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.

Description: Sediment community oxygen consumption (SCOC) represents an established bulk measure of benthic activity. It addresses rates of organic matter remineralization as a key function of seafloor ecosystems. SCOC is also explicitly recommended by the International Seabed Authority as a variable for baseline investigations by exploration license holders (document ISBA/25/LTC/6). In preparation for an assessment of environmental impacts associated with the first test of a manganese nodule collector pre-prototype, oxygen flux measurements took place in working areas that were located in the German and Belgian exploration license areas in the Clarion Clipperton Fracture Zone (CCZ) across a spatial scale of approx. 1000km. The study was carried out in the framework of the European collaborative project MiningImpact under the Joint Programming Initiative Healthy and Productive Seas and Oceans (JPI Oceans). During RV SONNE expedition SO268, rates of total (TOU) and diffusive oxygen uptake (DOU) were quantified in situ with benthic chambers and microprofilers manipulated by remotely operated vehicle (ROV). Within each of the license areas, replicate measurements were obtained at different sites across several kilometers distance. Based on this extensive data set, the presentation aims to assess the requirements (e.g., in terms of replication, relevant spatial scales, methodology) for oxygen uptake observations in the context of environmental baseline studies. Lateral variability in fluxes is addressed as well as differences between total and diffusive fluxes and relations to other biogeochemical data obtained in sediment and pore water samples (e.g., nutrients, organic matter, chloroplastic pigments). Given that a follow-up expedition (‚MANGAN 2021‘) is successfully conducted, first data on immediate effects of mining-related disturbances on benthic oxygen distribution and fluxes will be included in the presentation.

Description: Sediment community oxygen consumption is an established measure of benthic activity and recommended by the International Seabed Authority for baseline investigations as part of exploration activities (document ISBA/25/LTC/6). It addresses rates of organic matter remineralization as a key function of seafloor ecosystems. In 2019, oxygen flux measurements were carried out at locations approx. 1000 km apart in the Clarion Clipperton Fracture Zone (CCZ) within the German and Belgian exploration license areas. Rates of total (TOU) and diffusive oxygen uptake (DOU) were quantified in situ with benthic chambers and microprofilers manipulated by remotely operated vehicle (ROV). The study was carried out in the framework of the European collaborative project MiningImpact under the Joint Programming Initiative Healthy and Productive Seas and Oceans (JPI Oceans). The primary aim was to settle a baseline in preparation of a subsequent assessment of the environmental impacts associated with the first test of a manganese nodule collector pre-prototype in international waters. In both license areas, replicate measurements were obtained at different sites across several kilometers distance. Based on this extensive data set, the presentation aims to assess the requirements (e.g., in terms of replication, relevant spatial scales, methodology) for oxygen uptake observations in the context of environmental baseline studies. Lateral variability in fluxes is addressed as well as differences between total and diffusive fluxes and relations to other biogeochemical data obtained in sediment and pore water samples (e.g., nutrients, organic matter, chloroplastic pigments). In addition, first data on immediate effects of the recently performed pre-prototype collector test on oxygen distribution in the upper sediment layer are presented.

Description: Der Tiefseebergbau wird als eine neue Möglichkeit diskutiert, um wichtige Rohstoffe zu fördern. Entsprechende Technologien befinden sich in der Entwicklung. Wir sprechen mit Forschenden vom Max-Planck-Institut für Marine Mikrobiologie, vom Alfred-Wegener-Institut und vom GEOMAR, die sich mit den Auswirkungen des Abbaus von Rohstoffen in der Tiefsee auf das dortige Ökosystem befassen.

Description: Deep-seabed polymetallic nodule mining could have multiple adverse effects on benthic communities, such as permanent loss of habitat by removal of nodules and habitat modification of sediments. One tool to manage biodiversity risks is the mitigation hierarchy, including avoidance, minimization of impacts, rehabilitation/restoration, and offset. We initiated long-term restoration experiments at sites in polymetallic nodule exploration contract areas in the Clarion-Clipperton Zone that were (i) cleared of nodules by a pre-prototype mining vehicle, (ii) disturbed by dredge or sledge, (iii) undisturbed, and (iv) naturally devoid of nodules. To accommodate for habitat loss, we deployed 〉 2000 artificial ceramic nodules to study the possible effect of substrate provision on recovery of biota and its impact on sediment biogeochemistry. Seventy-five nodules were recovered after eight weeks and had not been colonized by any sessile epifauna. All other nodules will remain on the seafloor for several years before recovery. Furthermore, to account for habitat modification of the top sediment layer, sediment in an epibenthic sledge track was loosened by a metal rake to test the feasibility of sediment decompaction to facilitate soft-sediment recovery. Analyses of granulometry and nutrients one month after sediment decompaction revealed that sand fractions are proportionally lower within the decompacted samples, whereas TOC values are higher. Considering the slow natural recovery rates of deep-sea communities, these experiments represent the beginning of a ~30 year study during which we expect to gain insights into thenature and timing of the development of hard-substrate communities and the influence of nodules on recovery of disturbed sediment communities. Results will help to understand adverse long-term effects of nodule removal, providing an evidence base for setting criteria for the definition of “serious harm” to the environment. Furthermore, accompanying research is needed to define a robust ecosystem baseline in order to effectively identify restoration success.