Using Hydrographic Surveying For Investigating Deep Ocean Polymetallic Nodules Areas
The Global Ocean covers approximately 71% of the Earth’s surface, and although humans have been conducting marine activities for thousands of years, we still know very little about the ocean’s deepest portions due to inaccessibility and the lack of high-resolution data. Relatively recent scientific and technological advancements (e.g. sonar, plate tectonics theory) have allowed us to better understand the benthic environments, and to observe large scale morphology of the seabed. Thus we have been able to identify the resource potential of some remote areas.
As the Earth’s population continues to grow, so does our need for renewable energy, and the emergence of concepts such as the Blue Economy and the Blue Growth strategy offers the opportunity to explore and develop marine activities in a sustainable manner [1,2]. One example of such resources are the polymetallic nodules that cover large swaths of the ocean floor, and are proven to contain important quantities of critical metals. This article examines what these polymetallic nodules are, and how we can locate and quantify them using hydrographic surveying methods.
Deep-ocean polymetallic nodules are round to ovoid-shaped concretions of Fe and Mn oxyhydrates and oxides that occur as alternating laminae, which also adsorb metals such as Ni, Co, Cu, Mo and REE’s [1,2,3,5]. They were first discovered by the HMS Challenger expedition of 1872–1876 while studying the seabed at approximately 4500m depth. The nodules cover large areas of the seabed in the Pacific and Indian Oceans, with the highest quantities recorded in the Clarion-Clipperton Zone (CCZ) between Hawaii and Mexico (estimated 21 billion dry tonnes)[3]. The Atlantic Oceans is also thought to contain considerable volumes of these nodules, however more in-depth studies are to conducted in these regions [2]. The nodules, which range in size from 2 to 10 cm on the long axis, form by either hydrogenetic or diagenetic precipitation around a hard nucleus. Hydrogenetic precipitation occurs when Fe2+ and Mn2+ are oxidized in oxygen-rich waters, which makes growth rates of FeOOH and δMnO2 (vernadite) layers very slow (a few mm per million years). These are usually rich in Pt, Co and other REE’s. On the other hand, diagenetic precipitation occurs in the sediment when decaying organic matter leads to oxidized Mn that is released, which scavenges metal ions from the pore fluids to balance its negative charge deficit. They show much higher growth rates, of tens of mm per million years, and are rich in Ni, Cu and Li. Therefore it is possible to identify the place and method of formation from the exact composition of the nodules [3].
The polymetallic nodules appear to host significant quantities of metals with high applicability in a variety of industries, most notably in the production of electricity through “green” and sustainable methods. As the majority of the land-based reserves of such metals are located in potentially unstable countries (e.g. D.R. Congo and China control the cobalt and REE’s supplies, respectively), this has attracted the interest of industrialized countries (e.g. Germany, Japan, China, Republic of Korea etc.) who seek to avoid the risk of scarcity by developing this resource. The International Seabed Authority (ISA) has leased out 18 limited areas for exploration and research, to both private and state owned companies [1].
As shown above, the remoteness of the exploration areas and very deep waters (~4000–6000 m) are the main challenges that organizations have had to overcome. The ocean bottom is particularly little known, but due to recent technological advances, high-resolution charts can be created in order to outline the potential mining zones. Due to the complexity of the task, I have identified 3 key phases for deep seabed exploration, by using established hydrographic surveying equipment. Depending on logistics, they can be condensed into two phases.
Phase 1. The first phase in the exploration of the seabed would be the examination of the areas of interest through low-resolution bathymetry. This would provide an overview of the ocean floor morphology, outlining any potential large-scale obstacles on the bottom, such as seamounts. A hull-mounted multibeam echosounder (MBE) operating at low frequency in order to limit absorption of the sound wave in the water column (< 8 kHz) can be used for this operation. A MBE sends multiple sound signals that are reflected back by the ocean floor. The arrival time of the deflected wave is used to compute the distance travelled, i.e. the water depth. This method has the advantage of covering large swaths of the seafloor, which makes it quite cost effective [4].
