Competitive Ligand Exchange-adsorptive Cathodic Stripping Research Articles

Natural copper-binding ligands in the Arctic Ocean. The influence of the Transpolar Drift (GEOTRACES GN04)

The Arctic Ocean is a unique biogeochemical environment characterized by low salinity surface waters, extensive sea-ice coverage, high riverine inputs, large shelf extension and the long residence time of deep waters. These characteristics determine the distribution of dissolved bio-essential trace metals, such as copper (Cu), and the dissolved organic-binding ligands capable of complexing it. This work reports the concentrations and conditional stability constants of dissolved Cu-binding ligands (LCu and log KcondCu2+L) measured in samples from the Polarstern (PS94) expedition, as part of the international GEOTRACES program (cruise GN04). Full-depth profile stations from the Barents Sea, Nansen Basin, Amundsen Basin and Makarov Basin were analysed by competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV). The basins and water masses presented a wide range of LCu concentrations (range: 1.40 – 7.91 nM) and log KcondCu2+L values (range: 13.83 – 16.01). The highest variability of Cu-binding ligand concentrations was observed in surface waters (≤200 m), and mean concentrations increased from the Barents Sea and Nansen Basin (2.15 ± 0.31 nM and 1.93 ± 0.35 nM, respectively) to the Amundsen (3.84 ± 1.69 nM) and Makarov Basins (4.40± 2.03 nM). The influence of the Transpolar Drift (TDP) flow path was observed in the Amundsen and Makarov Basins, especially on Cu-binding ligand concentrations (LCu range: 3.96 – 7.91 nM). In contrast, deep waters (>200 m) showed no significant differences between basins and water masses in terms of LCu concentrations (range: 1.45 – 2.78 nM) and log KcondCu2+L (range: 14.02 – 15.46). The presence of strong Cu-binding ligands (log KcondCu2+L>13) in surface waters stabilises the excess of dissolved copper (dCu) transported in the TPD and favours its export to the Fram Strait and Nordic Seas.

Distribution of copper-binding ligands in Fram Strait and influences from the Greenland Shelf (GEOTRACES GN05)

The Fram Strait represents the major gateway of Arctic Ocean waters towards the Nordic Seas and North Atlantic Ocean and is a key region to study the impact of climate change on biogeochemical cycles. In the region, information about trace metal speciation, such as copper, is scarce. This manuscript presents the concentrations and conditional stability constants of copper-binding ligands (LCu and log KcondCu2+L) in the water column of Fram Strait and the Greenland shelf (GEOTRACES cruise GN05). Cu-binding ligands were analysed by Competitive Ligand Exchange-Adsorptive Cathodic Stripping Voltammetry (CLE-ACSV) using salicylaldoxime (SA) as competitive ligand. Based on water masses and the hydrodynamic influences, three provinces were considered (coast, shelf, and Fram Strait) and differences were observed between regions and water masses. The strongest variability was observed in surface waters, with increasing LCu concentrations (mean values: Fram Strait = 2.6 ± 1.0 nM; shelf = 5.2 ± 1.3 nM; coast = 6.4 ± 0.8 nM) and decreasing log KcondCu2+L values (mean values: Fram Strait = 15.7 ± 0.3; shelf = 15.2 ± 0.3; coast = 14.8 ± 0.3) towards the west. The surface LCu concentrations obtained above the Greenland shelf indicate a supply from the coastal environment to the Polar Surface Water (PSW) which is an addition to the ligand exported from the central Arctic to Fram Strait. The significant differences (in terms of LCu and log KcondCu2+L) between shelf and coastal samples were explained considering the processes which modify ligand concentrations and binding strengths, such as biological activity in sea-ice, phytoplankton bloom in surface waters, bacterial degradation, and meltwater discharge from 79NG glacier terminus. Overall, the ligand concentration exceeded those of dissolved Cu (dCu) and kept the free copper (Cu2+) concentrations at femtomolar levels (0.13–21.13 fM). This indicates that Cu2+ toxicity limits were not reached and dCu levels were stabilized in surface waters by organic complexes, which favoured its transport to the Nordic Seas and North Atlantic Ocean and the development of microorganism.

