In seafloor surprise, metal-rich chunks could generate deep-sea oxygen

But new experiments, both in chambers sitting on the Pacific Ocean seafloor and in the lab, show there may be other sources for that oxygen, Sweetman and his colleagues report July 22. Nature Geoscience.

Sweetman has been studying marine ecosystems thousands of meters deep in the Pacific for years. In vast areas there, metal-rich nodules that contain valuable minerals — and thus are targets for mining — litter the seabed.SNE: 2/21/14). On several expeditions, the team’s dissolved oxygen sensors strangely suggested that the substance, rather than simply being consumed by the organisms, was actually being produced as a whole. The researchers dismissed the readings as erroneous and then recalibrated the instruments for their next output.

After several such expeditions yielded similarly anomalous readings, the team developed a different method of measuring dissolved oxygen—which also showed that the gas was being generated.

The team’s data showed that the rogue oxygen did not come from bubbles trapped in their equipment, nor did it come from the polymer material used to make the test chambers. It was also not the result of the natural radioactivity of the metals in the joints separating the water molecules or the dissolution of the manganese oxide minerals in the joints. Laboratory tests under conditions that mimic the cold darkness of the Pacific seabed also showed that dissolved oxygen concentrations were increasing, not decreasing, in the presence of the nodules.

“That’s when we said, ‘My God, we have another source of oxygen,'” Sweetman says.

When team members tested the nodes further, they found that the bumps were acting like tiny batteries, producing up to 0.95 volts between several points on the surfaces of the bumps. Although it takes just over 1.5 volts to split seawater into hydrogen and oxygen, Sweetman suggests that under certain conditions, groups of nodes together can produce enough voltage to do the trick.

Oxygen production appears to be occurring on the surfaces of the joints, Sweetman says. In the team’s tests, the rate of oxygen production appeared to be related to the average surface area of ​​the joint, the researchers report.

“In the bigger picture, this is just one of many processes in the deep sea that we’re only now discovering,” says Lisa Levin, a biological oceanographer at the Scripps Institution of Oceanography in La Jolla, California. More than half of the biodiversity in these ecosystems lives in the nodules, taking advantage of the hard surfaces for bases, but also to access the oxygen that is created there. It is not clear, she notes, whether organisms living in underlying sediments also depend on this local source of oxygen.

“It’s surprising that we didn’t know about it [process] before, we overlooked it,” says Beth Orcutt, a geomicrobiologist at the Bigelow Laboratory for Ocean Sciences in East Boothbay, Maine.

Deep-sea mining of metal nodules would trigger plumes of sediment that could accumulate and suffocate nearby unmined areas. If so, mining could reduce oxygen production there, Orcutt adds, though it’s unclear what that might do to the wider ecosystem. This reduction would be above and beyond the amount resulting from removing the nodes themselves.

“At this point,” she notes, “we don’t know if oxygen production has an impact beyond the area around the joints.”