Archean biological activity evolved with environ mental changes of the earth's surface inferred from biomarkers and stable isotopic compositions

根据生物标志物和稳定同位素组成推断，太古宙生物活动随着地球表面环境变化而演化

基本信息

批准号：
12440154
负责人：
NARAOKA Hiroshi
金额：
$ 9.47万
依托单位：
TOKYO METROPOLITAN UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
2000
资助国家：
日本
起止时间：
2000 至 2002
项目状态：
已结题

项目摘要

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

从微化石的研究和有机物的碳同位素组成等化学化石的研究中，未发展的O_2的光自营养生物早在3.5 GA就已经活跃。据报道，真核生物标志物存在于大帝海洋中。然而，流行的理论是，进化的O_2被食用以氧化地球的陆地环境，直到2.2 GA之前，大气应该基本上无O_2。西澳大利亚州哈默斯利盆地的Jeerinah组（2.7 GA）主要由有机碳和黄铁矿富含黑色的黑色阴影组成。在这项研究中，确定了使用钻井核心部分，耶利那形成的沉积岩石样品的化学和同位素组成，以表征涉及海洋氧化还原条件的生物学活性。耶利娜·页岩中含有大量的有机碳（最高8 wt％）和黄铁矿硫硫（最高7.5 wt％）。有机碳各种系统上的同位素组成……在Δ^^<13> c中的地层截面应从-45到-35％。碳同位素偏移可以通过正常的光自养殖产生的同位素重成分与由甲烷概括的异育产生的光成分之间的有机物之间的两尾混合来解释。碳同位素偏移是通过硫同位素偏移来完成的，从δ^<34> s中的-10到 +10％。通过还原硫细菌（SRB），硫同位素分馏应超过20％。有机碳含量与氮含量呈正相关（原子量为N/C = 0.006）。大量氮同位素组成主要落在δ^<15> N中的0至 +5％的范围内；这些价值与现代海洋中常见生物的价值相似。具有最高有机碳含量的页岩具有δ^<15> n值 - ％，可能表明蓝细菌作为主要生产者的氮固定。但是，页岩具有中间δ^<13> C值（-40％），这可能是由于化学自由营养或异育体（如甲苯酚和SRB）的贡献所致。除两个样品外，钼浓度与有机碳含量正相关。 MO可以在氧气状态下风化，并用有机材料固定。在有机碳的δ^<13> C中的正偏移和硫化物的δ^<34> s的δ^<13> C中观察到负eu偏移。这种元素行为表明，MO和REE的风化，运输和固定机制应受到氧基条件的影响。与当前流行的观点相反，大海是完全缺氧的，我们研究的结果表明，哈默斯利盆地的氧化还原结构和生物学活性的变化要多得多。较少的