Influence of strong iron-binding ligands on cloud water oxidant capacity

Iron (Fe) plays a dual role in atmospheric chemistry: it is involved in chemical and photochemical reactivity and serves as a micronutrient for microorganisms that have recently been shown to produce strong organic ligands. These ligands control the reactivity, mobility, solubility and speciation of Fe, which have a potential impact on Fe bioavailability and cloud water oxidant capacity.In this work, the concentrations of Fe-binding ligands and the conditional stability constants were experimentally measured for the first time by Competitive Ligand Exchange-Adsorptive Cathodic Stripping Voltammetry (CLE-ACSV) technique in cloud water samples collected at puy de Dôme (France). The conditional stability constants, which indicate the strength of the Fe-ligand complexes, are higher than those considered until now in cloud chemistry (mainly Fe-oxalate). To understand the effect of Fe complexation on cloud water reactivity, we used the CLEPS cloud chemistry model. According to the model results, we found that Fe complexation impacts the hydroxyl radical formation rate: contrary to our expectations, Fe complexation by natural organic ligands led to an increase in hydroxyl radical production. These findings have important impacts on cloud chemistry and the global iron cycle.

Iron and copper complexation in Macaronesian coastal waters

Dissolved iron (dFe) and copper (dCu), the concentration and the conditional stability constants of organic binding ligands (LFe, LCu, log KcondFe3+L and log KcondCu2+L) were studied in the surface coastal waters of the Macaronesia region (Cape Verde, Canary Islands, and Madeira) using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV). Two oceanic stations were also studied: the Cape Verde Ocean Observatory (CVOO) and the European Station for Time Series in the Ocean in the Canary Islands (ESTOC). Dissolved Fe varied from 0.46 to 1.32 nM and LFe concentrations were between 0.56 and 2.96 nM. More than 98% of the total dFe was complexed with conditional stability constants (log KcondFe3+L) between 20.77 and 21.90 (L2-type ligands). Dissolved Cu concentrations ranged between 0.07 and 4.03 nM and the amount of LCu varied between 0.54 and 2.59 nM, with more than 99% of dCu organically complexed. The conditional stability constant (log KcondCu2+L) showed values between 13.40 and 14.42 (L1-type).Due to biological activity and water mixing induced by the wind around the islands, dissolved metals and ligand concentrations were greater at the coastal stations than in oceanic water. Variations were observed between the eastern and western parts of Fogo, Tenerife and Gran Canaria. On the east coasts, the increase in dissolved metals and ligand concentrations were related to wind-induced water mixing. The results of this study will contribute to the knowledge about the impact of coastal areas on the Fe and Cu biogeochemical cycles.

Spectrophotometric Determination of Fe(II) and Fe(III) Using 5-(4-Ethylphenyl azo ) 2-Hydroxybenzaldehyde Oxime Reagent

Spectrophotometric Determination of Fe(II) and Fe(III) Using 5-(4-Ethylphenyl azo ) 2-Hydroxybenzaldehyde Oxime Reagent

Wide-range detection of Cu-binding organic ligands in seawater using reverse titration

The toxicity and bioavailability of Cu in seawater were estimated using its chemical speciation. In this study, the concentrations and conditional stability constants (K'CuL) of Cu complexes with Cu-binding organic ligands (L) in seawater were determined using the reverse titration–competitive ligand exchange–adsorptive cathodic stripping voltammetry method. The concentrations of strong and weak ligands (L1 and L2, respectively) were determined using reverse titration via the addition of Cu to seawater samples to achieve ligand saturation. Our results revealed that the reverse titration method can be successfully used for the detection of high concentrations of L2. Using the reverse titration method, we detected L2 concentrations that exceeded 80 and 120 nM in seawater samples from the subtropical western North Pacific Ocean and Otsuchi Bay Japan, respectively. The K′CuL1 and K′CuL2 values obtained using the reverse titration method were comparable with those obtained using the forward titration method. Therefore, the reverse titration method can be used to determine the chemical speciation of Cu in the ocean and coastal regions as supplement to the more conventional forward titration method.

