Table of Contents

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

All reagents were prepared in acid-washed opaque low-density polyethylene (LDPE) bottles, to prevent trace metal contamination or degradation of the reagents by light. The luminol reagent was initially prepared as a concentrate, by mixing 0.796 grams (g) sodium luminol (Sigma), 250 milliliters (mL) Optima grade ammonium hydroxide solution (Fisher), high-purity water (18.2 megohms per centimeter (MΩ cm⁻¹)), and approximately 45 mL Optima grade hydrochloric acid (Fisher) to reach a final volume of 1 liter (L) and a pH of 10.25. To produce the working reagent, the concentrated mixture was diluted in high-purity water by a factor of four and heated at 50 degrees Celsius (°C) for 9 to 12 hours in either an oven or a hot water bath. DTPA was prepared in high-purity water with final concentrations of 50 millimolar (mM) DTPA (Sigma-Aldrich) and 200 mM NaOH (Aldrich, trace metals basis). EDTA was prepared in high-purity water with a final concentration of 50 mM EDTA (Aldrich, trace metals basis) and 100 mM NaOH (Aldrich, trace metals basis). The MOPS (3-(N-Morpholino)propanesulfonic acid) buffer was prepared with a final concentration of 100 mM MOPS (Sigma) and 50 mM NaOH (Aldrich, trace metals basis) in high-purity water, so that the final pH was 7.2. All reagents were used within 1 month of preparation.

Because iron(II) oxidizes rapidly in seawater, standards were prepared individually and analyzed immediately. A primary stock of 10⁻² M Fe(II) was prepared by dissolving ammonium iron(II) sulfate in high-purity water acidified to pH 2 using Optima grade hydrochloric acid (Fisher). The low pH slows oxidation to negligible levels, but the standard was replaced monthly. A working stock was prepared by diluting the primary stock by a factor of 10⁴ in high purity, creating a 10⁻⁶ M solution at pH 6. This solution would oxidize to a significant extent within several hours, so it was always prepared fresh immediately before a calibration curve or an experiment using standards. This working stock was then added to iron(II) free seawater, acquired by filtering seawater through a 0.2 micrometer (μm) filter and storing in an opaque bottle at room temperature for at least 24 hours.

Dissolved iron(II) concentrations were measured using a luminol chemiluminescence-based FeLume flow-injection system (Waterville Analytical), with data collection and integration controlled by Waterville Analytical software. The hardware was similar to that used in a number of previous open-ocean studies. A standard quartz flow cell was used in conjunction with a Hamamatsu HC135 photon counter. The flow rate through the peristaltic pump was 2 milliliters per minute (mL min⁻¹). The photon counter integration time was 200 milliseconds (ms), with two measurements per second for up to 360 seconds. All samples were introduced in acid-washed Teflon bottles.

Fe(II) was determined from raw photon counting data using linear regression, as described in the methods section above.

We estimated the difference between time of sampling and analysis. By combining our data with independent estimates of oxidation rates as a function of the very cold temperatures in the region, we can estimate the Fe(II) concentration at the point of sampling. The estimated Fe(II) concentrations at the time of collection are reported in the dataset.

- Converted original file, "NBP2401_Moffett_Fe(II)R2.xlsx" to CSV format, "NBP2401_Moffett_Fe(II)R2.csv".
- Imported "NBP2401_Moffett_Fe(II)R2.csv" into the BCO-DMO data processing system.
- Renamed fields to comply with BCO-DMO naming conventions.
- Removed the following empty columns: End_Date_UTC, End_Time_UTC, End_Latitude, End_Longitude, Rosette_Position.
- Created date-time column in ISO 8601 format.
- Saved the final file as "982741_v1_amundsen_sea_nbp2401_fe2.csv".

NSF Award Abstract:
The Amundsen Sea is one of the most rapidly changing areas along the Antarctic coast, with some of the fastest rates of glacial melting on the continent. There is great interest in the role of iron in controlling primary production and carbon and nutrient cycling within the Amundsen Sea, and it is probably an important source of iron to offshore waters of the Southern Ocean, which are rich in nutrients but iron limited. The work is part of the US GEOTRACES GP17-ANT cruise to the Amundsen Sea, a multi-investigator study of trace elements and isotope cycling. This work will study how iron is mobilized from sediments at the seafloor of the Amundsen basin. Such mobilization is influenced by two factors directly linked to climate change. Glacial melting increases ice-free areas known as polynyas, which experience large blooms of phytoplankton. Organic matter from these blooms eventually reaches the seafloor, creating low-oxygen conditions that accelerate iron transport into the overlying waters. Moreover, accelerated melting of glaciers will increase the transport of this iron to the surface via the buoyant “meltwater pump” along the glacier/ocean interface. These processes create feedbacks between climate and biological productivity that must be understood to develop models with a useful predictive capability. The broader impacts include partnering with University of Southern California Joint Educational Project and a PolarTREC teacher to create curriculum based on GEOTRACES Antarctic Expedition, which will be disseminated to 15 elementary school in Los Angeles area and develop online data exploration modules to encourage data-based learning in oceanography classes.

This collaborative project will investigate iron redox cycling between sediments and water column of the Amundsen Sea and exchange with the Southern Ocean by determining iron (II) concentrations and redox kinetics in the water column as well as fluxes of iron and other elements from the seafloor to the water column from porewater measurements. A major objective of the US GEOTRACES GP17-ANT cruise is to study the exchange of iron between the Amundsen Sea and the Southern Ocean. The proposed work is essential to identifying the sources of iron and controls on source fluxes as well as internal transformations that will determine its fate. The data product from the project will be integrated with the results of other investigators in a synthesis effort after the cruise, including dissolved and particulate iron and related metals like manganese, as well as important tracers of sediment sources like radium isotopes. Iron(II) oxidizes very slowly in these cold waters, and kinetics will be combined with iron(II) bottom water concentrations and benthic fluxes to evaluate the importance of the slow oxidation kinetics on iron transport away from the benthic boundary layer. Benthic-derived iodine (denoted as “excess iodine”), will be utilized as an important semi-conservative tracer of iron inputs. Iodine is a useful comparative element because both iron and iodine accumulate in sediments under oxidizing conditions and are released under reducing conditions. Iodine is of interest in its own right because Antarctic Shelf waters are a massive source of reactive iodine species to the atmosphere.

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.