IAEA Conducts Interlab Comparison Exercise for Lead Isotope Dating

Laboratories from fourteen different countries participated in an interlaboratory comparison (ILC) exercise regarding Pb-210 sediment dating techniques as a part of the International Atomic Energy Agency (IAEA) Coordinated Research Project.

Participating laboratories have different levels of expertise in performing radionuclide measurements and lead-210 isotope (Pb-210) techniques. Each of them was given aliquots from various strata of marine sedimentary core collected from the waters near Sao Paulo, Brazil. They were later asked to provide the mass activities of the radionuclides for each stratum of the sedimentary core, using radiometric results to create an age vs. depth assignment.

Challenges in Lead Isotope Dating

The exercise aptly demonstrated the skills of the participating laboratories in the area of conducting radiometric determination processes. However, the dating results were not as universally positive, explained by the largely varied dating experiences from these laboratories. Furthermore, the IAEA exercise allowed the agency to identify difficulties experienced by participating laboratories in conducting Pb-210 dating, also observing limitations in drawing reliable dating information.

An accompanying press release from the University of Seville noted that the use of the lead isotope sediment dating method "is far from a routine technique and requires expert knowledge and multidisciplinary experience."

However, it remains an effective method for accurately dating recent sedimentary cores, usually less than 100 years old, with the proper application of the technique being necessary for a wide variety of environmental studies. Unfortunately, dating Pb-210 is not as routine as other dating methods, with each core requiring a specific approach and enough additional data to support proposed dating results.

Lead 210 Dating

Lead 210 is a natural radionuclide that belongs to the uranium series with a half-life of 22.3 years. Generally, in lead 210 dating, its ratio is compared to the more stable isotope Pb-206. It is mostly used in studies related to the impact of anthropogenic activities on ecological, usually aquatic environments. It is often used to measure how pollutants accumulate over natural sedimentary samples, such as cores.

A 2011 study published in the journal Science of the Total Environment describes Pb-210 dating as a "valuable, widely used means of establishing recent chronologies for sediments and other accumulating natural deposits."

In sediment dating, persistent pollutants such as heavy metals, hydrocarbons, and dioxins are mostly detected - mainly because these materials tend to bind strongly to sediments. Estimating the age of the layer at which these pollutants are deposited also estimates the age of the substances.

In undisturbed locations, lead 210 decays exponentially from the topmost layer. Disturbed samples would also be apparent at the review of its lead isotope profile. Additionally, some cases require an additional computation to allow for corrections from additional lead 210 deposits, which might be produced from decaying Radon deposits on the sediment.

One application involved detecting polycyclic aromatic hydrocarbons (PAH) at different depths from the location near the infamous Exxon Valdez oil spill. Different sediment layers were dated with the lead 210 methods.