Table of Contents

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

Seawater sampling: Procedures used during GEOTRACES GP17-ANT were identical to those described for previous U.S. GEOTRACES cruises (Kadko, 2017; Kadko et al., 2020; He et al., 2025). Briefly, a centrifugal pump on deck was used to draw 500-750 liters (L) of seawater from predetermined depths via a 1.5-inch PVC hose to fill large plastic tanks on deck. Be-7 was then extracted by pumping this seawater through 200 grams of iron-impregnated acrylic fibers at a flowrate of ~10 L per minute (Lai et al., 1988; Krishnaswami et al., 1972; Lee et al., 1991). A temperature/depth recorder was attached close to the end of the hose to confirm an accurate sampling depth.

Aerosol sampling: Bulk aerosol samples were collected following procedures described for previous U.S. GEOTRACES cruises (Shelley et al., 2015; Buck et al., 2019; Marsay et al., 2018, 2022). Briefly, for each deployment, one of five Tisch Environmental high-volume (~1 cubic meter air per minute) aerosol samplers was loaded with 12 replicate 47-millimeter (mm) diameter Whatman 41 filters on open-face filter holders (Advantec MFS) installed on a PVC adaptor plate (Shelley et al., 2015). Filters were acid-washed before use to lower trace element blanks (Morton et al., 2013). Samplers were installed above the bridge of RVIB Nathaniel B. Palmer and operation was controlled by wind speed and direction through a Campbell Scientific CR800 data-logger interfaced with a sonic anemometer set up alongside the samplers. In this way, sampling was restricted to periods when relative wind speed and direction were >0.5 meters per second and from within ±60 degrees of the bow of the ship for at least five continuous minutes.

Analytical procedures: Samples were counted for Be-7 using low-background germanium gamma detectors at Florida International University (FIU). For aerosols, three replicate filters from each deployment were stacked in a petri dish for counting. For seawater, the fibers were dried and then ashed. The iron hydroxide powder remaining after ashing was pressed into a 5.8-centimeter (cm) diameter pellet and the pellet thickness measured, then placed in a petri dish for counting. Be-7 has a readily identifiable peak at 478 kiloelectron volts (keV), and the spectra were evaluated using Maestro (Ortec) or Genie 2000 Canberra software. The detectors were calibrated for each geometry by adding a commercially prepared mixed isotope solution of known gamma activities (Eckert & Ziegler Mixed Nuclide Solution 7500) to three W41 filters (for aerosol samples) and pellets of various thickness (for seawater samples) to derive calibration curves using peaks associated with the following isotopes: Sn-113 at 392 keV, Sr-85 at 514 keV, Cs-137 at 662 keV. Counting efficiencies of the four detectors at FIU ranged from 0.035304 to 0.05686 for Be-7 on aerosol filters, and from 0.07873 to 0.083779 for pellets derived from seawater samples.

Reported Be-7 activities were corrected to the time of sampling, using the following calculation:

[Be-7, dpm m^-3] = {cpm • ct • λ• exp(λ•d)}/{CE • PE • FE • V • [1-exp(-λ•ct)]}
where:

cpm = background corrected counts per minute for the region of interest

ct = count time (days)

λ = radioactive decay constant of Be-7 (0.013 d^-1)

d = time from sample collection to start of count (days)

CE = counting efficiency

PE = photon emission probability (0.104 for Be-7)

FE = fiber extraction efficiency (92±3%)

V = volume (m³)

Activity counts were converted from dpm to Bq for submission to BCO-CMO. The error (counts) associated with each measurement is the statistical counting error (σ) and the uncertainty in the blank, √(σ² + σ_b²), multiplied by {ct • λ • exp(λ•d)}/{CE • PE • FE • V • [1-exp(-λ•ct)]}. The uncertainty of the extraction efficiency (3%) and the detector efficiency (2%) was in all cases smaller than the statistical counting error.

Quality flags: Quality flags were applied following the GEOTRACES policy (https://www.geotraces.org/geotraces-quality-flag-policy/), which recommends the SeaDataNet Scheme:

0 = no quality control;
1 = good value;
2 = probably good value;
3 = probably bad value;
4 = bad value;
5 = changed value;
6 = value below detection;
7 = value in excess;
8 = interpolated value;
9 = missing value;
A = value phenomenon uncertain.

- Imported original file "NBP2401_Be7 data_BCODMO.xlsx" (sheet 1) into the BCO-DMO processing system.
- Treated "NaN" as missing values (missing values are empty/blank in the final CSV file).
- Renamed fields to comply with BCO-DMO naming conventions.
- Combined "Start_Date_UTC" (format %m-%d-%y) and "Start_Time_UTC" (%H:%M:%S) into a new ISO 8601 datetime column "Start_ISO_DateTime_UTC" (format %Y-%m-%dT%H:%M:%SZ, UTC).
- Combined "End_Date_UTC" and "End_Time_UTC" into a new ISO 8601 datetime column "End_ISO_DateTime_UTC" (format %Y-%m-%dT%H:%M:%SZ, UTC).
- Reformatted "Start_Date_UTC" from %m-%d-%y to %Y-%m-%d; reformatted "End_Date_UTC" similarly.
- Saved the final file as "1003205_v1_gp17-ant_aerosol_and_seawater_be7.csv".

NSF Award Abstract:
Trace Elements are defined by their low concentrations in the open ocean and their addition and removal rates are key uncertainties in our understanding of ocean chemistry. In spite of their scarcity, trace elements can play an outsized role in determining where living things thrive in the open ocean. Near the coasts, these elements may be found in abundance because of features like rivers which bring source material from the continents to the ocean. Far from land, however, trace elements are scarce. The most significant source of these elements to the open ocean may come from dust and other particles falling onto the surface water either directly, or within rain droplets. An improved understanding of this process will further knowledge of element cycling in the ocean, including the carbon cycle. This project will sample the atmosphere of the South Pacific Ocean and the Pacific sector of the Southern Ocean around Antarctica. Input from the atmosphere to these waters is not well known and the research results will provide much needed information to the scientific community. This project will support an early career researcher as well as a graduate student.

The deposition and subsequent dissolution of aerosols in surface waters remains a critical research area in the oceanographic community. Much of the ocean remains poorly sampled and improvements in flux estimates, chemical characterization, and fractional solubility estimates are necessary to improve the understanding of this important trace element source. This project will include a cruise in the South Pacific Ocean and one in the Amundsen Sea sector of the Southern Ocean, both as part of the GEOTRACES program. The cruise tracks will cover regions which lie downwind of dust producing regions of Australia and where atmospheric deposition could play a role in dictating distributions of trace elements in the water column. The flux of aerosols across the air-sea interface is a key research area within GEOTRACES and the broader oceanographic community. Flux from the atmosphere to the ocean is highly episodic making the capture of this important term difficult in the open ocean. Flux rates will be determined by the use of the cosmogenic isotope 7Be, providing a rate estimate for time scales longer than the cruise period. Aerosol trace element fractional solubility remains poorly constrained and is one of the reasons that biogeochemical models show poor agreement for marine trace element distributions. Aerosol dissolution will be tested in several solution of varying chemistry to provide a range of solubility estimates which are representative of conditions in precipitation, surface seawater, and in more aggressive environments which provide a high-end solubility estimate. Calculated terms like residence time are also poorly constrained due to this uncertainty. This project will address these questions by capturing bulk and size-fractionated aerosol samples as well as samples of precipitation. The study will advance understanding of dust and soluble aerosol trace element flux from the atmosphere to the ocean and provide contextual data for complimentary studies associated with these GEOTRACES section cruises.

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.