Table of Contents

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

Seawater depth profiles for iodine analyses was collected onboard through HYDRO-BIOS free flow water samplers on a SBE32 water sampler which were mounted on a CTD rosette (conductivity-temperature-depth profiler). The samples were filtered through 0.2µm Sterivex^TM filters and collected in 125ml or 60ml amber Nalgene bottles once on deck and were frozen immediately afterwards for iodine preservation (Campos, 1997; Farrenkopf and Luther, 2002; Hou et al., 2001). Iodine speciation analyses occurred at Michigan State University within 1 year of sampling, mostly much sooner.

Glass columns for separating iodine species were washed by ultra-pure water produced from the Milli-Q^® water purification system, followed by packing with PYREX glass wool and 1ml of Bio-rad AG ^® 1-X8 anion exchange resin. During the chromatography procedure, the I^- fraction is specifically adsorbed onto the surface of the resin and eluted following the addition of a 2M HNO₃ (Fisher Scientific, TraceMetal^TM Grade)-18% (v/v) Tetramethylammonium hydroxide (TMAH, Fisher Scientific) solution. The eluent containing IO₃^- and DOI fractions were collected separately during the elution steps prior to the I^- collection, followed by an acidification (pH <2) and reduction step which converts iodine to I^- by the addition of concentrated hydrochloric acid (HCl, Fisher Scientific) and sodium metabisulfite (Na₂S₂O₅, VWR Analytical) (Hardisty et al., 2020). The treated eluent was left overnight for reaction and was processed through the same chromatographic procedure to collect the I^- derived from the IO₃^- and DOI fractions. For independent measurement of total dissolved iodine (TI) and as a quality control, a separate split of seawater samples was treated with HCl/ Na₂S₂O₅ to reduce iodine completely to I^- and subsequently processed through the chromatographic method. The eluents from chromatography were collected in Savillex^® PFA Teflon vials and then diluted 20 times in 0.2M HNO₃-0.9% TMAH before instrumental analysis. The iodine concentrations were measured using a Thermo Scientific iCAP Q triple-quadrupole inductively coupled plasma mass spectrometer (ICP-MS-TQ) using Qtegra software version 2.10.3324.131 in both single-quad (SQ) and triple-quad (TQ) modes with O₂ reaction cell gas. 

For quality control, a 200ppb I^- solution, which was gravimetrically diluted from the Inorganic Ventures© 1000±4ug/ml I^- ICP-MS standard solution in 1% tetraethyl ammonium (TEA), was  processed through chromatography alongside the seawater samples to estimate the yield of I^-, DOI and TI fractions. Similarly, for estimating the yield of IO₃^- reduction and column separation, a 200ppb KIO₃ solution dissolved from gravimetrically weighed solid KIO₃ powder (Fisher Scientific Education) was reduced and chromatographically processed. Yields from standards were applied to corrections for unknown samples. For monitoring contamination, ultra-pure water was processed through the columns. Among each set of seawater samples (11 samples) which were processed at the same time, one of them was randomly selected as a replicate. 

The eluents from the chromatography were collected in Savillex PFA Teflon vials and then diluted 20 times (0.2M HNO₃-0.9% TMAH) before instrumental analysis. The iodine concentrations were measured using a Thermo Scientific iCAP Q triple-quadrupole inductively coupled plasma mass spectrometer (ICP-MS-TQ) using Qtegra software version 2.10.3324.131 in both single-quad (SQ) and triple-quad (TQ) mode with O₂ reaction cell gas. The samples were introduced into the ICP-MS via a Teledyne Cetac ASX-520 autosampler. We used 5ppb internal standard solution diluted from 1001±3ug/ml indium (In) in 2% HNO₃, 999±5ug/ml Rhodium (Rh) in 15% HCl, and 1000±4ug/ml Cesium 0.1% HNO₃ (Cs) elemental standards from Inorganic Ventures© to calibrate the intensity of iodine and to correct for instrumental drift during the analytical run. The internal standard was either spiked into each sample or was introduced into the ICP-MS after being mixed with sample within an inline mixing chamber. Detection limits for samples were determined as 10 nmol/kg iodine.

