Sediment accumulation and bio-diffusion mixing rates derived from excess Pb-210 and Cs-137 profiles in sediment cores of Mumbai Harbor Bay

摘要

Bio-diffusion mixing rates (D-b) were estimated from depth profiles of excess Pb-210 and Cs-137 in three sediment cores collected from Mumbai Harbour Bay (MHB) using a steady state vertical advection - diffusion model. The mean of Pb-210 and Cs-137 derived D-b values along the studied area were obtained to be about 23 and 36 cm(2)y(-1) respectively. These derived values were within the range of literature values reported for other equivalent environment internationally. The relatively higher D-b values for Cs-137 profiles demonstrated that particles have diffused more intensely within the surface layer of sediments over 1 year. Conversely, low D-b values for Pb-210 indicate slow mixing rates in the sediment profile which might be resulted from low Pb-210 flux and diffusion of Rn-222 to the seafloor. The significant differences between Pb-210 and Cs-137 derived D-b values among cores indicate that there appeared to be as regional differences in sediment properties and local variability in the intensity of seafloor mixing. Furthermore, D-b values also depend on differences in characteristic time and depth scales of radionuclides in cores, benthic fauna abundances, organic carbon flux to the sediments and primary production in overlying surface waters. Comparison of Pb-210 derived D-b values with those calculated from Cs-137 distributions reveals better agreement for core 2 than core 1 and 3. The agreement may be fortuitous because Cs-137 appears significantly deeper than Pb-210 in all cores. It was also observed that D-b values increases as sediment accumulation rate increases for both radionuclide.