In situ tracer (50–125 μm plastic particles) experiments conducted using the DSV Alvin over a two year period in the 1240 m deep Santa Catalina Basin (eastern Pacific) have yielded near-surface (0–1.5 cm) horizontal bioturbation rates of order 1–10 cm2yr–1. Vertical biodiffusivities obtained from the same and similar particulate tracers at the same site are approximately an order of magnitude less. Mixing of near-surface, coarse sediment in Santa Catalina Basin is anisotropic. Deeper within the sediment horizontal bioturbation is not diffusive on a two-year time scale, but would appear to be a form of mixing termed "nonlocal symmetric" by Boudreau and Imboden (1987), whereby particles are moved appreciable distances advectively. The finding that bioturbation in near-surface sediments is anisotropic in Santa Catalina Basin and the likelihood that this phenomenon is widespread in deep-ocean sediments calls into question the present parameterization of the effect sediment mixing has on various early diagenetic processes. Specifically, the contribution of bioturbation to organic carbon remineralization rates via microbial intermediaries may be underestimated. Bioturbation rates represent more than simply vertical mass transfer coefficients and should be incorporated into models of early diagenesis accordingly.