Paleoenvironmental reconstruction of the oldest microfossils from the 3.5Ga Pilbara craton, western Australia

本历年在澳大利亚皮尔巴拉地区完成了地质测绘和系统取样工作。它们分别是：(1)发现最古老微化石的3.5 Ga古北极地区；(2)发现最古老微化石的3.0 Ga古克利弗维尔地区；(3)发现浅海黑色页岩和叠层石的2.7 Ga古里士满地区。所收集的样品均被薄切片，在显微镜下观察。用XRF和激光ICP-MS分析了部分样品的主微量元素和稀土元素。分离的锆石用激光ICP-MS测定年代。确定了变质矿物组合并估计了它们的P-T条件。对流体包裹体进行了研究，以明确早期生命的物理化学环境。结果如下：(1)微化石单元是增生杂岩的一部分。(2)重建微化石生存的物化环境；深2000m，在富xco2的热液系统下。(3)微化石不是蓝藻，而是化学化石，早在3.5 Ga时就已经有一定程度的分化。(4)微化石沿着与热液流体循环有关的断裂体系形成了800m以下的海底群落。(5)在3.0 Ga Cleaverville地区发现了新的微化石。这些化石体积巨大且保存完好，表明真核生物可能诞生于3.0 Ga。(6)在3.5 Ga海山顶沉积物中也发现了新的微化石。这些可能是在浅海环境下开始光合作用的化石。(7)重建了2.7 Ga时叠层石单元的古环境（AGU, 2001）。(8) 2.7 Ga古沉积物从浅海到深海的古环境重建。这些微化石是在深海沉积物中新发现的。生命和地球的历史概要出版了。

Geologic mapping and systematic samplings have been completed in the Pilbara area, Australia in this calendar year. They are, (1) the 3.5 Ga old North Pole region where the oldest microfossils have been recognized, (2) 3.0 Ga old Cleaverville region, and (3) 2.7 Ga old Richmond region where shallow marine black shale and stromatolite are present. The collected samples were all thin-sectioned, and observed under the microscope. Some were chemically analyzed in major and trace elements by XRF, as well as REE by laser ICP-MS. Separated zircons were dated by laser ICP-MS at TIT. Metamorphic mineral assemblages were determined and their P-T conditions were estimated. Fluid inclusions were also investigated to make it clear the physico-chemical environment of early life. The results are as follows.(1) The microfossil-bearing unit forms a part of accretionary complex.(2) The physico-chemical environment where microfossils were survived was reconstructed ; it was at 2000m deep under XCO2-rich hyfrothermal system.(3) Microfossils are not cyanobacteria but chemo-fossils which had already differentiated to some extent by 3.5 Ga.(4) Microfossils formed a sub-seafloor community down to 800m along the fracture system related to hydrothermal fluid circulation.(5) New micro fossils were found from the 3.0 Ga Cleaverville region. Those are extremely large and well-preserved, suggesting a possibility of birth of eukaryotes at 3.0 Ga.(6) New microfossils were also identified from the 3.5 Ga seamount-top sediments. These may be fossils which may have begun photo-synthesis under the shallow marine environment.(7) Paleoenvironment of stromatolite-bearing unit at 2.7 Ga was reconstructed (presented at AGU, 2001 ).(8) Paleoenvironmental reconstruction of 2.7 Ga old sediments from shallow marine to deep-sea. The microfossils were newly found from those deep-sea sediments.(9) A brief summary of the history of life and the Earth was published.