The DORD multi-decadal time series of the deep-sea sediment environment in the Clarion-Clipperton Zone

Natural variability in the deep sea remains poorly understood, especially temporal variability. The growing likelihood of industrial activities and subsequent impacts occurring here necessitates using all available data to the fullest to address the issue. This study examines sediment characteristics and associated infaunal communities sampled over nearly 3 decades (1991–2019) in the Clarion-Clipperton Zone (CCZ) by Japan. Samples were collected in two regions: the location of the Japanese Deep Sea Impact Experiment (JET) in the southeast of the license area, and the high nodule abundance area (HAA) in the north/northwest. When examining temporal variability, abiotic parameters differed among most years in both regions. Microbial counts and abundances of several meiofaunal taxa also differed among years. Macrofaunal abundances were generally too low for robust analyses. Exploration of sites across the HAA revealed possible spatial heterogeneity from west to east in abiotic parameters including several heavy metals; however, these differences did not appear to result in community changes. Stations in the JET region were sampled over a much smaller area. While explanations for interannual differences are explored, rejecting the hypothesis that they are at least partially caused by methodological differences in sample collection and processing among times is not possible. Fortunately, samples throughout the approximately 30-year time series were collected by the same research group with consistent standard operating procedures. These results suggest that conditions in the CCZ seafloor may vary with time and highlight the need to sample deep-sea locations repeatedly over multiple years before industrial activities take place for accurate environmental impact assessments. This study also includes invaluable metadata for publicly available Deep Ocean Resources Development data in the CCZ and illustrates the need for strong and standardized metadata recording in the deep sea so that future studies and generations can utilize the deep-sea data that came before.