A 1979 mining test still leaves traces in Pacific seafloor sediments
· BehindLines newsroom
Forty-four years after a deep-sea mining trial, the sediments on the Pacific seabed still hold less organic matter, and of lower quality, than those of a neighbouring undisturbed area. This is the conclusion of a study led by Rachel Jeffreys of the University of Liverpool, which the website Phys.org presented on 2 November 2026. The trial itself is old (March 1979) and the samples date from 2023: the paper was published on 2 November 2026.
What the study shows
According to Phys.org, the work appeared in the journal Elementa: Science of the Anthropocene (volume 14, issue 1, article 00106) under the title "Long-term impacts of deep-sea mining on sediment organic matter quantity and quality" (DOI 10.1525/elementa.2025.00106). The full study, consulted for this correction, confirms publication on 2 November 2026.
The site studied lies in the Clarion-Clipperton Zone, in the heart of the Pacific, where the company Ocean Minerals Company (OMCO) carried out a mining trial in March 1979. According to Phys.org, a 100-tonne machine left tracks there that are still visible today. In 2023, the team spent about a month on site and took sediment cores at a depth of about 4,800 metres, using ROV ISIS in the tracks and plume area, and a Megacore sampler at the control site about 2 kilometres away.
The researchers measured the quantity and quality of organic matter particles through their carbon and nitrogen content, lipid biomarkers and stable-isotope analysis. The result: in the mined zone, organic matter is less abundant and of lower quality, with a composition and particle size different from the control site. Still and video images also show fewer large animals and fewer single-celled organisms in the disturbed sediments. The authors link these low abundances to the depletion of organic matter. Phys.org gives no percentages or counts.
Why it matters
On the ocean floor, organic matter sinking from the surface serves as food for organisms. According to Rachel Jeffreys, quoted by Phys.org, changes in the quantity and quality of this matter can affect the food available to seabed life, and therefore how communities recover after mining. She says the effects of a mining trial can still be detected more than four decades later and that the results underline the need for long-term monitoring.
A site already studied in 2025
The same area was the subject of a study published in Nature, presented by the University of Liverpool on 26 March 2025. Carried out within the NERC-funded SMARTEX project, with a 2023 cruise aboard the RRS James Cook, it was led by Daniel Jones of the National Oceanography Centre. According to that press release, the tracks still looked much as they did originally; animal numbers there were reduced, with signs of partial return for small mobile animals living on the sediment surface and for xenophyophores, large single-celled organisms. Large animals fixed to the seabed remained very rare. Sediment plumes, long considered a major threat, had had limited long-term physical effects and no detectable negative effect on numbers. Available food appeared comparable inside and outside the tracks, but patchier within them. The 2026 study extends those 2025 analyses; it does not measure a change between two cruises.
Limits
This is a single site and a single trial, carried out almost half a century ago; its results alone do not allow the effects of commercial mining to be predicted. According to the University of Liverpool, Adrian Glover, a co-author of the 2025 study, also stresses that the results do not settle the question of the social acceptability of deep-sea mining. The authors propose associations between organic matter and animal abundance; these alone do not demonstrate causation.