Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Curation Notes
  • Problem Description
• Parameters
• Instruments
• Deployments
• Project Information
• Program Information
• Funding

Water-column sensor data were collected during downcast deployments of a trace-metal clean CTD rosette ("GTC rosette") system. The GTC rosette casts were performed using the U.S. GEOTRACES trace-element rosette sampling system with a SeaBird Electronics (SBE) custom-built frame and carousel and 24 x 12 liter (L) GoFlo sampler bottles. The system included an SBE CTD unit with pressure sensor, temperature sensors, conductivity sensors and membrane oxygen sensor, as well as a transmissometer, chlorophyll-a fluorometer, altimeter, and turbidity sensor. The package was deployed on a sheathed conducting cable with a Vectran strength member using a Dynacon winch. Further details are provided in the GP17-ANT cruise report (Sedwick et al., 2024) and in the ODF cruise report (Scripps Institution of Oceanography Ocean Data Facility team, 2024).

Instrumentation Details:
The GTC rosette casts were performed with a package consisting of the U.S. GEOTRACES SBE trace-element rosette frame and carousel, and 24 x 12 L GoFlo sampler bottles. Underwater electronic components primarily consisted of an SBE-9plus housing unit with a Digiquartz pressure sensor, dual SeaBird SBE-3 temperature sensors and SBE-4C conductivity sensors, an SBE-43 membrane dissolved oxygen sensor, and a WetLabs C-Star transmissometer and ECO-FL-RTD chlorophyll-a fluorometer.

Sensors were calibrated during summer 2023, immediately prior to the GP17-ANT cruise, and the transmissometer was calibrated before the start and after the end of science operations. The GTC rosette also included a Benthos altimeter, which ceased functioning during the latter part of the cruise and was replaced with a Valeport altimeter borrowed from the shipboard USAP equipment, and a Seapoint turbidity sensor. The CTD data acquisition system consisted of an SBE-11plus deck unit and a networked generic PC workstation running Windows 10. SBE SeaSave7 v.7.26.7.121 software was used for data acquisition and to close bottles on the rosette. Further details are provided in the GP17-ANT cruise report (Sedwick et al., 2024) and in the ODF cruise report (Scripps Institution of Oceanography Ocean Data Facility team, 2024).

Note: Cruise reports are attached as Supplemental Files and available at https://usgeotraces.ldeo.columbia.edu/content/gp17-ant.

Shipboard CTD data processing was performed after deployment using SIO/ODF CTD processing software ctdcal v. 0.1.4b. The CTD acquisition data were copied onto a Macintosh OS X system, then processed, and a downcast pressure series of sensor measurements averaged over 2-decibar pressure bins was created. The pressure-series dataset was submitted for CTD data distribution after corrections were applied. The CTD data were examined at the completion of each deployment for clean corrected sensor response and any calibration shifts. As bottle salinity and oxygen results became available (see GEOTRACES GP17-ANT Bottle Data for Nutrients and Hydrographic Parameters), they were used to refine conductivity and oxygen sensor calibrations. Temperature, salinity, and dissolved oxygen comparisons were made between downcasts and upcasts, as well as between groups of adjacent deployments. Further details of the CTD data processing are provided in the ODF cruise report (Scripps Institution of Oceanography Ocean Data Facility team, 2024).

This section documents curation actions performed prior to publication review with the submitter, and additional information relevant to understanding and reusing this dataset. It distinguishes changes made to the submitted (meta)data from unresolved issues and/or enhancements that improve future reuse and interoperability.

CURATION ACTIONS PERFORMED ON DATA

- Imported all 35 CTD cast files (.csv) into the BCO-DMO data processing system.
- Treated row 13 as header, skipped row 14 (units row); captured metadata key value lines (rows above header) into new columns EXPOCODE, SECT_ID, STNNBR, CASTNO, DATE, TIME, LATITUDE, LONGITUDE, INSTRUMENT_ID, and cast_depth via regex extraction.
- Concatenated all 35 files into a single table, mapping identical column names across tables and adding a file_name column captured from each source file name.
- Filtered out END_DATA sentinel rows (removed rows where CTDPRS equals 'END_DATA').
- Converted combined DATE (format %Y%m%d) and TIME (format %H%M) columns into a new ISO_DateTime_UTC string column in ISO 8601 UTC format (%Y-%m-%dT%H:%MZ).
- Reordered columns.
- Saved the final file as "1009865_v1_gp17-ant_gtc_ctd.csv".

CURATION ACTIONS PERFORMED ON METADATA

- BCO-DMO's standard metadata entry and text formatting steps were performed. See: https://www.bco-dmo.org/how-to/standard-curation-edits

ISSUES POTENTIALLY IMPACTING REUSE

- N/A

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.

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.