Description: The release of potentially toxic metals, such as copper (Cu), into the water column is of concern during polymetallic nodule mining. The bioavailability and thus toxicity of Cu is strongly influenced by its speciation which is dominated by organic ligand (L) complexation in seawater, with L-complexes being considered less bioavailable than free Cu 2+ . The presence of CuL-complexes in deep-sea sediments has, however, not been systematically studied in the context of deep-sea mining. We thus analyzed the Cu-binding L concentration ([L]) in deep-sea pore waters of two polymetallic nodule provinces in the Pacific Ocean, the Peru Basin and the Clarion-Clipperton-Zone, using competitive ligand equilibration–adsorptive stripping voltammetry. The pore-water dissolved Cu concentration ([dCu]) ranged from 3 to 96 nM, generally exceeding bottom water concentrations (4–44 nM). Based on fitting results from ProMCC and Excel, Cu was predominantly complexed by L (3–313 nM) in bottom waters and undisturbed pore waters. We conclude that processes like deep-sea mining are unlikely to cause a release of toxic Cu 2+ concentrations ([Cu 2+ ]) to the seawater as 〉 99% Cu was organically complexed in pore waters and the [Cu 2+ ] was 〈 6 pM for 8 of 9 samples. Moreover, the excess of L found especially in shallow pore waters implied that even with a Cu release through mining activities, Cu 2+ likely remains beneath toxic thresholds.

Description: Only few studies exist that investigate the dynamics of deep-sea dissolved organic matter (DOM) derived from hydrothermal vents. In this study, we provide first insight into the molecular composition of DOM associated with Indian Ocean hydrothermal systems covering the full range from hot focussed endmember fluids over diffuse fluids to open ocean hydrothermal plumes and deep seawater. We combined geochemical analyses with molecular characterization of DOM using ultra-high resolution mass spectrometry (FT-ICR-MS). We studied two vent systems with fluids venting 〉330 °C and up to 97% of hydrothermal endmember: the Kairei vents (Central Indian Ridge) with brine phase separation, and the newly discovered Pelagia vents (South-East Indian Ridge). The hot fluids in both systems were highly enriched in dissolved Fe, Si, K, Li, Mn and Zn compared to seawater. The molecular composition of DOM from hot fluids differed substantially from that of diffuse fluids and plumes, in which the composition was highly dominated by the seawater DOM signature. Low O/C ratio average in hot fluids (〈0.38) indicated potential input of more reduced (O-poor) DOM compounds from the vents into the surrounding seawater, independently of the vent location and of whether the fluid has undergone phase separation. To test the importance of pure thermal degradation, we compared our samples to fluids subjected to abiotic thermal degradation under laboratory conditions and we observed that our natural samples largely differed (〉94% Bray Curtis dissimilarity) from the experimental ones, suggesting additional degradation processes of organic compounds at the Indian Ocean hydrothermal systems.

Description: Spatial distribution of nitrogen species across the Similan Islands chain from west to east directions were investigated during a low and high large amplitude internal waves (LAIW) season in December 2010 and March 2011. Periodic water sampling was performed on the west side Similan reef in a depth of about 15-17 meter together with an in situ temperature logged CTD. Temporal samplings were carried out to determine the dominant nitrogen species in the soliton water reaching the Similan island chain. The major inorganic nitrogen species were chosen in order to determine dissolved inorganic nitrogen (DIN) uptake in screlactenian corals. Pocillopora verrucosa (Ellis & Solander, 1786) was assigned as experimental species because of its common presence on both, west and east sides of the Similan Islands and Racha Island. 40 coral nubbins were collected in situ and reared in the PMBC aquarium for the experimental approaches. The corals were incubated in a Red Sea Salt® solution to control the organic and inorganic nutrient background. Normal warm reef temperature (28°C) and cold LAIW temperature (20°C) were applied in the incubations with different DIN concentrations (control N, 20 μM NO3 -, 6 μM NH4 +20 and 20 μM NO3 - & 6 μM NH4 +) under both dark and light conditions. The incubation period was 2 hours but the water samples were taken as time series after 0, 10, 30, 60, 90 and 120 minutes respectively. The DIN uptake rates were calculated per coral surface area and time period. Additionally, in order to get information on the micro scale of inorganic nitrogen uptake, labeled 15NO3 - and 15NH4 + were used in a further incubation run. Nitrate was found to be the dominant DIN in the soliton waters approaching the reefs at the Similan Islands. The NO3 - uptake experiments under low temperature conditions revealed a significant negative effect on the nitrate uptake rate in both light and dark. However, the concentration-dependent uptake behavior was found in both NH4 + and NO3 - enriched experiments. The concentration of DIN present in water had a higher effect on the uptake rates than temperature. High NH4 + concentration was shown to limit NO3 – uptake. NH4 + was more favorable to P.verrucosa than NO3 -. Light independent uptakes were discovered in both NH4 + and NO3 - enriched experiments. The highest uptake rates were found at the beginning of the incubation experiments (10 minutes after the start) in both NH4 + and NO3 - enriched experiments. The labeled DIN experiments showed that the symbiotic algae are the terminal station for both NO3 - and NH4 +. Delta 15N enrichment exhibits similar results in temperature dependence and concentration dependence as found in normal DIN uptake experiments. Interestingly, a higher uptake rate of NH4 + was found in the zooxanthellae under enriched NH4 + & NO3 - combined with low temperature conditions. LAIW seem to be favorable for the symbiotic corals in the Similans with a high inorganic nutrient supply. Even though the low temperature entrained by the LAIW to the reefs suppresses the optimum uptake the higher abundance of DIN has a greater effect on the corals. This denotes differences in the uptake behavior of corals growing on the exposed side of the Similan Islands (west) and the sheltered sides (east). With this study a possible explanation might be visualized to the differences of the framework formations found on the LAIW exposed and sheltered reefs.

