Description: Geochemical data (CH4, SO42−, I−, Cl−, particulate organic carbon (POC), δ13C-CH4, and δ13C-CO2) are presented from the upper 30 m of marine sediment on a tectonic submarine accretionary wedge offshore southwest Taiwan. The sampling stations covered three ridges (Tai-Nan, Yung-An, and Good Weather), each characterized by bottom simulating reflectors, acoustic turbidity, and different types of faulting and anticlines. Sulfate and iodide concentrations varied little from seawater-like values in the upper 1–3 m of sediment at all stations; a feature that is consistent with irrigation of seawater by gas bubbles rising through the soft surface sediments. Below this depth, sulfate was rapidly consumed within 5–10 m by anaerobic oxidation of methane (AOM) at the sulfate-methane transition. Carbon isotopic data imply a mainly biogenic methane source. A numerical transport-reaction model was used to identify the supply pathways of methane and estimate depth-integrated turnover rates at the three ridges. Methane gas ascending from deep layers, facilitated by thrusts and faults, was by far the dominant term in the methane budget at all sites. Differences in the proximity of the sampling sites to the faults and anticlines mainly accounted for the variability in gas fluxes and depth-integrated AOM rates. By comparison, methane produced in situ by POC degradation within the modeled sediment column was unimportant. This study demonstrates that the geochemical trends in the continental margins offshore SW Taiwan are closely related to the different geological settings.

Description: Gashydrate sind eisähnliche Verbindungen, in denen Hydratbildner, z.B. Methan, in hoher Dichte gespeichert werden können. Methanhydrate sind nur bei hohen Drücken und tiefen Temperaturen sowie in Anwesenheit hoher Methankonzentrationen stabil. Diese Stabilitätsbedingungen sind unter bestimmten Voraussetzungen in marinen Sedimenten erfüllt, in denen Methan durch den mikrobiellen Abbau von abgelagerter Biomasse entsteht oder aus größeren Tiefen zugeführt wird. Die globale Menge an Methan in marinen Gashydraten überschreitet die Menge an Erdgas in konventionellen Lagerstätten vermutlich um ein Mehrfaches. Eine potenzielle Nutzung von Gashydraten als zukünftige Energiequelle wird daher gegenwärtig weltweit untersucht. Erste Feldtests in Permafrostregionen und marinen Lagerstätten haben gezeigt, dass eine Produktion von Methan aus Gashydraten prinzipiell möglich ist. Eine Förderung von Methan aus Gashydraten kann technisch realisiert werden mittels Druckabsenkung, durch thermische Stimulation oder chemische Aktivierung. Die Injektion von CO2, ebenfalls ein Hydratbildner, kann eine solche Aktivierung der natürlichen Hydrate bewirken und das Methan in der Hydratstruktur ersetzen. Infolgedessen erscheint eine verfahrenstechnische Kombination von Hydratabbau und CO2-Speicherung als besonders sinnvoll, da im Idealfall eine emissionsarme bis -freie Energiegewinnung ermöglicht würde. Untersuchungen zur Aufklärung mechanistischer und fluiddynamischer Aspekte der CH4-CO2-Hydratumwandlung sowie zur Entwicklung eines technischen Verfahrens werden in unterschiedlichen Hochdruckanlagen auf verschiedenen Skalen durchgeführt. Diese speziellen Systeme bieten die Möglichkeit, marine Druck-, Temperatur- und Durchflussbedingungen zu simulieren. Sie sind mit verschiedenen Sensoren und Messsystemen (z.B. CTD, IR, Raman, MRI) ausgerüstet, um den Prozessverlauf störungsfrei zu überwachen. Basierend auf derzeitigen Ergebnissen erscheint die Injektion von erwärmtem, überkritischem CO2 als vielversprechender technischer Baustein für die Verfahrensentwicklung. Die Zuführung von Wärmeenergie bewirkt die initiale Destabilisierung der Gashydrate und die Freisetzung von CH4, während nach Abkühlung das CO2 seinerseits Hydrate bildet und als feste, immobile Phase im Sediment zurückgehalten wird. Sowohl Methanproduktion als auch CO2-Speicherung sind dabei abhängig von der Reservoirtemperatur, so dass die Prozesseffizienz und -ausbeute bei mittleren Temperaturen (8°C) höher ist als bei niedrigeren (2°C) und höheren Temperaturen (10°C). Dies deutet darauf hin, dass der Gesamtprozess durch die Raten der jeweiligen Teilreaktionen der Hydratzersetzung und Hydratneubildung stark beeinflusst wird. Der experimentelle Vergleich unterschiedlicher Injektionsmodi zeigt, dass eine alternierende CO2-Injektion bestehend aus Injektions- und Reaktionsintervallen höhere Ausbeuten erreicht als eine kontinuierliche Injektion.

