PARTICLE MIXING PROCESSES OF CHERNOBYL FALLOUT IN DEEP NORWEGIAN SEA SEDIMENTS - EVIDENCE FOR SEASONAL EFFECTS

A 1430 m deep station in the Norwegian Sea (Veering Plateau) was occup ied five times between May 1986 and February 1987 to investigate the s easonal variation in sediment mixing rates. Chernobyl-derived radioces ium, identified by its high proportion of short-lived Cs-134, was used as a tracer for mixing. Most of the nuclide input arrived at the sedi ment within a narrow time span in June/early July during the beginning of the seasonal biogenic sedimentation pulse. Measured Cs-137 profile s in the sediment over time were compared with modelled distributions calculated with a finite difference scheme. The input function of radi ocesium to the sea Boor was evaluated from the increase of the total i nventory with time. Time-invariant mixing coefficients did not provide reasonable fits to either summer or winter distributions. The best fi t was obtained with a rate of mixing proportional to the radiocesium i nput Bur, with an average enhancement factor of 6.6 during the two sum mer months. It appears that the benthic macrofauna are more active dur ing the food supply season and rapidly ingest/bury freshly sedimented materials.