KAKEGAWA Takeshi Institute of Mineralogy, Petrology, and Economic Geology, Faculty of Science ; R, 理学部, 助手 (60250669)
NARAOKA Hiroshi Faculty of Science, Tokyo Metropolitan University, Research Associate, 理学部, 助手 (20198386)
HAYASHI Ken-ichiro Institute of Mineralogy, Petrology, and Economic Geology, Faculty of Science ; A, 理学部, 助教授 (40124614)
The current "dominant" theory concerning the surface environments, biological activity, and global geochemical cycles of elements of the early Earth holds that they were quite different from those in the present time. The main objective of our research was to develop a new perspective of the Earth by critically examining this "dominant theory" and a new theory proposed by Ohmoto. The approaches taken in this research project, in contrast to researches carried out by previous investigators, were more detailed, systematic, and multidimensional studies of the geochemical nature of a large number of samples of soils, sedimentary rocks, igneous rocks, metamorphic rocks and ore deposits formed during the period of 3.8 to 1.8 b.y.ago in many different regions in the world.
Our geochemical investigations have resulted in many important discoveries concerning the biological and chemical evolution of the Earth : (1) the oxygen-producing photosynthetic plants and organisms have been active in ocea
n and on the continents since at least 3.5 b.y.ago ; (2) sulfate-reducing bacteria have been active in ocean since at least 3.8 b.y.ago, and methanogenic and methanotrophic bacteria since at least 2.8 b.y.ago ; (3) the atmosphere has been oxygen-rich since at least 3.5 b.y.ago ; (4) prior to -2.3 b.y.ago, the CO_2 content of the atmosphere was about 100 times greater and the surface temperature was about 30ﾟC higher than today ; (5) large-scale, deep, and anoxic basins developed periodically during the 2.8-2.3 b.y.period, and the basins became important sites for the accumulation of heavy metals ; (6) marine and terrestrial biota playd a major role in the geochemical cycles of C,O,S,Fe, U and many other elements in the early Earth in much the same ways as in today's Earth. We have developed a new theory that the biological and chemical evolution of the early Earth was more highly accelerated than previouly recognized, and that the elemental fluxes among the earth's various reservoirs (core, mantle, crust, ocean and atmosphere) have been at a steady state since at least -3.5 b.y.ago. Less