Description: In this work, multicomponent transport-reaction models were successfully applied to analyse and assess the effects of human interventions and natural, large-scale perturbations of the deep-sea floor. In particular, two scenarios were studied that are suitable as case studies for a variety of possible impacts. Firstly, the removal of the uppermost bioturbated sediment layer due to deepsea mining of manganese nodules in the Peru Basin is considered, and secondly, the disposal of highly reactive material on the deep-sea floor of the South China Sea, i.e. the Mount Pinatubo ash fallout of 1991, is evaluated. In addition, the current theoretical background for equilibrium calculations is expanded, and by this a mathematical tool is provided that, for example, may allow an evaluation of the influence of calcite dissolution and precipitation at the sea floor on the global C02 budget.

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: A geochemical model of the Peru Basin deep-sea floor, based on an extensive set of field data as well as on numerical simulations, is presented. The model takes into account the vertical oscillations of the redox zonation that occur in response to both long-term (glacial/interglacial) and short-term (El Niño Southern Oscillation (ENSO) time scale) variations in the depositional flux of organic matter. Field evidence of reaction between the pore water NO3− and an oxidizable fraction of the structural Fe(II) in the clay mineral content of the deep-sea sediments is provided. The conditions of formation and destruction of reactive clay Fe(II) layers in the sea floor are defined, whereby a new paleo-redox proxy is established. Transitional NO3− profile shapes are explained by periodic contractions and expansions of the oxic zone (ocean bottom respiration) on the ENSO time scale. The near-surface oscillations of the oxic–suboxic boundary constitute a redox pump mechanism of major importance with respect to diagenetic trace metal enrichments and manganese nodule formation, which may account for the particularly high nodule growth rates in this ocean basin. These conditions are due to the similar depth ranges of both the O2 penetration in the sea floor and the bioturbated high reactivity surface layer (HRSL), all against the background of ENSO-related large variations in depositional Corg flux. Removal of the HRSL in the course of deep-sea mining would result in a massive expansion of the oxic surface layer and, thus, the shut down of the near-surface redox pump for centuries, which is demonstrated by numerical modeling.

Description: Extensive methane hydrate layers are formed in the near-surface sediments of the Cascadia margin. An undissociated section of such a layer was recovered at the base of a gravity core (i.e. at a sediment depth of 120 cm) at the southern summit of Hydrate Ridge. As a result of salt exclusion during methane hydrate formation, the associated pore waters show a highly elevated chloride concentration of 809 mM. In comparison, the average background value is 543 mM. A simple transport-reaction model was developed to reproduce the Cl- observations and quantify processes such as hydrate formation, methane demand, and fluid flow. From this first field observation of a positive Cl- anomaly, high hydrate formation rates (0.15–1.08 mol cm-2 a-1) were calculated. Our model results also suggest that the fluid flow rate at the Cascadia accretionary margin is constrained to 45–300 cm a-1. The amount of methane needed to build up enough methane hydrate to produce the observed chloride enrichment exceeds the methane solubility in pore water. Thus, most of the gas hydrate was most likely formed from ascending methane gas bubbles rather than solely from CH4 dissolved in the pore water.

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.