The levels measured in these areas are the lowest ever recorded in Atlantic open waters, and are unsuitable for most marine animals, with only certain microorganisms able to survive.
"Before our study, it was thought that the open waters of the North Atlantic had minimum oxygen concentrations of about 40 micromol per litre of seawater, or about one millilitre of dissolved oxygen per litre of seawater," says lead-author Johannes Karstensen.
40 micromol per litre is low, but still allows most fish to survive. In contrast, the minimum levels of oxygen now measured are some 20 times lower than that, making the dead zones nearly void of all oxygen.
Dead zones are most common near inhabited coastlines where rivers often carry fertilisers and other chemical nutrients into the ocean, triggering algae blooms. As the algae die, they sink to the seafloor and are decomposed by bacteria, which use up oxygen in this process.
Currents in the ocean can carry these low-oxygen waters away from the coast, but a dead zone forming in the open ocean had not yet been discovered.
The newly discovered dead zones are unique in that they form within eddies, large masses of water spinning in a whirlpool pattern, but the areas around the dead-zone eddies remain rich in oxygen.
"The few eddies we observed in greater detail may be thought of as rotating cylinders of 100 to 150 km in diameter and a height of several hundred metres, with the dead zone taking up the upper 100 metres or so.
"The fast rotation of the eddies makes it very difficult to exchange oxygen across the boundary between the rotating current and the surrounding ocean," explained Mr Karstensen.
"From our measurements, we estimated that the oxygen consumption within the eddies is some five times larger than in normal ocean conditions."
The eddies studied form where a current that flows along the West African coast becomes unstable. They then move slowly to the west, for many months, due to the Earth's rotation.
The researchers have been conducting observations in the region off the West African coast and around the Cape Verde Islands for the past seven years, measuring not only oxygen concentrations in the ocean but also water movements, temperature and salinity.
To study the dead zones, they used several tools, including drifting floats that often got trapped within the eddies. To measure plant growth, they used satellite observations of ocean surface colour.
Their observations allowed them to study the impacts of the dead zones in the ecosystem.
Zooplankton - small animals that play an important role in marine food webs - usually come up to the surface at night to feed on plants and hide in the deeper, dark waters during the day to escape predators.
However, within the eddies, the researchers noticed that zooplankton remained at the surface, even during the day, not entering the low-oxygen environment underneath.
"Another aspect related to the ecosystem impact has a socioeconomic dimension," said Mr Karstensen.
"Given that the few dead zones we observed propagated less than 100 km north of the Cape Verde archipelago, it is not unlikely that an open-ocean dead zone will hit the islands at some point.
"This could cause the coast to be flooded with low-oxygen water, which may put severe stress on the coastal ecosystems and may even provoke fish kills and the die-off of other marine life."
Top image credit: NASA Earth Observatory
View the full report and author list in the journal Biogeosciences.