Description: Die Erde hält eine Vielzahl natürlicher mineralischer Rohstoffe bereit, die wir in unserer hochtechnisierten Gesellschaft benötigen. Zurzeit werden fast alle diese Rohstoffe auf einer Fläche abgebaut, die weniger als ein Drittel unseres Planeten ausmacht -der Landfläche. Hier wird es jedoch immer schwieriger, reiche Vorkommen zu finden. Dies zwingt die Bergbauindustrie dazu, Lagerstätten mit geringeren Konzentrationen abzubauen bzw. nach Lagerstätten in abgelegenen Regionen der Erde oder in großer Tiefe zu suchen. Dies ist jedoch mit einem erheblichen Anstieg des Flächenverbrauchs und mit zusätzlichen Umweltbeeinträchtigungen verbunden.

Description: Exploitation of mineral ores from the deep sea will impact the abyssal environment by removing the mineral deposits and sediments from the seafloor surface, where most deep-sea benthic life is found. Additional effects are expected from the blanketing of the mined area and the pristine surrounding seabed with sediments and/or mineral debris. As a consequence, seafloor integrity is lost in the impacted area, species densities and biodiversity are reduced, and ecosystem functions are negatively affected. Although a lot of open questions remain regarding, for example, indicator species, disturbance thresholds, and renaturation options, it is becoming increasingly clear that the induced environmental impacts last for at least many decades to centuries and affect all ecosystem compartments.

Description: Cruise SO268 is fully integrated into the second phase of the European collaborative JPI-Oceans project MiningImpact and is designed to assess the environmental impacts of deep-sea mining of polymetallic nodules in the Clarion-Clipperton Fracture Zone (CCZ). In particular, the cruise aimed at conducting an independent scientific monitoring of the first industrial test of a pre-protoype nodule collector by the Belgian company DEME-GSR. The work includes collecting the required baseline data in the designated trial and reference sites in the Belgian and German contract areas, a quantification of the spatial and temporal spread of the produced sediment plume during the trials as well as a first assessment of the generated environmental impacts. However, during SO268 Leg 1 DEME-GSR informed us that the collector trials would not take place as scheduled due to unresolvable technical problems. Thus, we adjusted our work plan accordingly by implementing our backup plan. This involved conducting a small-scale sediment plume experiment with a small chain dredge to quantify the spatial and temporal dispersal of the suspended sediment particles, their concentration in the plume as well as the spatial footprint and thickness of the deposited sediment blanket on the seabed. Leg 1 and 2 acquired detailed environmental baseline data in the designated collector trial and reference sites as well as the site of the small-scale sediment plume experiment. The plume experiment was monitored by an array of acoustic and optical sensors and the impacted area was investigated in order to develop standards and protocols for impact assessments and recommendations for marine policy and international legislation. A more technical aim of the cruise was to test tools, technologies, and a concept for the environmental monitoring of future deep-sea mining operations. This comprised oceanographic, biological, microbiological, biogeochemical, and geologic investigations which required the deployment of a multitude of seagoing equipment, such as ROV Kiel 6000 for sampling of sediments, nodules, and benthic fauna as well as carrying out in situ measurements and experiments, and the deployment of the plume sensor array. AUV ABYSS and ROV Kiel 6000 were used for high-resolution acoustic mapping of the seafloor using mounted multibeam systems and video/photo surveys of the manganese nodule habitat. This work was accompanied by video observations with the OFOS system. Several benthic landers and moorings with acoustic and optical sensors were deployed and recovered for the measurements of physical and chemical oceanographic variables. Coring devices (i.e., box corer, gravity corer, TV-guided multiple corer, ROV-operaten push cores) were used to collect sediment samples for biological, geochemical, and microbiological analyses, and a CTD rosette water sampler, in situ pumps, and a bottom water sampler sampled the water column. In addition, recolonization experiments for nodule-associated fauna were started by deploying artificial hard substrates on the seabed of the working areas.

Description: Deep-sea mining for polymetallic nodules is expected to have severe environmental impacts because not only nodules but also benthic fauna and the upper reactive sediment layer are removed through the mining operation and blanketed by resettling material from the suspended sediment plume. This study aims to provide a holistic assessment of the biogeochemical recovery after a disturbance event by applying prognostic simulations based on an updated diagenetic background model and validated against novel data on microbiological processes. It was found that the recovery strongly depends on the impact type; complete removal of the reactive surface sediment reduces benthic release of nutrients over centuries, while geochemical processes after resuspension and mixing of the surface sediment are near the pre-impact state 1 year after the disturbance. Furthermore, the geochemical impact in the DISturbance and reCOLonization (DISCOL) experiment area would be mitigated to some degree by a clay-bound Fe(II)-reaction layer, impeding the downward diffusion of oxygen, thus stabilizing the redox zonation of the sediment during transient post-impact recovery. The interdisciplinary (geochemical, numerical and biological) approach highlights the closely linked nature of benthic ecosystem functions, e.g. through bioturbation, microbial biomass and nutrient fluxes, which is also of great importance for the system recovery. It is, however, important to note that the nodule ecosystem may never recover to the pre-impact state without the essential hard substrate and will instead be dominated by different faunal communities, functions and services.

