Radioactivity in sediments of the Great Lakes: Post-depositional redistribution by deposit-feeding organisms

At two locations in southern Lake Huron (U.S.A.), twelve 35.5-cm diameter cores of fine-grained sediments were taken for comparison of the vertical distributions of²¹⁰Pb and falllout¹³⁷Cs with the distributions of benthic macroinvertebrates, mainly oligochaete worms (Tubificidae) and the amphipod,Pontoporeia affinis. Locations were selected on the basis of²¹⁰Pb distributions measured a year earlier which indicated contrasting depths of mixing of surface sediments. At one location the activity of²¹⁰Pb is uniform down to about 6 cm and 95% of total invertebrates occur within this zone; at the other location the zone of constant activity is only 3 cm deep but 90% of the invertebrates occur within it. Comparison of published tubificid defecation rates with sediment accumulation rates based on²¹⁰Pb shows that oligochaetes alone can account for mixing in one case while the effects of amphipods may be required in the case of shallower mixing. If mixing is represented as a diffusional process, eddy diffusion coefficients are at least 5.8 and 3.3 cm² yr^?1 at respective locations. In comparison with bioturbation, molecular diffusion is of minor importance in the post-depositional mobility of¹³⁷Cs. The necessity for introducing a diffusion coefficient varying continuously with depth is indicated by characteristics of the distribution of¹³⁷Cs. Biological reworking of near-surface sediments is an important process affecting radioactivity and chemical profiles in profundal deposits of this and probably other Great Lakes.