They used lab tests to determine microbes didn’t create the dark oxygen. Then they took a closer look at the lumpy potato-sized sedimentary rock formations created when metals like manganese and cobalt collect around shell fragments, sharks’ teeth, and similar substances commonly found on the ocean floor.
They couldn’t find any radioactive substances capable of splitting water molecules or decomposing minerals that contain oxygen, such as manganese oxide.
A chance viewing of a documentary on deep-sea mining while in Brazil gave Sweetman an epiphany when someone referred to the dark lumpy formations as a battery inside a rock.
Sweetman suspected an electrochemical process might create the oxygen they had measured in the ocean’s depths. Electricity splits water molecules through a process called electrolysis, which Sweetman’s team determined creates oxygen deep beneath the sea.
Source: Nature.com