Protactinium/thorium ratio

We will show below that the initial Pa/ U = zero assumption holds for a number of corals that have typical low Th concentrations. Initial Pa/ U values for most other carbonates have not been studied in detail. Furthermore, in contrast to thorium, there is no long-lived isotope of protactinium that can be used as an index isotope although some work has employed corrections for initial Pa. Such corrections essentially assume that Th is an isotope of protactinium and assume a bulk earth Th/ U ratio and secular equilibrium between Pa and U. The term for applying this correction is analogous to the initial Th term in Equation (3). 

Thorium-234 has 90 protons and 144 neutrons for a neutron to proton ratio greater than 1 (144 /90). Thorium-234 emits a beta particle. This particle is given off when a neutron breaks into a proton and an electron. The proton stays in the nucleus changing it to 91 protons which is Protactinium -234 while emitting the electron as a beta particle. 

Both protactinium and thorium are produced in the water column and scavenged by particles onto the seafloor. Because protactinium is scavenged less efficiently by marine particles (Anderson et al., 1983), much of the protactinium produced in the Atlantic Ocean is transported to the Southern Ocean by deep waters before it is scavenged onto the seafloor. This produces a low (below production) Pa/ °Th ratio in Atlantic Ocean and a high Pa/ °Th ratio in Atlantic Ocean in the Southern Ocean (Yu et al., 1996). Reconstruction of the Pa/ °Th ratio suggests... 

Protactinium is a radioactive element with over 20 isotopes, of which the one of interest is Pa with a half-life of 32,480 years and a corresponding decay constant of 2.134 x 10 a . It is a daughter of Th in the radioactive decay series. In oceans, its geochemical properties resemble those of thorium since the isotopes are removed from seawater either by adsorption on mineral grains or by incorporation into authigenic minerals. Hence, sediments deposited in oceans may contain not only excess unsupported Th, but also unsupported Pa. If both are removed from seawater equally efficiently, their activity ratios will change with time in a manner controlled by their decay constants. 

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