[Image above] Credit: cottonbro studio, Pexels
In the August 2024 Bulletin that publishes online tomorrow, we dive below the surface of the emerging yet controversial topic of deep-sea mining.
Deep-sea mining is one pathway being explored to fulfill the need for minerals as part of the green energy transition. It involves extracting valuable mineral resources from the deep seabed, which scientists warn may generate harmful, potentially irreparable environmental impacts to the poorly understood ecosystem.
The August cover story by University of Bern climate and environmental physics professor Thomas Frölicher and University of Lausanne geological sciences professor Samuel Jaccard thoughtfully lays out the potential impacts known so far and acknowledges the key scientific knowledge gaps that remain.
However, given that deep-sea scientific research is challenging as well as time and resource-intensive, “closing these gaps is likely to require substantial time and a capacity-intensive, coordinated scientific effort,” they write.
Last week, another piece of this complex puzzle came to light in an open-access paper published by an international team of researchers in the United States, the United Kingdom, and Germany. Specifically, they reported that the polymetallic nodules on the abyssal seafloor, which are a target of deep-sea mining activities, may play an integral role in “dark” oxygen production.
Dark oxygen refers to the oxygen that is prevalent in deep-sea surface sediments. Deep-seafloor organisms rely on this oxygen, but the mechanisms that produce this oxygen are unknown because it is too dark for the sun-driven method of photosynthesis.
In the recent study, the researchers placed benthic landers to measure abyssal sediment community oxygen consumption at various locations across the Clarion-Clipperton Zone, which is a focus of deep-sea mining efforts in the middle of the north Pacific Ocean. In contrast to previous deep-sea oxygen flux studies, they found that more oxygen was consistently accumulating in the landers than was being consumed, resulting in net oxygen production.
Because this finding contrasts with all published deep-sea benthic oxygen flux studies, the researchers closely investigated their experimental setup to determine if it was causing these unexpected results. But after ruling out experimental factors as well as biological mechanisms from deep-seafloor organisms, they considered the possibility that the polymetallic nodules had a hand in this situation, as dark oxygen production was measured in ex situ control chambers containing only polymetallic nodules.
The researchers determined that a small amount of charge—1.5 V, or an AA battery—is enough to make seawater undergo electrolysis, a reaction where water is split into hydrogen and oxygen. Analysis revealed that individual polymetallic nodules have voltage potentials up to 0.95 V on their surfaces, and so placing multiple nodules together could provide the zap needed to make oxygen out of seawater.
The researchers emphasize that this “geobattery” hypothesis is preliminary and many questions remain concerning the potential mechanism, including longevity of dark oxygen production, catalytic stabilities, electrochemical conditions on exposed versus buried nodules surfaces, and the influence of different chemistries within the nodule layers.
But if further investigation shows this hypothesis holds water, it invites “the urgent question of how sediment remobilization and distribution over large areas during deep-sea mining may influence [dark oxygen production],” the researchers write.
The open-access paper, published in Nature Geoscience, is “Evidence of dark oxygen production at the abyssal seafloor” (DOI: 10.1038/s41561-024-01480-8).