A particle introduction experiment in Santa Catalina Basin sediments: Testing the age-dependent mixing hypothesis

The occurrence of "age-dependent mixing," a process by which recently deposited, food-rich particles undergo more intense bioturbation than older, food-poor particles, could dramatically alter patterns of organic-matter diagenesis in deep-sea sediments. To explicitly test for age-dependent mixing, an in-situ particle introduction experiment was conducted on the bathyal Santa Catalina Basin floor. Mixtures of radioisotope-tagged particles representing a food quality gradient were dispersed in small amounts on the seafloor and sampled over periods of 0 to 594 days. Introduced particle types were all similar in size and included fresh diatoms ("young" particles), surface sediments ("intermediate-age" particles), and particles from 30-cm deep in the sediment column ("old" particles). This approach permitted evaluation of particle mixing intensity for several particle "ages" and provided an independent check on mixing coefficients determined from naturally occurring radioisotopes (234Thxs and 210Pbxs). All particles experienced rapid (