Immobility of Pb-210 in Black-Sea Sediments

We measured Pb-210 in a core from the deep Black Sea which contained a turbidite of low Pb-210 activity surrounded above the below by pelagic sediments of much higher activity to evaluate in situ Pb-210 mobility under the anoxic, moderately sulfidic conditions. By measuring at millimeter scales across the distinct laminae-turbidite contacts and modeling the observed profile, we estimated a lower limit of the K(d) of 10(7) (D(eff) = 5 X 10(-13)). This result increases confidence in sediment chronologies from the Black Sea based on Pb-210 because it implies that profiles will not be altered significantly by diffusive transport of Pb-210. Below the turbidite the Pb-210-inferred accumulation rate was 50 g/m2-a which is lower than laminae-based estimates by a factor of 3-4.