Seminar

Earth’s atmospheric oxygen was dramatically increased during 2.45 to 2.32 billion years ago. This is termed as great oxidation event(GOE). It is a collective result of early Earth’s biospheric and geospheric evolution. The redox states and biological environments of the late Archean to Paleoproterozoic oceans have still not yet been fully understood. As the main sedimentary rock majorly deposited between 3.8-1.8 Ga, banded iron formations may archive clues on Earth’s early biosphere and surface environments. Currently, microfossils, molecular fossils, elemental and isotopic geochemistry have been extensively used to reconstruct the surface environments and biospheric evolution on early Earth. However, mineralogical approaches are rarely reported. This thesis mainly focuses on high-resolution petrographic and mineralogical investigations on minerals from BIFs aged from >3.76 Ga to 2.2 Ga. With the combination of geochemical and synchrotron-base X-ray absorption fine structure and micro-X-ray fluorescence spectroscopy, the evolutionary histories of the minerals and material sources of the BIFs, the redox states of the atmosphere-ocean system and the microbial phosphatization in Archean to Paleoproterozoic are studied.

The results reveal that magnetite, hematite, carbonates and silicates could be mutually transformed to each other in post-depositional stages. Large euhedral magnetite crystals with clean and homogeneous interiors, submicrometer-sized euhedral hematite crystals in chert matrix and 3-5 nm hematite particle aggregated in iron-rich bands unlikely suffered external alterations, thus may reflect the primary environments when the BIF deposited. The latter two indicate the already existence of photosynthetic iron oxidization in late Archean to Paleoproterozoic seawaters. The Si and Fe in the BIFs were mainly sourced from seawaters and hydrothermal fluids respectively. The atmosphere-ocean system during 2.78 to 2.46 Ga was generally reduced with very weak oxidative weathering, though oxygen “oasis” might exist. By 2.2 Ga, the atmosphere-ocean system was more oxidized with much more intensive oxidative weathering.