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:
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.
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).
the Radium quartet
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. Shorter-lived 223Ra (11 d) and 224Ra (3.7 d halflife) trace rapid mixing processes near the coast whereas long-lived 226Ra (1600 y halflife) is often more conservative.
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.