Models of depth‐dependent bioturbation at MANOP Site H in the eastern equatorial Pacific

Abstract

The vertical distribution of excess 226Ra sorbed to manganese‐rich sediments from a box core taken at MANOP site H (6°34′N, 92°48′W) in the eastern equatorial Pacific can be modeled only if the rate of bioturbation decreases rapidly downward from the seafloor within the upper few centimeters of sediment. Independent bioturbation rates estimated by balancing dissolved and particulate diagenetic Mn fluxes are compatible with the values obtained from the radium model. Such results are consistent with 210Pb and 230Th measurements reported here and with MANOP lander and porewater observations that rapid organic carbon degradation in these sediments consumes oxygen within a few centimeters of the sediment‐seawater interface. The modeled decrease in bioturbation (expressed as a biodiffusion coefficient) is consistent with values of more than 25 cm2/kyr at the surface falling to less than 0.4 cm2/kyr at a depth of 14 cm.