Dating of marine sediments by using unsupported 210Pb method

In the framework of a more general study on marine species in sediments, 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, particularly, 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 high performance gamma spectrometry with a low-energy detector of 214Pb gamma emission (351.9 keV), after secular equilibrium has been reached. The activity of 210Pb was determined by its 46.5 keV gamma emission. As experimental test, the age of a column taken in the Palermo bay (Sicily) has been estimated being about 50 years and the average sedimentation rate as equal to about 0.6 cm y-1. Goodness of dating has been checked measuring the 137Cs activity in the same sediment column. 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.