Seminar

Banded iron formations (BIFs) are sedimentary records of the coevolutionary history of geosphere and biosphere of the early Earth, which deposited through the early Precambrian (3.8 Ga ~1.8 Ga). The Archean and early Palaeoproterozoic oceans were anoxic and enriched in soluble Fe(II). Iron-oxidizing bacteria (IOB), especially anoxygenic photosynthetic IOB are suggested to be responsible for the Fe(III) oxyhydroxide precipitation before the rise of atmospheric oxygen. Once deposited, the biogenic Fe(III) oxyhydroxide may transform to more stable Fe(III) minerals, such as magnetite or hematite through microbial reductions or diagenetic and metamorphic alternations. However, the diagenetic and metamorphic overprints on minerals brought difficulties in reconstructing the primary depositional environments of BIFs and blurred our understanding of the microbial processes in the early oceans. 

In order to interpret the biogenicity of BIFs, I conducted a series of geobiological experiments, which could be divided into 3 parts. 1. The impacts of diagenesis or low-grade metamorphism on the cell-mineral systems of microaerobic and nitrate-dependent IOB: the mineral transformation of biogenic Fe(III) mineral and the preservation of biogenic structures in the high P-T experiments. 2. Possible mineral evidence for the earliest IOB: The interaction between Gallionella biofilm and water soluble Fe(II) under the strictly anaerobic conditions suggest magnetite particles could be formed before lithification, via green rust as the intermediate product. The giant deposition of magnetite in BIFs could be considered as the mineral evidence of early emergence of anoxygenic photosynthetic IOB. 3. The comparative case study of iron nodules found in Western Australia: No sign of biological process could be detected by spectroscopic, geochemical and electron microscopic observations. The features of the quartz particles indicate these nodules were formed in situ. This study presents a reference for our understanding of iron nodules recently observed on Mars.