Humic acid like concentration in seawater samples, collected from the trace metal rosettes in the Southern Ocean during the austral summer of 2016/2017, on board the Antarctic Circumnavigation Expedition. 
 
***** Dataset abstract *****

Humic acid like concentration (abbreviated HA) measured with respect to the Suwannee River Fulvic acid standards (µmol SRFA equivalent per litre). 

Seawater samples were collected from trace metal rosette (TMR) deployments at different depths in the water column during the Antarctic Circumnavigation Expedition (ACE). Humic acid like data from legs 1 and 2, from TMR cast numbers 3 to 16, were analysed by electrochemistry following standard additions of Suwannee River Fulvic Acid (standard 1, IHSS). This data is to support iron ligands and iron bioavailability as well as hydrolysable saccharides (TPZT) data, also collected during ACE.

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

Samples were collected in duplicate using polyethylene (PE) or nitrile gloves in a clean room container. Before sampling, LDPE  acid-washed bottles and caps were rinsed three times with about 5-10 ml of gravity-filtered seawater (PALL cartridge of 0.2 µm). Samples from each station were gathered in a labelled ziplock bag in the ship’s walk-in freezer of -20 degrees C. Please refer to trace metal rosette sampling procedure for more information in the ACE Cruise Report (Walton and Thomas, 2018). Samples were stored upright in a freezer until analysis.

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

Analyses were carried out from September 2018 to November 2018 using a Methrom AUTOLAB PGSTAT101 with a VA663 hanging mercury drop electrode connected via a 663 IME interface to a PC. Linear sweep mode was used for the voltammetric scans. The mercury drop size was set to 3, purge time with N2 was 240 seconds and deposition time 240 seconds with a deposition potential of -0.1 V followed by an unstirred 10-second equilibration time. The scan was carried out between -0.1 V and -1.2 V with a scan rate of 50 mV/s and a step potential of 5 mV. Peak heights were calculated with ECDSoft. Analysis was performed using the analytical approach described in Cabanes et al., 2016. Briefly, seawater was defrosted in a fridge then left at reach room temperature. It was then complemented with FeCl3 (50 nM), a buffer (EPPS, 10 µM.) and a catalytic agent (KBrO3, 40 µM) and separated in 10 mL aliquots using PE tubes. Three aliquots were kept as is, and three to five aliquots were enriched in Suwanee River Fulvic acid (SRFA standard 1, IHSS) to determine in-situ concentration of humic acid like in-situ following standard addition. For this reason concentration is expressed in µg SRFA eq./L. Prior to analysis, samples were left to react at room temperature in the dark for 1.5-2 hours.  

Squared correlation coefficients (r^2) of linear regression of the SRFA additions ranged between 0.984 and 0.999. Limit of detection yielded 1.9 µg SRFA eq./L. 

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

The accuracy of the procedure was checked by the analysis of a solution of 95.0 µg/L SRFA (= 97.8 equiv. µg/L SRFA).

***** Further information for interpreting the data and using the dataset *****

Detection limit: Limit of detection yielded 1.9 µg SRFA eq./L.

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

ace_humics_data.csv, data file, comma-separated values
ace_humics_data_visual_summary.png, metadata, portable network graphics
data_file_header.txt, metadata, text format
README.txt, metadata, text format

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

Christel Hassler, University of Geneva, Department of F.-A. Forel for Environmental and Aquatic Sciences, Switzerland. ORCID: 0000-0002-8976-5469. Email: christel.hassler@unige.ch

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

This dataset can be cited as: 
Hassler, C. and Ellwood, M. (2021). Humic acid like concentration in seawater samples, collected from the trace metal rosettes 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.3250137

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

This humics 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/