Radiometric studies of sediment cores from Gulf of Mannar

Abstract

Gulf of Mannar sediments contain 19.2.to 61.4% CaCO_3, 1.9 to 5.2 % organic matter, 7.2 to 18.8 ppm U and 3.0 to 16.7 ppm ²³² Th,the last two on a calcium carbonate free basis. (²³⁴⁾U/⁽²³⁸⁾ U activity ratio ranges from 1.09 to 1.19 (mean 1.14 ± 0.02), indicating the authigenic nature of U in the sediments. High U concentrations prevail throughout the cores (length ~ 150cm) analysed in this study. The super(14)C - based deposition rates are 60.1 and 114 cm (10³ y) ^-1) for the shallow (water depth 1000 m) cores whereas they range from 2.2 to 11.4 cm (10 to the power of 3 y) super(-1) for the cores from the deeper (water depth> 2000m)regions. The authigenic deposition rates of U in the cores from shallow water regions are 130 and 314µg cm ^-2 (10 ³y)^-1 during the past < 3000 y which compare well with such rates derived from anoxic coastal regions of the Atlantic and Pacific oceans.