Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Water column sampling:
The full water column of 27 stations in the Amundsen Sea was sampled between 8 December 2023 and 13 January 2024 aboard the R/V IB Nathaniel B. Palmer. Depth profile samples were collected from the GEOTRACES trace metal clean rosette sampling system (GTC CTD) outfitted with 24 modified 12-liter (L) GO-FLO (General Oceanics) sampling bottles (Cutter and Bruland, 2012). Surface water (~2 meters) samples for the depth profiles were collected using a trace metal clean surface pump "towfish" system (FISH).

From the trace metal clean rosette and towfish, samples for humic-like substances were filtered (0.2 micrometer (µm), Acropak) inline and collected inside a trace metal clean sampling van. Samples were collected in acid-cleaned, Milli-Q-conditioned, and triple-rinsed narrow-mouth fluorinated high-density polyethylene bottles (FPE; Nalgene) and analyzed shipboard for dissolved iron speciation by Dr. Léo Mahieu in Dr. Kristen Buck's lab before freezing at -20 degrees Celsius (ºC) for shore-based humic-like substance analyses at Oregon State University.

Sample analyses – humic-like substances:
Humic-like substances were measured using cathodic stripping voltammetry following the method first developed by Laglera et al. (2007) and updated by Sukekava et al. (2018). All samples were analyzed using a polarographic 797 VA Computrace (Metrohm) stand equipped with a hanging mercury drop electrode, platinum rod auxiliary electrode, Ag/AgCl reference electrode, and an acid-cleaned Teflon analytical cell. Briefly, frozen samples were set out to thaw overnight at room temperature in the dark. The following day, 10 milliliter (mL) sub-samples were aliquoted into each of 2 paired acid-cleaned, pre-conditioned 50 mL polypropylene vials. The pH of the samples was then set to 8.2 with the addition of 50 microliters (μL) of 1.5 molar (M) boric acid buffer. Iron standard was added to one of the paired vials (vial B, final concentration 60 nanomolar (nM)) to saturate all the humic-like substance binding groups in the solution. Both aliquots were then left to equilibrate for a minimum of 14 hours. After the equilibration period, 500 μL of the 0.4M KBrO₃^- catalyst was dispensed into the cell with each aliquot immediately prior to analysis. Electrochemical analysis was performed using linear sweep voltammetry (-0.1 to -1 volts (V), scan rate 50 millivolts per second (mV s-1)) with a 120-second deposition period with stirring at -0.1V. Measurements were run in triplicate with a 300-second purge step with high-purity N₂ gas prior to analyzing each aliquot. For each sample, the Fe-free aliquot (vial A) was analyzed first, immediately followed by the Fe-saturated aliquot (vial B). Three standard additions of Fe-saturated SRFA standard (0.1, 0.2, and 0.3 milligrams (mg) SRFA per liter) were then added successively to the Fe-saturated aliquot to generate a calibration curve. A 70s N₂ purge was included after each addition to ensure oxygen removal and all additions were measured in triplicate. The Teflon analytical cell was rinsed thoroughly with Milli-Q water between samples to minimize carryover.

Peak heights were extracted from scans using ECDSOFT, then converted into the concentration of electroactive humic substances (micrograms (μg) eq SRFA^-1) using the slope of the curve generated by the three standard additions. The standard addition slope was applied to the peaks obtained for both vials A and B to obtain the ambient Fe-binding humic and total Fe-binding humic concentrations, respectively, in mg SRFA eq per liter. Units were then adjusted to nM Fe eq using the measured binding capacity of SRFA in seawater 14.6 ± 0.4 nM Fe mg SRFA^-1. Variability between triplicate scans averaged 0.02 nM Fe eq for both ambient and total humic results and variability between replicate samples averaged 0.05 (median 0.03) nM Fe eq and 0.06 (median 0.04) nM Fe eq for ambient and total humic-like substances, respectively. The limit of detection (LOD) was calculated as three times the standard deviation of triplicate scans in the lowest measured ambient Fe-binding humic concentration (0.06 ± 0.02 nM Fe eq) and was estimated to be 0.05 nM Fe eq. All measured values were above the LOD.

Data were flagged using the SeaDataNet quality flag scheme recommended by GEOTRACES (https://www.geotraces.org/geotraces-quality-flag-policy/) and described below. Notes specific to the application of these flags to this dataset are noted in brackets […].

0: No Quality Control: No quality control procedures have been applied to the data value. This is the initial status for all data values entering the working archive. [Not used].

1: Good Value: Good quality data value that is not part of any identified malfunction and has been verified as consistent with real phenomena during the quality control process. [Used for analyses that included replicates].

2: Probably Good Value: Data value that is probably consistent with real phenomena, but this is unconfirmed or data value forming part of a malfunction that is considered too small to affect the overall quality of the data object of which it is a part. [Used when no replicates were available to further verify the quality of the data].

3: Probably Bad Value: Data value recognized as unusual during quality control that forms part of a feature that is probably inconsistent with real phenomena. [Used for Fe bound to humics data when concentration of Fe bound to humics is greater than preliminary Fe concentrations by more than 0.03 nM (which is the median std dev of replicate analyses), or where no preliminary Fe data is available, but interpolated Fe values are greater than Fe-humic].

