Bioturbation and sedimentation in three basins of the California Borderland system, Santa Catalina, San Nicolas and San Clemente Basins, using naturally occurring radioisotopes (Th-234, Pb-210, C-14 and Th-230) and particle introduction experiments. C-14 sedimentation rates (12.1-13.7 cm/ky) were similar in the three basins, whereas particle traps that were deployed during periods of high flux (e.g., blooms, winter storms) yielded sedimentation rates 1.5-2x higher than sediment profile-derived rates. Particle-trap radioisotope fluxes generally were not significantly different from their sediment counterparts. Spatial variability in fluxes of the longer-lived radioisotopes (Pb-210 and Th-230) implied lateral transport of isotope-rich seawater from offshore and more intense boundary scavenging in San Clemente Basin.

Mechanisms and rates of bioturbation varied both spatially and temporally. On 100-day time scales, described by Th-234 profiles, biodiffusive mixing and non-local transport of surface material occurred simultaneously. Th-234 Db values were spatially heterogeneous, ranging from 0.8 cm^2/y (Clemente) to 18 cm^2/y (Nicolas) with variable diffusive penetration depths (1.5-7 cm). On 100-year time scales, described by Pb-210 profiles, the mixing regime was almost completely controlled by spatially homogeneous biodiffusion within the upper 7 cm (Pb-210-Db = 0.2-2.8 cm^2/y). The previously reported negative correlation between Db and radiotracer half-life (age-dependent mixing) was apparent when Th-234- and Pb-210-Db values were compared over the 7 cm mixed layer, but comparisons limited to the Th-234-diffusive front were indicative of depth-dependent rather than age-dependent mixing.

An in situ particle introduction experiment was conducted in Santa Catalina Basin to examine age-dependent mixing. Mixtures of radioisotope-tagged particles of varying food quality were placed in multiple plots on the seafloor and sampled over several time scales (0-594 days). 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 cm^2/y, Db at 520 d = 2.6 cm^2/y), and (2) The nature of bioturbation changed over this period with more efficient bioadvection and non-local exchange giving way to slower diffusive mixing. Both changes are consistent with age-dependent-mixing. Non-diffusive mixing exhibited distinct particle selectivity which decreased over time due to the decay of labile organic carbon associated with food-rich particles. In contrast, Db and time scale were negatively correlated, but biodiffusive mixing was not measurably selective. A nonlinear regression incorporating data from several continental-slope and deep-sea sites verified the global persistence of the Db-time scale relationship.