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.

Description: We investigated the effect of an artificial CO2 vent (0.0015−0.037 mol s−1), simulating a leak from a reservoir for carbon capture and storage (CCS), on the sediment geochemistry. CO2 was injected 3 m deep into the seafloor at 120 m depth. With increasing mass flow an increasing number of vents were observed, distributed over an area of approximately 3 m. In situ profiling with microsensors for pH, T, O2 and ORP showed the geochemical effects are localized in a small area around the vents and highly variable. In measurements remote from the vent, the pH reached a value of 7.6 at a depth of 0.06 m. In a CO2 venting channel, pH reduced to below 5. Steep temperature profiles were indicative of a heat source inside the sediment. Elevated total alkalinity and Ca2+ levels showed calcite dissolution. Venting decreased sulfate reduction rates, but not aerobic respiration. A transport-reaction model confirmed that a large fraction of the injected CO2 is transported laterally into the sediment and that the reactions between CO2 and sediment generate enough heat to elevate the temperature significantly. A CO2 leak will have only local consequences for sediment biogeochemistry, and only a small fraction of the escaped CO2 will reach the sediment surface.

Description: Carbon capture and storage (CCS) is a key technology to reduce carbon dioxide (CO2) emissions from industrial processes in a feasible, substantial, and timely manner. For geological CO2 storage to be safe, reliable, and accepted by society, robust strategies for CO2 leakage detection, quantification and management are crucial. The STEMM-CCS (Strategies for Environmental Monitoring of Marine Carbon Capture and Storage) project aimed to provide techniques and understanding to enable and inform cost-effective monitoring of CCS sites in the marine environment. A controlled CO2 release experiment was carried out in the central North Sea, designed to mimic an unintended emission of CO2 from a subsurface CO2 storage site to the seafloor. A total of 675 kg of CO2 were released into the shallow sediments (~3 m below seafloor), at flow rates between 6 and 143 kg/d. A combination of novel techniques, adapted versions of existing techniques, and well-proven standard techniques were used to detect, characterise and quantify gaseous and dissolved CO2 in the sediments and the overlying seawater. This paper provides an overview of this ambitious field experiment. We describe the preparatory work prior to the release experiment, the experimental layout and procedures, the methods tested, and summarise the main results and the lessons learnt.