Radioisotope dating

It is possible to identify changes in sedimentation rates, as well as the timing of key historical events, in coastal lake or estuary catchments through measurement of radio-active isotopes. Of these, analysis of lead-210 (²¹⁰Pb) is the only method capable of providing high-resolution ages for sediments over the last 150 to 200 years. It is thus an ideal technique for those investigating post-colonial anthropogenic impacts in Australia.

Lead-210, which is part of the uranium-238 (²³⁸U) decay series, is absorbed onto sediment particles deposited in lakes and estuaries. It consists of both supported and unsupported components. The former refers to that component which is in equilibrium with all members of the decay chain which precede it. It is derived from the in situ decay of radium-226 (226Ra) that has been directly washed into the system as part of eroded material (Figure 1). Unsupported 210Pb is derived from radon-222 (²²²Rn) which diffuses as gas through the soil interstitial pore space into the atmosphere, where it rapidly decays to 210Pb. The ²¹⁰Pb then attaches to aerosol particles and settles out of the atmosphere as dry fallout or is washed out in rainfall events. This ‘unsupported’ ²¹⁰Pb can fall directly onto the lake sediments or be washed in at a later time from elsewhere in the catchment (see Figure 1). In either event, once deposited and incorporated in the sediment, the activity of unsupported 210Pb will be solely a function of the amount present initially and its half-life (half-life = 22.6 year). Thus, a ²¹⁰Pb-chronology can be determined for a sediment core by measuring the down-core activities of unsupported ²¹⁰Pb and comparing these with that measured for the modern sediments at the top of the core. The activity of supported 210Pb can be determined indirectly by measuring the activity of ²²⁶Ra using either alpha or gamma spectrometry. Unsupported 210Pb cannot be measured directly and so is inferred from the activity of total ²¹⁰Pb minus the activity of supported 210Pb. The activity of total 210Pb can be determined by either measuring ²¹⁰Pb directly using gamma spectrometry or measuring (using alpha spectrometry) the progeny ²¹⁰Po with which it is assumed to be in secular equilibrium.

Figure 1. Pathways by which ²¹⁰Pb reaches aquatic sediments (after Oldfield and Appleby, 1984) [1].

1. Oldfield, F., Appleby, P.G., 1984. Empirical testing of 210Pb-dating models for lake sediments. In: Hayworth EY and Lund JWG (Eds). Lake Sediments and Environmental History. Leicester University Press, 93-124.

Authors

Kate Harle, CSIRO
Henk Heijnis, ANSTO