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We investigate the spatiotemporal evolution of radiocarbon (C-14) in the ocean over the 21st century under different scenarios for anthropogenic CO2 emissions and atmospheric CO2 and radiocarbon changes using a 3-D ocean carbon cycle model. Strong decreases in atmospheric C-14 in the high-emission scenario result in strong outgassing of C-14 over 2050-2100, causing C-14 spatial gradients in the surface ocean and vertical gradients between the surface and intermediate waters to reverse sign. Surface C-14 in the subtropical gyres is lower than C-14 in Pacific Deep Water and Southern Ocean surface water in 2100. In the low-emission scenario, ocean C-14 remains slightly higher than in 1950 and relatively constant over 2050-2100. Over the next 20years we find decadal changes in C-14 of -30 to +5 in the upper 2km of the ocean, which should be detectable with continued hydrographic surveys. Our simulations can help in planning future observations, and they provide a baseline for investigating natural or anthropogenic changes in ocean circulation using ocean C-14 observations and models.

Plain Language Summary The carbon content and acidity of the ocean are increasing as the ocean has absorbed roughly a third of the CO2 emitted by the burning of fossil fuels. Human activities are also changing the isotopic composition of carbon in the atmosphere and ocean. In the midtwentieth century, nuclear weapons testing produced a large amount of C-14 (radiocarbon), the heavy radioactive isotope of carbon that is extensively used for archeological dating. At the same time, the combustion of fossil fuels reduces the amount of C-14 relative to the more common isotope C-12, since fossil fuels have lost all their radiocarbon through radioactive decay. As time passes since the bomb testing, fossil fuel emissions are becoming an increasingly dominant influence on the carbon isotope composition of the atmosphere and ocean. Here we use a computer model to simulate how this composition is likely to evolve over the coming century in response to continued fossil fuel burning. Our results show that by the end of the century surface ocean waters will be more depleted in radiocarbon than deeper waters, a complete reversal of the pattern that prevailed before humans started changing the environment.