Natural Radionuclides: Tracers for transport and reaction rates 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 new decay charts of ²⁴¹Am, ²³⁸U, ²³⁵U and ²³²Th.

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

²³⁴Th. The depletion of ²³⁴Th (24 days half-life) with respect to its parent 238U in the surface ocean is used to determine the export rate of particles from the euphotic zone to the deep ocean. This method provides an independent way to determine the flux of carbon and other elements from the surface water to the deep ocean. In an analogous way it is also used for the study of resuspension rates near the deep-sea floor. The analytical techniques have recently been simplified. The wide interest in this isotope justified a special workshop on the future Applications of ²³⁴Thorium in the Environment (FATE) which gave birth to a special issue of Marine Chemistry (spring 2006). For further discussions visit cafe thorium of Ken Buesseler at WHOI.

²³⁰Th and ²³¹Pa. We know the production rate of ²³⁰Th (Half-life 75400 y) and ²³¹Pa (half-life 32500y) from Uranium dissolved in the ocean. Their activities in sediment traps and in the sediment cores are used to calibrate the collection efficiency of sediment traps and to determine to what extent sediments are redistributed before being buried at the ocean floor. Their distribution in the sediment helps us to determine the age of sediment layers and to find out whether the rain rate of particles to the seafloor has changed with the changes from glacial to interglacial climate.

²¹⁰Po and ²¹⁰Pb. We use Polonium-210 and Lead-210 as tracers for particle transport, too. They enable us to follow the transport of particles through the water column over a longer timescale (several months) due to their longer half life. The special chemistry of Po makes it a tracer for bacterial production, for the transfer of organic carbon to higher trophic levels, and perhaps for dimethyl sulfide (a product of algal decomposition).

²³²Th/²²⁸Ra. ²²⁸Ra is produced everywhere in marine sediments and diffuses into the bottom water. Especially water masses that have flowed over continental shelves are spiked with the nuclide and can be followed over several half-lives (5.8 y). 228Ra can therefore serve as tracer for inputs of e.g. iron from continental shelves to the open ocean.

²²⁷Ac. ²²⁷Ac is produced in marine sediments from the decay of ²³¹Pa. In contrast to ²²⁸Ra, the strongest sources are in deep-sea sediments. We attempt to use this new tracer as a tool to quantify the upwelling rates of deep water masses (including NADW) that enter the Southern Ocean from the north.

Follow links to a wide selection and reviews of Methods.