800,000 (Direct Cost : ¥12,800,000)
The primary objective of this project is to develop a quantitative model for the evolution of atmospheric oxygen and its connection to the evolution of organisms during the period between 3.5 Ga and 0.5 Ga, from detailed geochemical investigations of a variety of rocks formed during this period, including banded iron formations, paleosols, uraniferous quartz pebble conglomerates, shales, carbonates, and volcanic rocks. Field investigations of these types of rocks were carried out in South Africa, Namibia, and Australia in the summers of 1998 and 1999.
Various types of geochemical data obtained in this project are all consistent with the our model for the early development of an oxygenated atmosphere and of a diverse biosphere. The following are brief summaries of new findings.
The oldest paleosol sections that show a significant loss of Fe are those developed on the 2.8 Ga Mt. Roe basalt in western Australia. Nedachi et al., however, strongly suggest that the loss of Fe from the Mt. Roe
paleosols was caused by CH4-bearing hydrothermal solutions with temperatures of 〜200 - 〜100℃, rather than by the surface water during soil formation.
The advocates of the CWKH model have adapted the interpretation that the grains of uraninite and pyrite in pre-2.2 Ga quartz-pebble conglomerates are of detrital origin. Ono et al. (Paper #4) report the results of a small-scale, detailed analyses of textures, chemical composition, and isotopic composition of a variety of minerals in 〜2.4 Ga quartz-pebble conglomerates from the Elliot Lake district, Canada.
Based on new experimental data on the dissolution kinetics of uraninite, Ono and Ohmoto conclude that typical grains of uraninite can survive for more than 1000 years under the present atmospheric condition. Therefore, the presence or absence of detrital grains of uraninite may not be used to constrain the atmospheric pO2 level.
The same conclusions were obtained from the geochemistry of the Archean shale, increased Fe3+/Fe2+ ratios in the rims of pillow lava, origins of BIF, and evidence in BIFs for the early development of an oxygenated atmosphere Less