Magnetofossils reveal oxygen levels in past oceans

Magnetofossils, produced by magnetotactic bacteria, are widely found in marine sediments. These microscopic structures are composed of magnetic minerals and are related to oxygen concentrations at the bottom of the oceans. Researchers from Peking University and the Qingdao Marine Science and Technology Center developed a calibration in multiple ocean basins to correlate the characteristics of magnetofossils with oxygen levels at the seafloor ¹.

The discovery

The team analyzed more than 200 samples of modern sediments from various ocean basins. They discovered that both the coercivity fractions and the crystalline morphologies of magnetofossils have a linear correlation with oxygen levels at the seafloor, ranging from 0 to 230 μmol/kg. Furthermore, the abundance of magnetofossils is mainly controlled by organic carbon contents, but these contents have a weak influence on the morphologies of magnetofossils compared to oxygen levels ¹.

The researchers used a transmission electron microscope to document the shapes of more than 4,500 grains of magnetic minerals. They observed that samples from regions with lower oxygen levels had more bullet-shaped magnetite crystals, while more oxygenated waters favored the formation of magnetite cubes. This variation in crystal shape can be observed visually and also through the magnetic properties of the minerals ².

This discovery allows scientists to use magnetofossils as a tool to calculate absolute oxygen levels in past oceans. With this, it is possible to quickly and accurately reconstruct ocean oxygenation conditions and carbon storage at the seafloor during past climate transitions ¹.