Bacterial magnetite in sedimentary deposits and its geophysical and paleoecological implication

In the past two decades, natural remanent magnetization carried by marine sediments and sedimentary rocks has been used extensively to monitor the history of the geomagnetic field and to constrain motion of crustal plates. But the origin of the magnetic minerals contributing to observed remanent magnetization is only now being resolved. The main reason behind that is that representative magnetic extracts are hard to obtain from sediments for direct observation. Previous studies to separate the magnetic carriers in sediments and to examine their granulometry under the scanning electron microscope (SEM) have been limited in their ability to recognize the more magnetically stable and smaller single-domain fraction (;Several new occurrences of living magnetotactic bacteria have been discovered and investigated. Numerous deep sea core and other marine sediment samples have been examined to identify the presence of bacterial magnetite particles. To date, the oldest undoubtedly bacterial magnetite assemblage detected is from Nama sedimentary rocks of South Africa (approximately 700-600 My). A chain composed of single-domain magnetite particles with fuzzy outlines strongly resembling a bacterial magnetite assemblage has also been detected from the 2000 My Gunflint deposit. These findings reflect the necessity for organisms to develop mechanisms for acquiring and storing extracellular iron after the Global Ocean "Rusting" event drastically reduced the availability of dissolved iron (normally in the ferrous state) in the hydrosphere. The geologic record shows this event probably occurred around Early Proterozoic as represented by worldwide-spread Banded Iron Formation deposition at that time.