Editorial for Special Issue “Deep-Sea Ferromanganese Nodules and Related Mineral Resources: Genesis, Exploration, and Mining”

The development of deep-sea ferromanganese nodules has the potential to provide critical metals for the creation of high-tech and green-tech technologies and devices. Recent remarkable technological innovations have created an opportunity for obtaining and analyzing geological (or petrological) and engineering information about nodules, and the feasibility of extracting them. This Special Issue is our first attempt at gathering information on genetic models; exploration methods; and mining technologies of nodules, ferromanganese crust, and subseafloor sedimentary materials. Here, we introduce the eight papers in this Special Issue and provide our overarching conclusions.

A new mining model and technique were proposed by Yamazaki et al. [1] and Li et al. [2]. Yamazaki et al. [1] showed that ferromanganese nodules are an important contributor to increasing the feasibility of mining of pelagic mud enriched in rare-earth elements and yttrium (REY-rich mud) around Minamitorishima (Marcus) Island, western North Pacific. The authors discussed that, to economically collect REY-rich mud, the co-existing nodules are not a nuisance. It is preferable to extract them using a pulp-lifting system through a riser pipe. Their proposed unique and specific mining management plan for the combined mining of REY-rich mud and nodules provides important insights into the next step in the exploitation of the seafloor mineral resources. The verification of economic feasibility, as envisioned in this paper, also contributes to the realization of nodule exploitation. Li et al. [2] demonstrated that nodules degrade during transport from the seafloor to the surface. The particle size of nodules change with repeated impeller blade impact and fragmentation in the deep-sea multistage lifting motor pump, thereby affecting the design parameters of the processing equipment and the number of elements that can be recovered. This means that the economic feasibility of nodule mining depends critically on appropriate equipment design commensurate with actual geological conditions. To solve this problem, Li et al. [2] have proposed a new method to calculate the rate of degradation.

In addition to these specific cases, a comprehensive review of the development of nodule mining is provided by Kang and Liu [3]. They showed a possible facility design on the basis of the recent progress of mining trials in China and South Korea. Their design consists of a centrifugal pump, a hydraulic pipe lift, and a self-propelled tracked nodule collector and is as an alternative to an air lifting system.

The importance of investigations about other mineral resources related to ferromanganese nodules was highlighted by Yasukawa et al. [4], Hong et al. [5], and Pérez et al. [6]. Yasukawa et al. [4] focused on stratigraphic variations in geochemistry, morphology, and surficial textures of micronodules in REY-rich mud. Their results showed that micronodules are an important indicator of the environment of sedimentation in forming REY-rich mud at the seafloor surface and indicate the history of oxygen supply or consumption in response to the dynamics of bottom currents. Hong et al. [5] proposed an exploration method to recognize ferromanganese crusts using a combination of a well-designated parametric acoustic probe and a deep learning-based algorithm. Their method is suitable for real-time detection of the presence of ferromanganese crusts with a high recognition rate and small computational burden. This is expected to significantly increase the efficiency of surveys of the ferromanganese crust. Detailed lithological descriptions of sedimentary materials from the landward slope of the Chile trench, reported by Pérez et al. [6], provides information on the formation of mineral resources (hydrothermal sulfide and ferromanganese nodules) in the surrounding area.

Finally, the guest editors and their co-workers [7,8] propose a new strategy for studying ferromanganese nodules using non-destructive or less destructive analyses. Their comprehensive, exhaustive, and multidisciplinary approach is used to reveal the nature and genesis of nodules. Nakamura et al. [7] demonstrated a three-dimensional layered growth structure of 934 ferromanganese nodule samples using X-ray computed tomography. We examined almost all collected samples from regions around Minamitorishima Island. Therefore, Nakamura et al. [7] presented a general process of nodule growth in the region on the basis of a comprehensive data set of the layer structure. By using this data set and considering the representativeness of each sampling site, Machida et al. [8] investigated the fine-scale chemical structure of nodules using microfocus X-ray fluorescence. Both papers conclude that (1) the nodules grew equally in all directions regardless of the variations in shape and size and form a similar pattern of growth as vast as an entire nodule field and (2) the initiation of nodule growth was intermittent rather than simultaneous. Machida et al. [8] further proposed that the intermittent beginning of the growth of nodules was regulated by the dynamics of the Lower Circumpolar Deep Water (LCDW) flowing along the topographic flamework.

The guest editors acknowledge the important contributions of the authors of this Special Issue. Their contributions provided new techniques and points of view, as well as the direction we should aim for in the future studies. At the same time, it has also highlighted specific issues that need to be resolved for the future exploitation of these seafloor mineral resources. For example, Yamasaki et al. [1] demonstrated the economic importance of developing REY-rich mud and nodules together. We recommend additional studies for further refining development system design and economic evaluation, taking into account nodule degradation as revealed by Li et al. [2]. We look forward to seeing new exploration results using the methods proposed by Hong et al. [5] and Pérez et al. [6]. Although our model for the intermittent beginning of the growth of nodules [8] needs to be tested in other regions along the LCDW, we hope that our approach [7,8] will be a future standard of nodule investigation. Moreover, micronodules [4] could be a key material in investigating the genetic relationship between nodules and REY-rich muds in terms of the dynamics of bottom currents.

There is a need for further research and investment into ferromanganese nodules and related mineral resources by the minerals industry and by governmental sectors. We believe that this Special Issue will result in future important advances towards utilizing deep-sea ferromanganese nodules and related seafloor mineral resources.