14C and 210Pb in NE atlantic sediments: Evidence of biological reworking in the context of radioactive waste disposal

Abstract

Scavenging by the seabed and by sedimentary particles in the deep ocean may have a significant effect on the removal of artificial radionuclides released to the water column as a result of radioactive waste disposal operations. Biological activity in the upper layers of the sediment column will enhance the rate of removal for those particle-reactive radionuclides with a short half-life relative to the turnover time of the upper mixed layer. For longer-lived radionuclides the rate of sediment accumulation will determine the ultimate rate of removal. The rate of sediment accumulation and extent of biological mixing of deep-sea sediment from three areas of the NE Atlantic Ocean have been investigated using 14C and 210Pb data. Treatment of the radiocarbon with a simple box model provided estimates of sedimentation rate (ω), mixed layer depth (L), mixed layer age (T ML) and the age of material arriving at the surface (T 0), which were broadly similar to previously published values from other ocean basins. Box cores from the Iberia Abyssal Plain, Madeira Abyssal Plain and from the NEA low-level radioactive waste dumpsite yielded sedimentation rates in the range 0.8 to 2.2 cm ky −1 over the upper 16–25 cm. Continuous particle mixing appears to be taking place to a depth of 4 to 6 cm below the present sediment-water interface. Closely spaced vertical sampling of core 161-2 for 210Pb allowed a biodiffusion coefficient (D B) to be calculated (4 × 10 −9 cm 2 s −1), treating bioturbations as a diffusive term and sedimentations as an advective term in a simple mathematical model. In general, values of D B in deep ocean sediments fall within the range 1–10 × 10 −9 cm 2 s −1, two orders of magnitude lower than nearshore values. From a review of published data it is concluded that biological mixing takes place extensively in the deep ocean; it appears to be fairly constant on a basin-wide scale and amenable to incorporation in mathematical models of radionuclide behaviour in the water column.