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Abstract
To better understand temporal and particle size-dependent bioturbation processes, we conducted a study of sediment mixing in Massachusetts Bay using a newly developed deliberate tracer technique. Sediments from a 32-m, fine-grained site were collected and the 38–62 (“silt”) and 63–125 (“sand”) μm fractions isolated. These particle-size fractions were labeled with two different noble metals (Au: silt & Ag: sand) using a thermal diffusion technique. Mixtures of the tracers were spread onto the seafloor in April and July 1992 by divers and were tube-cored (3 replicates) ˜ 80 d later in each case. Vertical profiles of the tracers were measured at μg/g (Ag) and ng/g (Au) levels by instrumental neutron activation analysis. During the spring experiment, Au (silt) was mixed to depths \u3e 15 cm and displayed multiple subsurface maxima, whereas Ag (sand) was confined to the upper 5 cm of the bed and showed a near monotonic decrease in concentration with depth. In the fall experiment, the tracers displayed more congruent down-core profiles consisting of near-surface maxima and several subsurface peaks. Two nonlocal bioturbation modes are suggested by the tracer data: reverse conveyor-belt transport and head-down deposit feeding or excavation. A particle caching strategy by an unidentified macrofaunal species is postulated to explain the subsurface peaks, but remains conjectural without better species-level natural history information regarding solid-phase bioturbation
