Radiochemical dating for otoliths

Learn about elemental assays, radioactive decay and isotopic ratios in regard to otoliths.

On this page

• Elemental assays
• Radioactive decay
• Isotopic ratios
• Related links

Elemental assays

Age determination studies based on elemental or isotopic concentrations in the otolith are motivated by the ongoing requirement for accurate age-structured information to support fish studies. The objectivity of elemental assays is appealing in light of the subjectivity that can confound or invalidate the interpretation of annuli in otoliths or other structures.

As a result, chemically based age determination studies focus on the yearly scale. Virtually all have been used to validate more traditional, less expensive methods of age determination.

Radioactive decay

We have a long history of using radiochemical dating of calcified structures like corals and molluscs. The same underlying concepts apply to fish otoliths, and are based on well-established physical principles governing radioactive decay.

Radioisotopes are incorporated into fish otoliths in exactly the same way as are stable isotopes of any given element. Once incorporated into the otolith, the radioisotopes decay into radioactive daughter products, which are themselves retained within the acellular crystalline structure.

We know the fixed half-lives of the parent and daughter isotopes. We can use the ratio between them as an index of elapsed time since incorporation of the parent isotope into the otolith.

Secular equilibrium is approached, as the rate of loss (through decay) of the daughter comes to equal the rate of loss of the parent. Radioisotope ratios in whole otoliths can be interpreted if some rather problematic assumptions can be met.

However, the extracted otolith core reflects elapsed time since core formation, which is very similar to the age of the fish. Since interpretation of the otolith core also avoids the problematic assumptions, it’s widely acknowledged to provide more reliable results than would the whole otolith.

Isotopic ratios

We can age problematic fish species using 2 isotopic pairs that have proved popular: 

1. ²¹⁰Pb: ²²⁶Ra
2. ²²⁸Th: ²²⁸Ra

When based on analysis of the extracted core, the radiochemical approach appears to be both objective and accurate in the estimation of fish age. Nevertheless, the isotopic concentrations requiring measurement are exceedingly low, resulting in assay precisions which are often less than optimal.

Current discriminatory power is on the order of:

• 5 years for ²¹⁰Pb: ²²⁶Ra (0- to 40-year age range)
• 1 to 2 years for ²²⁸Th: ²²⁸Ra (0- to 8-year age range)

Therefore, this approach is best suited to species where the candidate age interpretations are widely divergent, such as in sebastes or hoplostethus.

Related links

• Otolith publications