Natural Radionuclides: Tracers for transport and reaction rates in the ocean

Particles fluxes in the ocean may vary enormously over time and space. Particle deposition and erosion often happen during events – during plankton blooms, river floods, due to tides, or during “benthic storms”. Ship-based observations can only cover a tiny part of these processes.

We use indirect evidence from elements that respond to the presence of particles- either by removal from the water, or by addition from particles. Especially helpful are naturally occurring radioactive elements, which provide time information on particle-water exchange, due to their known half-lives. This property helps us to determine integrated particle fluxes over a variety of time scales, from days to thousands of years. So even if the ship observation misses a particle flux event, it can be traced by the distribution of natural radionuclides in the ocean.

Three radioactive isotopes of the elements Uranium (238U and 235U) and Thorium (232Th) have survived the age of our solar system and are still ubiquitous in the ocean and in sediments. They are each at the top of a decay chain of radionuclides with a wide spectrum of half-lives. A fourth chain (241Am) has decayed in nature but can be produced artificially and provides a suite of useful tracers. A full overview of energies associated with these decay chains is given in Walter Geibert's decay charts of 241Am, 238U, 235U and 232Th.

Below is a chart that displays the elements of the natural U/Th decay series, with their relative particle reactivity, their isotopes, and their respective half lives.

We select nuclides of appropriate half-lives and chemistries as tools to determine transport and reaction rates in the ocean. Some examples: