Impact of small-scale disturbances on geochemical conditions, biogeochemical processes and element fluxes in surface sediments of the eastern Clarion–Clipperton Zone, Pacific Ocean

Abstract

Abstract. 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 re-establishment 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.

Unexpected high abyssal ophiuroid diversity in polymetallic nodule fields of the northeast Pacific Ocean and implications for conservation

“…The CCZ licence areas for exploration despite being all in a mesotrophic zone are not all the same. The POC flux in the CCZ shows a southeast-to-northwest gradient increasing towards its eastern edge (Vanreusel et al, 2016;Volz et al, 2018Volz et al, , 2020. Volz et al (2018Volz et al ( , 2020, after studying four of the CCZ areas studied herein and the APEI3, found that they differ in POC fluxes to the sea floor ranging from as low as 1 mg C org m −2 d −1 in APEI3 to 2 mg C org m −2 d −1 in the BGR area.…”

“…The CCZ is characterized by gradual changes in environmental conditions (e.g. differences in surface-water productivity, depth and sediment characteristics) across an east-west and a north-south axis that correspond to a variation in nodule size and coverage, as well as variations in faunal composition along these gradients (Wedding et al, 2013). Ophiuroid samples were collected from CCZ during six scientific cruises from five different exploration licence areas and one area protected from mining (APEI3).…”

“…the DIS-COL Experimental Area, DEA) (Mukhopadhyay et al, 2008;Hein et al, 2013;Miller et al, 2018). The CCZ lies in the Areas Beyond National Jurisdiction and thus falls under the legal mandate of the International Seabed Authority (ISA;Wedding et al, 2013). So far, 16 licence areas for the exploration of polymetallic nodules have been approved by the ISA within the CCZ, each up to 75 000 km 2 in size (Wedding et al, 2013).…”

“…The CCZ lies in the Areas Beyond National Jurisdiction and thus falls under the legal mandate of the International Seabed Authority (ISA;Wedding et al, 2013). So far, 16 licence areas for the exploration of polymetallic nodules have been approved by the ISA within the CCZ, each up to 75 000 km 2 in size (Wedding et al, 2013). In its environmental management plan for the CCZ (Lodge et al, 2014), the ISA adopted nine large protection areas defined as Areas of Particular Environmental Interest (APEIs), where mining will not be permitted (Lodge et al, 2014).…”

“…In its environmental management plan for the CCZ (Lodge et al, 2014), the ISA adopted nine large protection areas defined as Areas of Particular Environmental Interest (APEIs), where mining will not be permitted (Lodge et al, 2014). These APEIs are large enough (each of them 400 km × 400 km) and far enough away from potential mining areas that they will not be affected by deep-sea mining (Wedding et al, 2013). In order to be effective as source populations for the recolonization of impacted areas, however, APEIs should harbour a representative subset of the fauna found in the potential fields.…”