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 (<6 h) transport into the upper 2 cm of the seabed resulting from passive deposition down burrows or extremely rapid bioturbation. Intense bioturbation on 4-d time scales included both biodiffusive and nondiffusive (bioadvective and nonlocal) transport. Bioturbation of tracers exhibited time (or "age") dependence in two ways: (1) Diffusive mixing intensity for all tracer types decreased with time (4-d Db = 293 cm2 y-1, Db at 520 d = 2.6 cm2 y-1), and (2) The nature of bioturbation changed over this period with more efficient bioadvection and nonlocal exchange giving way to slower diffusive mixing. Both changes are consistent with the age-dependent-mixing hypothesis. Biodiffusive mixing was not measurably selective, with no significant preference for a single particle type. In contrast, nondiffusive transport, likely caused by deposit-feeding cirratulid polychaetes, exhibited distinct particle selectivity, especially over 4-d time scales, with the diatom tracer transported most rapidly to depth. Degradation of the labile organic carbon in diatoms most likely led to decreasing selection of diatoms by deposit feeders until diatoms and "old" sediment particles experienced comparable mixing intensities.