Tritium in Tree Rings

Supervisors: William J. Jenkins and Scott C. Doney, Marine Chemistry and Geochemistry Department, WHOI

Rationale: Tritium, the heaviest and only radioactive isotope of hydrogen was produced by atmospheric nuclear weapons testing in the 1950s and 1960s, and has spread into the hydrosphere and oceans in a characteristic pattern. Since it travels as part of the water molecule, it represents an ideal tracer of water movement. Further, it can be used with its decay product to perform tritium-helium dating. However, its value as a tool for studying the environment has been limited by how well we know the pathways it has travelled to the oceans. While there was an extensive land-based monitoring network for tritium in the environment during and after the bomb tests, there were very few measurements in the right places and times: that is, in the oceanic environment just after the biggest bomb tests, where we really needed to know the inputs.

Methodology: Measuring tritium in the cellulose of tree rings from ocean islands promises to provide us with an annually averaged record of tritium concentrations in rainfall over the ocean. This would allow us to measure the deposition of this isotope in oceanic regions and dramatically improve our knowledge of how tritium was delivered to the oceans. This improvement will enhance our ability to use tritium as a test of large-scale ocean models.

Wider Implications: Global scale models used for understanding the nature of biogeochemical and physical processes in the ocean, and for predicting how the oceans participate in and respond to climate change, are often "calibrated" or evaluated by comparing model simulations of tracer distributions with field observations. Transient (man-made) tracers such as tritium are especially useful for this activity because they are changing on the time scales relevant to anthropogenic global change.

Suitable For: students that have a good practical sense in the laboratory, are willing to go "in the field" (e.g., travelling to oceanic islands for sampling trees), and comfortable with computing and mathematics.

Training: Aside from learning about local and global scale hydrology plus oceanography, the student will learn chemical laboratory techniques and mass spectrometry, plus acquiring basic skills in computer analysis and modelling.

Additional Relevant Links: