Fossil iron bacteria may be preserved in Precambrian ferroan carbonate

Abstract

CONCLUSIVE evidence for microbial participation in the genesis of the massive sedimentary iron formations from the Precambrian is fragmentary. Structures resembling fossilised bacteria have been reported previously, and they tend to resemble modern iron bacteria such as Sphaerotilus, Gallionella and Metallogenium1,2. The atmosphere at that time is believed to have been almost devoid of oxygen3. The concentration of atmospheric oxygen probably ranged from 0.0001 to 0.001% of the present atmospheric level, and may have remained at this level for 2 × 109 yr after the emergence of algal photosynthesis4. The fossils described in this paper were found during a study of the mineralogy of the Lower Proterozoic Brockman Iron Formation (Hamersley Group) of Western Australia (about 2 × 109yr old5). Two specimens (PM9 and PM24) collected from the Whaleback Shale Member, at Wittenoom Gorge, were identified as dolomitic chert by X-ray diffraction. Optical microscopy showed the presence of fibrous stilpnomelane, and the staining technique suggested by Dickson6 distinguished the dolomite as ferroan.