Table of Contents

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

Total water-column Thorium-234
234Th was determined by the widely-adopted 4 L method (now reduced to a 2 L method, see Pike et al. 2005). (Buesseler et al., 2001), which has been utilized previously for other GEOTRACES efforts (e.g. Owens et al., 2015). An exact 1 mL aliquot of 230Th (50.03 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 mm discs, covered with a mylar layer and 2 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 (mBq kg⁻¹). Activities for 234Th are generally reported in dpm L-1, but have been converted here using a standard density of 1.025 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 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.

In order to calculate flux, Th-234 disequilibrium from its parent U-238 must be calculated. 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 (16-24 depths per station, see also PMT data from Lam group). All samples were analyzed using Risø Laboratory Anti-coincidence Beta Counters, using a helium/1% butane mixture. All Th-234 are measured 3 times for at least 12 hours or to <5% error. All data were decay corrected back to mid-pumping times.

>51 µm Th-234 (LPT, large particulate Thorium)
Mesh screens were provided by the Lam 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 406 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 Lam 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 41 L (1042 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, when the messenger was dropped for deep casts, and the time of collection for underway (fish tow) 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 %). Stations that were included to determine method efficiency were chosen to not include samples with potential coastal or hydrothermal influences.

Problem Report: Data were flagged with quality indicators: 1 = Good Value; 2 = Probably Good Value; Data is originally calculated in dpm/L and is converted to mBq/kg using the standard ocean rho = 1.0235 kg/L and 1 dpm = 16.667 mBq. If pumps failed during deployment, the data is either not included or marked with 'nd' for 'no data'.

Quality Flags: Data flags are according to the SeaDataNet scheme (https://www.geotraces.org/geotraces-quality-flag-policy/).

BCO-DMO Processing:
- renamed fields;
- added ISO8601 date/time fields;
- corrected years of 2017 and 2019 to 2018;
- Version history:
- 2021-05-05 (v2; current) - replaced entire dataset with version 2, which includes corrections to event numbers and quality flags.
- 2020-11-18 (v1) - version 1 processed and published.

A 60-day research cruise took place in 2018 along a transect form Alaska to Tahiti at 152° W. A description of the project titled "Collaborative Research: Management and implementation of the US GEOTRACES Pacific Meridional Transect", funded by NSF, is below. Further project information is available on the US GEOTRACES website and on the cruise blog. A detailed cruise report is also available as a PDF.

Description from NSF award abstract:
GEOTRACES is a global effort in the field of Chemical Oceanography in which the United States plays a major role. The goal of the GEOTRACES program is to understand the distributions of many elements and their isotopes in the ocean. Until quite recently, these elements could not be measured at a global scale. Understanding 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. For example, many "trace elements" (elements that are present in very low amounts) are also important for life, and their presence or absence can play a vital role in the population of marine ecosystems. This project will launch the next major U.S. GEOTRACES expedition in the Pacific Ocean between Alaska and Tahiti. The award made here would support all of the major infrastructure for this expedition, including the research vessel, the sampling equipment, and some of the core oceanographic measurements. This project will also support the personnel needed to lead the expedition and collect the samples.

This project would support the essential sampling operations and infrastructure for the U.S. GEOTRACES Pacific Meridional Transect along 152° W to support a large variety of individual science projects on trace element and isotope (TEI) biogeochemistry that will follow. Thus, the major objectives of this management proposal are: (1) plan and coordinate a 60 day research cruise in 2018; (2) obtain representative samples for a wide variety of TEIs using a conventional CTD/rosette, GEOTRACES Trace Element Sampling Systems, and in situ pumps; (3) acquire conventional CTD hydrographic data along with discrete samples for salinity, dissolved oxygen, algal pigments, and dissolved nutrients at micro- and nanomolar levels; (4) ensure that proper QA/QC protocols are followed and reported, as well as fulfilling all GEOTRACES intercalibration protocols; (5) prepare and deliver all hydrographic data to the GEOTRACES Data Assembly Centre (via the US BCO-DMO data center); and (6) coordinate all cruise communications between investigators, including preparation of a hydrographic report/publication. This project would also provide baseline measurements of TEIs in the Clarion-Clipperton fracture zone (~7.5°N-17°N, ~155°W-115°W) where large-scale deep sea mining is planned. Environmental impact assessments are underway in partnership with the mining industry, but the effect of mining activities on TEIs in the water column is one that could be uniquely assessed by the GEOTRACES community. In support of efforts to communicate the science to a wide audience the investigators will recruit an early career freelance science journalist with interests in marine science and oceanography to participate on the cruise and do public outreach, photography and/or videography, and social media from the ship, as well as to submit articles about the research to national media. The project would also support several graduate students.

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. A core approach of GEOTRACES involves coordinated expeditions that measure multiple trace elements and isotopes (TEIs) across key gradients in the ocean, such as changes in productivity, redox/oxygen, boundary scavenging intensity, and hydrothermal activity. However, trace elemental distributions are not enough to meet the GEOTRACES goal to "identify processes and quantify fluxes that control the distribution of TEIs in the ocean and to establish the sensitivity of these changing distributions to environmental conditions." To measure rates of processes requires the use of other measurements, such as naturally occurring radioactive isotopes. This project involves the measurement of two isotopes of the element thorium on a U.S. GEOTRACES expedition in the Pacific Ocean, from Alaska to Tahiti. Thorium has a tendency to become associated with particles in the oceans, and therefore it can be used to measure the sinking of other elements out of the ocean surface. Thorium isotopes with short half-lives can be used to track processes that occur on faster timescales (weeks or seasons), such as those related to biology in the surface waters of the ocean, while isotopes with longer half-lives are useful for studying processes that occur on longer time scales like that of ocean circulation. This project would use measurements of thorium-234 and thorium-228, with half-lives of 24.1 days and 1.9 years, respectively, to estimate sinking rates of thorium and other elements in the upper ocean.

The proposal is broken down into four main activities: 1) Use short-lived Th-234 to estimate particle export and remineralization rates; 2) use ratios of Th-234 to major biogenic phases (e.g., carbon, nitrogen, biogenic silica) and trace elements (e.g. iron, manganese, etc.) to derive flux profiles of other major and trace elements; 3) extend the export and remineralization rates in time/depth using Th-228; 4) examine how changes in #1-3 are related to expected gradients in particle concentration and type, plankton community shifts, net community production, aerosol inputs/dust, physical forcing (e.g. upwelling), hydrothermal plumes, and other features along this GEOTRACES transect. Profiles of total Th-234:U-238 provide quantitative estimates of Th-234 flux vs. depth, which along with parallel size-fractionated particulate profiles of Th-234 and TEIs allow us to empirically calculate the vertical profile of net particle export for any TEI. With high resolution vertical and spatial sampling, the application of Th-234 can be extended below the zone of net removal in the surface ocean to include quantification of particle export and remineralization in the upper several hundred meters. Based upon prior studies, the Th-234 flux model will need to be adjusted for physical transport, especially upwelling across the equator. Selected Th-228 profiles can extend these flux and remineralization estimates even further with depth and over longer time scales. The project will support a Ph.D. student.

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.