Phase 2. The scope of the second exploration phase would be to obtain better quality data that could help identify nodule-rich areas. Scientific studies and previous exploration surveys hint at flat abyssal plains (< 3 degrees slope) as the primary mining fields. This phase would therefore locate the flatter areas identified during phase 1, and conduct high-resolution bathymetry using a deep-towed “fish” or AUV (Autonomous Underwater Vehicle) at maximum 100 m above the seabed to allow higher frequency pulses (> 200 kHz). Alongside bathymetry, backscatter data can be collected in order to measure the hardness of the seabed, which could lead to identifying sectors with a high concentration of nodules. The AUV may also conduct a side-scan sonar (SSS) survey, which sends high frequency pulses (> 300 kHz) at oblique angles from 2 transducers, generating fan-shaped pulses that reflect and help identify any smaller scale objects on the ocean floor [4]. However, due to their small dimensions (2–10 cm) nodules would not be readily identifiable in the SSS data. As the nodules are typically located above, or are partially buried in the sediment layer, the use of a sub-bottom profiler might not be required, unless the exploration programme desires the acquisition of below-surface data for a better understanding of the ocean floor geology or the benthic habitat. (An interesting point is that, although the sedimentation rates at the bottom of the ocean are higher than the growth rate of both hydrogenetic and diagenetic nodules, they are rarely fully covered in sediment. More research needs to be conducted on this.) At the end of this phase, GIS software can be used to pinpoint areas with the desired slope value.
Phase 3. The final phase of the surveying programme would be the ground-truthing part. This is an essential part of every survey as it verifies the data gathered by MBE and SSS systems, by video imaging and sampling key areas. Sampling should be done by box corers, which are tools used in soft sediments and can penetrate approximately 50 cm into the seabed and obtain relatively undisturbed samples. This could be used to spot potential buried nodules, as well as provide a way to measure the quantity of nodules per square meter (usually 10 kg/m2 desired). With data acquired from multiple locations, kriging interpolation can be applied to predict the nodule abundance in the entire surveyed area, while accounting for local and regional variabilities. The abundance map obtained this way can be superimposed with the slope map obtained in Phase 2, to find the best possible mining zones.
The oceans are to be rich in deep-ocean polymetallic nodules that are a valuable source of useful metals and REE’s that modern society requires to achieve its energy goals. While the remoteness and ocean depth are major challenges for exploration programmes, the hydrographic surveying techniques MBE and SSS, as well as machine learning and GIS software can be used successfully to investigate the potential nodule-bearing areas. More research remains, however, to be conducted on the impact of mining activities on the pelagic and benthic habitats in the explored areas.
List of references:
1. Volkmann SE (2018) Blue Mining — Planning the Mining of Manganese Nodules (Doctoral dissertation, RWTH Aachen University, Aachen, Germany)
2. Rozemeijer M, Van den Burg S, Jak R, Lallier LE, Van Craenenbroek K (2018) Seabed Mining. In: Johson K, Dalton G, Masters I (eds) Building Industries at Sea: ’Blue Growth’ and the New Maritime Economy 2018. River Publishers
3. Hein JR, Koschinsky A, Kuhn T (2020) Deep-ocean Polymetallic Nodules as a Resource for Critical Materials. Nature Reviews Earth&Environment
4. Penrose JD, Siwabessy PJW, Gavrilov A, Parnum I, Hamilton LJ, Bickers A, Brooke B, Ryan DA, Kennedy P (2005) Acoustic Techniques for Seabed Classification. CRC for Coastal Zone, Estuary and Waterway Management
5. Hein JR, Koschinsky A (2014) Deep-ocean Ferromanganese Crusts and Nodules. In: Holland HD, Turekian KK (eds) Treatise of Geochemistry, second edition, v. 13, chapter 11, 2014. Elsevier
