Magnetite biomineralization in ferruginous waters and early Earth evolution
Evolution of Earth’s surface chemistry and biology are chronicled by minerals contained in rocks. For example, magnetite, an abundant iron mineral in Precambrian sedimentary iron formations (IFs), records the chemistry and biology of the ancient oceans and atmosphere. Mechanisms of magnetite deposition in IFs are uncertain, and thus so too are records of chemistry and biology in IFs. We find that magnetite forms unusual, raspberry-like, framboidal grains through microbial iron reduction in the waters of ancient ocean analogues, lakes Matano and Towuti (Indonesia). This magnetite is a major source of Fe to the underlying sediment, and the same mechanisms likely contributed to IF deposition. The conspicuous magnetite framboids may provide a biosignature on early Earth, Mars, and other planetary bodies.