Magnetic fossils reveal past ocean oxygen levels

Researchers have developed a new method to measure historical deep-ocean oxygen levels using microscopic magnetic fossils, or magnetofossils, left behind by bacteria. This technique could help predict how current ocean oxygen loss from climate change might progress. The study analyzed over 200 modern sediment samples from global ocean basins, linking magnetofossil shapes to present-day oxygen concentrations. Lower oxygen waters contained more bullet-shaped magnetite crystals, while higher oxygen favored cube-shaped ones, leading to a new equation for calculating past absolute oxygen levels. Published in Geophysical Research Letters, the findings hold across varying temperatures, salinities, and carbon levels, making magnetofossils a broadly applicable tool. Researchers can now use this yardstick to estimate deep-ocean oxygen under hotter past climates, offering insight into future ecosystem impacts.