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

通过对微生物化石、有机质碳同位素组成等化学化石的研究，发现早在3.5Ga以前，放氧光自养生物就已经活跃起来。据报道，太古宙海洋中也存在真核生物标志物。然而，流行的理论一直是，演化出的O2被消耗来氧化地球陆地环境，直到2.2Ga之前，大气基本上应该是无氧的。西澳大利亚州哈默斯利盆地的杰里纳组(2.7Ga)主要由有机碳和富硫铁矿的黑色页岩组成。在这项研究中，利用钻井岩心剖面，测定了杰里纳组沉积岩样品的化学和同位素组成，以表征与大洋氧化还原条件有关的生物活性。杰里纳页岩含有高达8wt%的有机碳和高达7.5wt%的黄铁矿硫。有机碳的同位素组成通过…系统地变化此外，在δ^&lt；13&gt；C的-45%~-35%的地层剖面上，碳同位素漂移可以用正常光自养产生的重同位素组分和甲烷同化异养生物产生的轻组分有机质的两端混合解释。碳同位素漂移伴随着硫同位素漂移，在δ^&lt；34&gt；S中，硫同位素漂移在-10%~+10%之间，硫还原菌的硫同位素分馏应大于20%。有机碳含量与氮素含量呈正相关(原子N/C=0.006)。δ^&lt；15&gt；N中的主体氮同位素组成主要在0-+5%的范围内；这些值与现代海洋中常见生物的值相似。有机碳含量最高的页岩具有δ^&lt；15&gt；N值-%，可能表明蓝藻作为主要生产者进行了固氮。然而，页岩具有中间的δ^&lt；13&gt；C值(-40%)，这可能是由于化学自养或异养生物的贡献，如甲烷和SRB。除两个样品外，钼含量与有机碳含量呈正相关。钼可以在氧化条件下风化，并固定在有机材料中。在有机碳的δ^&lt；13&gt；C和硫化物的δ^&lt；34&gt；S的正向漂移过程中，观察到Eu的负漂移。这些元素的行为表明，在氧化条件下，Mo和REE的风化、迁移和固定机制可能受到影响。与目前流行的太古宙海洋完全缺氧的观点相反，我们的研究结果表明，哈默斯利盆地的氧化还原结构和生物活性更具变异性。较少

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