Description: Competitive ligand exchange – adsorptive cathodic stripping voltammetry (CLE-AdCSV) is a widely used technique to determine dissolved iron (Fe) speciation in seawater, and involves competition for Fe of a known added ligand (AL) with natural organic ligands. Three different ALs were used, 2-(2-thiazolylazo)-p-cresol (TAC), salicylaldoxime (SA) and 1-nitroso-2-napthol (NN). The total ligand concentrations ([L t ]) and conditional stability constants (log K ′ Fe’L ) obtained using the different ALs are compared. The comparison was done on seawater samples from Fram Strait and northeast Greenland shelf region, including the Norske Trough, Nioghalvfjerdsfjorden (79N) Glacier front and Westwind Trough. Data interpretation using a one-ligand model resulted in [L t ] SA (2.72 ± 0.99 nM eq Fe) > [L t ] TAC (1.77 ± 0.57 nM eq Fe) > [L t ] NN (1.57 ± 0.58 nM eq Fe); with the mean of log K ′ Fe’L being the highest for TAC (log ′ K Fe’L(TAC) = 12.8 ± 0.5), followed by SA (log K ′ Fe’L(SA) = 10.9 ± 0.4) and NN (log K ′ Fe’L(NN) = 10.1 ± 0.6). These differences are only partly explained by the detection windows employed, and are probably due to uncertainties propagated from the calibration and the heterogeneity of the natural organic ligands. An almost constant ratio of [L t ] TAC /[L t ] SA = 0.5 – 0.6 was obtained in samples over the shelf, potentially related to contributions of humic acid-type ligands. In contrast, in Fram Strait [L t ] TAC /[L t ] SA varied considerably from 0.6 to 1, indicating the influence of other ligand types, which seemed to be detected to a different extent by the TAC and SA methods. Our results show that even though the SA, TAC and NN methods have different detection windows, the results of the one ligand model captured a similar trend in [L t ], increasing from Fram Strait to the Norske Trough to the Westwind Trough. Application of a two-ligand model confirms a previous suggestion that in Polar Surface Water and in water masses over the shelf, two ligand groups existed, a relatively strong and relatively weak ligand group. The relatively weak ligand group contributed less to the total complexation capacity, hence it could only keep part of Fe released from the 79N Glacier in the dissolved phase.

Description: Competitive ligand exchange–adsorptive cathodic stripping voltammetry (CLE-AdCSV) is used to determine the conditional concentration ([L]) and the conditional binding strength (logKcond) of dissolved organic Fe-binding ligands, which together influence the solubility of Fe in seawater. Electrochemical applications of Fe speciation measurements vary predominantly in the choice of the added competing ligand. Although different applications show the same trends, [L] and logKcond differ between the applications. In this study, binding of two added ligands in three different common applications to three known types of natural binding ligands is compared. The applications are (1) salicylaldoxime (SA) at 25 µM (SA25) and short waiting time, (2) SA at 5 µM (SA5), and (3) 2-(2-thiazolylazo)-ρ-cresol (TAC) at 10 µM, the latter two with overnight equilibration. The three applications were calibrated under the same conditions, although having different pH values, resulting in the detection window centers (D) DTAC 〉 DSA25 ≥ SA5 (as logD values with respect to Fe3+: 12.3 〉 11.2 ≥ 11). For the model ligands, there is no common trend in the results of logKcond. The values have a considerable spread, which indicates that the error in logKcond is large. The ligand concentrations of the nonhumic model ligands are overestimated by SA25, which we attribute to the lack of equilibrium between Fe-SA species in the SA25 application. The application TAC more often underestimated the ligand concentrations and the application SA5 over- and underestimated the ligand concentration. The extent of overestimation and underestimation differed per model ligand, and the three applications showed the same trend between the nonhumic model ligands, especially for SA5 and SA25. The estimated ligand concentrations for the humic and fulvic acids differed approximately 2-fold between TAC and SA5 and another factor of 2 between SA5 and SA25. The use of SA above 5 µM suffers from the formation of the species Fe(SA)x (x〉1) that is not electro-active as already suggested by Abualhaija and van den Berg (2014). Moreover, we found that the reaction between the electro-active and non-electro-active species is probably irreversible. This undermines the assumption of the CLE principle, causes overestimation of [L] and could result in a false distinction into more than one ligand group. For future electrochemical work it is recommended to take the above limitations of the applications into account. Overall, the uncertainties arising from the CLE-AdCSV approach mean we need to search for new ways to determine the organic complexation of Fe in seawater.

