Table of Contents

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

Sampling details:
Approximately 28 discrete total Th-234 samples were collected throughout the water column, a minimum of 12 depths were chosen in the upper 1000 meters (m).

Full stations: approximately 20 discrete total Th-234 samples were collected throughout the water column, a minimum of 12 depths were chosen in the upper 1000 m.

Demi and Shelf Stations: 13 discrete total Th-234 samples were collected. At shelf stations, depths spanned the entire water column, and at demi stations, the upper 1000 m was sampled.

The shipboard procedures follow Pike et al. (2005) and Clevenger et al., 2021.

Total water-column Thorium-234:
234Th was determined by following the methods of Pike, et al. (2005) and Clevenger, et al. (2021) on 2-liter (L) water column samples, which have been utilized previously for other GEOTRACES efforts (e.g. Owens et al., 2015). An exact 1-milliliter (mL) aliquot of 230Th (50.03 disintegrations per minute per gram (dpm g⁻¹)) was used as the yield monitor and added during initial acidification of the samples. QMAs were used to collect the precipitate from the 2 L process and immediately dried. Once dried, they were mounted onto plastic 25-millimeter (mm) discs, covered with a mylar layer and two layers of aluminum foil, and immediately beta counted at sea. The filters were counted again, 5 to 6 months later to quantify the background radioactivity due to the beta decay of long-lived natural radionuclides that are also precipitated. The mean value of the at-sea counts (decay-corrected to the time of collection) minus the background value for each filter is reported as the 234Th activity (milliBecquerels per kilogram (mBq kg⁻¹)). Activities for 234Th are generally reported in disintegrations per minute per liter (dpm L⁻¹) but have been converted here using a standard density of 1.028 kilogram per liter (kg L⁻¹) and 1 dpm = 16.667 mBq. Data are decay-corrected to the mid-point time between when the first and last bottles were fired for shallow casts, when the messenger was dropped for deep casts, and the time of collection for fish tow samples. Generally, shallow water column (< 1000 meters (m)) samples were collected from the ODF Rosette, and deep samples (>1000 m) were collected from Niskin bottles hung above in-situ pumps. All fish samples (namely intermediate surface and surface samples) were collected directly from the clean fish tow (denoted as either arriving or intermediate fish, depth 3 m). All data have been recovery-corrected using the 230Th /229Th recovery method (see References) to account for any loss of sample material during processing. All samples were analyzed using Risø Laboratory Anti-Coincidence Beta Counters, using a helium/1% butane mixture.

Uranium-238:
U-238 can be calculated via the equation described in Owens et al. (2011):

U-238 (dpm/L) = (Salinity * 0.0786) – 0.315

Salinity was measured on board the ship by the Scripps Ocean Data Facility (ODF).

Particulate Thorium-234:
Particulate material was collected using in situ McLane pumps (5-24 depths per station, see also GP17-OCE data from Ohnemus group). Approximately 24 discrete particulate Th-234 samples were collected throughout the water column for super stations before losing the pumps on December 25, 2022. The discrete particulate samples per station decreased to an average of 6 per station after December 25. All samples were analyzed using Risø Laboratory Anti-Coincidence Beta Counters, using a helium/1% butane mixture. All Th-234 are measured three times for at least 12 hours or to <5% error. All data were decay-corrected back to mid-pumping times.

>51-micrometer (µm) Th-234 (LPT, large particulate Thorium):
Mesh screens were provided by the Ohnemus group. A whole or partial (3/4) mesh screen was rinsed onto a 25 mm silver filter for beta counting. The mean volume pumped through the whole Supor mesh screens was 800 L.

1-51 µm Th-234 (SPT, small particulate Thorium):
Whole QMAs, located below a mesh screen in the filter head housing, were provided by the Ohnemus group and oven-dried upon recovery. A 25 mm subsample was taken from this whole filter for beta counting for Th-234. The mean effective volume for the 25 mm QMA subsample was 32 L (804 L for entire QMA).

Data are decay corrected to the mid-point time between when the first and last bottles were fired for shallow casts and when the messenger was dropped for deep casts samples. Overall method efficiency was determined by minimizing the percent difference between mean 238U and 234Th values for samples from >1000 m to >500 m from the bottom (44.18 %).

- Imported original file "RR2214_dataThorium Total and Particles.xlsx" into the BCO-DMO system.
- Marked "nd" as a missing data value (missing data are empty/blank in the final CSV file).
- Renamed fields to comply with BCO-DMO naming conventions.
- Created date-time columns in ISO 8601 format.
- Converted dates to YYYY-MM-DD format.
- Removed "N" before bottle numbers in the "Rosette_Position" column.
- Saved final file as "985536_v1_gp17-oce_th-234_total_and_particulate.csv".

NSF Award Abstract:
The overarching goal of the international GEOTRACES Program is to “identify processes and quantify fluxes that control the distributions of key trace elements and isotopes in the ocean, and to establish the sensitivity of these distributions to environmental conditions.” These trace elements exist at exceeding low concentrations in the ocean, yet play a key role in the growth and distribution of marine organisms, in particular marine phytoplankton. In addition, many trace elements are important to study as pollutants, such as copper and lead, which at high levels can be harmful to marine life. Thus, studies of the trace metal sources and sinks are needed. To meet this goal, the project is designed to measure how quickly these trace elements are cycled through marine food webs, in particular as they are transported by marine snow, i.e. slowly settling marine particles that carry these trace elements and carbon to the deep sea. The work will be conducted aboard two upcoming US GEOTRACES expeditions to the South Pacific, Southern Ocean, and Amundsen Sea. The project will provide training for two postdoctoral investigators, one focused on sample analysis and one on the modeling.

To carry out this study, this proposal will use the naturally occurring particle reactive radionuclide Thorium-234 (234Th, half-life = 24.1 d) to quantify variability in trace elements and isotopes scavenging, residence times, and particle export from the surface ocean and their attenuation into the deep sea. This project will sample across extreme biogeographical and trace elemental gradients that range from the clear waters of the low dust, low productivity South Pacific (GP17-OCE) to the “greenest” high productivity Amundsen Sea and its polynyas in the Southern Ocean (GP17-ANT). Assessing these spatial and vertical gradients requires 234Th sampling at every station (n=66 casts x 13 depths) and measuring the TEI/234Th ratio on particles (n=43 x 8 depths x 2 size classes). There results of this project will lead to an increased understanding of how the trace elements are modified by removal and regeneration associated with scavenging on to, and remineralization off of, sinking particles.

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.