Dating of a sediment column in the Palermo bay (Sicily, Italy)

In the framework of a more general study on foraminifers in sediments from the Palermo Bay (Sicily, Italy), it was necessary to obtain a time calibration of the different sections of the sampled sediment columns, both to estimate the pollution of the sediments and to study the migration of foraminifers within the column. The method of the unsupported 210PB, i.e. whose activity is not derived from the radioactive decay of its 226Ra parent, has been used. Since the activity of the supported 210PB can be determined from the activity of 226Ra, the unsupported 210PB activity can be calculated as the difference between 210PB total and 226Ra activities. The unsupported 210PB decays with its own half-life time, equal to 22.3 years, so being very suitable to the aim of dating the sediment column we are interested in. The 226Ra activity has been determined through the 214Pb, after secular equilibrium has been reached. High performance gamma spectrometry with low-energy detector has been used to determine 210PB (46.5 keV) and 214Pb (351.9 keV) activities. The column age has been estimated being about 50 years. The average sedimentation rate as equal to about 0.6 cm y-1. This result has been checked measuring the 137Cs activity in the same sediment columns. Using the above sedimentation rate, the maximum in cesium activity vs. year can be estimated to occur around the year 1986, obviously due the Chernobyl event. (Author)