Japan Deep-sea Impact Experiment (JET) is an in situ experiment aimed at evaluating environmental impacts and monitoring the recovery process arising from manganese nodule mining. The experiment has been done in 1994, and monitoring surveys have continued to 1996. However, due to short monitoring period, understandings of recovery process was insufficient. While 17–18 years after, the Deep Ocean Resources Development Co., Ltd revisited the site and surveyed the environmental condition. As a result, the environmental impacts, which was recognized immediately after and one year after the benthic disturbance were not recognized after 17–18 years. The present authors show the result and discuss the significance of long term monitoring survey.
With metal resources being run short, there has been increased attention given to the development of seabed mineral resources. At the same time, environmental impact assessments (EIA) have come to be required to be more rigorous than ever before (Fukushima and Nishijima, 2017). However, since there is no actual development being carried out, it's not easy to correctly assess the environmental impact. In 1972, the Lamont Doherty Geological Survey undertook a desk study in regard to the environmental impact of deep-sea mining (Ozturgut et al., 1997). Then the National Oceanic and Atmospheric Administration (NOAA) conducted a large-scale baseline study (DOMES: 1976–1980) (Ozturgut et al, 1978; Ozturgut, 1981); the results of that survey were used to list the factors that were thought to be potentially adverse impact to an environment in Programmatic Environmental Impact Statement (PEIS) (NOAA, 1980). The following year, NOAA narrowed the list down to two possibilities that were thought to be particularly serious (Marine Environmental Research Plan 1981–85) (NRC, 1984). The first possibility was that miner or collector could impact benthic organism directly (physically) while the other was that the resuspension and resettlement of marine sediment accompanying the operation of those machines could indirectly impact on benthic organisms in surrounding areas. Since then, NOAA has been tackling verification experiments such as the Acute Mortality Experiment (1987) (Smith et al., 1988) and QUAGMIRE Expedition II (1990) (Wilson, 1990), which are specific to the sediment's resuspension and resettlement (hereinafter, benthic disturbance). Then, in 1992, a new device aiming to create benthic disturbances (hereinafter, the disturber) was developed and the creation of benthic disturbance in the Clarion-Clipperton Fracture Zone (CCZ) was successful (Trueblood, 1993). This is known as the Benthic Impact Experiment (BIE) (1992). Subsequently, the same device was used to carry out the Japan Deep-sea Impact Experiment (JET) (1994) and IOM'BIE (1995) in the CCZ as well as the Indian Deep-sea Environment Experiment (INDEX) (1997) in the Indian Ocean (hereinafter, referred to as the BIE-type experiment) (Fukushima, 1994; Kotlinski, 1995; Sharma et al, 2000). As just described, until now, there has been a step-by-step consideration including desk studies, background surveys, a listing of potential effects, and the narrowing down of the same, followed by a verification experiment. Through four BIE-type experiments, it was revealed that benthic disturbance adverse effect on benthic organism, at least temporarily. However, the monitoring was discontinued one year later for BIE, two years later for JET, and five years later for IOMBIE, and three years later for INDEX (44 months). Therefore, the duration of the impact remained unknown.