Electrochemical evaluation of iron-binding ligands along the Australian GEOTRACES southwestern Pacific section (GP13)

In this work, we performed electrochemical investigations of Fe-binding ligands in water samples collected in autumn 2011 along the Australian GEOTRACES southwestern Pacific section (GP13, between 153°E and 170°W longitude along the 30°S line East of Australia, 0-1000 m depth). We determined the capacity of the bulk organic ligands to complex Fe using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV) with salicylaldoxime as the competing ligand. Two categories of organic ligands, humic substances (HS-like) and catalytically active polymers (Cat. P) were electrochemically quantified in order to better define the bulk of Fe-binding ligands. Finally, Fe speciation results have been linked to oceanographic data, phytoplankton biomass, and two groups of cyanobacteria (Prochlorococcus and Synechococcus) which are prominent members of the phototrophic community in the study region. Across the section, higher total ligand concentrations over dissolved Fe concentrations were observed, as well as the predominance of “weak” Fe-binding ligands (log K’Fe’L < 12). Highest “excess ligands” were mostly concentrated in the upper layer of the water column, suggesting a direct link with biological activity. None of the two groups of organic ligands measured (HS-like and Cat. P) accounted for the bulk of the total Fe-binding ligands concentration, hindering a better characterization of the nature of in-situ Fe-binding ligands. Cat. P concentrations showed statistically significant positive correlations with all biomarker pigments and the abundance of Prochlorococcus, suggesting that this material, resembling polysaccharides, could be a good parameter to probe organic compounds from specific biological origin. Amongst the biological parameters, only Prochlorococcus was related to Fe’ concentrations.

On the nature of dissolved copper ligands in the early buoyant plume of hydrothermal vents

Environmental contextCopper released by deep-sea hydrothermal vents has been recognised to be partly stabilised against precipitation by its complexation with strong Cu binding ligands. Yet, the sources and nature of these compounds in such environments are still not fully understood. This study shows that the Cu ligands detected are hydrothermally sourced and could be mainly inorganic sulfur species. AbstractThe apparent speciation of Cu in the early buoyant plume of two black smokers (Aisics and Y3) from the hydrothermal vent field Lucky Strike (Mid-Atlantic Ridge) was investigated using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE–AdCSV). We have assessed the apparent Cu-binding ligand concentration ([L]) and the corresponding conditional stability constant (log K′) for 24 samples. At the smoker Aisics, [L] ranged from 18.2 to 2970 nM. Log K′CuL ranged from 12.4 to 13.4. At Y3, the binding capacity of natural ligands was from 32.5 to 1020 nM, with Log K′CuL ranging from 12.5 to 13.1. Total dissolved Cu ranged from 7.0 to 770 nM and from 12.7 to 409 nM at Aisics and Y3, respectively. Our results show that the amount of ligand L increases with dissolved Mn (dMn) concentrations, suggesting a hydrothermal origin of the Cu-binding ligands detected. In addition, such high concentrations of Cu-binding ligands can only be explained by an additional abiotic source differing from organic processes. Based on the massive in situ concentrations of free sulfides (up to 300 µM) and on the striking similarities between our log K′CuL and the log K′Cu(HS) previously published, we infer that the Cu-binding ligands could be predominantly inorganic sulfur species in the early buoyant plume of the two vent sites studied.

