| NSF Org: |
OCE Division Of Ocean Sciences |
| Recipient: |
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| Initial Amendment Date: | August 30, 2017 |
| Latest Amendment Date: | June 20, 2019 |
| Award Number: | 1735445 |
| Award Instrument: | Continuing Grant |
| Program Manager: |
Henrietta Edmonds
hedmonds@nsf.gov (703)292-7427 OCE Division Of Ocean Sciences GEO Directorate for Geosciences |
| Start Date: | September 1, 2017 |
| End Date: | August 31, 2021 (Estimated) |
| Total Intended Award Amount: | $603,461.00 |
| Total Awarded Amount to Date: | $603,461.00 |
| Funds Obligated to Date: |
FY 2018 = $243,339.00 FY 2019 = $134,258.00 |
| History of Investigator: |
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| Recipient Sponsored Research Office: |
266 WOODS HOLE RD WOODS HOLE MA US 02543-1535 (508)289-3542 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
266 Woods Hole Road Woods Hole MA US 02543-0151 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Chemical Oceanography |
| Primary Program Source: |
01001819DB NSF RESEARCH & RELATED ACTIVIT 01001920DB NSF RESEARCH & RELATED ACTIVIT |
| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
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 natural ly 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.
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Project Outcomes Report
Award Number:1735445
Project Title: Quantifying Upper Ocean Export and Remineralization of Bioactive and Particle Reactive Trace Elements along the US GEOTRACES Tahiti to Alaska Transect
PI: Ken Buesseler, Woods Hole Oceanographic Institution
The ocean plays a major role in the cycling of carbon and other elements on Earth, including the transfer of materials from the surface ocean to the deep sea associated with sinking particles, or so-called marine snow. This proposal made key measurements of the amount and spatial distribution of these transport processes as part of a larger international oceanographic research program called GEOTRACES. In this case, samples were collected on a roughly 2-month research expedition between Alaska and Tahiti in 2018. We measured the differences between sites and at different depths using an approach pioneered by this lab, that takes advantage of a naturally occurring form of the element thorium, called thorium-234. Simply put, the lower the levels of thorium-234 we measure, the faster it is being carried to depth in association with the sinking marine snow particles. Since thorium-234 has a known source and loss from naturally occurring radioactive decay from uranium-238, we can use it as a clock to measure these loss rates. By measuring the amount of thorium-234 and other elements on marine snow particles (here we compare thorium-234 to carbon, nitrogen and several trace metals), we can quantify the removal of these other elements. The intellectual merit of the project includes the ability to make comparisons between these data and prior studies in order to have a better view of the cycling of particle associated elements in the ocean. Other GEOTRACES will be able to compare their results with this study to elucidate further specific controls on trace elemental cycling that is at the core of the GEOTRACES program.
This project was the main focus of a PhD thesis of Jennifer Kenyon, a WHOI/MIT graduate student. She and another undergraduate student were mentored as part of the broader impacts of this project. She also produced a graphic novel as part of a science-art collaboration at MIT that is now available that can be used to engage public audiences in the motivation and science behind the project. Results showing wide differences along the sampling transect in the amount of particles sinking out of the upper ocean have been presented at several virtual meetings and conferences, and are being brought to academic publication form as part of this PhD thesis.
Last Modified: 12/16/2021
Modified by: Ken O Buesseler
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