Project: Using Ocean Tritum-3He Observations to Quantify Thermocline Circulation and Biogeochemistry

Preliminary project description:
The overall goal of this project is to leverage the information on ocean circulation and mixing embedded in the wealth of historical tritium-3He tracer data to constrain better ocean oxygen and nutrient dynamics. A synthesis of ocean tritium-3He observations will be conducted 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-3He 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. Specific science objectives are: 1) characterize the spatial and temporal evolution of ocean tritium, excess 3He, and tritium-3He age; 2) diagnose in-situ oxygen utilization rates by integrating tritium-3He and oxygen data with a recently developed hybrid inverse approach; and 3) combine surface 3He data with subsurface 3He:nutrient distributions to quantify the magnitude of physical processes that resupply nutrients to the upper ocean and regulate biological new production.

NSF award 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.