Short-term impacts of polymetallic nodule mining on sediment remineralization activity: in situ observations of benthic oxygen fluxes after a nodule collector prototype trial in the Clarion Clipperton Fracture Zone

The vertical distribution of oxygen concentrations in sediment porewaters and oxygen fluxes across the sediment-water interface are key variables to assess benthic redox conditions and quantify seafloor community respiration. Oxygen fluxes serve as proxies for benthic activity and organic matter remineralization rates. Measurements of the distribution of oxygen in the sediment and sediment community oxygen consumption are also recommended by the International Seabed Authority (ISA) to establish ecological baselines and to identify effects of mining-related impacts (e.g., ISBA/25/LTC/6/Rev.2 and ISBA/27/C/11). This presentation addresses short-term impacts on benthic oxygen distribution and remineralization activity in response to primary (i.e., nodule removal and sediment reworking) and secondary impacts of nodule mining (i.e., blanketing of neighboring areas with resettling sediment plumes). Distribution of oxygen concentrations in porewaters and interfacial oxygen fluxes were investigated before and after trials of the pre-prototype nodule collector ‘PATANIA II’ carried out in 2019 by the Belgian Company DEME-GSR. Measurements were performed in situ in two exploration contract areas with microsensor profilers deployed by a Remotely Operated Vehicle (ROV). Additional measurements were obtained with benthic chamber systems. 1.5 years after the collector trial, diffusive oxygen fluxes were clearly reduced in ‘collector impact sites’ where nodules have been removed and surface sediments were redistributed and partly transported away. The opposite was found in neighboring ‘plume impact’ sites where sediments got covered by blanketing layers of different thickness. Here, fluxes were higher as compared to reference and undisturbed sites. The presentation will discuss possible explanations of the observed patterns including the redistribution of organic matter with resuspended sediments and possible effects of physical disturbances on metabolic activity of benthic communities. The study was carried out in the framework of the collaborative European project MiningImpact under the Joint Programming Initiative Healthy and Productive Seas and Oceans (JPI Oceans).