Sediment profiles of 210Pb and 226Ra, radio-isotopes from the 234U decay series with half-lives of respectively 22.3 and 1600 year, were measured in multicores collected from the DISCOL Experimental Area in the Peru Basin during SO242-1, for determining biological mixing rates integrated over a ~100 year time-scale. Total 210Pb and 226Ra activity were determined directly by gamma-spectrometry, whilst total 210Pb was also measured indirectly by alpha-spectrometry via its granddaughter isotope 210Po. Activities of anthropogenic 137Cs proved to be generally below detection level. Radionuclide activities are reported in the data base as mBq g-1 dry sediment.
For alpha spectrometry measurement of 210Po, 0.5 g of freeze-dried and homogenised sediment contained in a plastic reaction tube was spiked with 1 ml of a standard solution of 209Po in 2M HCl, and then leached for 6 h in 10 ml of concentrated HCl heated to 85ºC. After diluting the fluid with 45 ml of demineralised water and adding 5 ml of an aqueous solution of ascorbic acid (40 g L-1), natural 210Po and added 209Po were collected from the fluid by spontaneous electrochemical deposition on silver plates, which were suspended in the fluid for at least 16 h while the fluid was heated to 70ºC. The Po isotopes collected on the silver plates were then measured by alpha-spectrometry using Canberra Passivated Implanted Planar Silicon detectors, with a count time of 1 day sufficient to reduce counting error to < 3%. 210Pb activity was calculated from 210Po, assuming secular equilibrium and correcting for the time elapsed since collection of the samples.
