Towards Deep Sea Mining – Impact of mining activities on benthic pelagic coupling in the Clarion Clipperton Fracture Zone

As the worldwide metal demand is drastically increasing, and the metal supply are limited, countries and industries have prospected the deep-sea for their precious resources. Polymetallic nodules form on the seafloor and are rich in metals such as manganese, cobalt, and copper. Due to their economic potential, polymetallic nodules have been targeted for mining during the past three decades. The highest nodule’s abundance so far has been found in the Clarion Clipperton Zone (CCZ) where currently 16 license areas have been issued for exploration only. Very limited information is available to assess the anthropogenic impact of mining activities on this fragile ecosystem. The main objective of this dissertation is to evaluate the impact of sediment plumes released during mining activities on the benthic and pelagic environment.
The oceanographic description of the eastern German License area (CCZ) indicated that surface waters were influenced by nutrient-enriched water coming from the Mexican coastal area. During winter a depleted transitional zone appeared between these water masses and the highly productive water of the equatorial region. The vertical particle distribution was investigated using a combination of high definition camera images and CTD sensors. Specific water characteristics such as the oxygen minimum zone (OMZ) was identified and appeared to be strongly correlated to particulate matter alteration, distribution and transfer to greater depths. Overall, median particle sizes are small (77μm), and bigger particles (>300 μm) were rarely encountered.
Polymetallic nodules occur in the CCZ abyssal plains which are made of fine siliceous ooze sediments (d50= ± 20 μm). The anticipated impact of mining activities was tested by reproducing variable sediment plumes concentration (35–500 mg⋅L-1) and turbulence rate (0–10.4 s-1). The most rapid sediment plume flocculation was found using a high sediment discharge (500 mg⋅L-1) combined with a turbulent shear rate of ≥ 2.4 s-1. The modeling results simulating four days of plume release suggested that mining under “normal” flow conditions (± 4 cm⋅s-1) would result in a relatively fast deposition of particles, thus restricting the blanketing effect to a relatively small fall-out area (up to 4 km downstream). Setting such sediment plumes requirements (sediment concentration and turbulence rate) should, therefore, be considered for the design of the mining collector exhaust pipe. To date, the work in this thesis represents the most advanced and complete dataset freely available to everyone.
The bacterial community associated with the sediment plume was investigated using a cultivation-based approach. In total, 40 fast-growing bacterial strains belonging to 13 fast-growing bacterial species were identified. Based on their phenotypic characterization and heavy metals resistance, three potential organisms were found to be potential model systems for future ecological studies. Intra-species variabilities were not only found in phenotypic profiles but also in heavy metal tolerance although the taxonomic marker 16s rRNA sequences were almost identical. We proposed the use of API strips and partial-enrichment media that can easily be implemented on-board for rapid and cost-effective monitoring of deep-sea mining plume dispersion using microbial dissemination analyses. In this context, heavy-metal resistance analysis provides new scope for future research on Mn2+ resistance pathways and their role in microbial dispersion from anthropogenic impacts on deep-sea environments.
The exploitation of deep-sea resources is developing faster than policies are established and still requires extensive exploration. A precautionary approach based on environmental awareness and the history of similar activities suggest that deep-sea mining will have an impact on the marine environment. However, if mining is approved, the studied impacts will play a crucial role in regulating the mining activities. Overall, this thesis complements the scientific efforts made by describing the deep-sea environment and provide valuable knowledge on how to reduce the environmental impacts from sediment plumes released during mining activities in the CCZ.