Description: Recent studies, including many from the GEOTRACES program, have expanded our knowledge of trace metals in the Arctic Ocean, an isolated ocean dominated by continental shelf and riverine inputs. Here, we report a unique, pan-Arctic linear relationship between dissolved copper (Cu) and nickel (Ni) present north of 60°N that is absent in other oceans. The correlation is driven primarily by high Cu and Ni concentrations in the low salinity, river-influenced surface Arctic and low, homogeneous concentrations in Arctic deep waters, opposing their typical global distributions. Rivers are a major source of both metals, which is most evident within the central Arctic's Transpolar Drift. Local decoupling of the linear Cu-Ni relationship along the Chukchi Shelf and within the Canada Basin upper halocline reveals that Ni is additionally modified by biological cycling and shelf sediment processes, while Cu is mostly sourced from riverine inputs and influenced by mixing. This observation highlights differences in their chemistries: Cu is more prone to complexation with organic ligands, stabilizing its riverine source fluxes into the Arctic, while Ni is more labile and is dominated by biological processes. Within the Canadian Arctic Archipelago, an important source of Arctic water to the Atlantic Ocean, contributions of Cu and Ni from meteoric waters and the halocline are attenuated during transit to the Atlantic. Additionally, Cu and Ni in deep waters diminish with age due to isolation from surface sources, with higher concentrations in the younger Eastern Arctic basins and lower concentrations in the older Western Arctic basins.

Description: An insufficient supply of the micronutrient iron (Fe) limits phytoplankton growth across large parts of the ocean. Ambient Fe speciation and solubility are largely dependent on seawater physico-chemical properties. We calculated the apparent Fe solubility (SFe(III)app) at equilibrium for ambient conditions, where SFe(III)app is defined as the sum of aqueous inorganic Fe(III) species and Fe(III) bound to organic matter formed at a free Fe3+ concentration equal to the solubility of Fe hydroxide. We compared the SFe(III)app to measured dissolved Fe (dFe) in the Atlantic and Pacific Oceans. The SFe(III)app was overall ∼2 to 4-fold higher than observed dFe at depths less than 1000 m, ∼2-fold higher than the dFe between 1000-4000 m and ∼3-fold higher than dFe below 4000 m. Within the range of used parameters, our results showed that there was a similar trend in the vertical distributions of horizontally averaged SFe(III)app and dFe. Our results suggest that vertical dFe distributions are underpinned by changes in SFe(III)app which are driven by relative changes in ambient pH and temperature. Since both pH and temperature are essential parameters controlling ambient Fe speciation, these should be accounted for in investigations of changing Fe dynamics, particularly in the context of ocean acidification and warming. Key Points Apparent iron solubility is driven by ambient pH, temperature (T) and dissolved organic carbon (DOC), and showed a 6-fold variation between surface (pH= 8.05 on the total scale, DOC= 71.8 µmol L-1, T= 20.4 °C) and deep oceanic waters (pH= 7.82, DOC= 38.6 µmol L-1, T= 1.1°C). Higher values of apparent iron solubility were determined for deep Atlantic and Pacific waters, with lower values in subtropical gyres. Calculated apparent iron solubility showed a similar trend in vertical distribution to dissolved iron, highlighting the importance of considering the impact of changes in ambient physico-chemical conditions on seawater iron chemistry.

Description: During the Polarstern TransARC II cruise (PS94) expedition water samples were taken in the Arctic Ocean north of 71 °N, between 17 August and 14 October 2015. The cruise was part of the international GEOTRACES program. Samples for trace metal analysis were taken using 24 trace-metal clean polypropylene samplers of 24 L, each mounted on an all titanium frame with a SEABIRD 911 CTD system. In an ISO Class 6 clean room container the samples were immediately filtered over 〈 0.2 µm Sartobran 300 cartridges and acidified. The dissolved trace metals cadmium (Cd), cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), zinc (Zn) were analysed at the Netherlands Institute for Sea Research with a Thermo Finnigan High Resolution Inductively Coupled Plasma Mass Spectrometer (HR-ICP-MS) element 2 after pre-concentration and matrix removal. This study provides a baseline to assess future change, and additionally identifies processes driving bio-essential trace metal distributions.