Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Supplemental Files
• Related Publications
• Parameters
• Instruments
• Project Information
• Funding

Samples were collected using either a peristaltic pump or a well pump and the groundwater was passed through a 1 or 5 μm prefilter to remove suspended sediment before being filtered at <1 L/min onto Mn-coated acrylic fiber. The fiber was rinsed with Ra-free MilliQ water to remove any salts or sediment, then partially dried.

Samples with low activities (<0.4 dpm) were analyzed for 228Ra via 228Th ingrowth using a delayed coincidence counter (RaDeCC) (Moore, 2008), as described in Charette et al. (2015). Briefly, when 228Ra is extracted onto the Mn fiber, 228Th is extracted in parallel. Using the initial concentration of 228Th and the concentration of 228Th after 1 to 2 years, measured via RaDeCC, along with the decay constants of 228Th and 228Ra, the initial concentration of 228Ra can be calculated. Low activity samples were analyzed for 226Ra via 222Rn emanation (Key et al. 1979), as described in Charette et al. (2015). Fibers were placed in a fiber holder that was then flushed with He for 5 minutes at 250 mL/min, sealed, and left for two weeks before analysis via 222Rn ingrowth and scintillation counting. These two methods were used for low activity samples due to better method sensitivity than gamma counting (Charette et al., 2001), which was used for samples with high activities (>0.4 dpm per sample). For this method, the fibers were ashed (880 °C, 16 h), homogenized, capped with epoxy resin, and left for >3 weeks to obtain secular equilibrium between 226Ra and its daughter radionuclides. The samples were then counted in a well-type gamma spectrometer for 228Ra (via 228Ac at 338 keV) and 226Ra (via 214Pb at 351.9 keV) (Charette et al., 2001).

When not specifically listed, errors are 10%.

BCO-DMO Processing:
- renamed fields to comply with BCO-DMO naming conventions;
- removed commas and apostrophes from data values;
- re-organized original spreadsheet, moving sub-headings into the 'Source' column.

NSF Award Abstract:
The goal of the international GEOTRACES program is to understand the distributions of trace chemical elements and their isotopes in the oceans. Naturally occurring radioactive isotopes of the element radium can be used to measure the rates of important processes in the ocean. In turn, making these rate measurements at the same time as other trace element and isotope data are collected enables a more complete interpretation of these data. The investigators propose to measure the four isotopes of radium -- Ra-223, Ra-224, Ra-226, and Ra-228 -- on a U.S. GEOTRACES expedition from Alaska to Tahiti in 2018. The radium isotope data will be particularly useful in investigating trace element input and removal processes associated with ocean boundaries (rivers, continental shelves, and the ocean bottom) and with mid-ocean ridge hydrothermal vents and the long-range dispersal of their neutrally buoyant plumes. The investigators will also investigate the processes controlling the internal cycling of the longest-lived isotope, Ra-226, compared to the element barium, which has a very similar chemistry to radium.
 
The proposed work would address a number of key questions regarding trace element inputs from ocean boundaries and their potential impact on ocean productivity and biogeochemistry. As iron is an important nutrient for marine phytoplankton, the investigators will quantify the rates of lateral trace element transport from the Gulf of Alaska margin out to and including the offshore High Nutrient Low Chlorophyll region of the subarctic Northeast Pacific Ocean. In the ocean subsurface, they will seek to understand the trace element fluxes associated with high temperature hydrothermal venting, and the rate at which trace elements and isotopes are removed via scavenging along the hydrothermal plume. Lastly, the work will lead to an improved understanding of a marine carbonate sediment dating technique via an investigation of Ra-226 and barium fractionation processes in the upper ocean. The project will involve collaboration between two U.S. institutions and a partner in France who will analyze some of the samples. Two graduate students will participate in the project. Moore will supervise an undergraduate student through the South Carolina Alliance for Minority Participation, and will encourage this student to develop a senior thesis based on their participation in this project.