Atmospherically-derived radionuclides as tracers of sediment mixing and accumulation in near-shore marine and lake sediments: evidence from ⁷Be,²¹⁰Pb, and ^239,240Pu

Abstract

Cosmogenic ⁷Be(t_1/2 = 53.3days) has been used to estimate particle-mixing rates in the upper layers of lacustrine and near-shore marine sediments. Excess ²¹⁰Pb and/or ^239,240Pu have provided limits on rates of sediment accumulation in these environments and indices of the efficiency of the sediments as collectors of reactive nuclides over longer time scale. In sediment cores from Long Island Sound (marine) and Lake Whitney (fresh-water)⁷Be was measurable in the top 2-3 cm. Diffusion-analog particle-mixing coefficients calculated from these data are in the range of 10^-7 cm²/s. For Long Island Sound the coefficients are lower by factors of 3-6 than those estimated from the depth distributions of excess ²³⁴Th at the same stations [14]. For Lake Whitney the calculated mixing coefficient is an upper limit because of the possibility of a sampling artifact. Measurements of total (wet + dry) atmospheric deposition of ⁷Be in New Haven give an average flux of 0.07 dpm/cm² day during March-November, 1977; this is equivalent to a steady-state inventory of 5.4 dpm/cm² in a perfect collector. Sediment cores from Long Island Sound contain about half this ⁷Be inventory, consistent with either a mean residence time for ⁷Be in the water column of about one half-life or with post-depositional loss of ⁷Be from Long Island Sound sediments. The Lake Whitney cores contain about 5 dpm/cm², much nearer the atmospheric delivery. A higher inventory of ⁷Be in fresh-water, as compared to marine, sediments could be due either to a shorter mean residence time for ⁷Be in fresh water or to lateral transport processes in the lake or its catchment. High inventories of excess ²¹⁰Pb and ^239,240Pu in Lake Whitney sediments demonstrate the importance of lateral transport on longer time scales at least.