ALBERT

Description: Physical and chemical trace metal speciation are important for our understanding of metal cycling and potential toxicity to marine life. Trace metals can behave differently in diffusion processes or particle-solution interactions and have different bioavailabilities depending on their physical and chemical forms, which often depend on redox conditions. Here we investigated dissolved (〈 0.2 µm) and soluble (〈 0.02 µm) concentrations of Mn, Co, Ni, Fe, Cu, V, Mo, U, Cd, and As in oxic and suboxic deep-sea sediments of the central equatorial Pacific Ocean. Vanadium, Mo, U, As, and Cd showed no significant concentration differences between their dissolved and soluble forms, suggesting that they are present as inorganic ionic species or organic complexes in the truly dissolved or small colloidal fraction. In contrast, the colloidal fraction (〉 0.02 µm 〈 0.2 µm) of Mn, Co, Ni, and Cu increased with depth in oxic pore waters and Fe had the largest but variable colloidal pool. Soluble Mn, Co, and Ni were released in the uppermost 2-4 cm in the sediment because of reductive dissolution. The increasing colloidal fraction with depth suggests a decrease in the concentration of small organic ligands with depth, that are abundant in the surface sediment pore waters, and instead an increasing importance of larger (〉 0.02 µm) inorganic nanoparticles and colloids such as Mn and Fe (oxyhydr)oxides that control Mn, Fe, and Co cycling at depths 〉 10 cm. The distribution of Ni and Cu cannot be exclusively explained by inorganic nanoparticles and a shift from low to larger high molecular weight organic ligands might occur. These findings provide new insights into trace metal distributions in the dissolved phase, highlighting the diversity of metal complexes and the need to incorporate these in future calculations of benthic metal fluxes and ecotoxicity assessments, especially in oxic pore waters.

Description: Physical and chemical trace metal speciation are important for our understanding of metal cycling and potential toxicity to marine life. Trace metals can have different bioavailabilities and behave differently in diffusion processes or particle-solution interactions depending on their physical and chemical forms. Here we investigated dissolved (〈 0.2 µm) and soluble (〈 0.02 µm) concentrations of Mn, Fe, Co, Ni, Cu, V, Mo, U, Cd, and As in oxic and suboxic deep-sea sediments of the central equatorial Pacific. Additionally, surface sediments from selected cores were analyzed for solid-phase Mn to assess potential Mn (and associated metal) mobilization into the pore water from the solid phase during early diagenesis. Vanadium, Mo, U, Cd, and As showed no significant difference between the dissolved and soluble concentrations suggesting that they are present in the truly dissolved or small colloidal fraction. The same was true for Mn and Co in suboxic pore waters. In contrast, the colloidal fraction (〉 0.02 µm 〈 0.2 µm) of Cu increased with depth as well as the colloidal fraction of Mn, Co, and Ni in oxic pore waters. Fe had the largest but variable colloidal pool. Samples were taken during the SO268 cruise to the German and Belgian license areas for polymetallic nodule mining in the Clarion Clipperton Fracture Zone as part of the MiningImpact project. Sediment cores were collected with multicorers (MUC), ROV push cores (PUC), and gravity corers (GC). Pore water was extracted by means of centrifugation and sequential filtration using cleaned polyethersulfone (PES) syringe filters (0.2 µm) and Anopore syringe filters (0.02 µm). Pore water was preserved by acidification to ~ pH 1.8 with concentrated ultrapure HCl. Mn, Fe, Co, and Ni were measured by High Resolution Sector Field Inductively Coupled Plasma-Mass Spectrometry at GEOMAR, Kiel, Germany and Cu, V, Mo, U, Cd, and As as well as Mn and Co in the GCs by Inductively Coupled Plasma-Mass Spectrometry at Jacobs University Bremen, Germany (now Constructor University). Solid-phase samples from which the pore water was previously extracted were freeze-dried and acid pressure digested using HF and HClO4. Manganese was then analyzed in the digestion solutions with an Inductively Coupled Plasma Optical Emission Spectrometer at Jacobs University Bremen, Germany.