Award: OCE-1434000

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 Submitted by: William J Jenkins