Description: Uncertainties concerning deep-seabed mining relate to the expected impacts on the abyssal benthic and pelagic environment and its ecosystems but also include geopolitical, economic, societal and cultural uncertainty. The uncertain impacts from mining lead to anxiety and a low societal acceptance for the activity and are not the same for everybody at the same time. Hence, uncertainty is an important element of the risk involved in deep-seabed mining. This chapter describes the different risks involved, develops a methodology for risk assessment for the exploitation of marine mineral resources that takes into consideration the state of knowledge and evolving research on deep-sea ecosystems, and informs on possible environmental threshold values in relation to deep-seabed mining operations.

Description: Deep-seabed polymetallic nodule mining can 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 and/or 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 preprototype 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 the 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 total organic carbon 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 the nature and timing of the development of hard-substrate communities and the influence of nodules on the recovery of disturbed sediment communities. Results will help us 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.

Description: The deep-sea mining industry is currently at a point where large-sale, commercial polymetallic nodule exploitation is becoming a more realistic scenario. At the same time, certain aspects such as the spatiotemporal scale of impacts, sediment plume dispersion and the disturbance-related biological responses remain highly uncertain. In this paper, findings from a small-scale seabed disturbance experiment in the German contract area (Clarion-Clipperton Zone, CCZ) are described, with a focus on the soft-sediment ecosystem component. Despite the limited spatial scale of the induced disturbance on the seafloor, this experiment allowed us to evaluate how short-term (〈 1 month) soft-sediment changes can be assessed based on sediment characteristics (grain size, nutrients and pigments) and metazoan meiofaunal communities (morphological and metabarcoding analyses). Furthermore, we show how benthic measurements can be combined with numerical modelling of sediment transport to enhance our understanding of meiofaunal responses to increased sedimentation levels. The lessons learned within this study highlight the major issues of current deep-sea mining-related ecological research such as deficient baseline knowledge, unrepresentative impact intensity of mining simulations and challenges associated with sampling trade-offs (e.g., replication).

Description: Isolation and detection of microplastics (MP) in marine samples is extremely cost- and labor-intensive, limiting the speed and amount of data that can be collected. In the current work, we describe rapid measurement of net-collected MPs (net mesh size 300 µm) using a benchtop near-infrared hyperspectral imaging system during a research expedition to the subtropical North Atlantic gyre. Suspected plastic particles were identified microscopically and mounted on a black adhesive background. Particles were imaged with a Specim FX17 near-infrared linescan camera and a motorized stage. A particle mapping procedure was built on existing edge-finding algorithms and a polymer identification method developed using spectra from virgin polymer reference materials. This preliminary work focused on polyethylene, polypropylene, and polystyrene as they are less dense than seawater and therefore likely to be found floating in the open ocean. A total of 27 net tows sampled 2534 suspected MP particles that were imaged and analyzed at sea. Approximately 77.1% of particles were identified as polyethylene, followed by polypropylene (9.2%). A small fraction of polystyrene was detected only at one station. Approximately 13.6% of particles were either other plastic polymers or were natural materials visually misidentified as plastics. Particle size distributions for PE and PP particles with a length greater than 1 mm followed an approximate power law relationship with abundance. This method allowed at-sea, near real-time identification of MP polymer types and particle dimensions, and shows great promise for rapid field measurements of microplastics in net-collected samples.

Description: The crises of climate change and biodiversity loss are interlinked and must be addressed jointly. A proposed solution for reducing reliance on fossil fuels, and thus mitigating climate change, is the transition from conventional combustion-engine to electric vehicles. This transition currently requires additional mineral resources, such as nickel and cobalt used in car batteries, presently obtained from land-based mines. Most options to meet this demand are associated with some biodiversity loss. One proposal is to mine the deep seabed, a vast, relatively pristine and mostly unexplored region of our planet. Few comparisons of environmental impacts of solely expanding land-based mining versus extending mining to the deep seabed for the additional resources exist and for biodiversity only qualitative. Here, we present a framework that facilitates a holistic comparison of relative ecosystem impacts by mining, using empirical data from relevant environmental metrics. This framework (Environmental Impact Wheel) includes a suite of physicochemical and biological components, rather than a few selected metrics, surrogates, or proxies. It is modified from the “recovery wheel” presented in the International Standards for the Practice of Ecological Restoration to address impacts rather than recovery. The wheel includes six attributes (physical condition, community composition, structural diversity, ecosystem function, external exchanges and absence of threats). Each has 3–5 sub attributes, in turn measured with several indicators. The framework includes five steps: (1) identifying geographic scope; (2) identifying relevant spatiotemporal scales; (3) selecting relevant indicators for each sub-attribute; (4) aggregating changes in indicators to scores; and (5) generating Environmental Impact Wheels for targeted comparisons. To move forward comparisons of land-based with deep seabed mining, thresholds of the indicators that reflect the range in severity of environmental impacts are needed. Indicators should be based on clearly articulated environmental goals, with objectives and targets that are specific, measurable, achievable, relevant, and time bound.