| NSF Org: |
OCE Division Of Ocean Sciences |
| Recipient: |
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| Initial Amendment Date: | July 18, 2014 |
| Latest Amendment Date: | October 6, 2017 |
| Award Number: | 1434000 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Henrietta Edmonds
hedmonds@nsf.gov (703)292-7427 OCE Division Of Ocean Sciences GEO Directorate for Geosciences |
| Start Date: | September 1, 2014 |
| End Date: | August 31, 2019 (Estimated) |
| Total Intended Award Amount: | $759,390.00 |
| Total Awarded Amount to Date: | $759,390.00 |
| Funds Obligated to Date: |
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| 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: |
183 Oyster Pond Road Woods Hole MA US 02543-1501 |
| 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: |
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| Program Reference Code(s): | |
| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
In this study, researchers from the Woods Hole Oceanographic Institution will conduct the first systematic and coherent global-scale analysis of a considerable dataset of ocean tritium/helium-3 observations, and integrate findings into a global Earth System Model. Nuclear weapons tests in the late 1950s and early 1960s released into the atmosphere approximately 400 kilograms of tritium, a radioactive isotope of hydrogen. A large fraction of the tritium entered the Northern Hemisphere surface ocean via precipitation. The subsequent movement of tritium and helium-3 (produced when tritium undergoes radioactive decay) in the oceans has served as a powerful tracer of ocean circulation and mixing. For the past four decades, field measurements of the tritium/helium-3 ratio in worldwide oceanic waters have been collected, amassing more than 20,000 samples in the North Atlantic alone. Results from this study will provide a better understanding of oxygen and nutrient dynamics in the oceans, and will add new capabilities to the Community Earth System Model (CESM) ocean model, which is widely used for ocean physical, biogeochemical, and ecological research as well as climate change research. The researchers will also incorporate the results of the study into a graduate-level course.
Tritium released into the oceans has been a powerful tracer of ocean subduction, circulation, and mixing in the thermocline and newly formed deep and intermediate waters. Tritium also acts as a chemically and biologically inert analogue for nutrients and a unique tracer allowing the tracking and quantification of nutrient upwelling pathways and biological new production. This study will provide a synthesis of ocean tritium/helium-3 observations for the North Atlantic and North Pacific, bringing together more than four decades of field observations into a single, publicly available dataset. Oceanographic data analysis techniques combined with diagnostic and inverse modeling then will be used to estimate physical and biogeochemical rates. Tritium/helium-3 simulations from an advanced 3-D ocean circulation model will be used to resolve issues involving regional/temporal data gaps, physical transport mechanisms, and biogeochemical assumptions used in the diagnostic modeling. Overall results of the study will be a characterization of the spatial and temporal evolution of ocean tritium, a better understanding of oxygen utilization rates, and combined surface tritium data with subsurface tritium:nutrient distributions.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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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.
Chemical tracers provide powerful tools for tracking the patterns, rates, and variability of ocean circulation, turbulent mixing, and ocean-atmosphere exchange. This project focused on synthesizing and modeling ocean data for an important pair of ocean tracers, tritium and its radioactive decay product helium-3, a relatively rare isotope of the noble gas helium.
The project compiled into a publicly available database the wealth of historical global ocean tritium and helium-3 observations, along with related noble gas measurements. The database brings together data from about a dozen major ocean laboratories in the U.S. and internationally and contains roughly 58,000 data points for both tritium and helium-3, spanning field observations from the early 1950s to the present. The project also developed a new module for simulating the ocean distributions of tritium, helium-3, and noble gases in the Community Earth System Model, providing a valuable tool for the ocean and climate science communities.
Tritium is a radioactive isotope of hydrogen found in water molecules in seawater, and most of the tritium in the modern ocean was produced in the 1950s and 1960s by nuclear weapons tests in the atmosphere. Bomb-tritium deposition to the ocean occurred primarily in the northern hemisphere, the site of most of the atmospheric tests. In the intervening decades, tritium has been transported into the subsurface ocean by ocean currents and mixing, and the new database provides a window for documenting quantitatively the penetration of tritium into the ocean interior over time.
Tritium undergoes radioactive decay with a half-life of about 12 years to helium-3, a rare isotope of helium. Helium-3 is lost from the surface ocean to the atmosphere by gas exchange, and the combination of tritium and helium-3 provides a unique clock or age estimate for when a water parcel in the upper ocean was last at ocean surface. The ocean helium-3 observations show a distinct pattern with production from tritium decay in the ocean interior and loss at the surface. The helium-3 data also highlight deep-ocean helium-3 sources associated with subsurface volcanism and hydrothermal circulation. As a result, the tritium – helium-3 tracer pair provides an invaluable constraint for testing ocean circulation models.
Helium is the lightest of the stable noble gases, which also include neon, argon, krypton and the heaviest being xenon. Because they exhibit a range of physical-chemical properties and are biologically inert, the suite of noble gases provides good constraints on air-sea gas exchange and ocean interior mixing.
The new global ocean tracer database created as part of this project contributes to larger U.S. and international efforts such as the GO-SHIP Project, which is designed to observe the changing state of ocean physics and chemistry in response to climate variability and change.
Last Modified: 09/20/2019
Modified by: William J Jenkins
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