In situ iron isotope analyses of pyrite and organic carbon isotope ratios in the Fortescue Group: Metabolic variations of a Late Archean ecosystem
In situ iron isotope analyses of pyrite and organic carbon isotope ratios in the Fortescue Group: Metabolic variations of a Late Archean ecosystem
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DOI：
10.1016/j.precamres.2012.05.003
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发表时间：
2012-08
影响因子：
3.8
通讯作者：
Kazumi Yoshiya;M. Nishizawa;Y. Sawaki;Y. Ueno;T. Komiya;Keita Yamada;N. Yoshida;T. Hirata;H. Wada;S. Maruyama
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文献类型：
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作者：
Kazumi Yoshiya;M. Nishizawa;Y. Sawaki;Y. Ueno;T. Komiya;Keita Yamada;N. Yoshida;T. Hirata;H. Wada;S. Maruyama
晚太古代海洋的生物地球化学循环对于理解生物活动与大气和海洋氧合的关系具有重要意义。在对澳大利亚西部Pilbara克拉通Redmont地区Fortescue集团进行详细地质调查的基础上，我们精心挑选了砂岩、叠层石碳酸盐岩、泥岩/砂岩、泥岩和燧石等44个样品进行了铁同位素分析。对样品中210颗黄铁矿的δ56Fe值进行原位分析，结果表明δ56Fe值在−4.2‰~ +3.0‰之间变化较大。有机碳（δ13Corg:−51.8 ~−10.3‰）和无机碳（δ13Ccarb:−6.1 ~ 0.6‰）的碳同位素组成分别为128个和40个。显微观察表明，黄铁矿颗粒形态与铁同位素比值有明显的关系。大多数δ56Fe值为正的黄铁矿颗粒呈六角形、矩形和平行四边形，可能取代原有的铁氧化物晶系：赤铁矿、磁铁矿和针铁矿。δ56Fe值为负的黄铁矿中，有一半以上呈不规则形态。这种对比使我们有可能解决每粒黄铁矿的成因和形成过程。正δ56Fe值表明铁在缺氧环境中发生了部分氧化。有些黄铁矿的δ56Fe值非常低，低于−2.2‰，可能是由于微生物铁还原所致。黄铁矿伴生极轻有机碳（δ13Corg:−51.8‰~−40‰），为好氧或厌氧甲烷化产物。低δ56Fe值和低δ13C值在部分岩石中同时存在，表明甲烷存在铁还原缺氧氧化作用（AOM/IR）。铁和碳同位素反映了太古宙晚期浅海环境中微生物的代谢变化。
The biogeochemical cycle of the Late Archean ocean is important for understanding the relationships between biological activity and oxygenation of the atmosphere and ocean. Based on the detailed geological survey of the Fortescue Group in the Redmont area in Pilbara Craton, Western Australia, we carefully selected 44 samples for iron isotope analyses, which consist of sandstones, stromatolitic carbonate rocks, alternating mudstone/sandstone rocks, mudstones and cherts. Our in situ analyses of δ56Fe values of 210 pyrite grains in these samples show a large variation from −4.2‰ to +3.0‰. We also analyzed 128 and 40 carbon isotope compositions of organic (δ13Corg: −51.8 to −10.3‰) and inorganic (δ13Ccarb: −6.1 to 0.6‰) carbons, respectively. Microscopic observations show obvious relationships between pyrite grain morphology and iron isotope ratio. Most pyrite grains with positive δ56Fe values show hexagonal, rectangular, and parallelogram shapes, which may replace former iron-oxide crystal systems: hematite, magnetite, and goethite, respectively. In contrast, more than half the pyrite grains with negative δ56Fe values show irregular forms. The correlation allows the possibility to solve the origin and the formation process of each grain of pyrite. The positive δ56Fe values suggest the partial oxidation of iron in an oxygen-limited environment. Some pyrites show very lower δ56Fe values below −2.2‰ suggesting a biological origin, probably due to microbial iron reduction. On the other hand, the pyrite is accompanied by isotopically very light organic carbon (δ13Corg: −51.8‰ to −40‰), which indicates aerobic or anaerobic methanotrophy. The coexistence of the low δ56Fe values and low δ13C values in the some rocks suggests anoxic oxidation of methane by iron-reduction (AOM/IR). The iron and carbon isotopes demonstrate the metabolic variations of microorganisms in a Late Archean shallow marine environment.