Bioturbation and particle transport in Carolina slope sediments: A radiochemical approach

In situ tracer experiments investigated short-term sediment mixing processes at two Carolina continental margin sites (water depth = 850 m) characterized by different organic C fluxes, Th-234 mixing coefficients (D-b) and benthic assemblages. Phytoplankton, slope sediment, and sand-sized glass beads tagged with Pb-210, Sn-113, and Th-228, respectively, were placed via submersible at the sediment-water interface at both field sites (Site I off Cape Fear, and Site m off Cape Hatteras). Experimental plots were sampled at 0, 1.5 days, and 90 days after tracer emplacement to examine short-term, vertical transport. Both sites are initially dominated by nonlocal mixing. Transport to the bottom of the surface mixed layer at both sites occurs more rapidly than Th-234-based D-b values predict; after 1.5 days, tagged particles were observed 5 cm below the sediment-water interface at Site I and 12 cm below at Site III. Impulse tracer profiles after 90 days at Site m exhibit primarily diffusive distributions, most likely due to a large number of random, nonlocal mixing events. The D-b values determined from 90-day particle tagging experiments are comparable to those obtained from naturally occurring Th-234 profiles (similar to 100-day time scales) from nearby locations. The agreement between impulse tracer mixing coefficients and steady-state natural tracer mixing coefficients suggests that the diffusive analogue for bioturbation on monthly time scales is a realistic and useful approach. Tracer profiles from both sites exhibit some degree of particle selective mixing, but the preferential transport of the more labile carbon containing particles only occurred 30% of the time. Consequently, variations in the extent to which age-dependent mixing occurs in marine sediments may depend on factors such as faunal assemblage and organic carbon flux.