Measuring deep-ocean oxygen with microscopic magnetic fossils

Oxygen levels in deep ocean waters are in decline due to human activities, with the impact on marine ecosystems still unclear. To predict future oxygen changes, researchers are examining historical data by studying marine microorganisms and their microscopic fossils. Marine microorganisms leave behind tiny fossils that provide clues about past ocean conditions, but they typically indicate relative changes in oxygen levels rather than absolute concentrations.

Huang and his team investigated the use of magnetofossils, which are nanoscopic magnetic minerals left behind by bacteria, to establish a link between these fossils and present-day oxygen levels in the water. The researchers analyzed over 200 modern ocean sediment samples from various global basins to create a new equation that can calculate the absolute oxygen levels in ancient seas.

Bacteria living within the top few centimeters of the seafloor produce magnetite crystals to detect and respond to Earth's magnetic field. These crystals can be observed using a transmission electron microscope and their shapes can vary, depending on the oxygen levels in the water. Bulky, bullet-shaped magnetite crystals were more prevalent in low oxygen environments, while smaller, cube-shaped crystals were more common in higher oxygen conditions.

This pattern, with increasing cube formation as oxygen levels rise, holds true across different temperatures, salinity levels, and organic carbon concentrations in the seawater. By examining older seafloor samples and using the newly established relationship between magnetofossils and oxygen levels, researchers can estimate the depth of oxygen in the deep ocean during Earth's past warmer climates.