| Project/Area Number |
12440154
|
|---|
| Research Category |
Grant-in-Aid for Scientific Research (B)
|
| Allocation Type | Single-year Grants |
|---|
| Section | 一般 |
|---|
| Research Field |
地球化学
|
|---|
| Research Institution | TOKYO METROPOLITAN UNIVERSITY |
|---|
Principal Investigator |
NARAOKA Hiroshi TOKYO METROPOLITAN UNIVERSITY, Dept.of Chemistry, Assoc.Prof., 理学研究科, 助教授 (20198386)
|
|---|
| Co-Investigator(Kenkyū-buntansha) |
掛川 武 東北大学, 大学院・理学研究科, 助教授 (60250669)
ブリンカット デビット 東京都立大学, 理学研究科, 助手 (60301412)
|
|---|
| Project Period (FY) |
2000 – 2002
|
| Project Status |
Completed (Fiscal Year 2002)
|
| Budget Amount *help |
¥14,800,000 (Direct Cost: ¥14,800,000)
Fiscal Year 2002: ¥1,700,000 (Direct Cost: ¥1,700,000)
Fiscal Year 2001: ¥2,300,000 (Direct Cost: ¥2,300,000)
Fiscal Year 2000: ¥10,800,000 (Direct Cost: ¥10,800,000)
|
|---|
| Keywords | Archean / Biological Activity / Earth's Surface Environment / Biomarkers / Isotope Ratio / Organic Matter / Redox state / 生物多様性 / 炭素同位体比 / 窒素同位体比 / イオウ同位体比 / モリブデン / 炭素同位体 / 窒素同位体 / イオウ同位体 / 同位体組成 |
|---|
| Research Abstract |
Photoautotrophic organisms evolving O_2 were already active as early as 3.5 Ga ago from studies of microfossils, and chemical fossils such as carbon isotopic compositions of organic matter. Eukaryotic biomarkers were also reported to be present in the Archean ocean. The popular theory, however, has been that the evolved O_2 was consumed to oxidize the earth's terrestrial environment and the atmosphere should be essentially O_2-free until 2.2 Ga ago. The Jeerinah Formation ( 2.7 Ga) in the Hamersley Basin, Western Australia is mainly composed of organic carbon-and pyrite-rich black shales. In this study, using a drilling core section, chemical and isotopic compositions of sedimentary rock samples of the Jeerinah Formation were determined to characterize biological activites involving redox conditions of the ocean.
The Jeerinah shales contain high amounts of organic carbon (up to 8 wt%) and pyrite sulfur (up to 7.5 wt%). The isotopic composition of organic carbon varies systematically thr
… More
ough the stratigraphic section from -45 to -35% in δ^<13>C. The carbon isotope excursion could be explained by two-endmember mixing of organic matters between isotopically heavy component produced by normal photoautotrophs and light component produced by methane-assimilated heterotrophs. The carbon isotope excursions are accompanied with sulfur isotope excursions, from -10 to +10% in δ^<34>S. Sulfur isotope fractionation should be more than 20% by sulfur-reducing bacteria (SRB). The organic carbon contents are positively correlated with the nitorogen contents (N/C=0.006 by atomic). Bulk nitrogen isotopic composition falls mainly in a range from 0 to +5% in δ^<15>N ; these values are similar to those of common organisms in modern oceans. The shale with highest organic carbon content has a δ^<15>N value -%, probably indicating nitrogen fixation by cyanobacteria as a primary producer. However, the shale has an intermediate δ^<13>C value (-40%), possibly due to a contribution from chemoautotrophs or heterotrophs such as methanotrophs and SRB. Molybdenum concentrations are positively correlated with organic carbon content except two samples. The Mo could be weathered under oxic condition, and fixed in organic materials. A negative Eu shift is observed during positive shifts in δ^<13>C of organic carbon and δ^<34>S of sulfide. Such element behaviors suggest that weathering, transportation and fixation mechanism of Mo and REE should be affected under oxic condition. In contrast to a currently popular view that Archean oceans were entirely anoxic the results of our study suggest that the redox structure and biological activity in the Hamersley Basin were much more variable. Less
|