4: Bad Value: An obviously erroneous data value. [Used when anomalously high or low values of Fe-bound to humics or total Fe-binding capacity of humics is found in a depth profile with no correlation to existing physical or chemical data].

5: Changed Value: Data value adjusted during quality control. Best practice strongly recommends that the value before the change be preserved in the data or its accompanying metadata. [Not used].

6: Value Below Detection Limit: The level of the measured phenomenon was less than the limit of detection (LOD) for the method employed to measure it. [Not used].

7: Value in Excess: The level of the measured phenomenon was too large to be quantified by the technique employed to measure it. The accompanying value is the measurement limit for the technique. [Not used].

8: Interpolated Value: This value has been derived by interpolation from other values in the data object. [Not used].

9: Missing Value: The data value is missing. Any accompanying value will be a magic number representing absent data [When sample was not collected the notation 'na' for 'not applicable' was used].

A: Value Phenomenon Uncertain: There is uncertainty in the description of the measured phenomenon associated with the value such as chemical species or biological entity. [Not used]

- Imported original file "BCO_DMO_Moore_GP17ANT_HS-FeL.xlsx" into the BCO-DMO system.
- Marked "NA" as a missing data value (missing data are blank/empty in the final CSV file).
- Created date-time fields in ISO 8601 format.
- Saved the final file as "969502_v1_gp17-ant_hs-fel.csv".

NSF Award Abstract
Iron is an essential element for life and plays an important role in defining how much atmospheric carbon dioxide is taken up into the ocean by phytoplankton. However, iron cycling is closely governed by the chemistry of seawater; nearly all iron in seawater is associated with various unknown organic compounds, called iron-binding ligands, which impact whether and how iron is utilized by organisms and the distribution of iron throughout the ocean. Detail understanding of the cycling of organic iron-binding ligands is necessary to understand iron cycling in the oceans and the connections between iron cycling and atmospheric carbon. The proposed research will be carried out as a part of US GEOTRACES expedition to test the hypothesis that the Southern Ocean is a globally significant source of iron-binding organic ligands, and that different sources of these organic molecules lead to different iron-ligand characteristics. The US GEOTRACES program is a large collaborative effort to sample ocean systems at high resolution for a suite of key trace elements and isotopes. The South Pacific and Southern Ocean regions targeted by the upcoming US GEOTRACES GP17 cruises are important locations of water mass formation and the subsequent transport of carbon and nutrients to the global ocean. Organic ligands produced in these regions thus have important implications for the stabilization, reactivity, and residence time of iron along the path of global water mass circulation and could impact the global oceanic inventory of dissolved iron.

This project will measure the distribution of iron-binding organic ligands, and identify specific organic molecules that comprise these ligands, in field and experimental samples collected on upcoming US GEOTRACES cruises in the South Pacific (GP17-OCE) and Southern Ocean (GP17-ANT). These datasets will be utilized to conduct the first extensive intercalibration of the two most widely used approaches for characterizing iron-binding organic ligands, providing important insight into these datasets and how they can be synthesized to improve understanding of iron cycling in the oceans. All data from this project will be made publicly available. Project activities will provide educational and training opportunities for middle school, high school, undergraduate, and graduate students, and results will be shared with the public through the development of virtual reality modules and via local outreach events.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

NSF Award Abstract:
Iron is an essential nutrient in the global ocean, required by phytoplankton to perform several essential cellular functions. In the Southern Ocean, iron is especially scarce and exerts a primary control on primary production and resulting CO2 uptake by the surface ocean. Most dissolved iron in the Southern Ocean is bound to a complex mixture of organic iron-binding compounds, called ligands. The concentrations and identities of these ligands control iron bioavailability, reactivity, and transport. Exopolysaccharides (EPS) are a particularly important group of ligands that are produced in high concentrations by local microbial communities for a variety of functions, including as cryoprotectants. Despite the known importance of EPS, most iron-binding ligand studies focus on total ligand concentrations or siderophore identification. Siderophores, small iron-binding molecules produced as a microbial iron acquisition strategy, are thought to play a disproportionate role in iron cycling due to their unusually high iron-binding affinities. This project adds essential EPS concentration measurements to a subset of samples from the 2023 U.S. GEOTRACES GP17-ANT expedition to the Amundsen Sea. This project integrates EPS, siderophores and bulk ligand measurements for the first time, thereby offering unprecedented insights into how the composition of organic ligands governs the supply and fate of iron to the Southern Ocean.

The research plan involves a combination of field and experimental approaches. Field samples for electrochemical EPS analysis will be collected on a GP17-ANT transect spanning from the ice edge to open water. Previously funded paired samples of bulk ligand concentrations and siderophore identification will also be taken along the same transect. Results from the three analyses will be combined to assess the relative contributions of EPS and siderophores to the total ligand pool and interpret their impact on iron biogeochemical cycling in the region. Lab experiments will be performed to evaluate whether EPS alters the size fractionation of siderophores and iron in seawater, shuttling both species into the particulate phase. Experimental results will then be used to interpret the integrated field data and improve understanding of the impact of ligands on iron transport and bioavailability in the Amundsen Sea.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.