Description: The release of potentially toxic metals, such as copper (Cu), into the water column is of concern during polymetallic nodule mining. The bioavailability and thus toxicity of Cu is strongly influenced by its speciation which is dominated by organic ligand (L) complexation in seawater, with L-complexes being considered less bioavailable than free Cu2+. The presence of CuL-complexes in deep-sea sediments has, however, not been systematically studied in the context of deep-sea mining. We thus analyzed the dissolved Cu concentration ([dCu]), Cu-binding L concentration ([L]) and DOC concentration in deep-sea pore waters of two polymetallic nodule provinces in the Pacific Ocean, the Peru Basin and the Clarion-Clipperton-Zone. We compared undisturbed sites with 26-year-old disturbance track sites and one 5-week-old site where a small sediment plume resettled.

Description: Shallow submarine hydrothermal systems are extreme environments with strong redox gradients at the interface of hot, reduced fluids and cold, oxygenated seawater. Hydrothermal fluids are often depleted in sulfate when compared to surrounding seawater and can contain high concentrations of hydrogen sulfide (H2S). It is well known that sulfur in its various oxidation states plays an important role in processing and transformation of organic matter. However, the formation and the reactivity of dissolved organic sulfur (DOS) in the water column at hydrothermal systems are so far not well understood. We investigated DOS dynamics and its relation to the physicochemical environment by studying the molecular composition of dissolved organic matter (DOM) in three contrasting shallow hydrothermal systems off Milos (Eastern Mediterranean), Dominica (Caribbean Sea) and Iceland (North Atlantic). We used ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) to characterize the DOM on a molecular level. The molecular information was complemented with general geochemical data, quantitative dissolved organic carbon (DOC) and DOS analyses as well as isotopic measurements (d2H, d18O and F14C). In contrast to the predominantly meteoric fluids from Dominica and Iceland, hydrothermal fluids from Milos were mainly fed by recirculating seawater. The hydrothermal fluids from Milos were enriched in H2S and DOS, as indicated by high DOS/DOC ratios and by the fact that 〉90% of all assigned DOM formulas that were exclusively present in the fluids contained sulfur. In all three systems, DOS from hydrothermal fluids had on average lower O/C ratios (0.26?0.34) than surrounding surface seawater DOS (0.45?0.52), suggesting shallow hydrothermal systems as a source of reduced DOS, which will likely get oxidized upon contact with oxygenated seawater. Evaluation of hypothetical sulfurization reactions suggests DOM reduction and sulfurization during seawater recirculation in Milos seafloor. The four most effective potential sulfurization reactions were those exchanging an O atom by one S atom in the formula or the equivalent + H2S reaction, correspondingly exchanging H2O, H2 and/or O2 by a H2S molecule. Our study reveals novel insights into DOS dynamics in marine hydrothermal environments and provides a conceptual framework for molecular-scale mechanisms in organic sulfur geochemistry.

Description: This data set presents dissolved copper (dCu) and copper-binding ligand (L) data for deep-sea pore waters in the Peru Basin, south east equatorial Pacific Ocean and the Clarion Clipperton Zone (CCZ) in the central equatorial Pacific Ocean. Samples were taken during cruises SO242, SO262, and SO268 using rhizons with a pore size of 0.12-0.18 µm. Several depths from one multicorer liner were pooled to get sufficient sample volume. dCu was determined by either ICP-MS analysis in kinetic energy discrimination mode or by competitive ligand equilibration–adsorptive stripping voltammetry (CLE-AdCSV). Cu speciation in the samples was also determined with CLE-AdCSV using salicylaldoxime (SA) as the added ligand. For each titration, the concentration of complexing ligands ([L]) and corresponding conditional stability constants (logK) were determined using the fitting software named ProMCC. The free copper ion (Cu2+) and bioavailable Cu' (Cu2+ + inorganic copper) were also calculated. Data was fitted using both the one-ligand and two-ligand complete complexation-fitting model. The model with the best fit and thus the least fitting error was used for data interpretation - this was always the one-ligand model. Side reaction coefficients and stability constants of SA of each sample were provided by van den Berg's ion-pairing model for seawater (EXCEL worksheet - ion-pairing model for seawater, written by C.M.G. van den Berg in 2014) using sample specific salinity values measured in the lab, a SA concentration of 5 µM, an ambient temperature of 21˚C, and a pH of 8.1.