Description: Exploitation of mineral ores from the deep sea will impact the abyssal environment by removing the mineral deposits and sediments from the seafloor surface, where most deep-sea benthic life is found. Additional effects are expected from the blanketing of the mined area and the pristine surrounding seabed with sediments and/or mineral debris. As a consequence, seafloor integrity is lost in the impacted area, species densities and biodiversity are reduced, and ecosystem functions are negatively affected. Although a lot of open questions remain regarding, for example, indicator species, disturbance thresholds, and renaturation options, it is becoming increasingly clear that the induced environmental impacts last for at least many decades to centuries and affect all ecosystem compartments.

Description: Future supplies of rare minerals for global industries with high-tech products may depend on deep-sea mining. However, environmental standards for seafloor integrity and recovery from environmental impacts are missing. We revisited the only midsize deep-sea disturbance and recolonization experiment carried out in 1989 in the Peru Basin nodule field to compare habitat integrity, remineralization rates, and carbon flow with undisturbed sites. Plough tracks were still visible, indicating sites where sediment was either removed or compacted. Locally, microbial activity was reduced up to fourfold in the affected areas. Microbial cell numbers were reduced by ~50% in fresh “tracks” and by 〈30% in the old tracks. Growth estimates suggest that microbially mediated biogeochemical functions need over 50 years to return to undisturbed levels. This study contributes to developing environmental standards for deep-sea mining while addressing limits to maintaining and recovering ecological integrity during large-scale nodule mining.

Description: The thriving interest in harvesting deep-sea mineral resources, such as polymetallic nodules, calls for environmental impact studies and, ultimately, for regulations for environmental protection. Industrial-scale deep-sea mining of polymetallic nodules most likely has severe consequences for the natural environment. However, the effects of mining activities on deep-sea ecosystems, sediment geochemistry and element fluxes are still poorly understood. Predicting the environmental impact is challenging due to the scarcity of environmental baseline studies as well as the lack of mining trials with industrial mining equipment in the deep sea. Thus, currently we have to rely on small-scale disturbances simulating deep-sea mining activities as a first-order approximation to study the expected impacts on the abyssal environment. Here, we investigate surface sediments in disturbance tracks of seven small-scale benthic impact experiments, which have been performed in four European contract areas for the exploration of polymetallic nodules in the Clarion–Clipperton Zone (CCZ) in the NE Pacific. These small-scale disturbance experiments were performed 1 d to 37 years prior to our sampling program in the German, Polish, Belgian and French contract areas using different disturbance devices. We show that the depth distribution of solid-phase Mn in the upper 20 cm of the sediments in the CCZ provides a reliable tool for the determination of the disturbance depth, which has been proposed in a previous study from the SE Pacific (Paul et al., 2018). We found that the upper 5–15 cm of the sediments was removed during various small-scale disturbance experiments in the different exploration contract areas. Transient transport-reaction modeling for the Polish and German contract areas reveals that the removal of the surface sediments is associated with the loss of the reactive labile total organic carbon (TOC) fraction. As a result, oxygen consumption rates decrease significantly after the removal of the surface sediments, and, consequently, oxygen penetrates up to 10-fold deeper into the sediments, inhibiting denitrification and Mn(IV) reduction. Our model results show that the return to steady-state geochemical conditions after the disturbance is controlled by diffusion until the reactive labile TOC fraction in the surface sediments is partly re-established and the biogeochemical processes commence. While the reestablishment of bioturbation is essential, steady-state geochemical conditions are ultimately controlled by the delivery rate of organic matter to the seafloor. Hence, under current depositional conditions, new steady-state geochemical conditions in the sediments of the CCZ are reached only on a millennium scale even for these small-scale disturbances simulating deep-sea mining activities.

Description: Industrial-scale mining of deep-sea polymetallic nodules will remove nodules in large areas and impact the physical integrity of the seafloor. However, environmental standards for seafloor integrity and studies of recovery from environmental impacts are still largely missing. Further we have only a poor understanding of the role of nodules in shaping benthic microbial diversity and element cycles. We revisited the deep-sea disturbance and recolonization experiment carried out with a towed plough harrow in 1989 in the Peru Basin nodule field within a circular area of approx. 3.5 km diameter (〉4100 m water depth). In the experimental area, the 26 years old plough tracks were still visible and showed different types and levels of disturbance such as removal and compaction of surface sediments. Microbial communities and their diversity were studied in disturbance tracks and undisturbed sites and related to habitat integrity, remineralization rates, and carbon flow. Locally, microbial activity was reduced up to 4 times in the impacted areas. Microbial cell numbers were reduced by ~50% in fresh, and by 〈30% in the old tracks. Our data suggest that microbially-mediated biogeochemical functions need more than 50 years to return to undisturbed levels in the sediments. In areas with nodules (i.e., outside the disturbance tracks) microbial communities in the nodules themselves were studied. Nodule communities were distinct from sediments and showed a lower diversity and a higher proportion of sequences related to potential metal-cycling bacteria (i.e. Magnetospiraceae, Hyphomicrobiaceae), bacterial and archaeal nitrifiers (i.e. AqS1, unclassified Nitrosomonadaceae, Nitrosopumilus, Nitrospina, Nitrospira), as well as bacterial sequences typically found in ocean crust, hydrothermal deposits and sessile fauna. Our results confirm that nodules host specific microbial communities with potentially significant contributions to organic carbon remineralization and metal cycling. This study contributes to developing environmental standards for deep-sea mining and highlights the limits for maintaining and recovering ecological integrity and functions during large-scale nodule mining.