An Intercomparison of Dissolved Iron Speciation at the Bermuda Atlantic Time-series Study (BATS) Site: Results from GEOTRACES Crossover Station A

The organic complexation of dissolved iron (Fe) was determined in depth profile samples collected from GEOTRACES Crossover Station A, the Bermuda Atlantic Time-series Study (BATS) site, as part of the Dutch and U.S. GEOTRACES North Atlantic programs in June 2010 and November 2011, respectively. The two groups employed distinct competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV) methods, and resulting ligand concentrations and conditional stability constants from each profile were compared. Excellent agreement was found between the total ligand concentrations determined in June 2010 and the strongest, L1-type, ligand concentrations determined in November 2011. Yet a primary distinction between the datasets was the number of ligand classes observed: a single ligand class was characterized in the June 2010 profile while two ligand classes were observed in the November 2011 profile. To assess the role of differing interpretation approaches in determining final results, analysts exchanged titration data and accompanying parameters from the profiles for reinterpretation. The reinterpretation exercises highlighted the considerable influence of the sensitivity (S) parameter applied on interpretation results, consistent with recent intercalibration work on interpretation of copper speciation titrations. The potential role of titration data structure, humic-type substances, differing dissolved Fe concentrations, and seasonality are also discussed as possible drivers of the one versus two ligand class determinations between the two profiles, leading to recommendations for future studies of Fe-binding ligand cycling in the oceans.

Fe- and Cu-Complex Formation with Artificial Ligands Investigated by Ultra-High Resolution Fourier-Transform ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS): Implications for Natural Metal-Organic Complex Studies

In recent years, electrospray-ionization mass spectrometry (ESI-MS) has been increasingly used to complement the bulk determination of metal-ligand equilibria, for example via competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV). However, ESI-MS speciation analyses may be impacted by instrumental artefacts such as reduction reactions, fragmentation, and adduct formation at the ESI source, changes in the ionization efficiencies of the detected species in relation to sample matrix, and peak overlaps in response to increasing sample complexity. In our study, equilibria of the known artificial ligands citrate, ethylenediaminetetraacetic acid (EDTA), 1-nitroso-2-naphthol (NN), and salicylaldoxime (SA) with iron (Fe) and copper (Cu) were investigated by ultra-high resolution ESI-MS, Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), under a variety of sample matrix and ionization settings. The acquired mass spectra were compared with metal-ligand equilibrium data from the literature as well as an adapted speciation model. Overall, the mass spectra produced representative species mentioned in previous reports and predicted by the speciation calculations, such as Fe(Cit), Cu(Cit)2, Fe(EDTA), Cu(EDTA), Fe(NN)3, and Cu(SA)2. The analyses furthermore revealed new species which had been hypothesized but not measured directly using other methods, for example ternary complexes of citrate with Fe and Cu, Cu(SA) monomers, and the dimer Fe(SA)2. Finally, parallel measurements of a Cu+SA calibration series and a Cu+SA+EDTA competition series indicated that FT-ICR-MS can produce linear responses and low detection limits analogous to those of ACSV. We propose that ultra-high resolution FT-ICR-MS can be used as a representative tool to study interactions of trace metals with artificial as well as natural, unknown ligands at the molecular level.

Distribution and Speciation of Dissolved Iron in Jiaozhou Bay (Yellow Sea, China)

The distribution of total dissolved iron (DFe) and its chemical speciation were studied in vertical profiles of the shallow and semi-closed Jiaozhou Bay (JZB, China) during two contrasting periods: summer (July 19th, 2011) and spring (May 10th, 2012). Samples collected from the surface, middle and bottom layers were analyzed by competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-aCSV). The mean DFe concentration during the summer period (median 18.8 nM; average 20.7 nM) was higher than in the spring period (median 12.4 nM; average 16.9 nM), whereas the spatio-temporal variation in spring was larger than in summer. The DFe values showed distinct regional and seasonal differences, ranging from 5.6-107nM in spring period and 13.4-43.4 nM in summer period. In spring, the highest DFe values were observed in the eastern coastal region, especially near an industrial area (up to 107nM), whereas the DFe distribution in summer was relatively even. Due to a tide influence, the vertical variations in the DFe and Lt in both seasons were not significant. On average, the Lt concentration (one class of ligand was estimated in all samples), was higher in spring (35.2±23.4 nM) than in summer (31.1±10.3 nM). A statistically significant correlation was found between Lt and DFe concentrations, it was higher for the summer period than for the spring period. The conditional stability constants (logK') of organic complexes with iron were weaker in spring (11.7±0.3) than in summer (12.3±0.3). The concentrations of Lt were higher in comparison to DFe in all samples: the average [Lt]/[DFe] ratio in the spring and summer samples was 2.4 and 1.5, respectively. Speciation calculations showed that the DFe in the JZB existed predominantly (over 99.99%) in the form of strong organic complexes in both seasons.

