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: This dataset comprises gamma spectrometric measurements of radioisotopes in samples from polymetallic nodules, which were collected in the framework of the European collaborative JPI Oceans MiningImpact project onboard R/V SONNE cruise SO239 in 2015. Samples were taken from different parts of the nodules (topside surface (s), bottom surface (b), interior (i)). The gamma radiation was measured in a Canberra well-type HPGe detector. Emission lines at 46 keV (Pb-210), 63 keV (U-234 via Th-234), 67 keV (Th-230), 186 keV (Ra-226 in absence of U-235 interference), 295 keV (Ra-226 via Rn-222) and 402 keV (Pa-231 via Rn-219 with some uncertainty on the intermediate Pa-231/Ac-227 equilibrium) were evaluated. The diluted uranium ore standard RGU-1 and the uranium ore standard UREM-11 served as calibration and reference, respectively.

Description: This dataset comprises porosity data from undisturbed reference sites sampled by a multiple corer (MUC) and disturbed push-cores (PUC), which were retrieved using the ROV Kiel 6000 (GEOMAR) during RV SONNE expedition SO239 in 2015. During this cruise, different European contract areas for the exploration of polymetallic nodules in the area of the Clarion-Clipperton Zone (CCZ) were visited (BGR, IOM, GSR, IFREMER), which are currently being explored in light of potential future deep-sea mining. The PUCs were taken in several small-scale disturbance tracks, which were created 1 day to 37 years before sampling for the simulation of deep-sea mining activities in the different exploration areas. The sampled disturbance tracks include 1-day- and 3-year-old EBS tracks (created with an Epibenthic sledge), 8-months- and 37-year-old dredge tracks, and a 20-year-old track from the Benthic Impact Experiment in the IOM area (IOM-BIE). The dataset was produces in the laboratories of the Alfred Wegener Institute (AWI) Bremerhaven, Germany.