A new source of oxygen in the deep sea?

A discovery in the dark depths of the Pacific Ocean is challenging the scientific consensus of how oxygen is produced and has even called into question how life on Earth began. In a study published now in the scientific journal Nature Geoscience, a team of authors led by the Scottish Association for Marine Science (SAMS) show that oxygen is formed in complete darkness at the seafloor 4,000 metres below the ocean surface. They hypothesize that seawater electrolysis may contribute to this “dark oxygen” production. Expertise on specific sensors provided by GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany, helped to investigate the phenomenon which stands in surprising contrast to the established fact that oxygen is usually produced by photosynthetic organisms using energy from sunlight.

The discovery was made while sampling the seabed of the Clarion-Clipperton Zone to assess the possible impacts of deep-sea mining. This process would extract polymetallic nodules that contain metals such as manganese, nickel and cobalt, which may help to address the growing demand required to produce lithium-ion batteries for electric vehicles and mobile phones. The researchers found nodules to be carrying a very high electric charge, which could lead to the splitting of seawater into hydrogen and oxygen in a process called seawater electrolysis. Only a voltage of 1.5 Volt is needed for seawater electrolysis to occur – the same voltage as a typical AA battery. The team analysed multiple nodules and recorded readings of up to 0.95 volts on the surfaces of some, meaning that significant voltages can occur when the nodules are clustered together.

The researchers first thought their sensors were faulty, because every study ever done in the deep sea has only seen oxygen being consumed rather than produced. But despite recalibrations, the surprising oxygen readings kept showing up – and a back-up method was explored.

In this context, Tobias Hahn, then a doctoral researcher at GEOMAR Helmholtz Centre for Ocean Research Kiel, provided expertise on a specific type of oxygen optode sensors. To avoid methodological uncertainties and explore possibilities of bad sensor behaviour, Hahn conducted calibration procedures, data processing and data evaluation of these sensors. “Through our exchange, we assured the data quality which goes beyond the typical measuring uncertainties and includes the sensors individual drift and pressure behaviour,” the chemical oceanographer said. “I was astounded about the results, yet relieved to re-assure the quality and validity of the oxygen optode sensor data which were obtained during the first field campaigns. At the same time, I was able to provide further advice on how to handle this sensor during future deployments.”

Expedition leader Andrew Sweetman now says that more investigation into ‘dark oxygen’ production is required during deep-sea mineral extraction baseline investigations as well as an assessment of how sediment smothering during mining may alter the process. “Through this discovery, we have generated many unanswered questions and I think we have a lot to think about in terms of how we mine these nodules, which are effectively batteries in a rock.”

Original publication:

Sweetman, A.K., Smith, A.J., de Jonge, D.S.W. et al. (2024): Evidence of dark oxygen production at the abyssal seafloor. Nature Geoscience, doi: https://doi.org/10.1038/s41561-024-01480-8

To original GEOMAR press release