Fe chemical speciation collected using trace metal rosette in the Southern Ocean during the austral summer of 2016/2017, on board the Antarctic Circumnavigation Expedition.

***** Abstract *****

Fe chemical speciation of filtered seawater data are presented in this dataset, resulting from samples collected from a trace metal rosette on board the Antarctic Circumnavigation Expedition (ACE). During the austral summer of 2016/2017, seawater samples were collected from the Atlantic and Indian Ocean sectors of the Southern Ocean and dissolved Fe concentration, iron-binding organic ligands concentration and the conditional stability constant of Fe’ are presented here.

***** Original data collection *****

Seawater samples were collected from trace-metal clean bottles on a trace metal rosette. They were then filtered. The Fe speciation was determined by the CLE-AdCSV technique following the optimized method of Abualhaija and van den Berg (2014). After thawing, 10 mL aliquots of filtered seawater, subsamples were pipetted into 18 to 20 acid-cleaned low-density polyethylene 15 mL bottles (Nalgene, ThermoFisher). Each aliquot was then buffered to pH 8.2 with a 1 M borate buffer in which 12-13 known concentrations of Fe, between 0.2 and 15 nM, were added. In three of the tubes, no Fe was added. Following a 1 hour equilibration time at room temperature, 5 µM SA (competitive ligand) was then added to all the tubes, and samples were left to equilibrate overnight. Analysis was undertaken with the following parameters: 4-10 minute deposition time, 30 seconds air purging, 200 seconds of deposition time at 0 V and the CSV instrumental settings as per Abualhaija and van den Berg (2014).

Fe concentration values were originally published in Janssen et al. (2020; DOI: 10.5281/zenodo.3634411).

Instrumentation:

663 VA Stand electrode system equipped with hanging mercury drop electrode (HMDE) (Metrohm Switzerland; Metrohm Autolab, Utrecht, The Netherlands) as a working electrode, Ag/AgCl (3 M KCl) as a reference electrode and a glassy carbon rod as an auxiliary electrode. It was connected to a µAutolab potentiostat/galvanostat type III and to a PC via a 663 IME interface. The HDME surface area was set to 0.52 mm^2 and the solution was stirred during deposition step using a rotating PTFE rod set to stirring speed of 3000 rpm. The whole voltammetric system was kept in a laminar flow cabinet (600 PCR workstation, AirClean Systems) at ambient temperature.

***** Data processing *****

Voltammograms analysis was performed with ECDSoft software (Omanović et al., 2006), and the 4th derivative peak height was considering for further calculations. The proMCC software (Omanović et al., 2014), with auto-adjusted sensitivity (n = 3-5), was used to analyze the resulting Fe-binding ligand titration data as well as to quantify both total concentration of ligand (L) and their conditional stability constant (log(K’Fe'L)) for inorganic iron (Fe’). Both parameters were calculated assuming only one class of Fe-binding ligand. Indeed, the result from all different linear and non-linear mathematical approaches clearly showed the detection of a single class of ligand for the whole set of titration curves. 

Typical titration results and fitting can be seen in the accompanying figure 1 which displays an example of PROMCC fittings for TMR cast 8, at a depth of 125 m.

***** Quality checking *****

Calibration of the conditional stability constant for SA (5 µM) with Fe in UV-digested seawater from the studied region (K’(Fe’SA) and K’(FeSA2)) was determined by competition with ethylenediaminetetraacetic acid (EDTA). Both log(K’(Fe’SA)) (6.56) and log(K’(FeSA2)) (10.9) values were in accordance with the literature (Abualhaija and van den Berg, 2014; Mahmood et al., 2015) and correspond to a side reaction coefficient for 5 µM SA of 20.1 (αFe’SA). 

Addition of strong organic ligands (desferrioxamine B) was used to check our ability to detect this strong ligands concentration in synthetic inorganic seawater. Addition of 5nM DFB resulted in the detection of 4.61 ± 0.13 nM of organic ligands, and conditional staibility constant was 12.25 ± 0.04 M-1, in accordance with previous reports (e.g. Maldonado et al., 2005).

Finally, to check the ability to accurately detect accurate Fe concentration, SAFe S reference sample were measured (as per Abualhaija and van den Berg, 2014). The average dFe concentration of 0.101 ± 0.005 nM (n=3) measured was in good agreement with the certified value (0.093 ± 0.008 nM, SAFe Reference samples as of May 2013). 

***** Dataset contents *****

- ace_fe_chemical_speciation.csv, data file, comma-separated values
- figure1.pdf, metadata, portable document format
- data_file_header.txt, metadata, text
- README.txt, metadata, text

***** Dataset contact *****

Christel Hassler, Swiss Polar Institute, Switzerland. ORCID: 0000-0002-8976-5469. Email: christel.hassler@epfl.ch

***** Dataset license *****

This Fe chemical speciation dataset from ACE is made available under the Creative Commons Attribution 4.0 International License (CC BY 4.0) whose full text can be found at https://creativecommons.org/licenses/by/4.0/

***** Dataset citation *****

Please cite this dataset as :
Hassler, C., Cabanes, D., Ellwood, M., Conway, T. and Sieber, M. (2020). Fe chemical speciation collected using trace metal rosette in the Southern Ocean during the austral summer of 2016/2017, on board the Antarctic Circumnavigation Expedition. (Version 1.0) [Data set]. Zenodo. https://doi.org/10.5281/zenodo.3897170