Iron-Binding Ligands in the Southern California Current System: Mechanistic Studies

The distributions of dissolved iron and organic iron-binding ligands were examined in water column profiles and deckboard incubation experiments in the southern California Current System (sCCS) along a transition from coastal to semi-oligotrophic waters. Analysis of the iron-binding ligand pool by competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) using multiple analytical windows (MAWs) revealed three classes of iron-binding ligands present throughout the water column (L1-L3), whose distributions closely matched those of dissolved iron and nitrate. Despite significant biogeochemical gradients, ligand profiles were similar between stations, with surface minima in strong ligands (L1 and L2), and relatively constant concentrations of weaker ligands (L3) down to 500 m. A phytoplankton grow-out incubation, initiated from an iron-limited water mass, showed dynamic temporal cycling of iron-binding ligands. A biological iron model was able to capture the patterns of the strong ligands in the grow-out incubation relatively well with only the microbial community as a biological source. An experiment focused on remineralization of particulate organic matter showed production of both strong and weak iron-binding ligands by the heterotrophic community, supporting a mechanism for in-situ production of both strong and weak iron-binding ligands in the subsurface water column. Photochemical experiments showed a variable influence of sunlight on the degradation of natural iron-binding ligands, providing some evidence to explain differences in surface ligand concentrations between stations. Patterns in ligand distributions between profiles and in the incubation experiments were primarily related to macronutrient concentrations, suggesting microbial remineralization processes might dominate on longer time-scales over short-term changes associated with photochemistry or phytoplankton growth.

The organic complexation of dissolved iron along the U.S. GEOTRACES (GA03) North Atlantic Section

The organic complexation of dissolved iron was determined from full water column depth profile samples collected on the U.S. GEOTRACES North Atlantic Section cruises in 2010 and 2011 (GEOTRACES GA03). The concentrations of iron-binding ligands and their conditional stability constants were determined using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) with salicylaldoxime as the added competitive ligand. Across the basin, iron-binding ligands were found in excess of dissolved iron concentrations in all samples except those with the highest dissolved iron in the Trans-Atlantic Geotraverse (TAG) hydrothermal vent plume, where dissolved iron concentrations exceeded ligand concentrations. Ligand results were categorized based on conditional stability constants into three ligand classes (L1: logKFeLi,Fe′cond>12; L2: logKFeL2,Fe′cond=11–12; L3: logKFeL3,Fe′cond=10–11). The stronger L1-type ligand class tracked closely with dissolved iron, with the strongest ligands (i.e., highest logKFeL1,Fe′cond) found in the vicinity of the Trans-Atlantic Geotraverse (TAG) hydrothermal vent plume. All three ligand classes, including the stronger L1-type ligands, were observed through the water column. These measurements indicate that iron-binding ligands are indeed a ubiquitous feature of iron speciation in the North Atlantic.

The relevance of ligand exchange kinetics in the measurement of iron speciation by CLE–AdCSV in seawater

