Radionuclide tracing of water masses and processes in the water column and sediment in the Algerian Basin

Abstract

Caesium-137 and 239,240PU were analysed in the water column along the Algerian coast. The 137Cs activity concentration in surface water increased from the west to the east from 1.6 to 3.3 mBq L −1, documenting a presence of Modified Atlantic Water (MAW) in the region. Higher concentrations observed in deep waters may be due to an intrusion of Levantine Intermediate Water (LIW), which has been carrying higher levels of 137Cs from Chernobyl accident. The 239,240Pu sub-surface concentration peaked at about 250 m water depth as a result of biogeochemical processes in the water column. The observed 239,240Pu/ 137Cs activity ratio at the surface (0.003) was significantly lower than that in global fallout (0.04). This decrease exceeds that expected from radioactive decay of 137Cs, and confirms that Pu due to its adsorption on sinking particles is more effectively removed from surface layers than is 137Cs. An increase of the 239,240Pu/ 137Cs activity ratio with depth suggests that 239,240Pu, similarly as 137Cs, should be also transported by advection to maintain the observed ratios in deep waters. An intrusion of LIW may enhance therefore both the 137Cs and 239,240Pu concentrations in deep waters. The average 238Pu/ 239+240Pu activity ratio in seawater was 0.03 ± 0.02, confirming a global fallout origin of Pu in the Algerian Basin. Caesium-137 and 239,240Pu inventories in the water column were estimated to be from 2.7 ± 0.5 kBq m −2 to 3.8 ± 0.7 kBq m −2, and from 13.8 ± 2.6 Bq m −2 to 41 ± 7 Bq m −2, respectively. The 137Cs massic activities in surface sediment were almost constant, the average activity was 9.0 ± 0.8 Bq kg −1. Sedimentation rates obtained using the 210Pb method were from 0.1 to 0.7 cm y −1, and resulting penetration depths of 137Cs in the sediment cores were from 15 to over 40 cm. The 137Cs peaks found in the sediment cores were associated with the Chernobyl accident (1986) and global fallout (1964). The 137Cs inventories in the sediment were increasing from the west (180 Bq m −2) to the east (350 Bq m −2).