What is 'dark oxygen', that scientists have discovered 13,000 ft below ocean surface?

Scientists have discovered deep-sea oxygen, also known as 'dark oxygen', being produced in the deep ocean. The term 'dark oxygen' describes oxygen being produced electrochemically without photosynthesis at a depth of about 13,000 feet in the ocean, where no light can reach. The oxygen appears to be produced by naturally occurring metallic "nodules" which split seawater into hydrogen and oxygen.