Biogenesis of banded iron formations interpreted by geobiological experiments

Abstract

Banded iron formations (BIFs) are sedimentary records of the coevolutionary history of geosphere and biosphere of the early Earth 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 anoxic 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.

Some extracellular polymer structures, such as stalks of Gallionella, and encrusted cells of nitrate-dependent IOB are potential biosignatures of microorganisms, but the preservation of these biogenic structures in the sedimentary rocks after a long time is still an open question. In this study, high P-T experiments were conducted to simulate the impacts of diagenesis or low-grade metamorphism on the cell-mineral systems of microaerobic and nitrate-dependent IOB. The mineral transformation processes at 100 MPa/300⁰C of these two systems are similar: biogenic ferrihydrite colloid turns to hematite microcrystallines. Stalks, sheaths, iron oxide spheroidal aggregates and rod-like associations in the Gallionella’s biofilm can survive the treatments, while the globule aggregates and encrusted cells in the culture of Pseudogulbenkiania ferrooxidans strain 2002 can barely be observed after the P-T treatments.

Magnetite in BIFs has been suggested to be a secondary mineral formed during diagenesis or metamorphism, while the experimental results of 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 reaction between the soluble Fe(II) in seawaters and oxyhydroxides precipitated from the microbial oxidation of Fe(II), especially photoferrotrophy, might be responsible for the formation of primary magnetite in BIF all through its depositional history. Relatively high temperature and high concentrations of water soluble Fe(II) in the Archean ocean might be the favorable conditions for the deposition of high abundances of magnetite in the early BIFs.

The comparative case study of iron nodules found on the flat ground of vast Western Australia showed that these nodules are made of loosely-packed pisoids of a few to >10 mm in size. No sign of biological process could be detected by spectroscopic, geochemical and electron microscopic observations. Angular quartz particles, nano-sized goethite and hematite are found to be the major mineral phases in the nodules. No carbonate or amorphous silica implies that these nodules have experienced little underground fluid alteration. However, the quartz particles showing particularly three sections of fractal size-distributions, together with their extensive broken features and conchoidal fractures, strongly suggest an in situ fragmentation. Because of the environmental similarities between Western Australia and Mars, this study presents a reference for our understanding of iron nodules recently observed on Mars.