Table of Contents

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

U.S. GEOTRACES extended its meridional transect, initiated on the 2018 GP15 Alaska-Tahiti expedition, south to the Antarctic ice edge and then east to Chile with GP17-OCE (December 2022 - January 2023). Because of the potentially important trace elements and isotopes (TEIs) inputs and transformations occurring in Antarctic waters and shelves, GP17 also had a second leg, GP17-ANT (November 29, 2023 - January 30, 2024) into coastal and shelf waters of Antarctica’s Amundsen Sea. Further information is available on the US GEOTRACES website.

NSF Project Title: Collaborative Research: Management and Implementation of US GEOTRACES GP17 Section: Amundsen Sea Sector of the Antarctic Continental Margin (GP17-ANT)

NSF Award Abstract:
This project will support the management and implementation of a 60-day research cruise to the Amundsen Sea sector of the Antarctic continental margin to collect samples for measurements of a broad suite of trace elements and isotopes ('TEIs'), as part of the U.S. GEOTRACES program. GEOTRACES is a global effort in the field of Chemical Oceanography, the goal of which is to understand the distributions of trace elements and their isotopes in the ocean. Determining the distributions of these elements and isotopes will increase the understanding of processes that shape their distributions and also the processes that depend on these elements. Key TEIs include essential micronutrients such as iron and zinc; 'tracers' such as aluminum, manganese, and isotopes of nitrogen, thorium and neodymium that can be used to investigate modern and ancient ocean processes; and elements such as lead that are indicative of human activities. In the Southern Ocean, the Antarctic continental margins are important as sources of micronutrient trace elements such as iron, which is required to support biological production and carbon export over the Antarctic shelf and in offshore waters of the Antarctic Circumpolar Current. Moreover, these regions are experiencing rapid environmental changes that are expected to impact oceanic circulation and biogeochemical cycles, for which TEIs provide crucial data needed to test and refine numerical models of the Earth system. The Amundsen Sea sector holds particular interest because of the pronounced, decadal-scale increases in the melting rates of glacial ice shelves that border the region, driven by intrusions of warm Circumpolar Deep Water onto the continental shelf. This melting has potentially major impacts on global sea level, on the formation of Antarctic Bottom Water in the Ross Sea, and on the regional ecosystem.

The cruise will comprise essential sampling operations (collection and shipboard processing) and ancillary measurements (hydrography, nutrients, algal pigments) in support of multiple, individual science projects, following the successful model of previous U.S. GEOTRACES cruises in the Atlantic, Pacific and Arctic ocean basins. The cruise will sample the ocean region between 100°W and 135°W, with stations ranging from 67°S in the Antarctic Circumpolar Current southward to the Amundsen Sea continental shelf, including stations adjacent to several rapidly melting ice shelves and in highly-productive shelf polynyas. Water column samples will be collected using conventional and trace-metal clean CTD-rosette systems, in-situ high-volume pumps, and a towed fish sampler or small boat, using established methods. Sampling time will also be provided for collection of sea ice, floating glacial ice, and seafloor sediments. To facilitate coordination with a complementary open-ocean cruise and ensure access to the study region to document the impact of biological processes, the cruise is planned for late austral summer (late January-late March). Beyond the disciplinary contributions, the proposed research will contribute knowledge concerning the cryosphere and its impacts on global sea level and ocean circulation, regional ecosystems and biological processes, ocean-atmosphere interactions, and past and future environmental change. The project will contribute to STEM education and outreach through the participation of an NSF-funded PolarTREC education professional, and a K-12 STEM program for students from underserved and underrepresented schools run by Rutgers University education specialists. To foster public engagement, the investigators will partner with the UCSC Science Communication Program to engage freelance science journalists to profile research in this spectacular and harsh Antarctic environment.

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.

GEOTRACES is a SCOR sponsored program; and funding for program infrastructure development is provided by the U.S. National Science Foundation.

GEOTRACES gained momentum following a special symposium, S02: Biogeochemical cycling of trace elements and isotopes in the ocean and applications to constrain contemporary marine processes (GEOSECS II), at a 2003 Goldschmidt meeting convened in Japan. The GEOSECS II acronym referred to the Geochemical Ocean Section Studies To determine full water column distributions of selected trace elements and isotopes, including their concentration, chemical speciation, and physical form, along a sufficient number of sections in each ocean basin to establish the principal relationships between these distributions and with more traditional hydrographic parameters;

* To evaluate the sources, sinks, and internal cycling of these species and thereby characterize more completely the physical, chemical and biological processes regulating their distributions, and the sensitivity of these processes to global change; and

* To understand the processes that control the concentrations of geochemical species used for proxies of the past environment, both in the water column and in the substrates that reflect the water column.

GEOTRACES will be global in scope, consisting of ocean sections complemented by regional process studies. Sections and process studies will combine fieldwork, laboratory experiments and modelling. Beyond realizing the scientific objectives identified above, a natural outcome of this work will be to build a community of marine scientists who understand the processes regulating trace element cycles sufficiently well to exploit this knowledge reliably in future interdisciplinary studies.

Expand "Projects" below for information about and data resulting from individual US GEOTRACES research projects.