The effect of bioturbation and adsorption gradients on solid and dissolved radium profiles in sediments from the eastern equatorial Pacific

Remobilization of Mn resulting from the diagenetic oxidation of organic carbon causes steep adsorption gradients within hemipelagic sediments of the eastern equatorial Pacific. Transport of Ra along these gradients is capable of maintaining marked ²²⁶Ra-²³⁰Th disequilibrium in the surface sediments. Pore water and solid phase ²²⁶Ra profiles measured in cores from two sites in this area are modeled by adjusting the Ra adsorption coefficient as a function of the depth-varying Mn content. At MANOP site M, the high organic carbon accumulation rate causes a sharp redox gradient within the upper 10 cm of sediment. Biological mixing is constant and rapid (-500 cm²/kyr) in this upper layer, below which the Mn content, the Ra adsorption coefficient and the ²²⁶Ra ²³⁰Th ratio are all sharply reduced. At MANOP site H, the organic carbon accumulation rate is less and therefore the redox gradient is expanded over a greater depth scale. In the surface sediment, biological mixing is slower, decreases with depth and gradients in the adsorption coefficients and ²²⁶Ra ²³⁰Th ratio are less marked than at site M.