Mystery Oxygen Source Discovered on the Sea Floor

Something is pumping out large amounts of oxygen at the bottom of the Pacific Ocean, at depths where a total lack of sunlight makes photosynthesis impossible. Nature: The phenomenon was discovered in a region strewn with ancient, plum-sized formations called polymetallic nodules, which could play a part in the oxygen production by catalysing the splitting of water molecules, researchers suspect. The findings are published in Nature Geoscience. 'We have another source of oxygen on the planet, other than photosynthesis,' says study co-author Andrew Sweetman, a sea-floor ecologist at the Scottish Association for Marine Science in Oban, UK -- although the mechanism behind this oxygen production remains a mystery. The findings could also have implications for understanding how life began, he says, as well as for the possible impact of deep-sea mining in the region.

The observation is 'fascinating,' says Donald Canfield, a biogeochemist at the University of Southern Denmark in Odense. 'But I find it frustrating, because it raises a lot of questions and not very many answers.' Sweetman and his collaborators first noticed something amiss during field work in 2013. The researchers were studying sea-floor ecosystems in the Clarion-Clipperton Zone, an area between Hawaii and Mexico that is larger than India and a potential target for the mining of metal-rich nodules. During such expeditions, the team releases a module that sinks to the sea floor to perform automated experiments. Once there, the module drives cylindrical chambers down to close off small sections of the sea floor -- together with some seawater -- and create 'an enclosed microcosm of the seafloor,' the authors write. The lander then measures how the concentration of oxygen in the confined seawater changes over periods of up to several days.

Without any photosynthetic organisms releasing oxygen into the water, and with any other organisms consuming the gas, oxygen concentrations inside the chambers should slowly fall. Sweetman has seen that happen in studies he has conducted in areas of the Southern, Arctic and Indian oceans, and in the Atlantic. Around the world, sea-floor ecosystems owe their existence to oxygen carried by currents from the surface, and would quickly die if cut off. (Most of that oxygen originates in the North Atlantic and is carried to deep oceans around the world by a 'global conveyor belt.')

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