CLE–AdCSV (competitive ligand exchange–adsorptive cathodic stripping voltammetry) has proved to be a decisive technique in recognising the key role that organic matter plays in modulating the biogeochemical cycle of iron in aquatic ecosystems. As the method requires equilibrium among all the constituents of the solution before measurements are taken, accurate knowledge of the kinetics of the undergoing ligand exchange is essential. The kinetics of the process has been most frequently described in terms of a dissociative (or Eigen–Wilkins) mechanism, with some authors even claiming that, under such approach, iron complexes would dissociate during CLE so slowly that the method would be invalidated. The complexity of the processes—largely exceeding the simple mechanistic description of the Eigen–Wilkins mechanism—is discussed on the basis of a broad overview of possible reaction routes. Evidence of the role of matrix components of natural waters is presented and the impossibility of exporting kinetic constants to systems characterised by different molecular mechanics, mostly ignored in previous studies, is highlighted. Experimental evidence obtained from various methodological approaches allows inferring the range of CLE equilibration times to be applied in CLE–AdCSV measurements and further proves that the Eigen–Wilkins mechanism is just one of the many reaction routes applicable in natural waters. Studying iron CLE mechanisms for proposed competing ligands before designing any experiment using CLE–AdCSV is recommended.

The composition of dissolved iron in the dusty surface ocean: An exploration using size-fractionated iron-binding ligands

Abstract The size partitioning of dissolved iron and organic iron-binding ligands into soluble and colloidal phases was investigated in the upper 150 m of two stations along the GA03 U.S. GEOTRACES North Atlantic transect. The size fractionation was completed using cross-flow filtration methods, followed by analysis by isotope dilution inductively-coupled plasma mass spectrometry (ID-ICP-MS) for iron and competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) for iron-binding ligands. On average, 80% of the 0.1–0.65 nM dissolved iron ( 99% of dissolved iron in the ocean is organically complexed. Disentangling the chemical form of iron in the upper ocean has important implications for surface ocean biogeochemistry and may affect iron uptake by phytoplankton.

Distinct pools of dissolved iron‐binding ligands in the surface and benthic boundary layer of the California Current

Organic dissolved iron (dFe)—binding ligands were measured by competitive ligand exchange—adsorptive cathodic stripping voltammetry (CLE‐ACSV) at multiple analytical windows (side reaction coefficient of salicylaldoxime, αFe(SA)2 = 30, 60 and 100) in surface and benthic boundary layer (BBL) samples along the central California coast during spring and summer. The weakest ligands were detected in the BBL at the lowest analytical window with average log KcondFeL,Fe' = 10.2 ± 0.4 in the summer and 10.8 ± 0.2 in the spring. Between 3% and 18% of the dFe complexation in the BBL was accounted for by HS, which were measured separately in samples by ACSV and may indicate a source of dFe‐binding ligands from San Francisco Bay. The strongest ligands were found in nearshore spring surface waters at the highest analytical window with average log KcondFeL,Fe' = 11.9 ± 0.3, and the concentrations of these ligands declined rapidly offshore. The ligand pools in the surface and BBL waters were distinct from each other based on principal components analysis, with variances in the BBL ligand pool explained by sample location, and variance in surface waters explained by water mass. The use of multiple analytical window analysis elucidated several distinct iron‐binding ligand pools, each with unique distributions in the central California Current system.

Spatial variability of total dissolved copper and copper speciation in the inshore waters of Bermuda

Total dissolved copper (Cu) and Cu speciation were examined from inshore waters of Bermuda, in October 2009 and July–August 2010, to determine the relationship between total dissolved Cu, Cu-binding ligands and bioavailable, free, hydrated Cu2+ concentrations. Speciation was performed using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV). Mean total dissolved Cu concentrations ranged from 1.4nM to 19.2nM, with lowest concentrations at sites further from shore, consistent with previous measurements in the Sargasso Sea, and localized Cu enrichment inshore in enclosed harbors. Ligand concentrations exceeded dissolved [Cu] at most sites, and [Cu2+] were correspondingly low at those sites, typically <10−13 M. One site, Hamilton Harbour, was found to have [Cu] in excess of ligands, resulting in [Cu2+] of 10−10.7 M, and indicating that Cu may be toxic to phytoplankton here.

