Mass accumulation rates and fallout radionuclides 210Pb, 137Cs and 241Am inventories determined in radiometrically dated abyssal sediments of the Black Sea

Five abyssal (deep-sea) sediment cores collected during the two international cruises onboard the R/V 'Professor Vodyanitskiy' (RADEUX-1998 and RADEUX-2000) in the framework of the Regional Technical Co-operation Project RER/2/003 'Marine Environmental Assessment in the Black Sea Region' were subjected to detailed radiometric analysis. The sediments were dated using the radionuclides 210Pb, 137Cs and 241Am and the results used to calculate a number of key parameters, e.g. radionuclide inventories, fluxes and sediment accumulation rates. The sediment cores were collected using a MARK II-400 multi-corer (Bowers and Connelly) in both Western and Eastern sub-basins of the Black Sea. The cores were sliced on board with a resolution of 0.2-0.4 cm for the top 5 cm and 1-5 cm downward using an extruder that was specially designed to prevent loss of the uppermost fluff-layer, possible down-smearing and interlayer cross-contamination of the sediment. Dry bulk density (DBD) and cumulative dry mass (CDM) were calculated on a salt-free basis using direct determination of the salt contribution to the dry mass of the sediment. Calculations showed that in the near surface sediments, and particularly in the top fluff-layer, the salt dissolved in pore-water contributed up to 30-60% of the mass of dried sediments. Neglecting this correction could cause an erroneous interpretation of the 210Pb activity profile, resulting in overestimation of both the average sedimentation rate and its recent temporal changes. Sediment samples were analysed for 210Pb, 226Ra, 137Cs and 241Am by direct gamma assay in UHMI after 3 weeks equilibration in hermetically sealed plastic holders, using an EG and G Ortec (Ametek) HPGe GWL series well-type coaxial low background intrinsic germanium detector. Correction was made for the effect of self-absorption of low energy γ-rays within the sample using attenuation parameters determined in APPLEBY, P.G., RICHARDSON, N., NOLAN, P.J., Self-absorption corrections for well-type germanium detectors, Nucl. Inst. Meth. B 71 (1992) 228-233. Chronostratigraphical analysis of the data and sediment age calculation have been carried out by application of CRS and CIC dating models to unsupported 210Pb activity profiles which were calculated by subtracting 226Ra activity from the total 210Pb. The results obtained suggest that the recent deposition history of the deep-sea sediments (last 100-150 years) is recorded in the upper 10 cm, which corresponds to the 226Ra-210Pb equilibrium depth. There were no statistically significant changes in the mass accumulation rates (MAR) over the last century, which ranged between 35-85 g m-2 y-1 at various sites. 137Cs and 241Am activity profiles showed well resolved peaks for both bomb test and Chernobyl fallout, with the two maxima in activity corroborating that these deep-sea sediments are not being affected either by bioturbation or mechanical mixing. For the first time in a post-Chernobyl study 241Am records were detected in the upper sediments, revealing a previously underestimated impact of the nuclear fuel component of the Chernobyl fallout on the Black Sea. The 241Am inventory almost doubled in both the western and eastern sectors of the sea after Chernobyl, from 8-10 Bq m-2, to 16-17 Bq m-2 by the year 2000. Taking into account the ingrowth of 241Am due to the decay of its parent radionuclide 241Pu, a further double increase of the 241Am inventory in the following 60 years can be expected. The increase of the 137Cs inventory in Black Sea sediment after Chernobyl was up to 300-600 Bq m-2. Comparing the results of 210Pb chronology, 137Cs total inventory and the relative position of peaks within the sediment cores in the present and previous studies yields an estimate of 3 to 6 years for the time required by particles carrying radionuclides to sink from the sea surface to the bottom (∼2000 m)