As human activity drives up atmospheric carbon dioxide and warms the planet, the oceans are losing oxygen. But how far that loss might go remains unclear. To forecast the future, scientists are looking to the past, when CO2 levels and temperatures were higher than today. Most fossil evidence only shows relative changes in oxygen, not absolute concentrations. Now, a study in Geophysical Research Letters offers a way to measure actual oxygen levels in ancient deep seas using microscopic magnetic fossils left behind by bacteria.
The researchers examined more than 200 modern seafloor sediment samples from ocean basins worldwide. Using a transmission electron microscope, they documented the shapes of over 4,500 magnetic mineral grains, each barely wider than a thousandth of a human hair. They found a consistent pattern: sediments from low-oxygen waters contained more skinny, bullet-shaped magnetite crystals, while oxygen-rich waters favored tiny magnetite cubes. This relationship held across different temperatures, salinities, and organic carbon levels, making it a robust tool.
By applying this new equation to older seafloor samples, scientists can now estimate how much oxygen reached the deep ocean during warmer periods in Earth's history. That offers a yardstick for gauging the potential extent of current oxygen loss and its impact on marine ecosystems.