Researchers led by Hans Røy from the Center for Geomicrobiology at Aarhus University in Denmark, extracted samples from columns of sediment up to around 30 meters beneath the sea floor in the region of rotating currents north of Hawaii known as the North Pacific Gyre. The sediment columns, built up by deposition of clay, dead algae and crustaceans, and dust, can be as much as several kilometres thick, with the most ancient sediment at the bottom of the columns.

The team used sensitive oxygen sensors to measure the oxygen concentration in the sediment cores. Knowing how much oxygen should have been present at each level allowed them to determine if oxygen was “missing,” which meant it had been consumed by microbes. In most regions of seabed examined previously, all the oxygen is consumed within the first 10 cm of sediment.

They discovered that bacteria within the clay were slowly using the oxygen, and remained alive even at a depth of around 30 meters, even though they have not had access to fresh organic matter for millions of years.