Data were processed using Excel.

- Loaded CSV file "EMB276_iodine_depth_profiles-utf-8.csv" with header row 1; treated "", "nd", and "N/A" as missing values
- Applied find/replace on station_number column to correct a unicode-corrupted value, replacing "Far" with "Faro"
- Extracted non-numeric values (e.g., "<10") from iodate_concentration and DOI_concentration into new flag columns iodate_concentration_flag and DOI_concentration_flag, setting original columns to None where values were non-numeric
- Combined ISO_DateTime_collected_UTC (parsed as %m/%d/%y) and UTC_Time (parsed as %H:%M) into a single ISO 8601 datetime column ISO_DateTime_collected_UTC formatted as %Y-%m-%dT%H:%MZ
- Deleted now-redundant UTC_Time column
- Renamed Total I_concentration to Total_I_concentration to remove space from parameter and depth_m to depth to remove units from parameter to align with BCO-DMO guidance
- Output written to 1000767_v1_emb276_iodine_depth_profiles.csv

NSF Award Abstract:
The trace element manganese (Mn) is distributed widely throughout the global ocean where it cycles among three dominant oxidation states. Manganese in the higher oxidation states is highly reactive and thereby influences the cycling of nearly all other elemental cycles, including those of oxygen and nitrogen. The intermediate Mn species has only recently become recognized as an abundant component of the Mn pool, presenting now a previously unrecognized factor that may control the chemistry of the ocean. The Baltic Sea contains high Mn concentrations and preliminary investigations have pointed to the presence of an operationally defined "reactive" form of Mn but the composition and consequence of this Mn pool are unknown. This research will explore the cycling of Mn within the Baltic Sea enabled by an established collaboration with the Leibniz Institute for Baltic Sea Research in Warnemunde, Germany. By coupling field measurements and targeted shipboard incubations, this study will shed light on the processes controlling the Mn cycle and its link to the oxygen, iodine, and nitrogen cycles. This project will educate several undergraduate and graduate students and promote scientific exchange between research groups within the United States and Germany. Further, outreach efforts associated with this research will continue an existing collaboration between the PIs and the Boston Green Academy in South Boston to introduce high school students to chemical oceanography, and in particular biogeochemistry.

Manganese (Mn) is intricately linked to nearly all elemental cycles, and yet we know little about the processes governing its redox cycling within natural systems. Over the past decade a number of key scientific discoveries have provided greater insight into the diversity of processes and mechanisms involved in Mn redox cycling and introduced Mn(III) ligand complexes as important components of the dissolved Mn pool. The Baltic Sea is one of the most well studied stratified marine systems and reactive Mn has been implicated as a key factor in the formation and maintenance of suboxic zones. Thus, the goal of this research is to explore the cycling and elemental coupling of Mn within stratified basins of the Baltic Sea. The PIs predict that reactive Mn, as Mn(III) ligand complexes and Mn oxide particles, is a primary control on the redox landscape of stratified marine waters, particularly at redox boundaries and within the suboxic zone. The PIs propose fieldwork in a local permanently stratified brackish pond to refine experimental procedures followed by two cruises to suboxic basins in the Baltic Sea enabled by an established collaboration with the Leibniz Institute for Baltic Sea Research in Warnemunde. Field measurements will be obtained using a combination of in situ sensors and ship/lab-based instrumentation at several Baltic Sea sites to define the distribution of Mn species and the rates of Mn redox transformations spanning the redoxcline along with a suite of chemical information. Further, a matrix of shipboard incubations will be conducted to constrain the underlying (a)biotic processes responsible for the observed Mn profiles. Specifically, across oxygen and Mn gradients spanning the redoxcline, the PIs will interrogate the link between the Mn cycle and iodine and nitrogen species, which will ultimately help constrain current gaps in the mass balance of these elements in Baltic Sea models.

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.