New research shows deep-sea mining effects on the seafloor remain visible 44 years later. Scientists from the University of Liverpool studied a 1979 test site in the Pacific Ocean. They found that mining damaged organic matter, which supports life. This discovery suggests that deep-sea ecosystems struggle to recover after disturbance.

Little is known about the long-term effects of deep-sea mining on the organic matter that supports life on the seafloor. However, as interest in mining the deep sea for rare metals grows, there is concern that disturbing the seabed could affect the ecosystems of the deep ocean.

New research by the University of Liverpool published in Elementa: Science of the Anthropocene provides evidence that the effects of deep-sea mining on the organic matter that supports life on the seafloor can still be found more than four decades later.

In 2023, a team of researchers from the SMARTEX (Seabed Mining and Resilience To EXperimental Impact) project investigated an area of seafloor in the Clarion-Clipperton Zone of the Pacific Ocean that had been used as a test site for deep-sea mining by the Ocean Minerals Company in 1979. The tracks of the 100-ton mining machine are still visible on the seafloor.

The team spent a month collecting sediment samples from the seabed at a depth of about 4,800 meters (15,750 feet), both within and around the historical mining disturbance and at an undisturbed control site.

They analyzed the quantity and quality of particulate organic matter in the sediments. This organic matter is an important food source for organisms living on and within the seafloor and provides essential nutrients, such as omega-3 fatty acids, that support growth, development and reproduction.

The team assessed organic matter quality using a range of measurements, including carbon and nitrogen content, lipid biomarkers and stable isotope analysis.

The study reveals differences in organic matter composition and particle size, coupled with lower quantity and quality of organic matter at the historic mining site more than four decades after completion of the test mining.

Using still and video images from the area, the SMARTEX project identified lower abundances of both larger animals and single-celled organisms living in the sediments affected by the test mining. This study linked these lower organism abundances to the depleted organic matter observed in the mined area, suggesting implications for the recovery and resilience of deep-sea animals to mining impacts.

Implications for future deep-sea mining

The Clarion-Clipperton Zone is a huge area of ocean located in the eastern and central Pacific Ocean between Hawaii and Mexico. There is significant interest in mining its polymetallic nodules, which contain rare and critical metals used in modern electronics, such as rechargeable batteries and touch screens, and in manufacturing.

The researchers say their findings demonstrate the importance of considering long-term changes to the availability and quality of organic matter when assessing the environmental impacts of deep-sea mining.

Dr. Rachel Jeffreys is a marine biologist at the University of Liverpool and the lead author of this study. She said, "Deep-sea mining is likely to alter the seafloor by disturbing and moving sediments, but we still do not understand the full range of environmental impacts or how these disturbances will affect the species and ecosystems that live in the deep sea. Our study provides long-term evidence of changes in organic matter following a major disturbance to the seabed. Remarkably, we can still detect the effects of a mining test 44 years after it took place.

"Changes in the quantity and quality of organic matter could affect the food available to organisms living on the seafloor and, in turn, influence how ecosystems recover from disturbance. These findings are particularly important as proposals to advance commercial deep-sea mining continue to gather momentum.

"They highlight the need for long-term monitoring and for the potential impacts of changes to organic matter availability and quality to be considered in environmental management and regulation of deep-sea mining in the Clarion-Clipperton Zone. Further research is needed to understand how organic matter varies across the wider region and how these changes affect the recovery and resilience of deep-sea ecosystems."