Description: The manganese nodule belt within the Clarion and Clipperton Fracture Zones (CCZ) in the abyssal NE Pacific Ocean is characterized by numerous seamounts, low organic matter (OM) depositional fluxes and meter-scale oxygen penetration depths (OPD) into the sediment. The region hosts contract areas for the exploration of polymetallic nodules and Areas of Particular Environmental Interest (APEI) as protected areas. In order to assess the impact of potential mining on these deep-sea sediments and ecosystems, a thorough determination of the natural spatial variability of depositional and geochemical conditions as well as biogeochemical processes and element fluxes in the different exploration areas is required. Here, we present a comparative study on (1) sedimentation rates and bioturbation depths, (2) redox zonation of the sediments and element fluxes as well as (3) rates and pathways of biogeochemical reactions at six sites in the eastern CCZ. The sites are located in four European contract areas and in the APEI3. Our results demonstrate that the natural spatial variability of depositional and (bio)geochemical conditions in this deep-sea sedimentary environment is much larger than previously thought. We found that the OPD varies between 1 and 4.5 m, while the sediments at two sites are oxic throughout the sampled interval (7.5 m depth). Below the OPD, manganese and nitrate reduction occur concurrently in the suboxic zone with pore-water Mn2+ concentrations of up to 25 µM. The thickness of the suboxic zone extends over depth intervals of less than 3 m to more than 8 m. Our data and the applied transport-reaction model suggest that the extension of the oxic and suboxic zones is ultimately determined by the (1) low flux of particulate organic carbon (POC) of 1–2 mg Corg m−2 d−1 to the seafloor, (2) low sedimentation rates between 0.2 and 1.15 cm kyr−1 and (3) oxidation of pore-water Mn2+ at depth. The diagenetic model reveals that aerobic respiration is the main biogeochemical process driving OM degradation. Due to very low POC fluxes of 1 mg Corg m−2 d−1 to the seafloor at the site investigated in the protected APEI3 area, respiration rates are twofold lower than at the other study sites. Thus, the APEI3 site does not represent the (bio)geochemical conditions that prevail in the other investigated sites located in the European contract areas. Lateral variations in surface water productivity are generally reflected in the POC fluxes to the seafloor across the various areas but deviate from this trend at two of the study sites. We suggest that the observed spatial variations in depositional and (bio)geochemical conditions result from differences in the degree of degradation of OM in the water column and heterogeneous sedimentation patterns caused by the interaction of bottom water currents with seafloor topography.

Description: 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 conceived. 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). These small-scale disturbance experiments were performed 1 day 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 (Paul et al., 2018). We found that the upper 5–15 cm of the sediments were removed during various small-scale disturbance experiments in the different exploration contract areas. Transient transport-reaction modelling for the Polish and German contract areas reveals that the removal of the surface sediments is associated with the loss of reactive labile organic carbon. As a result, oxygen consumption rates decrease significantly after the removal of the surface sediments, and consequently, oxygen penetrates up to tenfold deeper into the sediments inhibiting denitrification and Mn(IV) reduction. Our model results show that the post-disturbance geochemical re-equilibration 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, the geochemical re-equilibration of the sediments is ultimately controlled by the burial rates of organic matter. Hence, under current depositional conditions, the new geochemical equilibrium in the sediments of the CCZ is reached only on a millennia scale even for these small-scale disturbances simulating deep-sea mining activities.