New approach to analysis of voltammetric ligand titration data improves understanding of metal speciation in natural waters

In this study, previously published multiple analytical window datasets of competitive ligand exchange—adsorptive cathodic stripping voltammetry titrations for dissolved copper with salicylaldoxime as the competing ligand were reanalyzed. A new numerical method was applied, the Sander‐Wells (S‐W) method, which is able to simultaneously analyze multiple analytical window titrations as a unified dataset and calculate parameters for one to three ligand classes in the samples. The two datasets used were from separate sampling events at Dumbarton Bridge, San Francisco Bay. The aim of this study was to see if discrete ligand classes can be resolved from these data, and how these compare with results obtained by applying commonly used manual calculations of individual analytical window titrations separately: Scatchard and van den Berg/Ružic linearizations, and the Gerringa non‐linear regression. The united multiple analytical window dataset could only be analyzed simultaneously by the S‐W method, and two or three discrete ligand classes were resolved for each sample. Applying the four different data analysis methods independently to each of the five analytical windows resulted in solutions spanning a continuum of conditional stability constants and ligand concentrations. It was shown that the sensitivity S, as well as data structure and iterative optimization, are the major reasons for different results. This study demonstrates the utility of simultaneous analysis of multiple analytical window titration curves to better determine the distribution of trace metal species in aqueous systems and the advantages of adopting such a ‘unified data set’ approach over fitting individual titration curves.

The stabilisation and transportation of dissolved iron from high temperature hydrothermal vent systems

Iron (Fe) binding phases in two hydrothermal plumes in the Southern Ocean were studied using a novel voltammetric technique. This approach, reverse titration–competitive ligand exchange–adsorptive cathodic stripping voltammetry, showed that on average 30±21% of dissolved Fe in the hydrothermal plumes was stabilised by chemically labile binding to ligands. The conditional stability constant (logK′FeL) of the observed complexes was 20.61±0.54 (mean±1 SD) for the two vent sites, intermediate between previous measurements of deep ocean ligands (21.4–23; Kondo et al., 2012) and dissolved weak estuarine ligands (<20; Gerringa et al., 2007).Our results indicate that approximately 7.5% of all hydrothermal Fe was stabilised by complexation with ligands. Furthermore, 47±26% of the dissolved Fe in the plume existed in the colloidal size range (0.02–0.2µm). Our data suggests that a portion (∼7.5%) of hydrothermal Fe is sufficiently stabilised in the dissolved size fraction (<0.2µm) to make an important impact on deep ocean Fe distributions. Lateral deep ocean currents transport this hydrothermal Fe as lenses of enhanced Fe concentrations away from mid ocean ridge spreading centres and back arc basins.

Measurement of iron chemical speciation in seawater at 4 °C: The use of competitive ligand exchange–adsorptive cathodic stripping voltammetry

Iron is mostly bound to poorly characterised organic ligands; thus, organic ligands are paramount in defining Fe biogeochemical cycling and its control on oceanic primary productivity. Since 1994, Fe chemical speciation has been determined by Competitive Ligand Exchange–Adsorptive Cathodic Stripping Voltammetry (CLE–AdCSV) at room temperature. However, chemical speciation is strongly dependent on temperature and some organic ligands can be temperature sensitive. Here, we compare the use of the CLE–AdCSV at room temperature and at 4°C—a temperature closer to that found in the Southern Ocean, one of the largest iron-limited regions. For both temperatures, similar detection limits and total Fe concentrations were found. However, at 4°C the analytical detection window (αFe(TAC)2) was shifted by 1.4-fold towards the detection of weaker ligands, resulting in up to 2-fold lower ligand concentrations as well as a 2- to 5-fold and 10- to70-fold lower conditional stability constants with inorganic Fe (Fe′) and Fe(III), respectively. As a result, the Fe′ concentration at 4°C was 2-fold greater, resulting in direct implication for Fe bioavailability. Results show that difference in Fe chemical speciation at 4°C was not solely explained by temperature effect on thermodynamics with the exchange ligands or the diffusion of the electroactive complex towards the Hg drop. Lowering analytical window during analysis at room temperature is proposed as a first estimate of temperature effect on iron chemical speciation.