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 gamma-spectrometry measurement of 210Pb and 226Ra, 2-5 ml of freeze-dried and homogenized sediment was weighed and packed in a 6-cm diameter plastic Petri dish which then was sealed with electrical tape and packed in a sealed plastic envelope. Calibration and monitor standards and samples were prepared in the same geometry. Sealed sediment samples were left for a minimum of four weeks to establish radioactive equilibrium between 222Rn and 226Ra. Gamma activity was measured with a Canberra Broad Energy Range High Purity Germanium Detector (BEGe), using the 46.5 keV line for 210Pb and 295.2, 351.9 and 609.3 keV lines for 226Ra. The detector, connected to a computer via a Digital Spectrum Analyser (DSA-1000), counted the radionuclide activities with Genie 2000 gamma spectroscopy software. Samples were counted for 2–5 days to obtain good counting statistics or relative standard deviation (RSD). A blank (empty petri dish) was counted during 23 days. The detector was externally calibrated with a Geological Certified Reference Material IAEA/RGU-1, with reference date of 01-01-1988. A monitor standard IAEA-300 provided quality control. Excess 210Pb activities were calculated by subtracting 226Ra activity averaged for the 295.2, 351.9 and 609.3 keV lines from the measured total 210Pb activity.
