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

通过对微体化石和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，光合自养生物就已经活跃。真核生物标志物也被报道存在于太古代海洋中。然而，流行的理论是，演化的O_2被消耗以氧化地球的陆地环境，直到2.2 Ga前，大气应该基本上是无O_2的。西澳大利亚哈默斯利盆地的Jeerinah组（2.7 Ga）主要由富含有机碳和黄铁矿的黑色页岩组成。在这项研究中，使用钻井岩心部分，化学和同位素组成的沉积岩样品的Jeerinah组进行了测定，以表征生物活动，涉及海洋的氧化还原conditions.The Jeerinah页岩含有大量的有机碳（高达8wt%）和黄铁矿硫（高达7.5wt%）。有机碳的同位素组成随时间的推移而发生系统性变化， ...更多信息 δ^ C为-45 ~-35%的地层剖面<13>。碳同位素漂移可以解释为有机质的同位素重组分由正常光合自养生物产生和轻组分由甲烷同化异养生物产生之间的双端元混合。碳同位素的偏移伴随着硫同位素的偏移，δ^ S的偏移范围为-10%到+10%<34>。硫还原菌（SRB）对土壤中硫同位素的分馏应大于20%。有机碳含量与氮含量呈显著正相关（N/C=0.006）。整体氮同位素组成福尔斯主要在δ^ N的0-+5%范围<15>内，这些值与现代海洋中常见生物的值相似。具有最高有机碳含量的页岩具有δ^<15>N值-%，可能表明蓝藻作为初级生产者的固氮作用。然而，页岩具有中间δ^<13>C值（~ 40%），这可能是由于来自化能自养生物或异养生物（例如甲烷氧化菌和SRB）的贡献。除两个样品外，钼含量与有机碳含量呈正相关。钼在好氧条件下可风化，并固定在有机质中。在有机碳δ^ C和硫化物δ^ S正移的过程中观察到负的Eu位移<13><34>。这些元素的行为表明，在氧化条件下，Mo和REE的风化、迁移和固定机制受到影响。与目前流行的太古代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变化更大。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