Long-term impacts of deep-sea mining on sediment organic matter quantity and quality

Particulate organic matter provides energy for life in the ocean and plays a critical role in the global carbon cycle. It is the primary food source for benthic fauna inhabiting abyssal plains (3,000–6,000 m water depth) and is a key determinant of deep-sea biodiversity and community structure. The Clarion-Clipperton Zone of the eastern equatorial Pacific abyssal plain has high densities of polymetallic nodules, and there is increased interest in deep-sea mining for nodule exploitation. The effects of deep-sea mining on sediment particulate organic matter are largely unknown. In this research, we determined the quantity, quality and distribution of particulate organic matter in sediments at the Ocean Minerals Company test mine site (known as the OMCO site, central Clarion-Clipperton Zone) within the collector tracks and adjacent plume area compared to a control site 44 years after the mining test. We determined organic matter quantity and quality using total organic carbon and nitrogen, stable carbon and nitrogen isotopes and lipid biomarkers. Forty-four years later, we found differences in the particle size distributions between the Ocean Minerals Company test tracks and surrounding area. We also found that the quantity and quality of organic matter in the mining track sediments were depressed and more heterogeneous compared to the surrounding area and that these patterns were linked to particle size distributions, demonstrating the long-term impact of the Ocean Minerals Company mining test. Faunal densities (foraminifera, xenophyophores and megafauna) were also linked to organic matter quality, being higher at the control and plume area where organic matter quality was highest, suggesting that organic matter quality plays a key role in community composition. These results suggest that the long-term impacts of mining on organic matter availability in sediments will influence the recovery and resilience of benthic biological communities.