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 depos ited, food-rich particles undergo more intense bioturbation than older, foo d-poor particles, could dramatically alter patterns of organic-matter diage nesis in deep-sea sediments. To explicitly test for age-dependent mixing, a n in-situ particle introduction experiment was conducted on the bathyal San ta Catalina Basin floor Mixtures of radioisotope-tagged particles represent ing 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 in dependent check on mixing coefficients determined from naturally occurring radioisotopes (Th-234(xs) and Pb-210(xs)). All particles experienced rapid (<6 h) transport into the upper 2 cm of the seabed resulting from passive d eposition down burrows or extremely rapid bioturbation. Intense bioturbatio n on 4-d time scales included both biodiffusive and nondiffusive (bioadvect ive and nonlocal) transport. Bioturbation of tracers exhibited time (or "ag e") dependence in two ways: (1) Diffusive mixing intensity for all tracer t ypes decreased with time (4-d D-b = 293 cm(2) y(-1), D-b at 520 d = 2.6 cm( 2) y(-1)), and (2) The nature of bioturbation changed over this period with more efficient bioadvection and nonlocal exchange giving way to slower dif fusive mixing. Both changes are consistent with the age-dependent-mixing hy pothesis. Biodiffusive mixing was not measurably selective, with no signifi cant preference for a single particle type. In contrast, nondiffusive trans port, likely caused by deposit-feeding cirratulid polychaetes, exhibited di stinct particle selectivity, especially over 4-d time scales, with the diat om tracer transported most rapidly to depth. Degradation of the labile orga nic carbon in diatoms most likely led to decreasing selection of diatoms by deposit feeders until diatoms and "old" sediment particles experienced com parable mixing intensities.