Description: Numerous studies have provided compelling evidence that the Pacific Ocean has experienced substantial glacial/interglacial changes in bottom-water oxygenation associated with enhanced carbon dioxide storage in the glacial deep ocean. Under postulated low glacial bottom-water oxygen concentrations (O), redox zonation, biogeochemical processes and element fluxes in the sediments must have been distinctively different during the last glacial period (LGP) compared to current well-oxygenated conditions. In this study, we have investigated six sites situated in various European contract areas for the exploration of polymetallic nodules within the Clarion-Clipperton Zone (CCZ) in the NE Pacific and one site located in a protected Area of Particular Environmental Interest (APEI3) north of the CCZ. We found bulk sediment Mn maxima of up to 1 wt% in the upper oxic 10 cm of the sediments at all sites except for the APEI3 site. The application of a combined leaching protocol for the extraction of sedimentary Mn and Fe minerals revealed that mobilizable Mn(IV) represents the dominant Mn(oxyhydr)oxide phase with more than 70% of bulk solid-phase Mn. Steady state transport-reaction modeling showed that at postulated glacial O of 35 μM, the oxic zone in the sediments was much more compressed than today where upward diffusing pore-water Mn2+ was oxidized and precipitated as authigenic Mn(IV) at the oxic-suboxic redox boundary in the upper 5 cm of the sediments. Transient transport-reaction modeling demonstrated that with increasing O during the last glacial termination to current levels of ∼ 150 μM, (1) the oxic-suboxic redox boundary migrated deeper into the sediments and (2) the authigenic Mn(IV) peak was continuously mixed into subsequently deposited sediments by bioturbation causing the observed mobilizable Mn(IV) enrichment in the surface sediments. Such a distinct mobilizable Mn(IV) maximum was not found in the surface sediments of the APEI3 site, which indicates that the oxic zone was not as condensed during the LGP at this site due to two- to threefold lower organic carbon burial rates. Leaching data for sedimentary Fe minerals suggest that Fe(III) has not been diagenetically redistributed during the LGP at any of the investigated sites. Our results demonstrate that the basin-wide deoxygenation in the NE Pacific during the LGP was associated with (1) a much more compressed oxic zone at sites with carbon burial fluxes higher than 1.5 mg Corg m−2 d−1, (2) the authigenic formation of a sub-surface mobilizable Mn(IV) maximum in the upper 5 cm of the sediments and (3) a possibly intensified suboxic-diagenetic growth of polymetallic nodules. As our study provides evidence that authigenic Mn(IV) precipitated in the surface sediments under postulated low glacial O, it contributes to resolving a long-standing controversy concerning the origin of widely observed Mn-rich layers in glacial/deglacial deep-sea sediments.

Description: Rare earth elements and yttrium (REY) are often used as proxies for (paleo)environmental conditions and for the reconstruction of element sources and transport pathways. Many geological systems are well described with respect to the behavior of REY but deep-sea sediments with their manifold processes impacting the sediment during early diagenesis leave some questions about the origin and development of the shale-normalized REY (REYSN) patterns unanswered. Here we report REY data for sediment solid phase and pore water from the upper 10 m of deep-sea sediments from the Clarion Clipperton Zone (CCZ) in the central equatorial Pacific. The solid-phase REY profiles show highest concentrations at depth below 5–8 m. The REYSN patterns show an enrichment in middle REY (MREY) (LaSN/GdSN between 0.35 and 0.60; GdSN/YbSN between 1.19 and 1.47) and either no or negative CeSN and YSN anomalies (i.e. chondritic to sub-chondritic Y/Ho ratios between 24.7 and 28.7). Based on correlation analyses of bulk sediment element concentrations and sequential extractions, we suggest that a Ca phosphate phase controls the distribution and the patterns of REY in these silty clay pelagic sediments rich in siliceous ooze. The MREY enrichment develops at the sediment-water interface and intensifies systematically with depth. The negative CeSN anomaly intensifies with depth possibly because Ce is mostly bound to Mn- and Fe-(oxyhydr)oxides. Therefore, Ce concentrations remain relatively constant throughout the sediment core, while its trivalent REY neighbors are mostly hosted by the Ca phosphate phase that continuously incorporates REY from ambient pore waters. The non-redox-sensitive trivalent REY concentrations increase with depth, producing or enhancing a negative CeSN anomaly through coupled substitution of REY3+ and Na+ for Ca2+. The solid-phase REYSN pattern is therefore determined by the pore-water REYSN pattern and not suitable for paleoceanographic interpretation. The similarity of the pore-water and solid-phase REYSN patterns suggests, however, that only minor fractionation occurs during REY incorporation into the Ca phosphate crystal structure.

Description: In search for critical elements, polymetallic nodules at the deep abyssal seafloor are targeted for mining operations. Nodules efficiently scavenge and retain several naturally occurring uranium-series radioisotopes, which predominantly emit alpha radiation during decay. Here, we present new data on the activity concentrations of thorium-230, radium-226, and protactinium-231, as well as on the release of radon-222 in and from nodules from the NE Pacific Ocean. In line with abundantly published data from historic studies, we demonstrate that the activity concentrations for several alpha emitters are often higher than 5 Bq g −1 at the surface of the nodules. These observed values can exceed current exemption levels by up to a factor of 1000, and even entire nodules commonly exceed these limits. Exemption levels are in place for naturally occurring radioactive materials (NORM) such as ores and slags, to protect the public and to ensure occupational health and radiation safety. In this context, we discuss three ways of radiation exposure from nodules, including the inhalation or ingestion of nodule fines, the inhalation of radon gas in enclosed spaces and the potential concentration of some radioisotopes during nodule processing. Seen in this light, inappropriate handling of polymetallic nodules poses serious health risks.